Extreme CFD workshop - User contributions [en]

Ecfd:ecfd 9th edition

2026-02-06T11:23:51Z

Benard: /* T8 - FSI-3D without deformation strategy for internal flows - P. Benez (Safran), H. Lam (LEGI) & P. Benard (CORIA) */

{{DISPLAYTITLE: ECFD workshop, 9th edition, 2026}}

== Description ==

* Event from '''19th of January to 30th of January 2026'''
* Location: [https://www.sport-normandie.fr/le-centre/le-site-de-houlgate Centre Sportif de Normandie], Houlgate, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* Participants from academics, HPC center/experts and industry are welcome
* The number of participants is limited to 80.


* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Voivenel (CORIA).

[[File:Logo_ECFD9.png|center|frameless|900px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_9th_edition]]



== News ==

* 22/09/2025: First announcement of the '''9th Extreme CFD Workshop & Hackathon''' !
* 15/11/2025: Deadline to submit your project

== Thematics / Mini-workshops ==

To be announced...

== Projects ==

=== Hackathon GENCI - P. Begou (LEGI), V. Moureau (CORIA) ===
This ECFD9 GENCI Hackathon was a rich event, involving 3 differents CFD codes (AVBP, SONICS and YALES2) using various paradigms (C++/cuda/hip, Fortran/OpenMP/OpenACC) with several SDKs (AMD, Cray/HPE, Nvidia, Gnu) on a large range of GPU architectures (Nvidia A100, GH100, AMD instinct Mi210, Mi250, Mi300). This two-week event benefited from a high level support from three HPC mentors, two on-site from AMD (J. Noudohouenou and A. Tsetoglou).

==== H3 - Hackathon SONICS - A. de Brauer (ONERA), B. Michel (ONERA), B. Berthoul (ONERA) & G. Staffelbach (ONERA) ====
CPU code generation for multispecies simulations – Code generation for multispecies simulations is currently being developed in the SoNICS code. The work carried out at ECFD9 focused on the vectorization of the generated code by code transformation : unrolling the species loops, rewriting if statements, and inverting do/while loops (arising from Newton type algorithms) used in the computation of thermodynamic quantities. The loop-unrolling and if statement rewriting have been profiled and show a speed-up of 2x for the vectorized generated code when computing the HLLC flux, compared with the hand-written implementation. The switch of do/while loops was prototyped on a test code and will be integrated into SoNICS. Code generation on GPU has been tested and validated, but a thorough performance profiling of the GPU version is still required.

Porting reactive multi-species terms to GPU – In 2025 multi-species reactive capabilities were introduced in SoNICS and tested on the Preccinsta case on CPU. Recently the multi-species components were ported to the GPU, so this activity concentrated on porting the reactive source terms. Tests on a 0D reactor show identical results on GPU and CPU. Work has also resumed on porting SoNICS to AMD GPU on the ADASTRA system from CINES/GENCI, where the hipGraphs implementation (AMD’s counterpart of cudaGraph) exhibited some issues. Our participation in ECFD9 allowed us to contact the AMD hipGraphs development team, opening the way to a collaboration. With their council, we updated the code to use rocm7.1.1 providing the first successful non reactive results on AMD GPU. Further work on reactive flows is ongoing.

Improving GPU residual calculations – Recent investigations show that SoNICS’s residual calculation on GPUs was about 100× slower than other GPU operations. The bottleneck was traced to the combination of cudaGraph and thrust::reduce, which prevented parallel execution. Replacing this with a hand-written hierarchical reduction kernel that works efficiently within the cudaGraph restores good scalability; the residual computation is now negligible compared with the other operations, as is the case on CPU. Additional timers were added to the cudaGraph kernels to quantify each operator’s cost relative to the CPU.

=== Numerics & User Interface - M. Bernard (LEGI), G. Lartigue (CORIA) & S. Mendez (IMAG) ===

==== N1 - Improving ICS robustness and accuracy - M. Bernard (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI) & T. Berthelon (LEGI) ====
Bad quality meshes generally lead to larger numerical errors when solving partial differential equations.
This project focused on improving the accuracy and robustness of the incompressible Navier-Stokes solver (ICS).
We investigated the sources of discrepancy introduced at each step of the algorithm, with particular attention to the consequences of the coexistence of two discrete velocity representations: (i) the convective flux <math>\vec{u}\cdot\vec{n}\,dS</math> and (ii) the transported nodal velocity field <math>u^n</math>.
Although these quantities are equivalent at the continuous level, this equivalence no longer holds in the discrete setting.
In particular, only the convective velocity strictly satisfies the divergence-free constraint after solving the Poisson problem for the pressure field.
During this two-week workshop, we developed a new correction strategy for the nodal velocity field in order to enforce consistency with the convective velocity and improve the overall behavior of the solver.

==== N3 - Shock & discontinuity treatment for Lattice-Boltzmann solvers - I. Tsetoglou (M2P2), W. Bessem (M2P2), H. Merley (M2P2) & S. Zhao (M2P2) ====
Lattice—Boltzmann methods (LBM) have traditionally been applied to weakly compressible flows; however, recent developments have extended their applicability to fully compressible regimes. In such flow configurations, shock waves and contact discontinuities naturally arise. To properly capture these features in a discretized framework, artificial diffusion mechanisms are commonly introduced to smooth discontinuities over a limited number of grid points.
In this project, the hybrid LBM solver ProLB was employed. In this framework, the mass and momentum equations are solved using an LBM formulation, while the total energy equation is discretized using a finite-volume (FV) approach with consistent spatial and temporal discretization. The primary objective of the work was to develop and implement an artificial diffusion strategy suitable for hybrid LBM/FV solvers.
Shock waves were detected with a Jameson-type pressure-based sensor, while contact discontinuities were identified with a temperature-based sensor. The pressure-based sensor was scaled to obtain a kinematic viscosity contribution, which was incorporated into the LBM collision relaxation time. Similarly, the temperature-based sensor was scaled to define an artificial thermal conductivity, which was added to the FV discretization of the total energy equation.
A set of validation cases—including the Sod shock tube at various pressure ratios, a 2D Riemann problem, and the interaction of a shock wave with a helium bubble in air—was performed. The results demonstrate that the hybrid LBM approach is capable of accurately capturing shocks and contact discontinuities, even on relatively coarse meshes, while avoiding spurious Gibbs oscillations.

==== N5 - Dorothy: Toward Fully Distributed Implementation - A. Vergnaud (LOMC), M. Roperch (LOMC) & G. Pinon (LOMC) ====

Dorothy is a Vortex Particle Method CFD code for turbine wakes. Its parallel performance needs to be improved when large number of particles is used (e.g. multi-turbines farm cases or far-wake studies). Several limitations are observed due to lacks in terms of memory, structure of data, parallel implementation, etc… To overcome these problems, the possibility of another code structure/architecture (fully parallel and scalable), even for large number of particles, needs to be investigated. The aim of this project is to explore the use of the library AMReX (https://amrex-codes.github.io/amrex/overview.html) which provides a large toolbox to manage massively parallel block-structured AMR applications (mesh data structure, particle data structure, load balancing, processors communications, etc...).

Some tests have been performed to study AMReX performances. In particular, a scalability test has been performed over a tutorial particle method case (Particles In Cells tutorial code), upgraded up to 134 millions of particles (which, for now, is much higher than the number of particles used with Dorothy). A good scalability has been measured, better than with Dorothy: over 75% on 800 cores (on CRIANN). These results are encouraging and suggest good performance when the AMReX library will be used to implement the Vortex Particle Method.

==== N6 - Relaxation of the IBM stability constraint - PL. Martin (IMAG) & S. Mendez (IMAG) ====
Many simulations done in the YALES2BIO framework involve fluid-structure interactions handled with the Immersed Boundary Method (IBM).
This model allows for the fluid/solid coupling, with the forces from the solid acting as a source term in the Navier-Stokes equations.
In some cases for red blood cells simulations, and for most cases for von Willebrand Factor simulations, the governing time step is the force time step. When this is the case, we also notice artifacts in the fluid velocity and pressure fields.
The robustness of our IBM implementation was improved for embedded surfaces by shifting our regularization/interpolation kernels away from the wall in case we work with an embedded solid.
Since these simulations are done at low Reynolds and CFL number (0.01 - 0.001), the stability constraint was relaxed by doing substeps without:
1. advancing the convective velocity, 2. correcting the velocity to make it divergence-free.
The artifacts showing when solids are a lot stiffer than the fluid viscous forces were reduced by projecting the regularized solid forces into a divergence-free space.

==== U1 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 - L. Korzeczek (GDTech), T-P. Luu (GDTech), S. Meynet (GDTech), M. Cailler (Safran), R. Letournel (Safran), G. Lartigue (CORIA)====

Yales2 features an initial version of a graphical interface. This version enables users to execute a series of processes on a local machine, covering data preparation, computation, and post-processing for basic aerodynamic and hydrodynamic calculations.

To facilitate industrialization and support advanced users in applying it to complex projects, it is essential to extend this interface to a broader range of physical applications. This includes enabling the implementation of coupled or chained calculations and allowing communication with remote servers.

The work conducted during this ECFD have significantly strengthened the current architecture, enhancing performance, modularity, and the capacity to accommodate complex scenarios. Additionally, new widgets have been developed, and an initial draft for connecting to a remote server has been initiated.

=== Turbulence - L. Voivenel (CORIA), P. Bénard, (CORIA) & T. Berthelon (LEGI) ===

==== T1 - Concurrent Precursor-Successor with Successor automated mesh convergence - P. Launay (CORIA), L. Voivenel (CORIA) & P. Benard (CORIA) ====

Using a periodic precursor simulation remains the more accurate method for generating realistic fully developed atmospheric turbulence for a successor simulation. However, it is also the most expensive one. Only the sequential method was implemented in YALES2, involving 2 separate simulation running one after the other, and relying on a lookup table as a link between the two. This project proposed to reduce the cost of the method by implementing a concurrent version where both simulations run in the mean time.

This was achieved using existing CWIPI developments. Another issue arising in such periodic precursors is the creation of spanwise inhomogeneities namely "streaks". This issue has been addressed using CWIPI by replacing the streamwise periodic boundary conditions by an internal coupling between an internal plane of the precursor and its inlet where it is being recycled. A spanwise shift of the velocity field is applied at the inlet preventing the generation of "streaks". A flow rate correction is also applied for preventing bulk velocity drift as the recycling procedure induces a 1 iteration delay. Note that this method is more efficient and more accurate than the Recycling method already existing in YALES2 and relying on particles. Finally, the method has been furthermore improved using Traction free outlet boundary conditions in both precursor and successor domains allowing the reduction of domain length.

Overall the cost of the whole workflow has been greatly reduced and the formation of streaks has been prevented.
The nature of the turbulent structures before and after this modification needs further investigation, as well as the use of other streamwise boundary conditions (INLET/INLET, ...), and are the subject of current work.

==== T2 - Discharge movement model for breakdown prediction - S. Wang (EM2C), T. Kabir (EM2C), E. Roger (EM2C), C. Laux (EM2C), B. Fiorina (EM2C), Y. Bechane (CORIA) & V. Moureau (CORIA) ====

A well-established approach for performing 3-D simulations of plasma-assisted combustion at reduced computational cost is the use of phenomenological models for Nanosecond Repetitively Pulsed (NRP) plasma discharges. So far, these models have assumed a static cylindrical shape for the discharge energy deposition region. However, the breakdown location is governed by the flow velocity, electron density, and reduced electric field, which are neither static nor uniform. As a result, the discharge may exhibit elongation, translation, or rotation. This project aimed to implement a simplified physics-based discharge movement model using the reduced electric field, electron mobility, and an electron density-like variable.
Most of the model was successfully implemented, except for the final step, in which the field line corresponding to the maximum restrike probability must be constructed to determine the new plasma restrike zone.

==== T3 - Vorticity model for discharge-induced flow dynamics - S. Wang (EM2C), T. Kabir (EM2C), E. Roger (EM2C), C. Laux (EM2C), B. Fiorina (EM2C), Y. Bechane (CORIA) & V. Moureau (CORIA) ====

A well-established approach for performing 3-D simulations of plasma-assisted combustion at reduced computational cost is the use of phenomenological models for Nanosecond Repetitively Pulsed (NRP) plasma discharges. These models can be implemented within a low-Mach number framework to further reduce the cost. However, doing so removes the acoustic necessary to resolve discharge-induced flow dynamics. Recently, Roger et al. (2025) proposed a model using physics-based vorticity patches to recover these flow dynamics. This project aimed to implement this model in the low-Mach number framework of YALES2 (YALES2-VDS). The model, formulated as an external forcing term in the momentum balance equation, was successfully implemented. However, simulations performed with YALES2-VDS without the vorticity model exhibit the formation of vortices in regions where none are expected. A possible source of error may be related to the treatment of the hydrodynamic pressure gradient and the associated baroclinic torque term in the vorticity equation. The behavior of this term requires further investigation before the viability of the vorticity model within a low-Mach number framework can be properly assessed.

==== T4 - Wind field reconstruction based on LiDAR measurements - T. Cousin (LMI), P. Benard (CORIA), G. Lartigue (CORIA) & JB. Lagaert (LMO) ====

Wind turbines experience significant loads due to the wind pressure exerted on their structure. Accurate prediction of wind turbine behavior is essential for effective management. Simulations use wind data as input, and their realism can be improved by incorporating wind profiles derived from on-site LiDAR measurements.
The scope of this project is to provide a suitable mathematical framework phrased as a minimization problem under incompressibility constraint to reconstruct the wind field from the LiDAR dataset. The entire framework has been developed using the YALES2 scalar solver, with the objective of extending it to the NS solver under the low-Mach number and constant-density approximation.

==== T5 – Hybrid RANS/LES based on dual mesh and LES of fluctuations - G. Balarac (LEGI), T. Berthelon (LEGI) & R. Letournel (Safran) ====

This project is devoted to a fully coupled hybrid RANS/LES strategy based on a dual-mesh framework, where the mean flow is solved by RANS on a mesh tailored for the mean field, while only the turbulent fluctuations are resolved by LES on a second mesh. In addition to deterministic drift (relaxation) terms that drive the resolved velocities in each model toward target fields provided by the other one (RANS mean for LES, LES statistics for RANS), a stochastic forcing built from RANS turbulent quantities is introduced in the LES of fluctuations. These combined forcing terms allow a controlled generation of fluctuations at the RANS/LES interface and reduce the sensitivity to interface location. Two-way coupling is achieved by feeding back the Reynolds stresses computed in the LES into the RANS equations in the resolved regions. The approach is demonstrated on turbulent pipe flows, including a fully coupled simulation at high Reynolds number (Re = 44,000), showing that the method enables wall-resolved hybrid simulations at a fraction of the cost of a full LES.

==== T6 - Injection of coherent structures for LES inlet condition - T. Berthelon (LEGI), G. Balarac (LEGI), R. Letournel (Safran), P. Launay (CORIA), L. Voivenel (CORIA) & P. Benard (CORIA) ====

The boundary conditions of an LES calculation play a key role in the predictability of simulations. In particular, the turbulence injected at the inlet can strongly influence the development of turbulence.
The aim of this project was to extend the turbulence injection capabilities of the YALES2 code. On the one hand, the historical strategy of injecting synthetic homogeneous isotropic turbulence calculated from a Passot-Pouquet spectrum model has been enhanced by enabling the generation of richer spectra (Pope and Von-Karman-Pao spectra model).
On the other hand, the Synthetic Eddy Method (SEM), proposed by Jarrin et al (2008), was implemented. This method consists of generating a coherent velocity field that respects a target Reynolds tensor and a characteristic size of the large turbulent scale. To do this, the velocity field is generated by summing the contributions of several eddies whose position is the result of a random process.
First, these new strategies were compared in the case of turbulent flow within a pipe. The SEM and the injection of a richer spectrum show a real gain in terms of the flow establishment length in this case.
Finally, the new SEM method was tested on an urban flow case and in a zonal RANS/LES coupling context.

==== T7 - Integration of a bending blade method with Dorothy - E. Mascrier (LOMC), M. Roperch (LOMC), A. Vergnaud (LOMC) & G. Pinon (LOMC) ====

The size of offshore wind turbine blades has been steadily increasing over the years. Longer blades result in larger structural displacements during operation. Blade deformation has therefore become a key design parameter for large rotors. In this context, the present project focuses on coupling an in-house structural beam solver, based on Timoshenko beam theory, with an in-house Lagrangian vortex particle solver called Dorothy.
The project was initiated during ECFD8, where static blade deformation was implemented. This year, Dorothy has been fully dynamically coupled with the structural solver.
The first results show good agreement with the literature in terms of blade deflection and aerodynamic forces for the NREL 5MW rotor.
This work will be continued after ECFD9, with additional simulations performed to verify the results against other numerical approaches, such as YALES2.

==== T8 - FSI-3D without deformation strategy for internal flows - P. Benez (Safran), H. Lam (LEGI) & P. Benard (CORIA) ====

The modeling of fluid–structure interactions (FSI) is a key element in many industrial applications. Prior to this ECFD9, several setups and strategies were implemented in YALES2, differing from one user to another. The objective of this ECFD9 was to test the new 'conformal_bodies' data structure in order to give a simplified and unified setup for handling FSI cases. The FSI method based on conformal bodies (relying on the computation of aerodynamic forces and torques on moving body fitted mesh surfaces), had previously been mainly tested in 2D. In this work, a 3D FSI case involving a sphere trapped in a cavity with multiple inlets and outlets has been performed, and encouraging qualitative results were obtained.

==== T9 - LES-based aero-servo-elastic simulation of wind turbines - E. Muller (CORIA & SGRE), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE), B. Duboc (SGRE) & H. Hamdani (GDTech) ====

The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine as well as the structural deformation when YALES2 is coupled to an external library/code. In the past years two coupling libraries have been developed, one to BHawC (SGRE certification tool) and one to OpenFast (NREL open access/open source tool). To improve the user and developer experience, a generalization and uniformization of the two coupling has been conducted in this project. Extensive tests and validations were performed to guarantee the non-regression.

The ALM and ADM (via ALADIN model) frameworks in the YALES2 code were thus enhanced to benefit from these couplings. Such method allows to take part of the external structural solver and controller in single and multiple turbines configurations. Updates were also initiated directly in the coupling libraries to benefit from the latest developments made in the servo-structural solvers, thus allowing to simulate modern academic wind turbines (with OpenFAST) or industrial flagships (with BHawC) in operation.

Furthermore, works on the Risoe Dynamic stall model, initiated during ECFD6, have been achieved. The implementation and integration of this model has been continued, ported to the parallel-optimized ALM framework, and tested and validated on different configurations.

Miscellaneous tasks related to the ALM code pipeline coverage and documentation have been improved.

==== T10 – Numerical simulation of engine rotors - L. Bricteux (UMONS), G. Balarac (LEGI), Y. Bechane (CORIA) & P. Benard (CORIA) ====
This project investigated the capability of the explicit compressible solver in YALES2 to simulate the fan stage of a turbofan engine. The selected configuration is the CATANA rotor, developed at École Centrale de Lyon, for which experimental data are available.
The mesh of this complex geometry was generated using Gmsh and YALES2 and consists of approximately 220 million tetrahedral elements. The setup of the simulation with a moving mesh framework was carried out during the research stay.
During this work, wall boundary conditions were improved, and it was identified that the near-wall turbulence modeling strategy could be enhanced by introducing a compressible wall model based on the work of Debroeyer et al (JFM 2024). Initial simulations have been performed and have produced promising results.
The next step will be to integrate mesh adaptation and the new compressible wall model, and to compare numerical diagnostics with experimental measurements in order to validate both the modeling approach and further validate the solver.

=== Combustion - Y. Bechane (CORIA), R. Letournel (Safran) & S. Dillon (Safran) ===

==== C3 - LES of the thermal degradation of a composite material - A. Grenouilloux (ONERA), K. Bioche (CORIA), N. Dellinger (ONERA) and R. Letournel (Safran) ====

In order to certify new composite materials for aerospace applications, it is essential to understand their degradation dynamics under severe thermal loads. The ONERA FIRE test bed was designed for this purpose. This burner generates a premixed air propane flame that reproduces a thermal flux consistent with certification standards near the impinging region. During tests, a strong emission of pyrolysis gases and a secondary diffusion flame are observed, and these gases can self ignite in regions not directly exposed to the primary flame. The project aimed to improve the modeling of this burner using Large-Eddy Simulation and reduce the overall computational cost. A reduced kinetic mechanism was derived with the Brookesia library, enabling the modeling of both premixed and diffusion flames to take into account appropriate chemistry at the front face. Used in FIRE simulations, this mechanism achieved a CPU speed-up of a factor of two compared with the previous scheme. A second reduced mechanism was generated to target auto ignition of pyrolysis gas mixtures that can occur at the rear face, and a dedicated test case was designed. Recent developments in the CWIPI interface allow for mesh adaptation during coupling between YALES2 and MoDeTheC solvers.

==== C4 - Flamelet-Progress Variable approach in LBM solvers - U. Chikkabikkodu (M2P2), D. Nouembissi (M2P2), I. Mir (M2P2), H. Meunier (M2P2), I. Tsetoglou (M2P2), S. Zhao (M2P2), P. Boivin (M2P2), J. L. Consalvi (IUSTI), R. Mercier (Safran) & S. Dillon (Safran) ====

This project extended the capabilities of ProLB to support flamelet-based combustion modelling by implementing the flamelet progress variable (FPV) approach together with FTACLES capabilities in our LBM solvers - closing a long-standing gap, since table generation and usage had never been available in ProLB.
During the workshop, transport of a passive scalar was implemented and the SDR was modelled using the passive-scalar gradient, which currently form the two control variables used in the flamelet approach. The implementation was verified through simulations of a 2D laminar methane-air jet diffusion flame.
In parallel, for FTACLES we successfully generated both premixed and non-premixed tables with TECERACT (thanks to Renaud and Samuel), and converted them into a format compatible with our code structure. A progress-variable transport equation was also implemented where the diffusion, source and correction terms were read directly from the tabulation. Validation was performed on a 1D CH4/air premixed flame with 10 sampling points within the filter width, accurately recovering the flame speed and demonstrating successful coupling between the LBM solver and the tabulated chemistry.

==== C5 - NOx prediction with a hybrid FTACLES-Virtual chemistry approach - É. Espada (EM2C), M. Préteseille (EM2C), N. Darabiha (EM2C), B. Fiorina (EM2C) ====

Filtered Tabulated Chemistry is a powerfull yet very cost efficient tool to compute flame structure and its stabilisation. However, it is unable to predict NO concentration wihtout adding additional coordinates in the manifold or by using premixed-flamelet based additional model and tabulation like NOMANI. Virtual chemistry on the other hand is a chemistry reduction method that uses machine learning algorithm to reduce drastically the number of species and reaction. This reduced scheme is then transported like any detailed chemistry mechanism. Although the method is also able to recover flame strucure and pollutants, unlike FTACLES, transported chemistry lacks a turbulent combustion model to be applied on realistics industrial LES mesh grids. This present works aims to couple FTACLES and virtual chemistry in a one way coupling: FTACLES will compute flame structure (density, Temperature, velocity field) thanks to its turbulent combusiton model, and will then feed a virtual mechanism with the "main" grid information in order to compute the pollutant informations.

==== C6 - Modelling laminar & turbulent flames with virtual chemistry - M. Préteseille (EM2C), É. Espada (EM2C), N. Darabiha (EM2C), B. Fiorina (EM2C), S. Dillon (Safran), M. Cailler (Safran) ====

A virtual chemistry framework yielding global-type mechanisms has recently been developed and validated, allowing accurate prediction of flame structures at a substantially reduced computational cost. By coupling virtual chemistry with Adaptive Mesh Refinement (AMR) strategies, this work assesses the ability to dynamically resolve reactive zones while maintaining affordable computational costs in high-fidelity LES of industrial burners. A second objective of ECFD9 was to disseminate the virtual schemes generated using SuperVision, a Python-based automated optimization tool built on Cantera. An optimized hydrogen virtual mechanism was successfully implemented and validated in the Lattice–Boltzmann solver ProLB, demonstrating the ease with which these standardized schemes can be integrated into existing reactive flow solvers, and the spread potential of this new chemistry reduction strategy in the combustion community. Finally, the NOx virtual submechanism for hydrogen combustion was improved to accurately capture both thermal and prompt NO formation in hydrogen flames.

==== C7 - TEMPERATURE BOUNDARY CONDITIONS FOR TABULATED CHEMISTRY - P. Illu,inqti (EM2C), R. Vicquelin (EM2C ====

Tabulated chemistry workframe usually rely on transporting an Enthalpy scalar to account for Heat Losses, resulting in tables that have a few dimension for the thermochemical state (e.g. Z, YC, etc) and a transported scalar for generic heat losses (e.g. ENTHALPY). In order to generalize the use of tabulated chemistry models for Heat Losses (Conjugate Heat Transfer, Radiation Heat Losses etc...) a new Boundary Condition has been developed that will allow the user to impose a tempearture on the wall and to retrieve accordingly the transported ENTHALPY value that enforces such condition. The boundary condition is available in the VDS solver, when the scalar of type ENTHALPY is being imposed. (to be merged)
As a side objective, the Robin condition for the Heat Transfer Solid has been expanded to account not only for convection, but also for radiation when the user specifies an emissivity and a blackbody temperature. (already merged)

==== C8 - Optimization of chemical source terms stiff integration - G. Lartigue (CORIA), Y. Bechane (CORIA), K. Bioche (CORIA), Q. Cerutti (CORIA), M. El Moatamid (CORIA), M. Laignel (CORIA) ====

Integration of chemical source terms remains computationally expensive in configurations that rely on detailed chemistry approach. This project aimed to reduce that cost by (1) modifying the CVODE integration strategy and (2) applying source-term clustering. A first attempt was to modify CVODE’s internal step-size control strategy but it produced only minor gains as some unnecessary integration steps still occurred, mainly in the unburned gases region. This has finally been addressed by enforcing an initial step based on the CFD time step which reduced the computational cost by a factor 2 in these regions. More importantly, relaxing the relative and absolute tolerances used to determine the accuracy of the method reduced the computational cost by approximately 40% while introducing negligible error in physical properties and flame topology for a 1D premixed flame. These results were confirmed on three methane flame configurations: a 1D premixed flame, a 2D triple flame, and the PRECCINSTA burner. Numerical experiments on the PRECCINSTA burner show a reduction in integration cost by a factor of 2.5 using the adjusted CVODE strategy and by a factor of 4.4 when that strategy is combined with clustering.

==== C11 - Optimization of decoupled approach for heat transfers - T.-P. Luu (GDTech), R. Letournel (Safran), M. Tripiciano (Safran) ====

Many industrial aerothermal applications involve strong interactions between fluid flow and solid thermal response, requiring an accurate representation of fluid–solid coupling to predict wall heat fluxes. Although fully two-way coupled simulations provide high fidelity, their complex numerical setup and high computational cost limit their applicability in industrial design loops. As a result, one-way decoupled approaches based on the estimation of heat transfer coefficients (HTCs) are usually preferred. The classical double-run method, which relies on two simulations with imposed wall temperatures to estimate HTC, remains workflow-intensive and highly sensitive to the choice of reference temperatures. In this project, a single-run methodology is proposed to reduce setup complexity and computational cost. The approach introduces an additional passive scalar representing the variation of the fluid sensible enthalpy induced by a change in imposed wall temperature. The associated transport equation is derived under the assumption that the thermophysical properties of the mixture remain weakly dependent on temperature variations. The method is validated on a canonical three-dimensional heated plate configuration and demonstrates promising results when applied to an industrial burner simulation.

=== Mesh adaptation - A. Grenouilloux (ONERA) & G. Balarac (LEGI) ===

==== M2 – Dynamic of SWBLI in Supersonic Propulsive Nozzle Under Hot Gas Conditions - F.A. Rojas Segovia (CORIA), Y. Bechane (CORIA) & L. Voivenel (CORIA) ====
In this project, a STBLE (Solution Thin Boundary Layer Equations) wall model was implemented in YALES2. The focus was to add and compare this model with the pre-existing wall models in the code, such as the logarithmic law and Duprat, in the context of supersonic nozzles. To achieve this, 2D simulations of supersonic compressible flow over a flat plate were conducted as an initial step and validation. These initial simulations provided good insights for future research on the dynamics of Shock Wave and Boundary Layer Interaction (SWBLI) in supersonic nozzles operating with both cold and hot gas conditions.

==== M3 – Criterion for dynamic mesh adaptation in LES - H.Lam (LEGI), G. Balarac (LEGI), V. Moureau (CORIA), R. Barbera (LEGI), P. Launay (CORIA) & L. Voivenel (CORIA) ====

This project proposes a new criterion for dynamic mesh adaptation in LES, designed to overcome the limitations of static LES mesh convergence (static AMC) strategies based on time-averaged quantities. In both static and dynamic contexts, a cell-based Reynolds number is first used as a DNS criterion to identify regions where all turbulent scales must be resolved. For LES, the DNS constraint is relaxed when the integral scale is sufficiently larger than the local cut-off scale, so that a meaningful GS/SGS separation exists. In static AMC, this condition can be evaluated from statistical quantities. In dynamic mesh adaptation, however, such statistics are not available. To overcome this limitation, the proposed approach relies on the assumption that the instantaneous dissipation is predominantly the turbulent dissipation. The integral scale is then estimated from local instantaneous quantities, allowing a dynamic evaluation of the scale-separation criterion. This provides a continuous transition between DNS-like and LES-like regions during the simulation. The method is complemented by a laminar–turbulent discrimination based on a "sigma-sensor" (inspired by the sigma SGS model), enabling the identification of purely laminar zones. The approach has been assessed on a turbulent jet and on flow around a three-dimensional cylinder. Ongoing work focuses on improving near-wall treatments, in particular through prismatic layers generation on boundaries coupled to mesh adaptation and the introduction of dedicated kernels to stabilize the wall mesh and limit excessive boundary motion.

==== M4 – Improve mesh adaptation tools - B. Andrieu (ONERA), C. Benazet (ONERA), N. Dellinger (ONERA), G. Janodet (ONERA) & B. Maugars (ONERA) ====

Building upon the foundations established during ECFD7 and ECFD8 — which focused on periodic CAD-based mesh generation in EGADS and periodic parallel metric gradation — our latest developments for ECFD9 mark a significant step toward a fully automated, CAD-based periodic remeshing algorithm.
First, the parallel hierarchical remeshing algorithm prototype was improved by using a more elaborate ownership system in ParaDiGM to drive the mechanism that merges/dissociates the periodic interface mesh before/after the remeshing pass.
Second, the ability of the refine library (developed at NASA) to remesh non-manifold 3D configurations was investigated. Changes have been made to refine's operators to unlock remeshing near the merged periodic interface in 3D, which yielded promising results, but more work is needed to achieve industrial robustness. To enable CAD-based projections on both sides of the merged periodic interface, an algorithm for building a coherent periodized CAD model was implemented in the EGADS library.
This CAD-based periodic remeshing algorithm was validated in serial through a numerical simulation of the 2D LS89 turbine blade using the SoNICS solver. The results demonstrate that the mesh effectively adapts to capture the strongly anisotropic flow features while strictly respecting the periodic constraints and the geometric support.
Non-manifold mesh adaptation was applied to the ablation of a plate up to burnthrough, first in 2D and then in 3D. The burnthrough detection workflow was improved by developing a Python mini-toolbox for basic geometric queries, allowing the removal of non-physical solid fragments in the middle of the hole after burnthrough. The MMG library was also evaluated for its ability to handle non-manifold meshes, and it appears more suitable than the Refine library for this configuration. The workflow is satisfactory in 2D but needs improvement in 3D to continue the simulation after burnthrough.

==== M5 – Anisotropic mesh adaptation for multiphase flows - Robin Barbera (LEGI), Manuel Bernard (LEGI), Giovanni Ghigliotti (LEGI) & Roxane Letrounel (Safran) ====

This project investigates anisotropic mesh adaptation strategies for multiphase flows, with the objective of reducing computational cost while preserving an accurate representation of fluid interfaces. The approach relies on curvature-based anisotropic remeshing, where mesh anisotropy is locally controlled from interface geometry to ensure a prescribed discretization angle. A key limitation of anisotropic coarsening along interfaces is mass loss induced by interpolation during remeshing, which increases with tangential coarsening and therefore directly conflicts with anisotropic strategies. During ECFD9, this issue was addressed by introducing a high-order interpolation scheme for interface variables, replacing the default linear interpolation. The results show that high-order interpolation significantly reduces mass loss, allowing for much higher mesh anisotropy at the interface, at the cost of a limited computational overhead. In addition, the curvature-based adaptation strategy was extended from mean curvature to the full curvature tensor, enabling the mesh to align with the two principal curvatures of three-dimensional interfaces. The approach was demonstrated on canonical multiphase configurations, including droplet advection and rising bubble cases, showing substantial reductions in mesh size compared to isotropic simulations. Ongoing perspectives include coupling curvature-based adaptation with feature-based anisotropic remeshing to better capture turbulent structures away from the interface.

==== M7 – Increased mesh anisotropy for laminar and RANS applications - R. Barbera (LEGI), J.-B. Lagaert (LMO), T. Berthelon (LEGI), R. Letournel (Safran), M. Bernard (LEGI) & G. Balarac (LEGI) ====

This project addresses the limited level of mesh anisotropy obtained with current feature-based anisotropic remeshing criteria in steady laminar and RANS simulations in YALES2. While recent development of anisotropic mesh adaptation have significantly reduced computational cost, the achieved aspect ratios remain moderate (AR < 50), well below the levels commonly reported in the RANS literature (AR > 100). The objective of the project was to identify the main mechanisms that limit anisotropy in practice, including numerical noise in the resolved quantities, inaccuracies in Hessian computation, the formulation of the criterion itself, ... During ECFD9, the current anisotropic criterion applied to a vectorial quantity of interest (QOI) implemented in YALES2 was reformulated as the minimization of a residual-based error estimator. A Newton optimization strategy was introduced to assess whether the theoretical optimum of the criterion differs from criterion use in practice.The approach was analyzed on the Kovasznay flow, and the optimal solution was shown to be very close to the criterion currently used in YALES2. Comparisons with alternative criteria from the literature and based on scalar QOI further demonstrated similar mesh convergence, highlighting the robustness of the YALES2 approach and its main advantage: a flow-independent, non-dimensional target error. Ongoing investigations focus on quantifying the influence of numerical noise in the resolved quantities and Hessian discretization on the achievable mesh aspect ratios.

=== Two-phase flows - J. Carmona (CORIA), N. Gasnier (Safran) & I. Tsetoglou (M2P2) ===

==== TP1 - Simulation of core shifting during investment casting - Y. Mayi (Safran), M. Cailler (Safran), S. Meynet (GDTech) ====

==== TP2 - Lattice Boltzmann method for free-surface two-phase flow - J. Lu (M2P2), Y. Mediene (M2P2), I. Tsetoglou (M2P2) & S. Zhao (M2P2) ====

The project aims to reproduce and improve a two-equation free-surface Lattice Boltzmann Method (LBM) model for two-phase flows. The original free-surface model features a sharp interface and good numerical stability, but it neglects the gas phase and is therefore limited to two-phase flows in which gas effects are negligible. The recently developed two-equation free-surface LBM model (Liu Y., Sun D., Zhang Z., et al., Physics of Fluids, 2024, 36(3)) incorporates the gas phase, enabling interactions between the two phases. However, this model suffers from a lack of mass conservation and insufficient accuracy in curvature computation.
To overcome these limitations, an auxiliary distribution function is introduced to track mass evolution, thereby decoupling mass conservation from pressure evolution and restoring global mass conservation. In parallel, a pseudo-smoothing step is implemented to achieve more accurate calculations of interface normals and curvature. These improvements are validated through two benchmark test cases. (1) A Laplace test involving both static and advected droplets. It demonstrates exact mass conservation and a significant enhancement in surface tension modeling. (2) A two-phase Poiseuille flow. It shows good agreement with theoretical predictions, validating the viscous coupling between the two fluids.
Future work will focus on improving information exchange across the interface to reduce numerical oscillations and enhance numerical stability, as well as on conducting more complex validation cases.

==== TP3 - Modeling of a gear wheel immersed in an oil bath ====

==== TP4 - Implementation of a granular temperature model - T. Ndereyimana (Université de Sherbrooke), S. Moreau (Université de Sherbrooke), Y. Dufresne (Enerkem) ====

In gas–solid systems such as fluidized beds, clusters of particles naturally appear. These clusters tend to exhibit a Gaussian velocity distribution around an average velocity, with a spread that depends on the local environment of the cluster.
In coarse-grained DEM simulations, real particles are replaced by numerical parcels representing groups of particles in order to reduce the computational cost associated with a large number of particles. In this approach, all particles within a parcel are assumed to move at the same velocity; consequently, no velocity distribution is represented.
This project focuses on comparing two approaches to model the standard deviation of the velocity distribution within a parcel: (1) a local averaging method and (2) a kinetic-theory-of-granular-flow-based methodology. The former computes the standard deviation based on the velocities of surrounding parcels, while the latter relies on two-phase flow theory in which this standard deviation is explicitly modeled.
Both methodologies predict a high standard deviation in the vicinity of gas bubbles in the fluidized bed and lower values in very dense and very dilute regimes. However, the local averaging method tends to increase the computational cost by requiring the detection of neighboring parcels for each parcel.

==== TP5 - Jet-A1 cavitation modeling - P. Benez (Safran), J. Carmona (CORIA) ====

==== TP6 - Comparison of JICF models for turbulent reactive applications, S Puggelli (SAE), L Carbajal Carrasco (SAE), E Charles (CERFACS), L Gicquel (CERFACS) ====
Simulating jet-in-crossflow (JICF) configurations using LES for industrial applications is complex due to the high computational cost associated with fully resolving jets, especially for computations in reactive conditions. Within a purely Lagrangian approach, certain physical phenomena—such as surface or column break-up—are not effectively captured. This project aims to assess the Lucid model (Charles et al., International Conference on Numerical Combustion, 2025), which introduces a liquid column representation into the Lagrangian framework, by comparing its performance against a fully resolved SPS simulation of a JICF case validated by experimental data. Additionally, a pure Lagrangian injection case, namely without any ad-hoc model for the jet, is also analyzed. The results indicate that the Lucid model better captures surface breakup dynamics when compared to the pure Lagrangian approach. Nevertheless, both Lagrangian-based methods overlook aerodynamic blockage effects of the liquid column, potentially influencing the downstream distribution of droplets.

==== TP7 - Validation and extension of PCS solver for cryo tanks ====

==== TP8 - Jet-in-crossflow simulation with the Hybrid SPH-FVM solver ====

==== TP9 - Multi-physics effects modeling in film flows - N. Gasnier (Safran), P. Portais (CORIA/Safran), L. Voivenel (CORIA), E. Bourrel (CORIA), M. Cailler (Safran) ====
The project aimed at improving the multi scale model for parietal film flows implemented in the YALES2 platform. This model, based on the Shallow Water equations, allows to describe the dynamics of a thin liquid layer spreading over dry walls at a reduced computational cost. First, the numerical method designed during ECFD8 to convert impinging Lagrangian droplets into film data has been extended to account for droplet splashing and rebound phenomena. Then, a sensitivity analysis has been initiated to determine the influence of inlet conditions on the properties of the droplets generated by film atomization. Preliminary results showed that an increase in the gas velocity causes a significant increase in the number of droplet generated, and a large decrease in the drops diameter. Then, the film dynamics model has been extended to rotating walls by including inertial and Coriolis forces in the momentum conservation equation. A first validation of the implementation has been conducted by analyzing the spreading of a liquid film generated by impinging droplets over a rotating disk at high angular speed, which gave promising results. Finally, a film temperature equation has been added to include thermal effects in the thin film model, this additional equation describes the temporal evolution of the surface temperature of the liquid, which is primarily affected by the temperatures of the wall and of the surrounding air. The influence of thermal effects on the dynamics of the liquid is taken into account through the temperature-dependency of the surface tension, which is likely to cause the onset of Marangoni currents due to heating discrepancies.

==== TP10 - Solid-Fluid Coupling for Nucleate Boiling Simulations - M. Umair (LEGI), G. Ghigliotti (LPSC), H. Lam (LEGI), M. Bernard (LEGI), R. Barbera (LEGI), G. Balarac (LEGI) ====

Ecfd:ecfd 9th edition

2026-02-06T07:03:04Z

Benard: /* Hackathon GENCI - P. Begou (LEGI), V. Moureau (CORIA) */

{{DISPLAYTITLE: ECFD workshop, 9th edition, 2026}}

== Description ==

* Event from '''19th of January to 30th of January 2026'''
* Location: [https://www.sport-normandie.fr/le-centre/le-site-de-houlgate Centre Sportif de Normandie], Houlgate, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* Participants from academics, HPC center/experts and industry are welcome
* The number of participants is limited to 80.


* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Voivenel (CORIA).

[[File:Logo_ECFD9.png|center|frameless|900px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_9th_edition]]



== News ==

* 22/09/2025: First announcement of the '''9th Extreme CFD Workshop & Hackathon''' !
* 15/11/2025: Deadline to submit your project

== Thematics / Mini-workshops ==

To be announced...

== Projects ==

=== Hackathon GENCI - P. Begou (LEGI), V. Moureau (CORIA) ===
This ECFD9 GENCI Hackathon was a rich event, involving 3 differents CFD codes (AVBP, SONICS and YALES2) using various paradigms (C++/cuda/hip, Fortran/OpenMP/OpenACC) with several SDKs (AMD, Cray/HPE, Nvidia, Gnu) on a large range of GPU architectures (Nvidia A100, GH100, AMD instinct Mi210, Mi250, Mi300). This two-week event benefited from a high level support from three HPC mentors, two on-site from AMD (J. Noudohouenou and A. Tsetoglou).

==== H3 - Hackathon SONICS - A. de Brauer (ONERA), B. Michel (ONERA), B. Berthoul (ONERA) & G. Staffelbach (ONERA) ====
CPU code generation for multispecies simulations – Code generation for multispecies simulations is currently being developed in the SoNICS code. The work carried out at ECFD9 focused on the vectorization of the generated code by code transformation : unrolling the species loops, rewriting if statements, and inverting do/while loops (arising from Newton type algorithms) used in the computation of thermodynamic quantities. The loop-unrolling and if statement rewriting have been profiled and show a speed-up of 2x for the vectorized generated code when computing the HLLC flux, compared with the hand-written implementation. The switch of do/while loops was prototyped on a test code and will be integrated into SoNICS. Code generation on GPU has been tested and validated, but a thorough performance profiling of the GPU version is still required.

Porting reactive multi-species terms to GPU – In 2025 multi-species reactive capabilities were introduced in SoNICS and tested on the Preccinsta case on CPU. Recently the multi-species components were ported to the GPU, so this activity concentrated on porting the reactive source terms. Tests on a 0D reactor show identical results on GPU and CPU. Work has also resumed on porting SoNICS to AMD GPU on the ADASTRA system from CINES/GENCI, where the hipGraphs implementation (AMD’s counterpart of cudaGraph) exhibited some issues. Our participation in ECFD9 allowed us to contact the AMD hipGraphs development team, opening the way to a collaboration. With their council, we updated the code to use rocm7.1.1 providing the first successful non reactive results on AMD GPU. Further work on reactive flows is ongoing.

Improving GPU residual calculations – Recent investigations show that SoNICS’s residual calculation on GPUs was about 100× slower than other GPU operations. The bottleneck was traced to the combination of cudaGraph and thrust::reduce, which prevented parallel execution. Replacing this with a hand-written hierarchical reduction kernel that works efficiently within the cudaGraph restores good scalability; the residual computation is now negligible compared with the other operations, as is the case on CPU. Additional timers were added to the cudaGraph kernels to quantify each operator’s cost relative to the CPU.

=== Numerics & User Interface - M. Bernard (LEGI), G. Lartigue (CORIA) & S. Mendez (IMAG) ===

==== N1 - Improving ICS robustness and accuracy - M. Bernard (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI) & T. Berthelon (LEGI) ====
Bad quality meshes generally lead to larger numerical errors when solving partial differential equations.
This project focused on improving the accuracy and robustness of the incompressible Navier-Stokes solver (ICS).
We investigated the sources of discrepancy introduced at each step of the algorithm, with particular attention to the consequences of the coexistence of two discrete velocity representations: (i) the convective flux <math>\vec{u}\cdot\vec{n}\,dS</math> and (ii) the transported nodal velocity field <math>u^n</math>.
Although these quantities are equivalent at the continuous level, this equivalence no longer holds in the discrete setting.
In particular, only the convective velocity strictly satisfies the divergence-free constraint after solving the Poisson problem for the pressure field.
During this two-week workshop, we developed a new correction strategy for the nodal velocity field in order to enforce consistency with the convective velocity and improve the overall behavior of the solver.

==== N3 - Shock & discontinuity treatment for Lattice-Boltzmann solvers - I. Tsetoglou (M2P2), W. Bessem (M2P2), H. Merley (M2P2) & S. Zhao (M2P2) ====
Lattice—Boltzmann methods (LBM) have traditionally been applied to weakly compressible flows; however, recent developments have extended their applicability to fully compressible regimes. In such flow configurations, shock waves and contact discontinuities naturally arise. To properly capture these features in a discretized framework, artificial diffusion mechanisms are commonly introduced to smooth discontinuities over a limited number of grid points.
In this project, the hybrid LBM solver ProLB was employed. In this framework, the mass and momentum equations are solved using an LBM formulation, while the total energy equation is discretized using a finite-volume (FV) approach with consistent spatial and temporal discretization. The primary objective of the work was to develop and implement an artificial diffusion strategy suitable for hybrid LBM/FV solvers.
Shock waves were detected with a Jameson-type pressure-based sensor, while contact discontinuities were identified with a temperature-based sensor. The pressure-based sensor was scaled to obtain a kinematic viscosity contribution, which was incorporated into the LBM collision relaxation time. Similarly, the temperature-based sensor was scaled to define an artificial thermal conductivity, which was added to the FV discretization of the total energy equation.
A set of validation cases—including the Sod shock tube at various pressure ratios, a 2D Riemann problem, and the interaction of a shock wave with a helium bubble in air—was performed. The results demonstrate that the hybrid LBM approach is capable of accurately capturing shocks and contact discontinuities, even on relatively coarse meshes, while avoiding spurious Gibbs oscillations.

==== N5 - Dorothy: Toward Fully Distributed Implementation - A. Vergnaud (LOMC), M. Roperch (LOMC) & G. Pinon (LOMC) ====

Dorothy is a Vortex Particle Method CFD code for turbine wakes. Its parallel performance needs to be improved when large number of particles is used (e.g. multi-turbines farm cases or far-wake studies). Several limitations are observed due to lacks in terms of memory, structure of data, parallel implementation, etc… To overcome these problems, the possibility of another code structure/architecture (fully parallel and scalable), even for large number of particles, needs to be investigated. The aim of this project is to explore the use of the library AMReX (https://amrex-codes.github.io/amrex/overview.html) which provides a large toolbox to manage massively parallel block-structured AMR applications (mesh data structure, particle data structure, load balancing, processors communications, etc...).

Some tests have been performed to study AMReX performances. In particular, a scalability test has been performed over a tutorial particle method case (Particles In Cells tutorial code), upgraded up to 134 millions of particles (which, for now, is much higher than the number of particles used with Dorothy). A good scalability has been measured, better than with Dorothy: over 75% on 800 cores (on CRIANN). These results are encouraging and suggest good performance when the AMReX library will be used to implement the Vortex Particle Method.

==== N6 - Relaxation of the IBM stability constraint - PL. Martin (IMAG) & S. Mendez (IMAG) ====
Many simulations done in the YALES2BIO framework involve fluid-structure interactions handled with the Immersed Boundary Method (IBM).
This model allows for the fluid/solid coupling, with the forces from the solid acting as a source term in the Navier-Stokes equations.
In some cases for red blood cells simulations, and for most cases for von Willebrand Factor simulations, the governing time step is the force time step. When this is the case, we also notice artifacts in the fluid velocity and pressure fields.
The robustness of our IBM implementation was improved for embedded surfaces by shifting our regularization/interpolation kernels away from the wall in case we work with an embedded solid.
Since these simulations are done at low Reynolds and CFL number (0.01 - 0.001), the stability constraint was relaxed by doing substeps without:
1. advancing the convective velocity, 2. correcting the velocity to make it divergence-free.
The artifacts showing when solids are a lot stiffer than the fluid viscous forces were reduced by projecting the regularized solid forces into a divergence-free space.

==== U1 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 - L. Korzeczek (GDTech), T-P. Luu (GDTech), S. Meynet (GDTech), M. Cailler (Safran), R. Letournel (Safran), G. Lartigue (CORIA)====

Yales2 features an initial version of a graphical interface. This version enables users to execute a series of processes on a local machine, covering data preparation, computation, and post-processing for basic aerodynamic and hydrodynamic calculations.

To facilitate industrialization and support advanced users in applying it to complex projects, it is essential to extend this interface to a broader range of physical applications. This includes enabling the implementation of coupled or chained calculations and allowing communication with remote servers.

The work conducted during this ECFD have significantly strengthened the current architecture, enhancing performance, modularity, and the capacity to accommodate complex scenarios. Additionally, new widgets have been developed, and an initial draft for connecting to a remote server has been initiated.

=== Turbulence - L. Voivenel (CORIA), P. Bénard, (CORIA) & T. Berthelon (LEGI) ===

==== T1 - Concurrent Precursor-Successor with Successor automated mesh convergence - P. Launay (CORIA), L. Voivenel (CORIA) & P. Benard (CORIA) ====

Using a periodic precursor simulation remains the more accurate method for generating realistic fully developed atmospheric turbulence for a successor simulation. However, it is also the most expensive one. Only the sequential method was implemented in YALES2, involving 2 separate simulation running one after the other, and relying on a lookup table as a link between the two. This project proposed to reduce the cost of the method by implementing a concurrent version where both simulations run in the mean time.

This was achieved using existing CWIPI developments. Another issue arising in such periodic precursors is the creation of spanwise inhomogeneities namely "streaks". This issue has been addressed using CWIPI by replacing the streamwise periodic boundary conditions by an internal coupling between an internal plane of the precursor and its inlet where it is being recycled. A spanwise shift of the velocity field is applied at the inlet preventing the generation of "streaks". A flow rate correction is also applied for preventing bulk velocity drift as the recycling procedure induces a 1 iteration delay. Note that this method is more efficient and more accurate than the Recycling method already existing in YALES2 and relying on particles. Finally, the method has been furthermore improved using Traction free outlet boundary conditions in both precursor and successor domains allowing the reduction of domain length.

Overall the cost of the whole workflow has been greatly reduced and the formation of streaks has been prevented.
The nature of the turbulent structures before and after this modification needs further investigation, as well as the use of other streamwise boundary conditions (INLET/INLET, ...), and are the subject of current work.

==== T2 - Discharge movement model for breakdown prediction - S. Wang (EM2C), T. Kabir (EM2C), E. Roger (EM2C), C. Laux (EM2C), B. Fiorina (EM2C), Y. Bechane (CORIA) & V. Moureau (CORIA) ====

A well-established approach for performing 3-D simulations of plasma-assisted combustion at reduced computational cost is the use of phenomenological models for Nanosecond Repetitively Pulsed (NRP) plasma discharges. So far, these models have assumed a static cylindrical shape for the discharge energy deposition region. However, the breakdown location is governed by the flow velocity, electron density, and reduced electric field, which are neither static nor uniform. As a result, the discharge may exhibit elongation, translation, or rotation. This project aimed to implement a simplified physics-based discharge movement model using the reduced electric field, electron mobility, and an electron density-like variable.
Most of the model was successfully implemented, except for the final step, in which the field line corresponding to the maximum restrike probability must be constructed to determine the new plasma restrike zone.

==== T3 - Vorticity model for discharge-induced flow dynamics - S. Wang (EM2C), T. Kabir (EM2C), E. Roger (EM2C), C. Laux (EM2C), B. Fiorina (EM2C), Y. Bechane (CORIA) & V. Moureau (CORIA) ====

A well-established approach for performing 3-D simulations of plasma-assisted combustion at reduced computational cost is the use of phenomenological models for Nanosecond Repetitively Pulsed (NRP) plasma discharges. These models can be implemented within a low-Mach number framework to further reduce the cost. However, doing so removes the acoustic necessary to resolve discharge-induced flow dynamics. Recently, Roger et al. (2025) proposed a model using physics-based vorticity patches to recover these flow dynamics. This project aimed to implement this model in the low-Mach number framework of YALES2 (YALES2-VDS). The model, formulated as an external forcing term in the momentum balance equation, was successfully implemented. However, simulations performed with YALES2-VDS without the vorticity model exhibit the formation of vortices in regions where none are expected. A possible source of error may be related to the treatment of the hydrodynamic pressure gradient and the associated baroclinic torque term in the vorticity equation. The behavior of this term requires further investigation before the viability of the vorticity model within a low-Mach number framework can be properly assessed.

==== T4 - Wind field reconstruction based on LiDAR measurements - T. Cousin (LMI), P. Benard (CORIA), G. Lartigue (CORIA) & JB. Lagaert (LMO) ====

Wind turbines experience significant loads due to the wind pressure exerted on their structure. Accurate prediction of wind turbine behavior is essential for effective management. Simulations use wind data as input, and their realism can be improved by incorporating wind profiles derived from on-site LiDAR measurements.
The scope of this project is to provide a suitable mathematical framework phrased as a minimization problem under incompressibility constraint to reconstruct the wind field from the LiDAR dataset. The entire framework has been developed using the YALES2 scalar solver, with the objective of extending it to the NS solver under the low-Mach number and constant-density approximation.

==== T5 – Hybrid RANS/LES based on dual mesh and LES of fluctuations - G. Balarac (LEGI), T. Berthelon (LEGI) & R. Letournel (Safran) ====

This project is devoted to a fully coupled hybrid RANS/LES strategy based on a dual-mesh framework, where the mean flow is solved by RANS on a mesh tailored for the mean field, while only the turbulent fluctuations are resolved by LES on a second mesh. In addition to deterministic drift (relaxation) terms that drive the resolved velocities in each model toward target fields provided by the other one (RANS mean for LES, LES statistics for RANS), a stochastic forcing built from RANS turbulent quantities is introduced in the LES of fluctuations. These combined forcing terms allow a controlled generation of fluctuations at the RANS/LES interface and reduce the sensitivity to interface location. Two-way coupling is achieved by feeding back the Reynolds stresses computed in the LES into the RANS equations in the resolved regions. The approach is demonstrated on turbulent pipe flows, including a fully coupled simulation at high Reynolds number (Re = 44,000), showing that the method enables wall-resolved hybrid simulations at a fraction of the cost of a full LES.

==== T6 - Injection of coherent structures for LES inlet condition - T. Berthelon (LEGI), G. Balarac (LEGI), R. Letournel (Safran), P. Launay (CORIA), L. Voivenel (CORIA) & P. Benard (CORIA) ====

The boundary conditions of an LES calculation play a key role in the predictability of simulations. In particular, the turbulence injected at the inlet can strongly influence the development of turbulence.
The aim of this project was to extend the turbulence injection capabilities of the YALES2 code. On the one hand, the historical strategy of injecting synthetic homogeneous isotropic turbulence calculated from a Passot-Pouquet spectrum model has been enhanced by enabling the generation of richer spectra (Pope and Von-Karman-Pao spectra model).
On the other hand, the Synthetic Eddy Method (SEM), proposed by Jarrin et al (2008), was implemented. This method consists of generating a coherent velocity field that respects a target Reynolds tensor and a characteristic size of the large turbulent scale. To do this, the velocity field is generated by summing the contributions of several eddies whose position is the result of a random process.
First, these new strategies were compared in the case of turbulent flow within a pipe. The SEM and the injection of a richer spectrum show a real gain in terms of the flow establishment length in this case.
Finally, the new SEM method was tested on an urban flow case and in a zonal RANS/LES coupling context.

==== T7 - Integration of a bending blade method with Dorothy - E. Mascrier (LOMC), M. Roperch (LOMC), A. Vergnaud (LOMC) & G. Pinon (LOMC) ====

The size of offshore wind turbine blades has been steadily increasing over the years. Longer blades result in larger structural displacements during operation. Blade deformation has therefore become a key design parameter for large rotors. In this context, the present project focuses on coupling an in-house structural beam solver, based on Timoshenko beam theory, with an in-house Lagrangian vortex particle solver called Dorothy.
The project was initiated during ECFD8, where static blade deformation was implemented. This year, Dorothy has been fully dynamically coupled with the structural solver.
The first results show good agreement with the literature in terms of blade deflection and aerodynamic forces for the NREL 5MW rotor.
This work will be continued after ECFD9, with additional simulations performed to verify the results against other numerical approaches, such as YALES2.

==== T8 - FSI-3D without deformation strategy for internal flows - P. Benez (Safran), H. Lam (LEGI) & P. Benard (CORIA) ====

==== T9 - LES-based aero-servo-elastic simulation of wind turbines - E. Muller (CORIA & SGRE), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE), B. Duboc (SGRE) & H. Hamdani (GDTech) ====

The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine as well as the structural deformation when YALES2 is coupled to an external library/code. In the past years two coupling libraries have been developed, one to BHawC (SGRE certification tool) and one to OpenFast (NREL open access/open source tool). To improve the user and developer experience, a generalization and uniformization of the two coupling has been conducted in this project. Extensive tests and validations were performed to guarantee the non-regression.

The ALM and ADM (via ALADIN model) frameworks in the YALES2 code were thus enhanced to benefit from these couplings. Such method allows to take part of the external structural solver and controller in single and multiple turbines configurations. Updates were also initiated directly in the coupling libraries to benefit from the latest developments made in the servo-structural solvers, thus allowing to simulate modern academic wind turbines (with OpenFAST) or industrial flagships (with BHawC) in operation.

Furthermore, works on the Risoe Dynamic stall model, initiated during ECFD6, have been achieved. The implementation and integration of this model has been continued, ported to the parallel-optimized ALM framework, and tested and validated on different configurations.

Miscellaneous tasks related to the ALM code pipeline coverage and documentation have been improved.

==== T10 – Numerical simulation of engine rotors - L. Bricteux (UMONS), G. Balarac (LEGI), Y. Bechane (CORIA) & P. Benard (CORIA) ====
This project investigated the capability of the explicit compressible solver in YALES2 to simulate the fan stage of a turbofan engine. The selected configuration is the CATANA rotor, developed at École Centrale de Lyon, for which experimental data are available.
The mesh of this complex geometry was generated using Gmsh and YALES2 and consists of approximately 220 million tetrahedral elements. The setup of the simulation with a moving mesh framework was carried out during the research stay.
During this work, wall boundary conditions were improved, and it was identified that the near-wall turbulence modeling strategy could be enhanced by introducing a compressible wall model based on the work of Debroeyer et al (JFM 2024). Initial simulations have been performed and have produced promising results.
The next step will be to integrate mesh adaptation and the new compressible wall model, and to compare numerical diagnostics with experimental measurements in order to validate both the modeling approach and further validate the solver.

=== Combustion - Y. Bechane (CORIA), R. Letournel (Safran) & S. Dillon (Safran) ===

==== C3 - LES of the thermal degradation of a composite material - A. Grenouilloux (ONERA), K. Bioche (CORIA), N. Dellinger (ONERA) and R. Letournel (Safran) ====

In order to certify new composite materials for aerospace applications, it is essential to understand their degradation dynamics under severe thermal loads. The ONERA FIRE test bed was designed for this purpose. This burner generates a premixed air propane flame that reproduces a thermal flux consistent with certification standards near the impinging region. During tests, a strong emission of pyrolysis gases and a secondary diffusion flame are observed, and these gases can self ignite in regions not directly exposed to the primary flame. The project aimed to improve the modeling of this burner using Large-Eddy Simulation and reduce the overall computational cost. A reduced kinetic mechanism was derived with the Brookesia library, enabling the modeling of both premixed and diffusion flames to take into account appropriate chemistry at the front face. Used in FIRE simulations, this mechanism achieved a CPU speed-up of a factor of two compared with the previous scheme. A second reduced mechanism was generated to target auto ignition of pyrolysis gas mixtures that can occur at the rear face, and a dedicated test case was designed. Recent developments in the CWIPI interface allow for mesh adaptation during coupling between YALES2 and MoDeTheC solvers.

==== C4 - Flamelet-Progress Variable approach in LBM solvers - U. Chikkabikkodu (M2P2), D. Nouembissi (M2P2), I. Mir (M2P2), H. Meunier (M2P2), I. Tsetoglou (M2P2), S. Zhao (M2P2), P. Boivin (M2P2), J. L. Consalvi (IUSTI), R. Mercier (Safran) & S. Dillon (Safran) ====

This project extended the capabilities of ProLB to support flamelet-based combustion modelling by implementing the flamelet progress variable (FPV) approach together with FTACLES capabilities in our LBM solvers - closing a long-standing gap, since table generation and usage had never been available in ProLB.
During the workshop, transport of a passive scalar was implemented and the SDR was modelled using the passive-scalar gradient, which currently form the two control variables used in the flamelet approach. The implementation was verified through simulations of a 2D laminar methane-air jet diffusion flame.
In parallel, for FTACLES we successfully generated both premixed and non-premixed tables with TECERACT (thanks to Renaud and Samuel), and converted them into a format compatible with our code structure. A progress-variable transport equation was also implemented where the diffusion, source and correction terms were read directly from the tabulation. Validation was performed on a 1D CH4/air premixed flame with 10 sampling points within the filter width, accurately recovering the flame speed and demonstrating successful coupling between the LBM solver and the tabulated chemistry.

==== C5 - NOx prediction with a hybrid FTACLES-Virtual chemistry approach - É. Espada (EM2C), M. Préteseille (EM2C), N. Darabiha (EM2C), B. Fiorina (EM2C) ====

Filtered Tabulated Chemistry is a powerfull yet very cost efficient tool to compute flame structure and its stabilisation. However, it is unable to predict NO concentration wihtout adding additional coordinates in the manifold or by using premixed-flamelet based additional model and tabulation like NOMANI. Virtual chemistry on the other hand is a chemistry reduction method that uses machine learning algorithm to reduce drastically the number of species and reaction. This reduced scheme is then transported like any detailed chemistry mechanism. Although the method is also able to recover flame strucure and pollutants, unlike FTACLES, transported chemistry lacks a turbulent combustion model to be applied on realistics industrial LES mesh grids. This present works aims to couple FTACLES and virtual chemistry in a one way coupling: FTACLES will compute flame structure (density, Temperature, velocity field) thanks to its turbulent combusiton model, and will then feed a virtual mechanism with the "main" grid information in order to compute the pollutant informations.

==== C6 - Modelling laminar & turbulent flames with virtual chemistry - M. Préteseille (EM2C), É. Espada (EM2C), N. Darabiha (EM2C), B. Fiorina (EM2C), S. Dillon (Safran), M. Cailler (Safran) ====

A virtual chemistry framework yielding global-type mechanisms has recently been developed and validated, allowing accurate prediction of flame structures at a substantially reduced computational cost. By coupling virtual chemistry with Adaptive Mesh Refinement (AMR) strategies, this work assesses the ability to dynamically resolve reactive zones while maintaining affordable computational costs in high-fidelity LES of industrial burners. A second objective of ECFD9 was to disseminate the virtual schemes generated using SuperVision, a Python-based automated optimization tool built on Cantera. An optimized hydrogen virtual mechanism was successfully implemented and validated in the Lattice–Boltzmann solver ProLB, demonstrating the ease with which these standardized schemes can be integrated into existing reactive flow solvers, and the spread potential of this new chemistry reduction strategy in the combustion community. Finally, the NOx virtual submechanism for hydrogen combustion was improved to accurately capture both thermal and prompt NO formation in hydrogen flames.

==== C8 - Optimization of chemical source terms stiff integration - G. Lartigue (CORIA), Y. Bechane (CORIA), K. Bioche (CORIA), Q. Cerutti (CORIA), M. El Moatamid (CORIA), M. Laignel (CORIA) ====

Integration of chemical source terms remains computationally expensive in configurations that rely on detailed chemistry approach. This project aimed to reduce that cost by (1) modifying the CVODE integration strategy and (2) applying source-term clustering. A first attempt was to modify CVODE’s internal step-size control strategy but it produced only minor gains as some unnecessary integration steps still occurred, mainly in the unburned gases region. This has finally been addressed by enforcing an initial step based on the CFD time step which reduced the computational cost by a factor 2 in these regions. More importantly, relaxing the relative and absolute tolerances used to determine the accuracy of the method reduced the computational cost by approximately 40% while introducing negligible error in physical properties and flame topology for a 1D premixed flame. These results were confirmed on three methane flame configurations: a 1D premixed flame, a 2D triple flame, and the PRECCINSTA burner. Numerical experiments on the PRECCINSTA burner show a reduction in integration cost by a factor of 2.5 using the adjusted CVODE strategy and by a factor of 4.4 when that strategy is combined with clustering.

==== C11 - Optimization of decoupled approach for heat transfers - T.-P. Luu (GDTech), R. Letournel (Safran), M. Tripiciano (Safran) ====

Many industrial aerothermal applications involve strong interactions between fluid flow and solid thermal response, requiring an accurate representation of fluid–solid coupling to predict wall heat fluxes. Although fully two-way coupled simulations provide high fidelity, their complex numerical setup and high computational cost limit their applicability in industrial design loops. As a result, one-way decoupled approaches based on the estimation of heat transfer coefficients (HTCs) are usually preferred. The classical double-run method, which relies on two simulations with imposed wall temperatures to estimate HTC, remains workflow-intensive and highly sensitive to the choice of reference temperatures. In this project, a single-run methodology is proposed to reduce setup complexity and computational cost. The approach introduces an additional passive scalar representing the variation of the fluid sensible enthalpy induced by a change in imposed wall temperature. The associated transport equation is derived under the assumption that the thermophysical properties of the mixture remain weakly dependent on temperature variations. The method is validated on a canonical three-dimensional heated plate configuration and demonstrates promising results when applied to an industrial burner simulation.

=== Mesh adaptation - A. Grenouilloux (ONERA) & G. Balarac (LEGI) ===

==== M2 – Dynamic of SWBLI in Supersonic Propulsive Nozzle Under Hot Gas Conditions - F.A. Rojas Segovia (CORIA), Y. Bechane (CORIA) & L. Voivenel (CORIA) ====
In this project, a STBLE (Solution Thin Boundary Layer Equations) wall model was implemented in YALES2. The focus was to add and compare this model with the pre-existing wall models in the code, such as the logarithmic law and Duprat, in the context of supersonic nozzles. To achieve this, 2D simulations of supersonic compressible flow over a flat plate were conducted as an initial step and validation. These initial simulations provided good insights for future research on the dynamics of Shock Wave and Boundary Layer Interaction (SWBLI) in supersonic nozzles operating with both cold and hot gas conditions.

==== M3 – Criterion for dynamic mesh adaptation in LES - H.Lam (LEGI), G. Balarac (LEGI), V. Moureau (CORIA), R. Barbera (LEGI), P. Launay (CORIA) & L. Voivenel (CORIA) ====

This project proposes a new criterion for dynamic mesh adaptation in LES, designed to overcome the limitations of static LES mesh convergence (static AMC) strategies based on time-averaged quantities. In both static and dynamic contexts, a cell-based Reynolds number is first used as a DNS criterion to identify regions where all turbulent scales must be resolved. For LES, the DNS constraint is relaxed when the integral scale is sufficiently larger than the local cut-off scale, so that a meaningful GS/SGS separation exists. In static AMC, this condition can be evaluated from statistical quantities. In dynamic mesh adaptation, however, such statistics are not available. To overcome this limitation, the proposed approach relies on the assumption that the instantaneous dissipation is predominantly the turbulent dissipation. The integral scale is then estimated from local instantaneous quantities, allowing a dynamic evaluation of the scale-separation criterion. This provides a continuous transition between DNS-like and LES-like regions during the simulation. The method is complemented by a laminar–turbulent discrimination based on a "sigma-sensor" (inspired by the sigma SGS model), enabling the identification of purely laminar zones. The approach has been assessed on a turbulent jet and on flow around a three-dimensional cylinder. Ongoing work focuses on improving near-wall treatments, in particular through prismatic layers generation on boundaries coupled to mesh adaptation and the introduction of dedicated kernels to stabilize the wall mesh and limit excessive boundary motion.

==== M4 – Improve mesh adaptation tools - B. Andrieu (ONERA), C. Benazet (ONERA), N. Dellinger (ONERA), G. Janodet (ONERA) & B. Maugars (ONERA) ====

Building upon the foundations established during ECFD7 and ECFD8 — which focused on periodic CAD-based mesh generation in EGADS and periodic parallel metric gradation — our latest developments for ECFD9 mark a significant step toward a fully automated, CAD-based periodic remeshing algorithm.
First, the parallel hierarchical remeshing algorithm prototype was improved by using a more elaborate ownership system in ParaDiGM to drive the mechanism that merges/dissociates the periodic interface mesh before/after the remeshing pass.
Second, the ability of the refine library (developed at NASA) to remesh non-manifold 3D configurations was investigated. Changes have been made to refine's operators to unlock remeshing near the merged periodic interface in 3D, which yielded promising results, but more work is needed to achieve industrial robustness. To enable CAD-based projections on both sides of the merged periodic interface, an algorithm for building a coherent periodized CAD model was implemented in the EGADS library.
This CAD-based periodic remeshing algorithm was validated in serial through a numerical simulation of the 2D LS89 turbine blade using the SoNICS solver. The results demonstrate that the mesh effectively adapts to capture the strongly anisotropic flow features while strictly respecting the periodic constraints and the geometric support.
Non-manifold mesh adaptation was applied to the ablation of a plate up to burnthrough, first in 2D and then in 3D. The burnthrough detection workflow was improved by developing a Python mini-toolbox for basic geometric queries, allowing the removal of non-physical solid fragments in the middle of the hole after burnthrough. The MMG library was also evaluated for its ability to handle non-manifold meshes, and it appears more suitable than the Refine library for this configuration. The workflow is satisfactory in 2D but needs improvement in 3D to continue the simulation after burnthrough.

==== M5 – Anisotropic mesh adaptation for multiphase flows - Robin Barbera (LEGI), Manuel Bernard (LEGI), Giovanni Ghigliotti (LEGI) & Roxane Letrounel (Safran) ====

This project investigates anisotropic mesh adaptation strategies for multiphase flows, with the objective of reducing computational cost while preserving an accurate representation of fluid interfaces. The approach relies on curvature-based anisotropic remeshing, where mesh anisotropy is locally controlled from interface geometry to ensure a prescribed discretization angle. A key limitation of anisotropic coarsening along interfaces is mass loss induced by interpolation during remeshing, which increases with tangential coarsening and therefore directly conflicts with anisotropic strategies. During ECFD9, this issue was addressed by introducing a high-order interpolation scheme for interface variables, replacing the default linear interpolation. The results show that high-order interpolation significantly reduces mass loss, allowing for much higher mesh anisotropy at the interface, at the cost of a limited computational overhead. In addition, the curvature-based adaptation strategy was extended from mean curvature to the full curvature tensor, enabling the mesh to align with the two principal curvatures of three-dimensional interfaces. The approach was demonstrated on canonical multiphase configurations, including droplet advection and rising bubble cases, showing substantial reductions in mesh size compared to isotropic simulations. Ongoing perspectives include coupling curvature-based adaptation with feature-based anisotropic remeshing to better capture turbulent structures away from the interface.

==== M7 – Increased mesh anisotropy for laminar and RANS applications - R. Barbera (LEGI), J.-B. Lagaert (LMO), T. Berthelon (LEGI), R. Letournel (Safran), M. Bernard (LEGI) & G. Balarac (LEGI) ====

This project addresses the limited level of mesh anisotropy obtained with current feature-based anisotropic remeshing criteria in steady laminar and RANS simulations in YALES2. While recent development of anisotropic mesh adaptation have significantly reduced computational cost, the achieved aspect ratios remain moderate (AR < 50), well below the levels commonly reported in the RANS literature (AR > 100). The objective of the project was to identify the main mechanisms that limit anisotropy in practice, including numerical noise in the resolved quantities, inaccuracies in Hessian computation, the formulation of the criterion itself, ... During ECFD9, the current anisotropic criterion applied to a vectorial quantity of interest (QOI) implemented in YALES2 was reformulated as the minimization of a residual-based error estimator. A Newton optimization strategy was introduced to assess whether the theoretical optimum of the criterion differs from criterion use in practice.The approach was analyzed on the Kovasznay flow, and the optimal solution was shown to be very close to the criterion currently used in YALES2. Comparisons with alternative criteria from the literature and based on scalar QOI further demonstrated similar mesh convergence, highlighting the robustness of the YALES2 approach and its main advantage: a flow-independent, non-dimensional target error. Ongoing investigations focus on quantifying the influence of numerical noise in the resolved quantities and Hessian discretization on the achievable mesh aspect ratios.

=== Two-phase flows - J. Carmona (CORIA), N. Gasnier (Safran) & I. Tsetoglou (M2P2) ===

==== TP1 - Simulation of core shifting during investment casting - Y. Mayi (Safran), M. Cailler (Safran), S. Meynet (GDTech) ====

==== TP2 - Lattice Boltzmann method for free-surface two-phase flow - J. Lu (M2P2), Y. Mediene (M2P2), I. Tsetoglou (M2P2) & S. Zhao (M2P2) ====

The project aims to reproduce and improve a two-equation free-surface Lattice Boltzmann Method (LBM) model for two-phase flows. The original free-surface model features a sharp interface and good numerical stability, but it neglects the gas phase and is therefore limited to two-phase flows in which gas effects are negligible. The recently developed two-equation free-surface LBM model (Liu Y., Sun D., Zhang Z., et al., Physics of Fluids, 2024, 36(3)) incorporates the gas phase, enabling interactions between the two phases. However, this model suffers from a lack of mass conservation and insufficient accuracy in curvature computation.
To overcome these limitations, an auxiliary distribution function is introduced to track mass evolution, thereby decoupling mass conservation from pressure evolution and restoring global mass conservation. In parallel, a pseudo-smoothing step is implemented to achieve more accurate calculations of interface normals and curvature. These improvements are validated through two benchmark test cases. (1) A Laplace test involving both static and advected droplets. It demonstrates exact mass conservation and a significant enhancement in surface tension modeling. (2) A two-phase Poiseuille flow. It shows good agreement with theoretical predictions, validating the viscous coupling between the two fluids.
Future work will focus on improving information exchange across the interface to reduce numerical oscillations and enhance numerical stability, as well as on conducting more complex validation cases.

==== TP3 - Modeling of a gear wheel immersed in an oil bath ====

==== TP4 - Implementation of a granular temperature model - T. Ndereyimana (Université de Sherbrooke), S. Moreau (Université de Sherbrooke), Y. Dufresne (Enerkem) ====

In gas–solid systems such as fluidized beds, clusters of particles naturally appear. These clusters tend to exhibit a Gaussian velocity distribution around an average velocity, with a spread that depends on the local environment of the cluster.
In coarse-grained DEM simulations, real particles are replaced by numerical parcels representing groups of particles in order to reduce the computational cost associated with a large number of particles. In this approach, all particles within a parcel are assumed to move at the same velocity; consequently, no velocity distribution is represented.
This project focuses on comparing two approaches to model the standard deviation of the velocity distribution within a parcel: (1) a local averaging method and (2) a kinetic-theory-of-granular-flow-based methodology. The former computes the standard deviation based on the velocities of surrounding parcels, while the latter relies on two-phase flow theory in which this standard deviation is explicitly modeled.
Both methodologies predict a high standard deviation in the vicinity of gas bubbles in the fluidized bed and lower values in very dense and very dilute regimes. However, the local averaging method tends to increase the computational cost by requiring the detection of neighboring parcels for each parcel.

==== TP5 - Jet-A1 cavitation modeling - P. Benez (Safran), J. Carmona (CORIA) ====

==== TP6 - Comparison of JICF models ====

==== TP7 - Validation and extension of PCS solver for cryo tanks ====

==== TP8 - Jet-in-crossflow simulation with the Hybrid SPH-FVM solver ====

==== TP9 - Multi-physics effects modeling in film flows - N. Gasnier (Safran), P. Portais (CORIA/Safran), L. Voivenel (CORIA), E. Bourrel (CORIA), M. Cailler (Safran) ====
The project aimed at improving the multi scale model for parietal film flows implemented in the YALES2 platform. This model, based on the Shallow Water equations, allows to describe the dynamics of a thin liquid layer spreading over dry walls at a reduced computational cost. First, the numerical method designed during ECFD8 to convert impinging Lagrangian droplets into film data has been extended to account for droplet splashing and rebound phenomena. Then, a sensitivity analysis has been initiated to determine the influence of inlet conditions on the properties of the droplets generated by film atomization. Preliminary results showed that an increase in the gas velocity causes a significant increase in the number of droplet generated, and a large decrease in the drops diameter. Then, the film dynamics model has been extended to rotating walls by including inertial and Coriolis forces in the momentum conservation equation. A first validation of the implementation has been conducted by analyzing the spreading of a liquid film generated by impinging droplets over a rotating disk at high angular speed, which gave promising results. Finally, a film temperature equation has been added to include thermal effects in the thin film model, this additional equation describes the temporal evolution of the surface temperature of the liquid, which is primarily affected by the temperatures of the wall and of the surrounding air. The influence of thermal effects on the dynamics of the liquid is taken into account through the temperature-dependency of the surface tension, which is likely to cause the onset of Marangoni currents due to heating discrepancies.

==== TP10 - Solid-Fluid Coupling for Nucleate Boiling Simulations - M. Umair (LEGI), G. Ghigliotti (LPSC), H. Lam (LEGI), M. Bernard (LEGI), R. Barbera (LEGI), G. Balarac (LEGI) ====

Ecfd:ecfd 9th edition

2026-02-06T07:02:22Z

Benard: /* Projects */

{{DISPLAYTITLE: ECFD workshop, 9th edition, 2026}}

== Description ==

* Event from '''19th of January to 30th of January 2026'''
* Location: [https://www.sport-normandie.fr/le-centre/le-site-de-houlgate Centre Sportif de Normandie], Houlgate, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* Participants from academics, HPC center/experts and industry are welcome
* The number of participants is limited to 80.


* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Voivenel (CORIA).

[[File:Logo_ECFD9.png|center|frameless|900px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_9th_edition]]



== News ==

* 22/09/2025: First announcement of the '''9th Extreme CFD Workshop & Hackathon''' !
* 15/11/2025: Deadline to submit your project

== Thematics / Mini-workshops ==

To be announced...

== Projects ==

=== Hackathon GENCI - P. Begou (LEGI), V. Moureau (CORIA) ===
This ECFD8 GENCI Hackathon was a rich event, involving 4 differents CFD codes (AVBP, ParaDIGM, SONICS and YALES2) using various paradigms (C++/cuda/hip, Fortran/OpenMP/OpenACC) with several SDKs (AMD, Cray/HPE, Nvidia, Gnu) on a large range of GPU architectures (Nvidia A100, GH100, AMD instinct Mi210, Mi250, Mi300). This two-week event benefited from a high level support from three HPC mentors, two on-site from AMD (J. Noudohouenou and A. Tsetoglou) and one remote from CINES (M. Boudaoud).

==== H3 - Hackathon SONICS - A. de Brauer (ONERA), B. Michel (ONERA), B. Berthoul (ONERA) & G. Staffelbach (ONERA) ====
CPU code generation for multispecies simulations – Code generation for multispecies simulations is currently being developed in the SoNICS code. The work carried out at ECFD9 focused on the vectorization of the generated code by code transformation : unrolling the species loops, rewriting if statements, and inverting do/while loops (arising from Newton type algorithms) used in the computation of thermodynamic quantities. The loop-unrolling and if statement rewriting have been profiled and show a speed-up of 2x for the vectorized generated code when computing the HLLC flux, compared with the hand-written implementation. The switch of do/while loops was prototyped on a test code and will be integrated into SoNICS. Code generation on GPU has been tested and validated, but a thorough performance profiling of the GPU version is still required.

Porting reactive multi-species terms to GPU – In 2025 multi-species reactive capabilities were introduced in SoNICS and tested on the Preccinsta case on CPU. Recently the multi-species components were ported to the GPU, so this activity concentrated on porting the reactive source terms. Tests on a 0D reactor show identical results on GPU and CPU. Work has also resumed on porting SoNICS to AMD GPU on the ADASTRA system from CINES/GENCI, where the hipGraphs implementation (AMD’s counterpart of cudaGraph) exhibited some issues. Our participation in ECFD9 allowed us to contact the AMD hipGraphs development team, opening the way to a collaboration. With their council, we updated the code to use rocm7.1.1 providing the first successful non reactive results on AMD GPU. Further work on reactive flows is ongoing.

Improving GPU residual calculations – Recent investigations show that SoNICS’s residual calculation on GPUs was about 100× slower than other GPU operations. The bottleneck was traced to the combination of cudaGraph and thrust::reduce, which prevented parallel execution. Replacing this with a hand-written hierarchical reduction kernel that works efficiently within the cudaGraph restores good scalability; the residual computation is now negligible compared with the other operations, as is the case on CPU. Additional timers were added to the cudaGraph kernels to quantify each operator’s cost relative to the CPU.

=== Numerics & User Interface - M. Bernard (LEGI), G. Lartigue (CORIA) & S. Mendez (IMAG) ===

==== N1 - Improving ICS robustness and accuracy - M. Bernard (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI) & T. Berthelon (LEGI) ====
Bad quality meshes generally lead to larger numerical errors when solving partial differential equations.
This project focused on improving the accuracy and robustness of the incompressible Navier-Stokes solver (ICS).
We investigated the sources of discrepancy introduced at each step of the algorithm, with particular attention to the consequences of the coexistence of two discrete velocity representations: (i) the convective flux <math>\vec{u}\cdot\vec{n}\,dS</math> and (ii) the transported nodal velocity field <math>u^n</math>.
Although these quantities are equivalent at the continuous level, this equivalence no longer holds in the discrete setting.
In particular, only the convective velocity strictly satisfies the divergence-free constraint after solving the Poisson problem for the pressure field.
During this two-week workshop, we developed a new correction strategy for the nodal velocity field in order to enforce consistency with the convective velocity and improve the overall behavior of the solver.

==== N3 - Shock & discontinuity treatment for Lattice-Boltzmann solvers - I. Tsetoglou (M2P2), W. Bessem (M2P2), H. Merley (M2P2) & S. Zhao (M2P2) ====
Lattice—Boltzmann methods (LBM) have traditionally been applied to weakly compressible flows; however, recent developments have extended their applicability to fully compressible regimes. In such flow configurations, shock waves and contact discontinuities naturally arise. To properly capture these features in a discretized framework, artificial diffusion mechanisms are commonly introduced to smooth discontinuities over a limited number of grid points.
In this project, the hybrid LBM solver ProLB was employed. In this framework, the mass and momentum equations are solved using an LBM formulation, while the total energy equation is discretized using a finite-volume (FV) approach with consistent spatial and temporal discretization. The primary objective of the work was to develop and implement an artificial diffusion strategy suitable for hybrid LBM/FV solvers.
Shock waves were detected with a Jameson-type pressure-based sensor, while contact discontinuities were identified with a temperature-based sensor. The pressure-based sensor was scaled to obtain a kinematic viscosity contribution, which was incorporated into the LBM collision relaxation time. Similarly, the temperature-based sensor was scaled to define an artificial thermal conductivity, which was added to the FV discretization of the total energy equation.
A set of validation cases—including the Sod shock tube at various pressure ratios, a 2D Riemann problem, and the interaction of a shock wave with a helium bubble in air—was performed. The results demonstrate that the hybrid LBM approach is capable of accurately capturing shocks and contact discontinuities, even on relatively coarse meshes, while avoiding spurious Gibbs oscillations.

==== N5 - Dorothy: Toward Fully Distributed Implementation - A. Vergnaud (LOMC), M. Roperch (LOMC) & G. Pinon (LOMC) ====

Dorothy is a Vortex Particle Method CFD code for turbine wakes. Its parallel performance needs to be improved when large number of particles is used (e.g. multi-turbines farm cases or far-wake studies). Several limitations are observed due to lacks in terms of memory, structure of data, parallel implementation, etc… To overcome these problems, the possibility of another code structure/architecture (fully parallel and scalable), even for large number of particles, needs to be investigated. The aim of this project is to explore the use of the library AMReX (https://amrex-codes.github.io/amrex/overview.html) which provides a large toolbox to manage massively parallel block-structured AMR applications (mesh data structure, particle data structure, load balancing, processors communications, etc...).

Some tests have been performed to study AMReX performances. In particular, a scalability test has been performed over a tutorial particle method case (Particles In Cells tutorial code), upgraded up to 134 millions of particles (which, for now, is much higher than the number of particles used with Dorothy). A good scalability has been measured, better than with Dorothy: over 75% on 800 cores (on CRIANN). These results are encouraging and suggest good performance when the AMReX library will be used to implement the Vortex Particle Method.

==== N6 - Relaxation of the IBM stability constraint - PL. Martin (IMAG) & S. Mendez (IMAG) ====
Many simulations done in the YALES2BIO framework involve fluid-structure interactions handled with the Immersed Boundary Method (IBM).
This model allows for the fluid/solid coupling, with the forces from the solid acting as a source term in the Navier-Stokes equations.
In some cases for red blood cells simulations, and for most cases for von Willebrand Factor simulations, the governing time step is the force time step. When this is the case, we also notice artifacts in the fluid velocity and pressure fields.
The robustness of our IBM implementation was improved for embedded surfaces by shifting our regularization/interpolation kernels away from the wall in case we work with an embedded solid.
Since these simulations are done at low Reynolds and CFL number (0.01 - 0.001), the stability constraint was relaxed by doing substeps without:
1. advancing the convective velocity, 2. correcting the velocity to make it divergence-free.
The artifacts showing when solids are a lot stiffer than the fluid viscous forces were reduced by projecting the regularized solid forces into a divergence-free space.

==== U1 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 - L. Korzeczek (GDTech), T-P. Luu (GDTech), S. Meynet (GDTech), M. Cailler (Safran), R. Letournel (Safran), G. Lartigue (CORIA)====

Yales2 features an initial version of a graphical interface. This version enables users to execute a series of processes on a local machine, covering data preparation, computation, and post-processing for basic aerodynamic and hydrodynamic calculations.

To facilitate industrialization and support advanced users in applying it to complex projects, it is essential to extend this interface to a broader range of physical applications. This includes enabling the implementation of coupled or chained calculations and allowing communication with remote servers.

The work conducted during this ECFD have significantly strengthened the current architecture, enhancing performance, modularity, and the capacity to accommodate complex scenarios. Additionally, new widgets have been developed, and an initial draft for connecting to a remote server has been initiated.

=== Turbulence - L. Voivenel (CORIA), P. Bénard, (CORIA) & T. Berthelon (LEGI) ===

==== T1 - Concurrent Precursor-Successor with Successor automated mesh convergence - P. Launay (CORIA), L. Voivenel (CORIA) & P. Benard (CORIA) ====

Using a periodic precursor simulation remains the more accurate method for generating realistic fully developed atmospheric turbulence for a successor simulation. However, it is also the most expensive one. Only the sequential method was implemented in YALES2, involving 2 separate simulation running one after the other, and relying on a lookup table as a link between the two. This project proposed to reduce the cost of the method by implementing a concurrent version where both simulations run in the mean time.

This was achieved using existing CWIPI developments. Another issue arising in such periodic precursors is the creation of spanwise inhomogeneities namely "streaks". This issue has been addressed using CWIPI by replacing the streamwise periodic boundary conditions by an internal coupling between an internal plane of the precursor and its inlet where it is being recycled. A spanwise shift of the velocity field is applied at the inlet preventing the generation of "streaks". A flow rate correction is also applied for preventing bulk velocity drift as the recycling procedure induces a 1 iteration delay. Note that this method is more efficient and more accurate than the Recycling method already existing in YALES2 and relying on particles. Finally, the method has been furthermore improved using Traction free outlet boundary conditions in both precursor and successor domains allowing the reduction of domain length.

Overall the cost of the whole workflow has been greatly reduced and the formation of streaks has been prevented.
The nature of the turbulent structures before and after this modification needs further investigation, as well as the use of other streamwise boundary conditions (INLET/INLET, ...), and are the subject of current work.

==== T2 - Discharge movement model for breakdown prediction - S. Wang (EM2C), T. Kabir (EM2C), E. Roger (EM2C), C. Laux (EM2C), B. Fiorina (EM2C), Y. Bechane (CORIA) & V. Moureau (CORIA) ====

A well-established approach for performing 3-D simulations of plasma-assisted combustion at reduced computational cost is the use of phenomenological models for Nanosecond Repetitively Pulsed (NRP) plasma discharges. So far, these models have assumed a static cylindrical shape for the discharge energy deposition region. However, the breakdown location is governed by the flow velocity, electron density, and reduced electric field, which are neither static nor uniform. As a result, the discharge may exhibit elongation, translation, or rotation. This project aimed to implement a simplified physics-based discharge movement model using the reduced electric field, electron mobility, and an electron density-like variable.
Most of the model was successfully implemented, except for the final step, in which the field line corresponding to the maximum restrike probability must be constructed to determine the new plasma restrike zone.

==== T3 - Vorticity model for discharge-induced flow dynamics - S. Wang (EM2C), T. Kabir (EM2C), E. Roger (EM2C), C. Laux (EM2C), B. Fiorina (EM2C), Y. Bechane (CORIA) & V. Moureau (CORIA) ====

A well-established approach for performing 3-D simulations of plasma-assisted combustion at reduced computational cost is the use of phenomenological models for Nanosecond Repetitively Pulsed (NRP) plasma discharges. These models can be implemented within a low-Mach number framework to further reduce the cost. However, doing so removes the acoustic necessary to resolve discharge-induced flow dynamics. Recently, Roger et al. (2025) proposed a model using physics-based vorticity patches to recover these flow dynamics. This project aimed to implement this model in the low-Mach number framework of YALES2 (YALES2-VDS). The model, formulated as an external forcing term in the momentum balance equation, was successfully implemented. However, simulations performed with YALES2-VDS without the vorticity model exhibit the formation of vortices in regions where none are expected. A possible source of error may be related to the treatment of the hydrodynamic pressure gradient and the associated baroclinic torque term in the vorticity equation. The behavior of this term requires further investigation before the viability of the vorticity model within a low-Mach number framework can be properly assessed.

==== T4 - Wind field reconstruction based on LiDAR measurements - T. Cousin (LMI), P. Benard (CORIA), G. Lartigue (CORIA) & JB. Lagaert (LMO) ====

Wind turbines experience significant loads due to the wind pressure exerted on their structure. Accurate prediction of wind turbine behavior is essential for effective management. Simulations use wind data as input, and their realism can be improved by incorporating wind profiles derived from on-site LiDAR measurements.
The scope of this project is to provide a suitable mathematical framework phrased as a minimization problem under incompressibility constraint to reconstruct the wind field from the LiDAR dataset. The entire framework has been developed using the YALES2 scalar solver, with the objective of extending it to the NS solver under the low-Mach number and constant-density approximation.

==== T5 – Hybrid RANS/LES based on dual mesh and LES of fluctuations - G. Balarac (LEGI), T. Berthelon (LEGI) & R. Letournel (Safran) ====

This project is devoted to a fully coupled hybrid RANS/LES strategy based on a dual-mesh framework, where the mean flow is solved by RANS on a mesh tailored for the mean field, while only the turbulent fluctuations are resolved by LES on a second mesh. In addition to deterministic drift (relaxation) terms that drive the resolved velocities in each model toward target fields provided by the other one (RANS mean for LES, LES statistics for RANS), a stochastic forcing built from RANS turbulent quantities is introduced in the LES of fluctuations. These combined forcing terms allow a controlled generation of fluctuations at the RANS/LES interface and reduce the sensitivity to interface location. Two-way coupling is achieved by feeding back the Reynolds stresses computed in the LES into the RANS equations in the resolved regions. The approach is demonstrated on turbulent pipe flows, including a fully coupled simulation at high Reynolds number (Re = 44,000), showing that the method enables wall-resolved hybrid simulations at a fraction of the cost of a full LES.

==== T6 - Injection of coherent structures for LES inlet condition - T. Berthelon (LEGI), G. Balarac (LEGI), R. Letournel (Safran), P. Launay (CORIA), L. Voivenel (CORIA) & P. Benard (CORIA) ====

The boundary conditions of an LES calculation play a key role in the predictability of simulations. In particular, the turbulence injected at the inlet can strongly influence the development of turbulence.
The aim of this project was to extend the turbulence injection capabilities of the YALES2 code. On the one hand, the historical strategy of injecting synthetic homogeneous isotropic turbulence calculated from a Passot-Pouquet spectrum model has been enhanced by enabling the generation of richer spectra (Pope and Von-Karman-Pao spectra model).
On the other hand, the Synthetic Eddy Method (SEM), proposed by Jarrin et al (2008), was implemented. This method consists of generating a coherent velocity field that respects a target Reynolds tensor and a characteristic size of the large turbulent scale. To do this, the velocity field is generated by summing the contributions of several eddies whose position is the result of a random process.
First, these new strategies were compared in the case of turbulent flow within a pipe. The SEM and the injection of a richer spectrum show a real gain in terms of the flow establishment length in this case.
Finally, the new SEM method was tested on an urban flow case and in a zonal RANS/LES coupling context.

==== T7 - Integration of a bending blade method with Dorothy - E. Mascrier (LOMC), M. Roperch (LOMC), A. Vergnaud (LOMC) & G. Pinon (LOMC) ====

The size of offshore wind turbine blades has been steadily increasing over the years. Longer blades result in larger structural displacements during operation. Blade deformation has therefore become a key design parameter for large rotors. In this context, the present project focuses on coupling an in-house structural beam solver, based on Timoshenko beam theory, with an in-house Lagrangian vortex particle solver called Dorothy.
The project was initiated during ECFD8, where static blade deformation was implemented. This year, Dorothy has been fully dynamically coupled with the structural solver.
The first results show good agreement with the literature in terms of blade deflection and aerodynamic forces for the NREL 5MW rotor.
This work will be continued after ECFD9, with additional simulations performed to verify the results against other numerical approaches, such as YALES2.

==== T8 - FSI-3D without deformation strategy for internal flows - P. Benez (Safran), H. Lam (LEGI) & P. Benard (CORIA) ====

==== T9 - LES-based aero-servo-elastic simulation of wind turbines - E. Muller (CORIA & SGRE), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE), B. Duboc (SGRE) & H. Hamdani (GDTech) ====

The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine as well as the structural deformation when YALES2 is coupled to an external library/code. In the past years two coupling libraries have been developed, one to BHawC (SGRE certification tool) and one to OpenFast (NREL open access/open source tool). To improve the user and developer experience, a generalization and uniformization of the two coupling has been conducted in this project. Extensive tests and validations were performed to guarantee the non-regression.

The ALM and ADM (via ALADIN model) frameworks in the YALES2 code were thus enhanced to benefit from these couplings. Such method allows to take part of the external structural solver and controller in single and multiple turbines configurations. Updates were also initiated directly in the coupling libraries to benefit from the latest developments made in the servo-structural solvers, thus allowing to simulate modern academic wind turbines (with OpenFAST) or industrial flagships (with BHawC) in operation.

Furthermore, works on the Risoe Dynamic stall model, initiated during ECFD6, have been achieved. The implementation and integration of this model has been continued, ported to the parallel-optimized ALM framework, and tested and validated on different configurations.

Miscellaneous tasks related to the ALM code pipeline coverage and documentation have been improved.

==== T10 – Numerical simulation of engine rotors - L. Bricteux (UMONS), G. Balarac (LEGI), Y. Bechane (CORIA) & P. Benard (CORIA) ====
This project investigated the capability of the explicit compressible solver in YALES2 to simulate the fan stage of a turbofan engine. The selected configuration is the CATANA rotor, developed at École Centrale de Lyon, for which experimental data are available.
The mesh of this complex geometry was generated using Gmsh and YALES2 and consists of approximately 220 million tetrahedral elements. The setup of the simulation with a moving mesh framework was carried out during the research stay.
During this work, wall boundary conditions were improved, and it was identified that the near-wall turbulence modeling strategy could be enhanced by introducing a compressible wall model based on the work of Debroeyer et al (JFM 2024). Initial simulations have been performed and have produced promising results.
The next step will be to integrate mesh adaptation and the new compressible wall model, and to compare numerical diagnostics with experimental measurements in order to validate both the modeling approach and further validate the solver.

=== Combustion - Y. Bechane (CORIA), R. Letournel (Safran) & S. Dillon (Safran) ===

==== C3 - LES of the thermal degradation of a composite material - A. Grenouilloux (ONERA), K. Bioche (CORIA), N. Dellinger (ONERA) and R. Letournel (Safran) ====

In order to certify new composite materials for aerospace applications, it is essential to understand their degradation dynamics under severe thermal loads. The ONERA FIRE test bed was designed for this purpose. This burner generates a premixed air propane flame that reproduces a thermal flux consistent with certification standards near the impinging region. During tests, a strong emission of pyrolysis gases and a secondary diffusion flame are observed, and these gases can self ignite in regions not directly exposed to the primary flame. The project aimed to improve the modeling of this burner using Large-Eddy Simulation and reduce the overall computational cost. A reduced kinetic mechanism was derived with the Brookesia library, enabling the modeling of both premixed and diffusion flames to take into account appropriate chemistry at the front face. Used in FIRE simulations, this mechanism achieved a CPU speed-up of a factor of two compared with the previous scheme. A second reduced mechanism was generated to target auto ignition of pyrolysis gas mixtures that can occur at the rear face, and a dedicated test case was designed. Recent developments in the CWIPI interface allow for mesh adaptation during coupling between YALES2 and MoDeTheC solvers.

==== C4 - Flamelet-Progress Variable approach in LBM solvers - U. Chikkabikkodu (M2P2), D. Nouembissi (M2P2), I. Mir (M2P2), H. Meunier (M2P2), I. Tsetoglou (M2P2), S. Zhao (M2P2), P. Boivin (M2P2), J. L. Consalvi (IUSTI), R. Mercier (Safran) & S. Dillon (Safran) ====

This project extended the capabilities of ProLB to support flamelet-based combustion modelling by implementing the flamelet progress variable (FPV) approach together with FTACLES capabilities in our LBM solvers - closing a long-standing gap, since table generation and usage had never been available in ProLB.
During the workshop, transport of a passive scalar was implemented and the SDR was modelled using the passive-scalar gradient, which currently form the two control variables used in the flamelet approach. The implementation was verified through simulations of a 2D laminar methane-air jet diffusion flame.
In parallel, for FTACLES we successfully generated both premixed and non-premixed tables with TECERACT (thanks to Renaud and Samuel), and converted them into a format compatible with our code structure. A progress-variable transport equation was also implemented where the diffusion, source and correction terms were read directly from the tabulation. Validation was performed on a 1D CH4/air premixed flame with 10 sampling points within the filter width, accurately recovering the flame speed and demonstrating successful coupling between the LBM solver and the tabulated chemistry.

==== C5 - NOx prediction with a hybrid FTACLES-Virtual chemistry approach - É. Espada (EM2C), M. Préteseille (EM2C), N. Darabiha (EM2C), B. Fiorina (EM2C) ====

Filtered Tabulated Chemistry is a powerfull yet very cost efficient tool to compute flame structure and its stabilisation. However, it is unable to predict NO concentration wihtout adding additional coordinates in the manifold or by using premixed-flamelet based additional model and tabulation like NOMANI. Virtual chemistry on the other hand is a chemistry reduction method that uses machine learning algorithm to reduce drastically the number of species and reaction. This reduced scheme is then transported like any detailed chemistry mechanism. Although the method is also able to recover flame strucure and pollutants, unlike FTACLES, transported chemistry lacks a turbulent combustion model to be applied on realistics industrial LES mesh grids. This present works aims to couple FTACLES and virtual chemistry in a one way coupling: FTACLES will compute flame structure (density, Temperature, velocity field) thanks to its turbulent combusiton model, and will then feed a virtual mechanism with the "main" grid information in order to compute the pollutant informations.

==== C6 - Modelling laminar & turbulent flames with virtual chemistry - M. Préteseille (EM2C), É. Espada (EM2C), N. Darabiha (EM2C), B. Fiorina (EM2C), S. Dillon (Safran), M. Cailler (Safran) ====

A virtual chemistry framework yielding global-type mechanisms has recently been developed and validated, allowing accurate prediction of flame structures at a substantially reduced computational cost. By coupling virtual chemistry with Adaptive Mesh Refinement (AMR) strategies, this work assesses the ability to dynamically resolve reactive zones while maintaining affordable computational costs in high-fidelity LES of industrial burners. A second objective of ECFD9 was to disseminate the virtual schemes generated using SuperVision, a Python-based automated optimization tool built on Cantera. An optimized hydrogen virtual mechanism was successfully implemented and validated in the Lattice–Boltzmann solver ProLB, demonstrating the ease with which these standardized schemes can be integrated into existing reactive flow solvers, and the spread potential of this new chemistry reduction strategy in the combustion community. Finally, the NOx virtual submechanism for hydrogen combustion was improved to accurately capture both thermal and prompt NO formation in hydrogen flames.

==== C8 - Optimization of chemical source terms stiff integration - G. Lartigue (CORIA), Y. Bechane (CORIA), K. Bioche (CORIA), Q. Cerutti (CORIA), M. El Moatamid (CORIA), M. Laignel (CORIA) ====

Integration of chemical source terms remains computationally expensive in configurations that rely on detailed chemistry approach. This project aimed to reduce that cost by (1) modifying the CVODE integration strategy and (2) applying source-term clustering. A first attempt was to modify CVODE’s internal step-size control strategy but it produced only minor gains as some unnecessary integration steps still occurred, mainly in the unburned gases region. This has finally been addressed by enforcing an initial step based on the CFD time step which reduced the computational cost by a factor 2 in these regions. More importantly, relaxing the relative and absolute tolerances used to determine the accuracy of the method reduced the computational cost by approximately 40% while introducing negligible error in physical properties and flame topology for a 1D premixed flame. These results were confirmed on three methane flame configurations: a 1D premixed flame, a 2D triple flame, and the PRECCINSTA burner. Numerical experiments on the PRECCINSTA burner show a reduction in integration cost by a factor of 2.5 using the adjusted CVODE strategy and by a factor of 4.4 when that strategy is combined with clustering.

==== C11 - Optimization of decoupled approach for heat transfers - T.-P. Luu (GDTech), R. Letournel (Safran), M. Tripiciano (Safran) ====

Many industrial aerothermal applications involve strong interactions between fluid flow and solid thermal response, requiring an accurate representation of fluid–solid coupling to predict wall heat fluxes. Although fully two-way coupled simulations provide high fidelity, their complex numerical setup and high computational cost limit their applicability in industrial design loops. As a result, one-way decoupled approaches based on the estimation of heat transfer coefficients (HTCs) are usually preferred. The classical double-run method, which relies on two simulations with imposed wall temperatures to estimate HTC, remains workflow-intensive and highly sensitive to the choice of reference temperatures. In this project, a single-run methodology is proposed to reduce setup complexity and computational cost. The approach introduces an additional passive scalar representing the variation of the fluid sensible enthalpy induced by a change in imposed wall temperature. The associated transport equation is derived under the assumption that the thermophysical properties of the mixture remain weakly dependent on temperature variations. The method is validated on a canonical three-dimensional heated plate configuration and demonstrates promising results when applied to an industrial burner simulation.

=== Mesh adaptation - A. Grenouilloux (ONERA) & G. Balarac (LEGI) ===

==== M2 – Dynamic of SWBLI in Supersonic Propulsive Nozzle Under Hot Gas Conditions - F.A. Rojas Segovia (CORIA), Y. Bechane (CORIA) & L. Voivenel (CORIA) ====
In this project, a STBLE (Solution Thin Boundary Layer Equations) wall model was implemented in YALES2. The focus was to add and compare this model with the pre-existing wall models in the code, such as the logarithmic law and Duprat, in the context of supersonic nozzles. To achieve this, 2D simulations of supersonic compressible flow over a flat plate were conducted as an initial step and validation. These initial simulations provided good insights for future research on the dynamics of Shock Wave and Boundary Layer Interaction (SWBLI) in supersonic nozzles operating with both cold and hot gas conditions.

==== M3 – Criterion for dynamic mesh adaptation in LES - H.Lam (LEGI), G. Balarac (LEGI), V. Moureau (CORIA), R. Barbera (LEGI), P. Launay (CORIA) & L. Voivenel (CORIA) ====

This project proposes a new criterion for dynamic mesh adaptation in LES, designed to overcome the limitations of static LES mesh convergence (static AMC) strategies based on time-averaged quantities. In both static and dynamic contexts, a cell-based Reynolds number is first used as a DNS criterion to identify regions where all turbulent scales must be resolved. For LES, the DNS constraint is relaxed when the integral scale is sufficiently larger than the local cut-off scale, so that a meaningful GS/SGS separation exists. In static AMC, this condition can be evaluated from statistical quantities. In dynamic mesh adaptation, however, such statistics are not available. To overcome this limitation, the proposed approach relies on the assumption that the instantaneous dissipation is predominantly the turbulent dissipation. The integral scale is then estimated from local instantaneous quantities, allowing a dynamic evaluation of the scale-separation criterion. This provides a continuous transition between DNS-like and LES-like regions during the simulation. The method is complemented by a laminar–turbulent discrimination based on a "sigma-sensor" (inspired by the sigma SGS model), enabling the identification of purely laminar zones. The approach has been assessed on a turbulent jet and on flow around a three-dimensional cylinder. Ongoing work focuses on improving near-wall treatments, in particular through prismatic layers generation on boundaries coupled to mesh adaptation and the introduction of dedicated kernels to stabilize the wall mesh and limit excessive boundary motion.

==== M4 – Improve mesh adaptation tools - B. Andrieu (ONERA), C. Benazet (ONERA), N. Dellinger (ONERA), G. Janodet (ONERA) & B. Maugars (ONERA) ====

Building upon the foundations established during ECFD7 and ECFD8 — which focused on periodic CAD-based mesh generation in EGADS and periodic parallel metric gradation — our latest developments for ECFD9 mark a significant step toward a fully automated, CAD-based periodic remeshing algorithm.
First, the parallel hierarchical remeshing algorithm prototype was improved by using a more elaborate ownership system in ParaDiGM to drive the mechanism that merges/dissociates the periodic interface mesh before/after the remeshing pass.
Second, the ability of the refine library (developed at NASA) to remesh non-manifold 3D configurations was investigated. Changes have been made to refine's operators to unlock remeshing near the merged periodic interface in 3D, which yielded promising results, but more work is needed to achieve industrial robustness. To enable CAD-based projections on both sides of the merged periodic interface, an algorithm for building a coherent periodized CAD model was implemented in the EGADS library.
This CAD-based periodic remeshing algorithm was validated in serial through a numerical simulation of the 2D LS89 turbine blade using the SoNICS solver. The results demonstrate that the mesh effectively adapts to capture the strongly anisotropic flow features while strictly respecting the periodic constraints and the geometric support.
Non-manifold mesh adaptation was applied to the ablation of a plate up to burnthrough, first in 2D and then in 3D. The burnthrough detection workflow was improved by developing a Python mini-toolbox for basic geometric queries, allowing the removal of non-physical solid fragments in the middle of the hole after burnthrough. The MMG library was also evaluated for its ability to handle non-manifold meshes, and it appears more suitable than the Refine library for this configuration. The workflow is satisfactory in 2D but needs improvement in 3D to continue the simulation after burnthrough.

==== M5 – Anisotropic mesh adaptation for multiphase flows - Robin Barbera (LEGI), Manuel Bernard (LEGI), Giovanni Ghigliotti (LEGI) & Roxane Letrounel (Safran) ====

This project investigates anisotropic mesh adaptation strategies for multiphase flows, with the objective of reducing computational cost while preserving an accurate representation of fluid interfaces. The approach relies on curvature-based anisotropic remeshing, where mesh anisotropy is locally controlled from interface geometry to ensure a prescribed discretization angle. A key limitation of anisotropic coarsening along interfaces is mass loss induced by interpolation during remeshing, which increases with tangential coarsening and therefore directly conflicts with anisotropic strategies. During ECFD9, this issue was addressed by introducing a high-order interpolation scheme for interface variables, replacing the default linear interpolation. The results show that high-order interpolation significantly reduces mass loss, allowing for much higher mesh anisotropy at the interface, at the cost of a limited computational overhead. In addition, the curvature-based adaptation strategy was extended from mean curvature to the full curvature tensor, enabling the mesh to align with the two principal curvatures of three-dimensional interfaces. The approach was demonstrated on canonical multiphase configurations, including droplet advection and rising bubble cases, showing substantial reductions in mesh size compared to isotropic simulations. Ongoing perspectives include coupling curvature-based adaptation with feature-based anisotropic remeshing to better capture turbulent structures away from the interface.

==== M7 – Increased mesh anisotropy for laminar and RANS applications - R. Barbera (LEGI), J.-B. Lagaert (LMO), T. Berthelon (LEGI), R. Letournel (Safran), M. Bernard (LEGI) & G. Balarac (LEGI) ====

This project addresses the limited level of mesh anisotropy obtained with current feature-based anisotropic remeshing criteria in steady laminar and RANS simulations in YALES2. While recent development of anisotropic mesh adaptation have significantly reduced computational cost, the achieved aspect ratios remain moderate (AR < 50), well below the levels commonly reported in the RANS literature (AR > 100). The objective of the project was to identify the main mechanisms that limit anisotropy in practice, including numerical noise in the resolved quantities, inaccuracies in Hessian computation, the formulation of the criterion itself, ... During ECFD9, the current anisotropic criterion applied to a vectorial quantity of interest (QOI) implemented in YALES2 was reformulated as the minimization of a residual-based error estimator. A Newton optimization strategy was introduced to assess whether the theoretical optimum of the criterion differs from criterion use in practice.The approach was analyzed on the Kovasznay flow, and the optimal solution was shown to be very close to the criterion currently used in YALES2. Comparisons with alternative criteria from the literature and based on scalar QOI further demonstrated similar mesh convergence, highlighting the robustness of the YALES2 approach and its main advantage: a flow-independent, non-dimensional target error. Ongoing investigations focus on quantifying the influence of numerical noise in the resolved quantities and Hessian discretization on the achievable mesh aspect ratios.

=== Two-phase flows - J. Carmona (CORIA), N. Gasnier (Safran) & I. Tsetoglou (M2P2) ===

==== TP1 - Simulation of core shifting during investment casting - Y. Mayi (Safran), M. Cailler (Safran), S. Meynet (GDTech) ====

==== TP2 - Lattice Boltzmann method for free-surface two-phase flow - J. Lu (M2P2), Y. Mediene (M2P2), I. Tsetoglou (M2P2) & S. Zhao (M2P2) ====

The project aims to reproduce and improve a two-equation free-surface Lattice Boltzmann Method (LBM) model for two-phase flows. The original free-surface model features a sharp interface and good numerical stability, but it neglects the gas phase and is therefore limited to two-phase flows in which gas effects are negligible. The recently developed two-equation free-surface LBM model (Liu Y., Sun D., Zhang Z., et al., Physics of Fluids, 2024, 36(3)) incorporates the gas phase, enabling interactions between the two phases. However, this model suffers from a lack of mass conservation and insufficient accuracy in curvature computation.
To overcome these limitations, an auxiliary distribution function is introduced to track mass evolution, thereby decoupling mass conservation from pressure evolution and restoring global mass conservation. In parallel, a pseudo-smoothing step is implemented to achieve more accurate calculations of interface normals and curvature. These improvements are validated through two benchmark test cases. (1) A Laplace test involving both static and advected droplets. It demonstrates exact mass conservation and a significant enhancement in surface tension modeling. (2) A two-phase Poiseuille flow. It shows good agreement with theoretical predictions, validating the viscous coupling between the two fluids.
Future work will focus on improving information exchange across the interface to reduce numerical oscillations and enhance numerical stability, as well as on conducting more complex validation cases.

==== TP3 - Modeling of a gear wheel immersed in an oil bath ====

==== TP4 - Implementation of a granular temperature model - T. Ndereyimana (Université de Sherbrooke), S. Moreau (Université de Sherbrooke), Y. Dufresne (Enerkem) ====

In gas–solid systems such as fluidized beds, clusters of particles naturally appear. These clusters tend to exhibit a Gaussian velocity distribution around an average velocity, with a spread that depends on the local environment of the cluster.
In coarse-grained DEM simulations, real particles are replaced by numerical parcels representing groups of particles in order to reduce the computational cost associated with a large number of particles. In this approach, all particles within a parcel are assumed to move at the same velocity; consequently, no velocity distribution is represented.
This project focuses on comparing two approaches to model the standard deviation of the velocity distribution within a parcel: (1) a local averaging method and (2) a kinetic-theory-of-granular-flow-based methodology. The former computes the standard deviation based on the velocities of surrounding parcels, while the latter relies on two-phase flow theory in which this standard deviation is explicitly modeled.
Both methodologies predict a high standard deviation in the vicinity of gas bubbles in the fluidized bed and lower values in very dense and very dilute regimes. However, the local averaging method tends to increase the computational cost by requiring the detection of neighboring parcels for each parcel.

==== TP5 - Jet-A1 cavitation modeling - P. Benez (Safran), J. Carmona (CORIA) ====

==== TP6 - Comparison of JICF models ====

==== TP7 - Validation and extension of PCS solver for cryo tanks ====

==== TP8 - Jet-in-crossflow simulation with the Hybrid SPH-FVM solver ====

==== TP9 - Multi-physics effects modeling in film flows - N. Gasnier (Safran), P. Portais (CORIA/Safran), L. Voivenel (CORIA), E. Bourrel (CORIA), M. Cailler (Safran) ====
The project aimed at improving the multi scale model for parietal film flows implemented in the YALES2 platform. This model, based on the Shallow Water equations, allows to describe the dynamics of a thin liquid layer spreading over dry walls at a reduced computational cost. First, the numerical method designed during ECFD8 to convert impinging Lagrangian droplets into film data has been extended to account for droplet splashing and rebound phenomena. Then, a sensitivity analysis has been initiated to determine the influence of inlet conditions on the properties of the droplets generated by film atomization. Preliminary results showed that an increase in the gas velocity causes a significant increase in the number of droplet generated, and a large decrease in the drops diameter. Then, the film dynamics model has been extended to rotating walls by including inertial and Coriolis forces in the momentum conservation equation. A first validation of the implementation has been conducted by analyzing the spreading of a liquid film generated by impinging droplets over a rotating disk at high angular speed, which gave promising results. Finally, a film temperature equation has been added to include thermal effects in the thin film model, this additional equation describes the temporal evolution of the surface temperature of the liquid, which is primarily affected by the temperatures of the wall and of the surrounding air. The influence of thermal effects on the dynamics of the liquid is taken into account through the temperature-dependency of the surface tension, which is likely to cause the onset of Marangoni currents due to heating discrepancies.

==== TP10 - Solid-Fluid Coupling for Nucleate Boiling Simulations - M. Umair (LEGI), G. Ghigliotti (LPSC), H. Lam (LEGI), M. Bernard (LEGI), R. Barbera (LEGI), G. Balarac (LEGI) ====

Ecfd:ecfd 9th edition

2026-02-05T12:52:24Z

Benard: /* Turbulence - L. Voivenel (CORIA), P. Bénard, (CORIA) & T. Berthelon (LEGI) */

{{DISPLAYTITLE: ECFD workshop, 9th edition, 2026}}

== Description ==

* Event from '''19th of January to 30th of January 2026'''
* Location: [https://www.sport-normandie.fr/le-centre/le-site-de-houlgate Centre Sportif de Normandie], Houlgate, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* Participants from academics, HPC center/experts and industry are welcome
* The number of participants is limited to 80.


* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Voivenel (CORIA).

[[File:Logo_ECFD9.png|center|frameless|900px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_9th_edition]]



== News ==

* 22/09/2025: First announcement of the '''9th Extreme CFD Workshop & Hackathon''' !
* 15/11/2025: Deadline to submit your project

== Thematics / Mini-workshops ==

To be announced...

== Projects ==

The projects will be selected after the end of the submission phase (end of November).

=== Numerics & User Interface - M. Bernard (LEGI), G. Lartigue (CORIA) & S. Mendez (IMAG) ===

==== N1 - Improving ICS robustness and accuracy - M. Bernard (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI) & T. Berthelon (LEGI) ====
Bad quality meshes generally lead to larger numerical errors when solving partial differential equations.
This project focused on improving the accuracy and robustness of the incompressible Navier-Stokes solver (ICS).
We investigated the sources of discrepancy introduced at each step of the algorithm, with particular attention to the consequences of the coexistence of two discrete velocity representations: (i) the convective flux <math>\vec{u}\cdot\vec{n}\,dS</math> and (ii) the transported nodal velocity field <math>u^n</math>.
Although these quantities are equivalent at the continuous level, this equivalence no longer holds in the discrete setting.
In particular, only the convective velocity strictly satisfies the divergence-free constraint after solving the Poisson problem for the pressure field.
During this two-week workshop, we developed a new correction strategy for the nodal velocity field in order to enforce consistency with the convective velocity and improve the overall behavior of the solver.

==== N3 - Shock & discontinuity treatment for Lattice-Boltzmann solvers - I. Tsetoglou (M2P2), W. Bessem (M2P2), H. Merley (M2P2) & S. Zhao (M2P2) ====
Lattice—Boltzmann methods (LBM) have traditionally been applied to weakly compressible flows; however, recent developments have extended their applicability to fully compressible regimes. In such flow configurations, shock waves and contact discontinuities naturally arise. To properly capture these features in a discretized framework, artificial diffusion mechanisms are commonly introduced to smooth discontinuities over a limited number of grid points.
In this project, the hybrid LBM solver ProLB was employed. In this framework, the mass and momentum equations are solved using an LBM formulation, while the total energy equation is discretized using a finite-volume (FV) approach with consistent spatial and temporal discretization. The primary objective of the work was to develop and implement an artificial diffusion strategy suitable for hybrid LBM/FV solvers.
Shock waves were detected with a Jameson-type pressure-based sensor, while contact discontinuities were identified with a temperature-based sensor. The pressure-based sensor was scaled to obtain a kinematic viscosity contribution, which was incorporated into the LBM collision relaxation time. Similarly, the temperature-based sensor was scaled to define an artificial thermal conductivity, which was added to the FV discretization of the total energy equation.
A set of validation cases—including the Sod shock tube at various pressure ratios, a 2D Riemann problem, and the interaction of a shock wave with a helium bubble in air—was performed. The results demonstrate that the hybrid LBM approach is capable of accurately capturing shocks and contact discontinuities, even on relatively coarse meshes, while avoiding spurious Gibbs oscillations.

==== N5 - Dorothy: Toward Fully Distributed Implementation - A. Vergnaud (LOMC), M. Roperch (LOMC) & G. Pinon (LOMC) ====

Dorothy is a Vortex Particle Method CFD code for turbine wakes. Its parallel performance needs to be improved when large number of particles is used (e.g. multi-turbines farm cases or far-wake studies). Several limitations are observed due to lacks in terms of memory, structure of data, parallel implementation, etc… To overcome these problems, the possibility of another code structure/architecture (fully parallel and scalable), even for large number of particles, needs to be investigated. The aim of this project is to explore the use of the library AMReX (https://amrex-codes.github.io/amrex/overview.html) which provides a large toolbox to manage massively parallel block-structured AMR applications (mesh data structure, particle data structure, load balancing, processors communications, etc...).

Some tests have been performed to study AMReX performances. In particular, a scalability test has been performed over a tutorial particle method case (Particles In Cells tutorial code), upgraded up to 134 millions of particles (which, for now, is much higher than the number of particles used with Dorothy). A good scalability has been measured, better than with Dorothy: over 75% on 800 cores (on CRIANN). These results are encouraging and suggest good performance when the AMReX library will be used to implement the Vortex Particle Method.

==== N6 - Relaxation of the IBM stability constraint - PL. Martin (IMAG) & S. Mendez (IMAG) ====
Many simulations done in the YALES2BIO framework involve fluid-structure interactions handled with the Immersed Boundary Method (IBM).
This model allows for the fluid/solid coupling, with the forces from the solid acting as a source term in the Navier-Stokes equations.
In some cases for red blood cells simulations, and for most cases for von Willebrand Factor simulations, the governing time step is the force time step. When this is the case, we also notice artifacts in the fluid velocity and pressure fields.
The robustness of our IBM implementation was improved for embedded surfaces by shifting our regularization/interpolation kernels away from the wall in case we work with an embedded solid.
Since these simulations are done at low Reynolds and CFL number (0.01 - 0.001), the stability constraint was relaxed by doing substeps without:
1. advancing the convective velocity, 2. correcting the velocity to make it divergence-free.
The artifacts showing when solids are a lot stiffer than the fluid viscous forces were reduced by projecting the regularized solid forces into a divergence-free space.

==== U1 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 - L. Korzeczek (GDTech), T-P. Luu (GDTech), S. Meynet (GDTech), M. Cailler (Safran), R. Letournel (Safran), G. Lartigue (CORIA)====

Yales2 features an initial version of a graphical interface. This version enables users to execute a series of processes on a local machine, covering data preparation, computation, and post-processing for basic aerodynamic and hydrodynamic calculations.

To facilitate industrialization and support advanced users in applying it to complex projects, it is essential to extend this interface to a broader range of physical applications. This includes enabling the implementation of coupled or chained calculations and allowing communication with remote servers.

The work conducted during this ECFD have significantly strengthened the current architecture, enhancing performance, modularity, and the capacity to accommodate complex scenarios. Additionally, new widgets have been developed, and an initial draft for connecting to a remote server has been initiated.

=== Turbulence - L. Voivenel (CORIA), P. Bénard, (CORIA) & T. Berthelon (LEGI) ===

==== T1 - Concurrent Precursor-Successor with Successor automated mesh convergence - P. Launay (CORIA), L. Voivenel (CORIA) & P. Benard (CORIA) ====

Using a periodic precursor simulation remains the more accurate method for generating realistic fully developed atmospheric turbulence for a successor simulation. However, it is also the most expensive one. Only the sequential method was implemented in YALES2, involving 2 separate simulation running one after the other, and relying on a lookup table as a link between the two. This project proposed to reduce the cost of the method by implementing a concurrent version where both simulations run in the mean time.

This was achieved using existing CWIPI developments. Another issue arising in such periodic precursors is the creation of spanwise inhomogeneities namely "streaks". This issue has been addressed using CWIPI by replacing the streamwise periodic boundary conditions by an internal coupling between an internal plane of the precursor and its inlet where it is being recycled. A spanwise shift of the velocity field is applied at the inlet preventing the generation of "streaks". A flow rate correction is also applied for preventing bulk velocity drift as the recycling procedure induces a 1 iteration delay. Note that this method is more efficient and more accurate than the Recycling method already existing in YALES2 and relying on particles. Finally, the method has been furthermore improved using Traction free outlet boundary conditions in both precursor and successor domains allowing the reduction of domain length.

Overall the cost of the whole workflow has been greatly reduced and the formation of streaks has been prevented.
The nature of the turbulent structures before and after this modification needs further investigation, as well as the use of other streamwise boundary conditions (INLET/INLET, ...), and are the subject of current work.

==== T2 - Discharge movement model for breakdown prediction - S. Wang (EM2C), T. Kabir (EM2C), E. Roger (EM2C), C. Laux (EM2C), B. Fiorina (EM2C), Y. Bechane (CORIA) & V. Moureau (CORIA) ====

A well-established approach for performing 3-D simulations of plasma-assisted combustion at reduced computational cost is the use of phenomenological models for Nanosecond Repetitively Pulsed (NRP) plasma discharges. So far, these models have assumed a static cylindrical shape for the discharge energy deposition region. However, the breakdown location is governed by the flow velocity, electron density, and reduced electric field, which are neither static nor uniform. As a result, the discharge may exhibit elongation, translation, or rotation. This project aimed to implement a simplified physics-based discharge movement model using the reduced electric field, electron mobility, and an electron density-like variable.
Most of the model was successfully implemented, except for the final step, in which the field line corresponding to the maximum restrike probability must be constructed to determine the new plasma restrike zone.

==== T3 - Vorticity model for discharge-induced flow dynamics - S. Wang (EM2C), T. Kabir (EM2C), E. Roger (EM2C), C. Laux (EM2C), B. Fiorina (EM2C), Y. Bechane (CORIA) & V. Moureau (CORIA) ====

A well-established approach for performing 3-D simulations of plasma-assisted combustion at reduced computational cost is the use of phenomenological models for Nanosecond Repetitively Pulsed (NRP) plasma discharges. These models can be implemented within a low-Mach number framework to further reduce the cost. However, doing so removes the acoustic necessary to resolve discharge-induced flow dynamics. Recently, Roger et al. (2025) proposed a model using physics-based vorticity patches to recover these flow dynamics. This project aimed to implement this model in the low-Mach number framework of YALES2 (YALES2-VDS). The model, formulated as an external forcing term in the momentum balance equation, was successfully implemented. However, simulations performed with YALES2-VDS without the vorticity model exhibit the formation of vortices in regions where none are expected. A possible source of error may be related to the treatment of the hydrodynamic pressure gradient and the associated baroclinic torque term in the vorticity equation. The behavior of this term requires further investigation before the viability of the vorticity model within a low-Mach number framework can be properly assessed.

==== T4 - Wind field reconstruction based on LiDAR measurements - T. Cousin (LMI), P. Benard (CORIA), G. Lartigue (CORIA) & JB. Lagaert (LMO) ====

Wind turbines experience significant loads due to the wind pressure exerted on their structure. Accurate prediction of wind turbine behavior is essential for effective management. Simulations use wind data as input, and their realism can be improved by incorporating wind profiles derived from on-site LiDAR measurements.
The scope of this project is to provide a suitable mathematical framework phrased as a minimization problem under incompressibility constraint to reconstruct the wind field from the LiDAR dataset. The entire framework has been developed using the YALES2 scalar solver, with the objective of extending it to the NS solver under the low-Mach number and constant-density approximation.

==== T5 – Hybrid RANS/LES based on dual mesh and LES of fluctuations - G. Balarac (LEGI), T. Berthelon (LEGI) & R. Letournel (Safran) ====

This project is devoted to a fully coupled hybrid RANS/LES strategy based on a dual-mesh framework, where the mean flow is solved by RANS on a mesh tailored for the mean field, while only the turbulent fluctuations are resolved by LES on a second mesh. In addition to deterministic drift (relaxation) terms that drive the resolved velocities in each model toward target fields provided by the other one (RANS mean for LES, LES statistics for RANS), a stochastic forcing built from RANS turbulent quantities is introduced in the LES of fluctuations. These combined forcing terms allow a controlled generation of fluctuations at the RANS/LES interface and reduce the sensitivity to interface location. Two-way coupling is achieved by feeding back the Reynolds stresses computed in the LES into the RANS equations in the resolved regions. The approach is demonstrated on turbulent pipe flows, including a fully coupled simulation at high Reynolds number (Re = 44,000), showing that the method enables wall-resolved hybrid simulations at a fraction of the cost of a full LES.

==== T6 - Injection of coherent structures for LES inlet condition - T. Berthelon (LEGI), G. Balarac (LEGI), R. Letournel (Safran), P. Launay (CORIA), L. Voivenel (CORIA) & P. Benard (CORIA) ====

The boundary conditions of an LES calculation play a key role in the predictability of simulations. In particular, the turbulence injected at the inlet can strongly influence the development of turbulence.
The aim of this project was to extend the turbulence injection capabilities of the YALES2 code. On the one hand, the historical strategy of injecting synthetic homogeneous isotropic turbulence calculated from a Passot-Pouquet spectrum model has been enhanced by enabling the generation of richer spectra (Pope and Von-Karman-Pao spectra model).
On the other hand, the Synthetic Eddy Method (SEM), proposed by Jarrin et al (2008), was implemented. This method consists of generating a coherent velocity field that respects a target Reynolds tensor and a characteristic size of the large turbulent scale. To do this, the velocity field is generated by summing the contributions of several eddies whose position is the result of a random process.
First, these new strategies were compared in the case of turbulent flow within a pipe. The SEM and the injection of a richer spectrum show a real gain in terms of the flow establishment length in this case.
Finally, the new SEM method was tested on an urban flow case and in a zonal RANS/LES coupling context.

==== T7 - Integration of a bending blade method with Dorothy - E. Mascrier (LOMC), M. Roperch (LOMC), A. Vergnaud (LOMC) & G. Pinon (LOMC) ====

The size of offshore wind turbine blades has been steadily increasing over the years. Longer blades result in larger structural displacements during operation. Blade deformation has therefore become a key design parameter for large rotors. In this context, the present project focuses on coupling an in-house structural beam solver, based on Timoshenko beam theory, with an in-house Lagrangian vortex particle solver called Dorothy.
The project was initiated during ECFD8, where static blade deformation was implemented. This year, Dorothy has been fully dynamically coupled with the structural solver.
The first results show good agreement with the literature in terms of blade deflection and aerodynamic forces for the NREL 5MW rotor.
This work will be continued after ECFD9, with additional simulations performed to verify the results against other numerical approaches, such as YALES2.

==== T8 - FSI-3D without deformation strategy for internal flows - P. Benez (Safran), H. Lam (LEGI) & P. Benard (CORIA) ====

==== T9 - LES-based aero-servo-elastic simulation of wind turbines - E. Muller (CORIA & SGRE), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE), B. Duboc (SGRE) & H. Hamdani (GDTech) ====

The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine as well as the structural deformation when YALES2 is coupled to an external library/code. In the past years two coupling libraries have been developed, one to BHawC (SGRE certification tool) and one to OpenFast (NREL open access/open source tool). To improve the user and developer experience, a generalization and uniformization of the two coupling has been conducted in this project. Extensive tests and validations were performed to guarantee the non-regression.

The ALM and ADM (via ALADIN model) frameworks in the YALES2 code were thus enhanced to benefit from these couplings. Such method allows to take part of the external structural solver and controller in single and multiple turbines configurations. Updates were also initiated directly in the coupling libraries to benefit from the latest developments made in the servo-structural solvers, thus allowing to simulate modern academic wind turbines (with OpenFAST) or industrial flagships (with BHawC) in operation.

Furthermore, works on the Risoe Dynamic stall model, initiated during ECFD6, have been achieved. The implementation and integration of this model has been continued, ported to the parallel-optimized ALM framework, and tested and validated on different configurations.

Miscellaneous tasks related to the ALM code pipeline coverage and documentation have been improved.

==== T10 – Numerical simulation of engine rotors - L. Bricteux (UMONS), G. Balarac (LEGI), Y. Bechane (CORIA) & P. Benard (CORIA) ====
This project investigated the capability of the explicit compressible solver in YALES2 to simulate the fan stage of a turbofan engine. The selected configuration is the CATANA rotor, developed at École Centrale de Lyon, for which experimental data are available.
The mesh of this complex geometry was generated using Gmsh and YALES2 and consists of approximately 220 million tetrahedral elements. The setup of the simulation with a moving mesh framework was carried out during the research stay.
During this work, wall boundary conditions were improved, and it was identified that the near-wall turbulence modeling strategy could be enhanced by introducing a compressible wall model based on the work of Debroeyer et al (JFM 2024). Initial simulations have been performed and have produced promising results.
The next step will be to integrate mesh adaptation and the new compressible wall model, and to compare numerical diagnostics with experimental measurements in order to validate both the modeling approach and further validate the solver.

=== Combustion - Y. Bechane (CORIA), R. Letournel (Safran) & S. Dillon (Safran) ===

==== C3 - LES of the thermal degradation of a composite material - A. Grenouilloux (ONERA), K. Bioche (CORIA), N. Dellinger (ONERA) and R. Letournel (Safran) ====

In order to certify new composite materials for aerospace applications, it is essential to understand their degradation dynamics under severe thermal loads. The ONERA FIRE test bed was designed for this purpose. This burner generates a premixed air propane flame that reproduces a thermal flux consistent with certification standards near the impinging region. During tests, a strong emission of pyrolysis gases and a secondary diffusion flame are observed, and these gases can self ignite in regions not directly exposed to the primary flame. The project aimed to improve the modeling of this burner using Large-Eddy Simulation and reduce the overall computational cost. A reduced kinetic mechanism was derived with the Brookesia library, enabling the modeling of both premixed and diffusion flames to take into account appropriate chemistry at the front face. Used in FIRE simulations, this mechanism achieved a CPU speed-up of a factor of two compared with the previous scheme. A second reduced mechanism was generated to target auto ignition of pyrolysis gas mixtures that can occur at the rear face, and a dedicated test case was designed. Recent developments in the CWIPI interface allow for mesh adaptation during coupling between YALES2 and MoDeTheC solvers.

==== C4 - Flamelet-Progress Variable approach in LBM solvers - U. Chikkabikkodu (M2P2), D. Nouembissi (M2P2), I. Mir (M2P2), H. Meunier (M2P2), I. Tsetoglou (M2P2), S. Zhao (M2P2), P. Boivin (M2P2), J. L. Consalvi (IUSTI), R. Mercier (Safran) & S. Dillon (Safran) ====

This project extended the capabilities of ProLB to support flamelet-based combustion modelling by implementing the flamelet progress variable (FPV) approach together with FTACLES capabilities in our LBM solvers - closing a long-standing gap, since table generation and usage had never been available in ProLB.
During the workshop, transport of a passive scalar was implemented and the SDR was modelled using the passive-scalar gradient, which currently form the two control variables used in the flamelet approach. The implementation was verified through simulations of a 2D laminar methane-air jet diffusion flame.
In parallel, for FTACLES we successfully generated both premixed and non-premixed tables with TECERACT (thanks to Renaud and Samuel), and converted them into a format compatible with our code structure. A progress-variable transport equation was also implemented where the diffusion, source and correction terms were read directly from the tabulation. Validation was performed on a 1D CH4/air premixed flame with 10 sampling points within the filter width, accurately recovering the flame speed and demonstrating successful coupling between the LBM solver and the tabulated chemistry.

==== C5 - NOx prediction with a hybrid FTACLES-Virtual chemistry approach - É. Espada (EM2C), M. Préteseille (EM2C), N. Darabiha (EM2C), B. Fiorina (EM2C) ====

Filtered Tabulated Chemistry is a powerfull yet very cost efficient tool to compute flame structure and its stabilisation. However, it is unable to predict NO concentration wihtout adding additional coordinates in the manifold or by using premixed-flamelet based additional model and tabulation like NOMANI. Virtual chemistry on the other hand is a chemistry reduction method that uses machine learning algorithm to reduce drastically the number of species and reaction. This reduced scheme is then transported like any detailed chemistry mechanism. Although the method is also able to recover flame strucure and pollutants, unlike FTACLES, transported chemistry lacks a turbulent combustion model to be applied on realistics industrial LES mesh grids. This present works aims to couple FTACLES and virtual chemistry in a one way coupling: FTACLES will compute flame structure (density, Temperature, velocity field) thanks to its turbulent combusiton model, and will then feed a virtual mechanism with the "main" grid information in order to compute the pollutant informations.

==== C6 - Modelling laminar & turbulent flames with virtual chemistry - M. Préteseille (EM2C), É. Espada (EM2C), N. Darabiha (EM2C), B. Fiorina (EM2C), S. Dillon (Safran), M. Cailler (Safran) ====

A virtual chemistry framework yielding global-type mechanisms has recently been developed and validated, allowing accurate prediction of flame structures at a substantially reduced computational cost. By coupling virtual chemistry with Adaptive Mesh Refinement (AMR) strategies, this work assesses the ability to dynamically resolve reactive zones while maintaining affordable computational costs in high-fidelity LES of industrial burners. A second objective of ECFD9 was to disseminate the virtual schemes generated using SuperVision, a Python-based automated optimization tool built on Cantera. An optimized hydrogen virtual mechanism was successfully implemented and validated in the Lattice–Boltzmann solver ProLB, demonstrating the ease with which these standardized schemes can be integrated into existing reactive flow solvers, and the spread potential of this new chemistry reduction strategy in the combustion community. Finally, the NOx virtual submechanism for hydrogen combustion was improved to accurately capture both thermal and prompt NO formation in hydrogen flames.

==== C8 - Optimization of chemical source terms stiff integration - G. Lartigue (CORIA), Y. Bechane (CORIA), K. Bioche (CORIA), Q. Cerutti (CORIA), M. El Moatamid (CORIA), M. Laignel (CORIA) ====

Integration of chemical source terms remains computationally expensive in configurations that rely on detailed chemistry approach. This project aimed to reduce that cost by (1) modifying the CVODE integration strategy and (2) applying source-term clustering. A first attempt was to modify CVODE’s internal step-size control strategy but it produced only minor gains as some unnecessary integration steps still occurred, mainly in the unburned gases region. This has finally been addressed by enforcing an initial step based on the CFD time step which reduced the computational cost by a factor 2 in these regions. More importantly, relaxing the relative and absolute tolerances used to determine the accuracy of the method reduced the computational cost by approximately 40% while introducing negligible error in physical properties and flame topology for a 1D premixed flame. These results were confirmed on three methane flame configurations: a 1D premixed flame, a 2D triple flame, and the PRECCINSTA burner. Numerical experiments on the PRECCINSTA burner show a reduction in integration cost by a factor of 2.5 using the adjusted CVODE strategy and by a factor of 4.4 when that strategy is combined with clustering.

=== Mesh adaptation - A. Grenouilloux (ONERA) & G. Balarac (LEGI) ===

==== M2 – Dynamic of SWBLI in Supersonic Propulsive Nozzle Under Hot Gas Conditions - F.A. Rojas Segovia (CORIA), Y. Bechane (CORIA) & L. Voivenel (CORIA) ====
In this project, a STBLE (Solution Thin Boundary Layer Equations) wall model was implemented in YALES2. The focus was to add and compare this model with the pre-existing wall models in the code, such as the logarithmic law and Duprat, in the context of supersonic nozzles. To achieve this, 2D simulations of supersonic compressible flow over a flat plate were conducted as an initial step and validation. These initial simulations provided good insights for future research on the dynamics of Shock Wave and Boundary Layer Interaction (SWBLI) in supersonic nozzles operating with both cold and hot gas conditions.

==== M3 – Criterion for dynamic mesh adaptation in LES - H.Lam (LEGI), G. Balarac (LEGI), V. Moureau (CORIA), R. Barbera (LEGI), P. Launay (CORIA) & L. Voivenel (CORIA) ====

This project proposes a new criterion for dynamic mesh adaptation in LES, designed to overcome the limitations of static LES mesh convergence (static AMC) strategies based on time-averaged quantities. In both static and dynamic contexts, a cell-based Reynolds number is first used as a DNS criterion to identify regions where all turbulent scales must be resolved. For LES, the DNS constraint is relaxed when the integral scale is sufficiently larger than the local cut-off scale, so that a meaningful GS/SGS separation exists. In static AMC, this condition can be evaluated from statistical quantities. In dynamic mesh adaptation, however, such statistics are not available. To overcome this limitation, the proposed approach relies on the assumption that the instantaneous dissipation is predominantly the turbulent dissipation. The integral scale is then estimated from local instantaneous quantities, allowing a dynamic evaluation of the scale-separation criterion. This provides a continuous transition between DNS-like and LES-like regions during the simulation. The method is complemented by a laminar–turbulent discrimination based on a "sigma-sensor" (inspired by the sigma SGS model), enabling the identification of purely laminar zones. The approach has been assessed on a turbulent jet and on flow around a three-dimensional cylinder. Ongoing work focuses on improving near-wall treatments, in particular through prismatic layers generation on boundaries coupled to mesh adaptation and the introduction of dedicated kernels to stabilize the wall mesh and limit excessive boundary motion.

==== M4 – Improve mesh adaptation tools - B. Andrieu (ONERA), C. Benazet (ONERA), N. Dellinger (ONERA), G. Janodet (ONERA) & B. Maugars (ONERA) ====

Building upon the foundations established during ECFD7 and ECFD8 — which focused on periodic CAD-based mesh generation in EGADS and periodic parallel metric gradation — our latest developments for ECFD9 mark a significant step toward a fully automated, CAD-based periodic remeshing algorithm.
First, the parallel hierarchical remeshing algorithm prototype was improved by using a more elaborate ownership system in ParaDiGM to drive the mechanism that merges/dissociates the periodic interface mesh before/after the remeshing pass.
Second, the ability of the refine library (developed at NASA) to remesh non-manifold 3D configurations was investigated. Changes have been made to refine's operators to unlock remeshing near the merged periodic interface in 3D, which yielded promising results, but more work is needed to achieve industrial robustness. To enable CAD-based projections on both sides of the merged periodic interface, an algorithm for building a coherent periodized CAD model was implemented in the EGADS library.
This CAD-based periodic remeshing algorithm was validated in serial through a numerical simulation of the 2D LS89 turbine blade using the SoNICS solver. The results demonstrate that the mesh effectively adapts to capture the strongly anisotropic flow features while strictly respecting the periodic constraints and the geometric support.
Non-manifold mesh adaptation was applied to the ablation of a plate up to burnthrough, first in 2D and then in 3D. The burnthrough detection workflow was improved by developing a Python mini-toolbox for basic geometric queries, allowing the removal of non-physical solid fragments in the middle of the hole after burnthrough. The MMG library was also evaluated for its ability to handle non-manifold meshes, and it appears more suitable than the Refine library for this configuration. The workflow is satisfactory in 2D but needs improvement in 3D to continue the simulation after burnthrough.

==== M5 – Anisotropic mesh adaptation for multiphase flows - Robin Barbera (LEGI), Manuel Bernard (LEGI), Giovanni Ghigliotti (LEGI) & Roxane Letrounel (Safran) ====

This project investigates anisotropic mesh adaptation strategies for multiphase flows, with the objective of reducing computational cost while preserving an accurate representation of fluid interfaces. The approach relies on curvature-based anisotropic remeshing, where mesh anisotropy is locally controlled from interface geometry to ensure a prescribed discretization angle. A key limitation of anisotropic coarsening along interfaces is mass loss induced by interpolation during remeshing, which increases with tangential coarsening and therefore directly conflicts with anisotropic strategies. During ECFD9, this issue was addressed by introducing a high-order interpolation scheme for interface variables, replacing the default linear interpolation. The results show that high-order interpolation significantly reduces mass loss, allowing for much higher mesh anisotropy at the interface, at the cost of a limited computational overhead. In addition, the curvature-based adaptation strategy was extended from mean curvature to the full curvature tensor, enabling the mesh to align with the two principal curvatures of three-dimensional interfaces. The approach was demonstrated on canonical multiphase configurations, including droplet advection and rising bubble cases, showing substantial reductions in mesh size compared to isotropic simulations. Ongoing perspectives include coupling curvature-based adaptation with feature-based anisotropic remeshing to better capture turbulent structures away from the interface.

==== M7 – Increased mesh anisotropy for laminar and RANS applications - R. Barbera (LEGI), J.-B. Lagaert (LMO), T. Berthelon (LEGI), R. Letournel (Safran), M. Bernard (LEGI) & G. Balarac (LEGI) ====

This project addresses the limited level of mesh anisotropy obtained with current feature-based anisotropic remeshing criteria in steady laminar and RANS simulations in YALES2. While recent development of anisotropic mesh adaptation have significantly reduced computational cost, the achieved aspect ratios remain moderate (AR < 50), well below the levels commonly reported in the RANS literature (AR > 100). The objective of the project was to identify the main mechanisms that limit anisotropy in practice, including numerical noise in the resolved quantities, inaccuracies in Hessian computation, the formulation of the criterion itself, ... During ECFD9, the current anisotropic criterion applied to a vectorial quantity of interest (QOI) implemented in YALES2 was reformulated as the minimization of a residual-based error estimator. A Newton optimization strategy was introduced to assess whether the theoretical optimum of the criterion differs from criterion use in practice.The approach was analyzed on the Kovasznay flow, and the optimal solution was shown to be very close to the criterion currently used in YALES2. Comparisons with alternative criteria from the literature and based on scalar QOI further demonstrated similar mesh convergence, highlighting the robustness of the YALES2 approach and its main advantage: a flow-independent, non-dimensional target error. Ongoing investigations focus on quantifying the influence of numerical noise in the resolved quantities and Hessian discretization on the achievable mesh aspect ratios.

=== Two-phase flows - J. Carmona (CORIA), N. Gasnier (Safran) & I. Tsetoglou (M2P2) ===

==== TP1 - Simulation of core shifting during investment casting - Y. Mayi (Safran), M. Cailler (Safran), S. Meynet (GDTech) ====

==== TP2 - Lattice Boltzmann method for free-surface two-phase flow - J. Lu (M2P2), Y. Mediene (M2P2), I. Tsetoglou (M2P2) & S. Zhao (M2P2) ====

The project aims to reproduce and improve a two-equation free-surface Lattice Boltzmann Method (LBM) model for two-phase flows. The original free-surface model features a sharp interface and good numerical stability, but it neglects the gas phase and is therefore limited to two-phase flows in which gas effects are negligible. The recently developed two-equation free-surface LBM model (Liu Y., Sun D., Zhang Z., et al., Physics of Fluids, 2024, 36(3)) incorporates the gas phase, enabling interactions between the two phases. However, this model suffers from a lack of mass conservation and insufficient accuracy in curvature computation.
To overcome these limitations, an auxiliary distribution function is introduced to track mass evolution, thereby decoupling mass conservation from pressure evolution and restoring global mass conservation. In parallel, a pseudo-smoothing step is implemented to achieve more accurate calculations of interface normals and curvature. These improvements are validated through two benchmark test cases. (1) A Laplace test involving both static and advected droplets. It demonstrates exact mass conservation and a significant enhancement in surface tension modeling. (2) A two-phase Poiseuille flow. It shows good agreement with theoretical predictions, validating the viscous coupling between the two fluids.
Future work will focus on improving information exchange across the interface to reduce numerical oscillations and enhance numerical stability, as well as on conducting more complex validation cases.

==== TP3 - Modeling of a gear wheel immersed in an oil bath ====

==== TP4 - Implementation of a granular temperature model - T. Ndereyimana (Université de Sherbrooke), S. Moreau (Université de Sherbrooke), Y. Dufresne (Enerkem) ====

==== TP5 - Jet-A1 cavitation modeling - P. Benez (Safran), J. Carmona (CORIA) ====

==== TP6 - Comparison of JICF models ====

==== TP7 - Validation and extension of PCS solver for cryo tanks ====

==== TP8 - Jet-in-crossflow simulation with the Hybrid SPH-FVM solver ====

==== TP9 - Multi-physics effects modeling in film flows - N. Gasnier (Safran), P. Portais (CORIA/Safran), L. Voivenel (CORIA), E. Bourrel (CORIA), M. Cailler (Safran) ====

==== TP10 - Solid-Fluid Coupling for Nucleate Boiling Simulations - M. Umair (LEGI), G. Ghigliotti (LPSC), H. Lam (LEGI), M. Bernard (LEGI), R. Barbera (LEGI), G. Balarac (LEGI) ====

Ecfd:ecfd 9th edition

2026-02-04T14:20:56Z

Benard: /* T6 - Injection of coherent structures for LES inlet condition - T. Berthelon (LEGI), G. Balarac (LEGI), R. Letournel (SAFRAN), P. Launay (CORIA), L. Voivenel (CORIA) & P. Benard (CORIA) */

Ecfd:ecfd 9th edition

2026-02-03T10:38:18Z

Benard: /* Turbulence - L. Voivenel (CORIA), P. Bénard, (CORIA) & T. Berthelon (LEGI) */

Ecfd:ecfd 9th edition

2026-02-03T10:36:53Z

Benard: /* T1 - Concurrent Precursor-Successor with Successor automated mesh convergence - P. Launay (CORIA), L. Voivenel (CORIA), P. Benard (CORIA) */

Ecfd:ecfd 9th edition

2026-02-03T10:36:04Z

Benard: /* T4 - Wind field reconstruction based on LiDAR measurements - T. Cousin (LMI), P. Benard (CORIA), G. Lartigue (CORIA), JB. Lagaert (LMO) */

Ecfd:ecfd 9th edition

2026-02-03T10:35:20Z

Benard: /* T9 - LES-based aero-servo-elastic simulation of wind turbines - E. Muller (CORIA & SGRE), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE), B. Duboc (SGRE) & H. Hamdani (GDTech) */

Ecfd:ecfd 9th edition

2026-02-03T08:46:07Z

Benard: /* Turbulence - L. Voivenel (CORIA), P. Bénard, (CORIA) & T. Berthelon (LEGI) */

Ecfd:ecfd 9th edition

2026-02-03T08:44:27Z

Benard: /* T1 - Concurrent Precursor-Successor with Successor automated� mesh convergence - P. Launay (CORIA), L. Voivenel (CORIA), P. Benard (CORIA) */

Ecfd:ecfd 9th edition

2026-02-03T08:44:05Z

Benard: /* Turbulence - L. Voivenel (CORIA), P. Bénard, CORIA & T. Berthelon (LEGI) */

Ecfd:ecfd 9th edition

2026-02-03T08:34:20Z

Benard: /* T9 - LES-based aero-servo-elastic simulation of wind turbines - E. Muller (CORIA & SGRE), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE), B. Duboc (SGRE) & H. Hamdani (GDTech) */

Ecfd:ecfd 8th edition

2025-02-15T08:26:32Z

Benard: /* T6 - FSI model in Dorothy - Enzo MASCRIER (LOMC), Grégory PINON (LOMC) */

{{DISPLAYTITLE: ECFD workshop, 8th edition, 2025}}

== Description ==

* Event from '''27th of January to 7th of February 2025'''
* Location: [https://www.sport-normandie.fr/le-centre/le-site-de-houlgate Centre Sportif de Normandie], Houlgate, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* Participants from academics, HPC center/experts and industry are welcome
* The number of participants is limited to 68.

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes
* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Voivenel (CORIA).

[[File:ecfd8.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_8th_edition]]

[[File:Acknowledgments_ecfd8.png|text-bottom|600px]]

== News ==

* 23/10/2024: First announcement of the '''8th Extreme CFD Workshop & Hackathon''' !
* 22/11/2024: Deadline to submit your project

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

To come...

== Projects ==

=== Hackathon GENCI - P. Begou, LEGI ===
This ECFD8 GENCI Hackathon was a rich event, involving 4 differents CFD codes (AVBP, ParaDIGM, SONICS and YALES2) using various paradigms (C++/cuda/hip, Fortran/OpenMP/OpenACC) with several SDKs (AMD, Cray/HPE, Nvidia, Gnu) on a large range of GPU architectures (Nvidia A100, GH100, AMD instinct Mi210, Mi250, Mi300). This two-week event benefited from a high level support from three HPC mentors, two on-site from AMD (J. Noudohouenou and A. Tsetoglou) and one remote from CINES (M. Boudaoud).

==== H1 - ParaDIGM and SONICS on GPU, B. Maugars, G. Staffelbach, R.Cazalbou and B. Michel (ONERA)====

==== H2 - AVBP GPU offloading based on OpenMP, M.Ghenai, L. Legaux and A. Dauptain (CERFACS) ====

This hackathon provided a valuable opportunity to work on GPU offloading for AVBP. In the past, significant efforts were made to offload the entire AVBP code to GPUs. OpenACC was the primary strategy chosen, mainly due to access to NVIDIA's support, along with the availability of both software and hardware. This approach achieved good scalability performance.
Recently, with the deployment of new supercomputers like ADASTRA at CINES, some issues have emerged when running AVBP on AMD GPUs, including both MI250 and MI300. The closed-source nature of the Cray environment has also prevented CERFACS from deploying AVBP on local MI210 GPUs.
This hackathon was a great opportunity to address these challenges by exploring a new approach using OpenMP. An automatic translation tool was used to convert approximately 2,700 OpenACC directives to OpenMP, with each directive manually verified and fine-tuned afterward. AVBP with OpenMP had already been tested on NVIDIA GPUs, and during this hackathon, the focus was on extending support to AMD GPUs.
Two compilers were used: Cray and the newly released AFAR from AMD. With the support of AMD and CINES, a working environment for compiling AVBP was set up, and performance-related issues were identified. Additionally, two mini-apps were used for benchmarking. One of them achieved a 2.5× speedup when compiled with AFAR compared to Cray.
The next steps involve adapting the code to address necessary modifications, such as fixing issues related to Fortran indirections, and continuing performance evaluations with mini-apps. Further comparisons will be conducted using both compilers against results obtained with NVIDIA’s NVHPC.

==== H3 - YALES2 GPU from OpenACC to OpenMP, P. Bégou (LEGI), V. Moureau, G. Lartigue (CORIA) and R. Dubois (IMAG) ====
This Hackathon focuses on running Yales2 code on AMD Instinct Mi250 and Mi300 GPUs of the Adastra supercomputer (CINES).
Previously, a first solver in the Yales2 CFD code was successfully ported on the GPU accelerators of the Jean-Zay supercomputer (IDRIS) using Nvidia SDK but difficulties remain on Adastra AMD GPUs, mainly related to the available development tools. High compilation time and the impossibility to use debug flags at compile time as soon as OpenACC is enabled are a real challenge when tracking errors. The current project is to evaluate a freshly deployed version (at the begining of the workshop) of the AMD Fortran compiler. This requires moving to OpenMP paradigm, starting from scratch since the OpenACC branch has largely diverged from the master one while tracking spurious remaining bugs.
If the AMD compiler is able to build the cpu version of Yales2 "out of the box" (wich is not the case for Cray Fortran), the compilation time for each file is significantly higher. However, setting up a 2 stages dynamic compilation process allows for high parallelism that is not possible with Cray Fortran 18 and the library build time drops from nearly 2 hours (Cray Fortran 18) to 17 minutes (Amd Fortran compiler).
Large kernels have been ported from OpenACC to OpenMP, raising some difficulties when offloading intrinsics functions or using strutures attributes in kernels loops. These limitations were also known in the previous OpenACC work. The goal was mainly to check the correctness of the results. The offloading of the complex data structure of Yales2 code was then investigated. Here again some limitations of the "young" compiler were discovered and workarounds were implemented. Several reproducers were built during this ECFD8 and provided to developpers by the 2 on-site AMD engineers.
Preliminary tests on micro-applications show good performances of the generated binaries proving that this compiler could be a serious alternative on AMD GPUs and the goal is now to focus on this SDK in an OpenMP strategy while checking the portablility of this new implementation in Nvidia, Cray/HPE (and Gnu ?) environments.

=== Mesh adaptation - A. Grenouilloux, ONERA & G. Balarac, LEGI ===

==== M2 - Enhancement of mesh adaptation algorithms, B. Maugars (ONERA), B. Andrieu (ONERA), C. Benazet (ONERA), N. Dellinger (ONERA), G. Janodet (ONERA), G. Staffelbach (ONERA) ====
Mesh adaptation has become a crucial tool in order to automate industrial numerical simulations. ECDF7 allowed us to investigate the refine and EGADS libraries as tools for parallel mesh generation and adaptation using CAD as a geometric support. Since then, we fortified the workflow but some of our targeted industrial applications such as turbomachinery involve periodic boundary conditions. To manage these cases, the mesh generation and adaptation procedures must maintain matching periodic boundaries.
During ECFD8, we addressed multiple topics : periodic mesh generation from CAD model in EGADS, parallel and periodic metric gradation in ParaDiGM, making our parallel remeshing algorithm more generic to support non-manifold and periodic meshes. All these topics were covered, but there is still some work to be done to fully manage periodic mesh adaptation.
Non-manifold mesh adaptation was applied to changing mesh topologies. Here, an internal surface was described by a level-set and deformed up to hole creation to mimic ablation.

==== M3 - Development and definition of a new Automatic Mesh Convergence (AMC) driver for automating static mesh convergence in YALES2 (C. Papagiannis (LEGI/INRIA), G. Balarac (LEGI), O. LeMaître (INRIA), P.M. Congedo (INRIA)) ====
Static mesh adaptation requires re-adapting an existing (usually coarse) mesh, to conform to some quality metric that satisfies some optimality criterion. This involves statistics (Mean and RMS fields) that need to be adequately converged. We proposed and developed a novel AMC strategy that can converge an initial coarse mesh to an adequately refined one by taking into account the quality of the calculated statistics before each adaptation. Specifically, so far, the moment (in terms of integrating the LES equations) at which to launch the adaptation was unknown and had to be set apriori as a parameter of the AMC, dependent on the flow physics and therefore on the case at hand. Our method is automatically performing adaptations when the quality of the volume averaged mean field estimate (characterized by its RMSE) reaches the levels of the bias error that this field has, due to the mesh being far from the desired one (dictated by the quality criterion). This was achieved through drawing inspiration from a theoretical model that treats the AMC as a "conditionally" contractive mapping with a fixed point.

=== Numerics - M. Bernard, LEGI & G. Lartigue, CORIA ===

==== N1 - Traction open boundary condition G. Balarac (LEGI), J.-B. Lagaert (IMO) ====

In simulations, artificial boundaries need to be introduced due to the limited size of computational domains. At these boundaries, flow variables need to be calculated in a way that will not induce any perturbation of the interior solution. During previous ECFD, more generic outlet boundaries conditions has been implemented, allowing to prescribe for instance an arbitrary traction at the boundary. This year, the focus was on setting up a traction model to determine those values. This model extrapolates edge tension from upstream tension within the calculation domain. It has been validated on an initial series of test cases where the theoretical solution is known, enabling to compare the traction-model accuracy both with the imposition of exact theoretical traction and with other more conventional boundary conditions.

==== N2 - Treatment of Inlet conditions in High-Order solver. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====
In the context of node-centered Finite Volumes Method, spacial accuracy of a numerical scheme depends on ability to evaluate accurately fluxes through interface of each control volume (CV). Such accurate evaluation is not straightforward, especially when dealing with distorted grids. This project follows the work of [1] where fluxes use pointwise quantities, which are reconstructed from integrated quantities advanced in time. During the previous edition of the ECFD, a new data structure has been developed to store data at location of the boundary conditions facelets, with application to wall boundary conditions. During this 8th edition of the ECFD, we used the same data structure, but dedicated to the treatment of inlet conditions.
The inlet condition is then either imposed directly at facelets center, or at nodes position them extrapolated to facelets center by use of Taylor expansion. For this later solution, high-order treatment requires the successive derivatives to be computed in the plane of the boundary condition. This is not done yet, leading for the moment to low accuracy results but the framework is ready for upcoming implementation.

[1] ''A framework to perform high-order deconvolution for finite-volume method on simplicial meshes, , Bernard et. al., IJNMF 2020''

==== N3 - Conservative mesh-to-mesh interpolation. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====

Mesh to mesh interpolations occur quite often in CFD simulations : in the context of adaptative mesh convergence or in the case of dynamic mesh adaptation for for example.
Quality of the solution on the destination grid will depend on the characteristics of the interpolation method.
In this project, we did not focus on accuracy of the interpolation method but rather on conservativity characteristics.
A conservative interpolation ensures that the integral of the data on the source grid is exactly retrieved on the destination grid.
This property is highly interesting when dealing with scalar quantities or phase indicators, whose values should remained bounded.
In the context of nodes centered Finite Volume schemes, the methodology we used consists in (i) reconstructing element quantity from average nodal quantities on source grid.
Then, for a cell of the destination mesh, (ii) computing the geometrical intersection between cells of source and destination meshes to evaluate to evaluate the rate of quantities they.
Eventually, (iii) redistributing the solution from elements to control volumes of the destination mesh.
The overall process is fully conservative as it is based on geometrical intersection of locally integrated quantities.
The methodology as been implemented and tested on a few basic configurations and the conservativity is retrieved.

==== N4 - Determination of timestep in semi-implicit solver. T. Berthelon (LEGI), G. Balarac (LEGI), H. Lam (LEGI), M. El Moatamid (CORIA) ====
In order to reduce the computation time associated with incompressible LES simulations, an implicit time integration, based on BDF schemes, has been developed within the ICS solver. This integration eliminates the stability constraints associated with explicit schemes, and therefore opens up the question of the appropriate choice of time step.
In parallel, recent work has been carried out on meshing criteria in LES. The strategy in place consists of adapting the mesh by distinguishing two zones:
- "DNS" zones, where the meshing criterion is based on an estimate of the adimensioned spatial error.
- "LES" zones, where the meshing criterion is based on Kolmorogov theory.
During this project, the spatial criteria were extended to include temporal criteria. In the "DNS" zones, the time step is chosen using an estimate of the temporal error of the BDF scheme judiciously scaled to match the spatial error. In the "LES" zones, the time step is chosen using a scaling law associated with fully developed turbulence.
The new time step selection strategy has been tested on the case of a turbulent jet and leads to an accuracy equivalent to the explicit case while reducing the simulation return time by a factor of nearly 3.

Another aspect of this project was to integrate certain implicit temporal schemes (C-N and SDIRK) recently developed by Mr. El Moatamid into the incompressible solver.

==== N5 - Local timestep. T. Berthelon (LEGI), M. Bernard (LEGI), G. Balarac (LEGI) ====
RANS modelling has recently been developed within the YALES2 library. With this modeling strategy, the objective is to reach as quick as possible a steady state.
During this project, we investigate the use of a local time step to reduce the time to solution of steady computation in the incompressible solver.
This implies solving a variable-coefficient Poisson equation. Encouraging results were obtained in the simple case of "Couette plan" flow artificially constrained by a mesh variation. In fact, the use of local time-step reduce drastically the time to solution on this configuration. This method needs to be tested on real RANS case.

==== N6 - Distributed version of DOROTHY. M. Roperch, G. Pinon (LOMC) ====
Dorothy is a Lagrangian code using vortex particle method. This method is based on the discretisation of the fluid into vorticity carrying particles. Before ECFD7, all the processors knew all the particles. Since ECFD7, we have gradually moved to a fully distributed version. At ECFD8, the goal was to get a domain decomposition without knowing all the particles. This decomposition is done by dividing the domain by prime factors so that each subdomain has the same number of particles. Prior to this ECFD, a Python draft of this module was created. At ECFD8, this new algorithm was implemented and will soon be operational. The next step is to continue this work for the velocity calculation, as each particle affects the velocity at a given position, and calculating the velocity without knowing all the particles requires more MPI communication.

==== N7 - Implicit time advancement for low-Reynolds number flows with particles. S. Mendez, C. Raveleau (IMAG), M. El Moatamid, V. Moureau (CORIA) ====
IMAG runs numerous simulations of red blood cells under flow. Those simulations are at low Reynolds number (0.001 to 1.0, typically). Splitting of the time advancement is used to treat the diffusion terms implicitly, albeit with an important numerical cost: implicit diffusion is 50 to 60% of the computational cost. Recently, M. El Moatamid implemented a genral framework to deal with implicit time advancement for scalars. In this project, the general method has been transposed to the advancement of the velocity field in the ICS and RBC solvers of YALES2/YALES2BIO. This enables testing various linear solvers (GMRES based). However, such solvers do not decrease the CPU time compared to the existing implementation. However, while working on this, it was identified that residual recycling was not activated in the current implementation of the implicit diffusion. This sped up the implicit diffusion cost by 35%, for a total gain of 20% for the computation. In addition to this achievement, moving to the framework coded by Moncef will have other beneficial side effects: we anticipate simplifying the implementation, with an easier merging between YALES2BIO and YALES2. The method will also be implemented in the electrosatic solver, for which the Poisson problem should benefit from the new GMRES-based solvers. In addition, this project highlights the importance of improving the treatment of stiff source terms in the red blood cells simulations, to be able to overcome the current limitation in time step due to those term and have a chance to benefit from higher-order time schemes, efficient at high Fourier numbers.

==== N8 - Boundary Element Method in YALES2. B. Thibaud, S. Mendez (IMAG), G. Lartigue, P. Benard (CORIA), F. Nicoud (IMAG) ====
In the context of microfluidic systems for diagnosis, the Boundary Element Method alows to solve linear PDE such as electrostatic or Stokes. With well chosen kernel functions and the divergence theorem, this method allows to write on the boundary condition only the initial volumic problem. This project aimed at exploring the feasibility of the BEM in the context of massively parallel unstructured solver like YALES2 by developping a Julia demonstrator. The first step have been to implement and validate the method on simple configurations for the Laplace's equation. Only Neumann problems were considered (Dirichlet boundary conditions imposed). In a second time, the multi-domain approach has been identified to be the most suited in the framework of YALES. The inner domain is partitioned on each processor, each having a part of the physical boundary and interfaces between them. Every processor solve its own boundary problem and a parallel Dirichlet-Dirichlet fixed-point is used to converge the interface problem on the all domain. Applied to the ring case, with one interface, we managed to reproduce the linear convergence of the P-DD method.

=== Turbulence - L. Voivenel, CORIA & P. Bénard, CORIA ===

==== T1 - FSI-1D strategy for internal flows - Pierre BENEZ (SAFRAN Aerosystem), Renaud MERCIER (SafranTech), Yacine BECHANE (CORIA) ====

Many applications developed at Safran Aerosystem are based on internal turbulent flows coupled to a moving body. 2 cases were studied during this ECFD:

'''Case 1 (Incompressible flow)''': Translation of a piston subjected to a pressure difference in a pipe.

The challenges of this case are twofold: the small gap between the piston and the pipe and the large pressure gradient across the piston (>100bar). During the 1st week of ECFD, the CLIB (Conservative Lagrangian Immersed Boundary) solver was tested on this case. The study showed that the solver was unable to ensure the impermeability of the solid under these pressure conditions. In the rest of the study, a porous medium following Darcy's law will be added to the penalty force of the immersed solid to fully satisfy the impermeability of the piston.

'''Case 2 (Compressible flow)''': Rotation of a butterfly in a discharge vane.

The coupling between the ECS (Explicit Compressible Solver) and ALE (Arbitrary Lagrangian Solver) solvers having recently been implemented, this strategy was tested to model the opening of the valve by rotation of the butterfly. The challenge here lies in the small gap between the bottom of the butterfly and the vane casing. To limit the simulation cost, the gap is meshed with 1 element. In this case, MMG succeeded in adapting the mesh up to a critical angle at which the gap becomes too small (Work In Progress).

==== T2 - Dynamic Smagorinsky in Dorothy - Maëlenn ROPERCH (LOMC), Grégory PINON (LOMC) ====
Dorothy is a Lagrangian code using vortex particle method (VPM). This method is based on the discretisation of the fluid into vorticity carrying particles. In some cases, there is a perturbation in the wake that needs to be diffused. Two LES model for a turbulent viscosity exist currently in the code: a standard Smagorinsky model and a Mansour model. Both model apply the same LES constant everywhere. The aim of this ECFD is to add and compare two dynamic and local Smagorinsky models. The first version is a classical dynamic Smagorinsky model in VPM, quantities are filtered with a sum over all particles. Since the values of the sum terms are lower for more distant particles, a second version reduces the sum to neighbouring particles. Preliminary results shows some differences between the two method. The next step will be to validate and compare the results with the case of two vortex ring colliding.

==== T3 - Turbulence injection strategy for compressible flows - Patrick TENE HEDJE (UMONS), Laurent Bricteur (UMONS), Yacine BECHANE (CORIA), Pierre BENARD (CORIA) ====
Taking realistic turbulence into account in turbomachinery simulations for aeronautical applications remains a major challenge, particularly as regards the management of non-reflexive boundary conditions. The implementation of the actuator line method (ALM) in the YALES2 Library has enabled us to set up wind tunnel test reproduction setups, such as the modeling of turbulence grids. During the ECFD8, this project aimed to (WP1) finalize the implementation of this method in the Explicit Compressible Solver (ECS) of YALES 2 and (WP2) use this same method to model the interaction of the upstream stator wake on the downstream rotor wheels. The setups implemented have been successfully tested on simple channel flow cases. The aim is to continue the validation process on real turbomachinery geometry cases.

==== T4 - Improve wind farm modeling and simulation workflow - Pierre BENARD (CORIA), Ulysse VIGNY (UMONS), Etienne MULLER (CORIA & SGRE), Hakim HAMDANI (GDTech), Félix HOUTIN-MONGROLLE (SGRE), Anand PARINAM (CORIA & TUDelft), Hari MULAKALOORI (CORIA), Léa VOIVENEL (CORIA) ====
The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine. Still, applying this method to investigate a wind farm flow can be challenging, both in terms of computational cost and simulation setup. For instance, an inadequate management of the wind turbine individual modeling parts in a HPC context can end up being the main bottleneck of the simulation. From another perspective, a wind farm is usually composed of more than 50 wind turbines. For such a case, setting up all YALES2 required inputs manually can be very tedious and error-prone. This project thus mainly aimed to optimize the YALES2 ALM implementation and the user experience around it. Additionally, a cost-effective alternative to the ALM when modeling wind farm flows, namely the Rotating Actuator Disk Method (ADM-R), has been implemented for further investigations.

'''WP1''': Improve Actuator set rotor modelling
* Parallel processing of the ''actuator sets'' used to model the wind turbines
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified during the 5th iteration of the ECFD: (i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges. (ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications. During this workshop edition, the work focused on the re-synchronisation of the algorithm steps by allowing some packing an unpacking of the object to allow the transfert of the object inbetween workers. This is of major importance to enable load balancing and mesh adaptation during the temporal loop. This work required the refactoring of the involved oject structures.

* Rotating Actuator Disk Method (ADM-R):
According to the usual guidelines, the mesh requirements of the ALM, to profit entirely from its reachable accuracy, can be difficult to achieve or even unaffordable when simulating a wind farm flow, especially from the industrial point of view. Alternatives are available in the literature for this kind of application. Likely, the methods from the Actuator Disk family are the most prominent ones. Several kinds of implementation exist, which mostly differ by their capability to include the wake rotation. During the workshop, a new method from the Rotating Actuator Disk kind has been implemented and underwent an early validation on a single turbine setup. Applications to wind farm flows will follow.

'''WP2''': Improve tools User Experience

Three Python tools have been developed or improved :
*The first tool is the wind farm previsualisation tool, 'y2_wind_previsualization', which is used before the calculation run. This provides an interactive HTML interface for viewing global data for each turbine on the farm (position, hub height, yaw angle, etc.). The tool traces all of these via the parsing of the input file.
* The second tool is for duplicating rotor templates for a wind farm (`y2_wind_duplication`). This tool was developed in the previous ECFD, but this time it has been refactored and incorporated into the y2tools package.
* The third and final tool is a post-processing tool for the temporal processing of global wind turbine simulation metrics (Thrust, Power, etc.), `y2_post_wind`. This tool generates an interactive HTML plot of time-dependent global quantities.

==== T5 - Improve atmospheric inflow turbulence - Ulysse VIGNY (UMONS), Pierre BENARD (CORIA), Etienne MULLER (CORIA & SGRE), Hakim HAMDANI (GDTech), Félix HOUTIN-MONGROLLE (SGRE), Anand PARINAM (CORIA & TUDelft), Hari MULAKALOORI (CORIA), Léa VOIVENEL (CORIA) ====
Atmospheric inflow turbulence is generated using the precursor database method. A half-channel flow driven by a pressure gradient is used to obtain the inflow which is used as inlet boundary condition for the wind turbine simulation domain. This project aimed to improve the whole methodology, from generation to injection.

* WP1: Improve inflow generation

The inflow generation using a velocity controller implementation was initially unstructured and lacked robustness. This PID-based velocity controller allows users to impose a specified velocity and direction at a given height, primarily for atmospheric flow simulations in wind farms. During the workshop, the controller was integrated into the source code by applying velocity forcing as a source term in the flow field. The approach utilizes a target velocity, interior boundary and specification of the ground boundary. Additionally, when using the logarithmic law in atmospheric flows, inconsistencies between the computed and the intended wall shear stress for the target velocity sometimes led to diverging velocity profiles. This was due to mismatches in the source terms. To address this, the wall shear stress is now computed for the target velocity and imposed as a source term, ensuring consistency and stability.

* WP2: Improve injection methodology (method A)
The previous workflow used plane probes in the ASCII format to sample the flow. The COWIT2 toolbox was used to convert the file into turbulence box (.man format). While functioning, this methodology had two major flaws. First the probe files are heavy ~O(10Go). Second, the method requires a lot of human effort, allowing numerous sources of errors.
During this workshop, a new methodology has been developed. First, the probes are generated using the HDF5 format (now available for all probe types), leading to lighter file ~O(1Go). Second, Y2_tools is used to read HDF5 format (working for probes and temporals). HDF5 file is then converted into a Look-up Table. Finally, the Look-up Table is read directly by YALES2 as a boundary conditions.

* WP3: Improve injection methodology (method B)
Even though improvements achieved in WP2 prove to be very handy while removing many potential human errors, injecting a turbulent inflow through wind boxes ('offline' precursor approach) can sometimes remain cumbersome for several reasons: (1) no periodicity is enforced in the streamwise direction of those boxes, (2) potential high memory consumption, and (3) the boxes need to be moved to other cores whenever a mesh adaptation occurs. An alternative consists in co-simulating the precursor flow and the flow of interest (refered as the 'successor' simulation) at the same time ('online' precursor approach). The inlet boundary condition for the successor flow is then obtained by mapping the outflow of the precursor domain. During the workshop, some work has been initiated to implement this kind of coupling using the CWIPI library, for which YALES2 provides already an interface.

==== T6 - FSI model in Dorothy - Enzo MASCRIER (LOMC), Grégory PINON (LOMC) ====

The objective of the project was to enhance the numerical code Dorothy (developed at LOMC laboratory) to perform fluid-structure interaction studies. To achieve this goal, wind turbine blades must be capable of deforming in response to the incoming flow. The prediction of the blade deformation is modeled by a Timoshenko beam approach.

The code is presently written in a way that keeps the blades always in the rotor plane. Additionally, particles are currently emitted based on a local frame at each blade section. In a near future, when the blade will deform, this will change the orientation of the particles. Therefore, the particle emission must be updated accordingly to reflect these deformations.

During ECFD8, a forced blade motion case was coded and tested, allowing us to verify particle’s emission. This work will be further developed to deform the blade dynamically using the structural solver code at each time step.

=== Two Phase Flow - J. Leparoux, SAFRAN & J. Carmona, CORIA ===

==== TP1 - Towards very small contact angles in Nucleate boiling ====

Participants: Henri Lam (LEGI), Mohammad Umair (LEGI), Manuel Bernard (LEGI), Robin Barbera (LEGI) and Giovanni Ghigliotti (LPSC)

The boiling solver (BOI) was not able to accurately impose a contact angle (angle formed by the two-phase interface on the wall) at values lower than 30°. This angle is needed when simulating nucleate boiling. A similar limitation in contact angle value was applying to the spray (SPS) solver. A modified version of the level set reinitialization has been implemented during ECFD8, based on a different normal vector in the blind spot region around the contact line, vector now chosen to be a zero-order extension from outside the blind spot. This modification, that implied other modifications to the level set reinitialization in the blind spot, has been tested successfully on the spray solver (where no phase change occurs). Then, this new reinitialisation has been tested in the boiling solver for nucleate boiling, with great improvements. Now simulations of nucleate boiling at very small contact angle (10°) can be accurately performed.
In the meanwhile, the level set reinitialization algorithm has been streamlined and the computational cost greatly reduced, resolving a computational cost issue that appeared when using the contact angle imposition both in the spray and boiling solvers, and that hampered its use in industrial configurations.

==== TP2 - Modeling spray-film interactions ====

Participants: Nicolas Gasnier (EM2C-SafranTech), Julien Leparoux (SafranTech), Mehdi Helal (CORIA-SafranTech) and Julien Carmona (CORIA)

The numerical simulation of sheared liquid films over walls with conventional high-fidelity methods requires fine meshes to depict accurately the dynamics of the fluids, as well as the gas-liquid interactions at the phase interface. In the context of thin film flows (with a thickness h ~ 1.0E-4 m), the spatial resolution required to ensure the validity of the simulations can be computationally prohibitive. To tackle this issue, a reduced-order model of thin film dynamics based on the Saint Venant equations has been implemented in the YALES2 platform over the last months. This numerical model is able to reproduce accurately the dynamics of strongly sheared film flows over partially wetted surfaces, and to take into account capillary effects. The objective of the ECFD8 was to couple the thin film framework with the pre-existing multiphase methods of the YALES2 platform: the Eulerian multiphase solver - based on the ACLS -, and the Lagrangian solver. This coupling was performed in three steps: first, a numerical method was designed to convert impinging Lagrangian droplets into source terms for the thin film model. Then, a phenomenological model was implemented to depict the atomization of films at sharp edges under the action of a high speed gas flow. The liquid film is converted into Lagragian droplets, whose dimensions are computed based on the PAMELA model. Finally, a numerical method has been designed to convert Eulerian droplets into source terms for the film model. This method is based on a transfer of the liquid properties from the nodes to the boundaries by using a set of fictitious particles. It ensures conservation of the liquid mass, and it is more robust than the first attempts that had been made during ECFD7.

==== TP3 - High-fidelity two-phase flow simulations of the purge of a fuel feed line ====

Participants: Thomas LAROCHE (Safran HE), Romain JANODET (Safran AE), Julien Leparoux (Safran Tech) and Melody Cailler (Safran Tech)

During the second week of the ECFD8, the fuel feed line purge process has been numerically investigated. In the context of aeronautical engines, the fuel feed line—carrying the fuel from the fuel tank to the injectors within the combustion chamber—needs to be purged at engine shutdown. This is intended to prevent fuel stagnation near hot metal parts, which could lead to coke formation and therefore decrease engine performance. Since this complex phenomenon is mainly driven by two-phase flow physics, the spray solver (SPS) of the YALES2 library has been considered in order to understand the physics of such process. The numerical setup was first converged on a simplified test case: the possibility of driving the flow dynamics with inlet and outlet pressure conditions was tested beforehand on a single-phase, incompressible case, and then on a two-phase flow problem. The setup has then been successfully applied to an industrial configuration: a pressure-swirl injector connected to a reduced portion of the fuel feed line. Due to the large scale of the domain, the interface resolution was set to 50μm, which is intentionally coarse for such problem. This initial computation successfully ran up to 3ms of physical time during the workshop, proving YALES2's capability to model the fuel purge. The computation is to be continued and analyzed further even after the workshop.

==== TP4 - Volume of Fluid solver in YALES2 ====

Participants: Léa Voivenel (CORIA), Julien Carmona (CORIA), Mehdi Helal (CORIA), Pierre Portais (CORIA), Julien Leparoux (Safran Tech), Mélody Cailler (Safran Tech) and Nicolas Gasnier (EM2C / Safran Tech)

The aim of this project was to develop an alternative strategy to the ACLS method already implemented in Yales2 to investigate incompressible two phase flows. The Volume of Fluid (VoF) methods were chosen as they show excellent performances, with interesting qualities such as inherent conservation of mass. Nevertheless, they remain challenging in an unstructured mesh framework.
The work conducted during ECFD8 can be divided into two parts.

First of all, we aimed to robustify and improve the new Volume of Fluid Solver (VFS) where a liquid volume fraction is transported with an imposed velocity field. In this simplified framework several resharpening strategies to counteract the liquid volume fraction numerical diffusion were explored. On one hand, a compressive velocity only acting at the interface was implemented as a source term in the Volume Fraction transport equation, <math> \frac{\partial \alpha}{\partial t}+\nabla \cdot(\alpha \boldsymbol{u})=\nabla .\left(\begin{array}{l}
\alpha(1-\alpha)^{\begin{array}{c}
\end{array}} C_\gamma|\boldsymbol{u}| \boldsymbol{n}_{\Gamma}
\end{array}\right)</math>. While satisfactory results were obtained, several improvements were identified: compute the source term at pairs instead of nodes and to investigate the influence of <math> C_\gamma </math> depending on the numerical scheme.
On the other hand, a strategy based on the solving of a "resharpening equation" at each timestep <math> \partial_\tau \alpha+\underline{\nabla} \cdot(\alpha(1-\alpha) \underline{n}-D(\underline{\nabla \alpha} \cdot \underline{n}) \underline{n})=0</math> was also explored. This second approach seemed more efficient but its cost must still be evaluated.

Finally, the second axis of this work aimed at coupling the liquid volume fraction transport with the momentum equation. A first working version of this new solver was proposed at the end of the 2 weeks. The next steps are the integration of the resharpening methods presented in the above section, the investigation of the problem of Poisson convergence for higher order schemes and the addition of the surface tension.

==== TP5 - Implement a local operator to distribute the solid volume of a particle over multiple cells ====

Participants: Théo Ndereyimana (Université de Sherbrooke), Stéphane Moreau (Université de Sherbrooke)

In the CFD-DEM, the cell size is required to be larger than the particle size for stability condition and keep feasible solid volume fraction. However, some applications require a cell size smaller than the particle. During this 8th edition of the ECFD, the use of operators to distribute the particle volume over multiple cells, ensuring a feasible solid volume particle has been tested on a fluidized bed configuration. The main operators tested (gather-scatter filter and gaussian filter) showed a tendency to blur the void structure interfaces. The equivalence of the gaussian filter of bandwidth <math>b</math> and the resolution of a diffusion equation over a pseudo-time <math>T=b^2/4</math> has been verified.
One anisotropic diffusion constant has been tested and shows a possibility to adress the sharpness requirements.

Another objective was to develop a post-processing tool to detect and track the void structures (bubbles) in a fluidized bed. Based on previous work from J. Carmona, a tool to track the bubbles has been initiated.

==== TP6 - Complex thermodynamics in sloshing tanks ====

Participants: C. Merlin (AGS), D. Fouquet (CORIA), V. Moureau (CORIA), J. Carmona (CORIA) and G. Lartigue (CORIA)

During this ECFD, the phase change solver was improved. After introduction of conservative transport for enthalpy and energy, a new framework for multi fluid two phase flow was used that relies on a new reinitialization of the conservative level set, the transport of discontinuous scalars like temperature, energy or enthalpy and the transport of weight to ensure consistency. The pressure loop was improved to ensure mass and energy conservation while meeting the low Mach hypothesis. Many 1D test cases were performed with only the gas phase and two phase flow to validate the pressure loop. The new framework for the transport of discontinuous scalar was first derived for temperature with constant properties in each phase. A first step was to extend it to sensible enthalpy with ideal gas behavior for the gas phase. Then, the framework was extended to NIST tabulated gas & liquid with ideal or real gas behavior. After some 2D cases, a tank pressurization test was investigated as well as a sloshing test without phase change (just the pressure drop due to the increase of the thermal exchange between the liquid and the gas phase).

=== Combustion - Y. Bechane, CORIA & S. Dillon, SAFRAN & K. Bioche, CORIA ===

==== C1 - LES of the thermal degradation of a composite material ====
Participants: A. Grenouilloux (ONERA), K. Bioche (CORIA), N. Dellinger (ONERA) and R. Letournel (SafranTech)

The FIRE test bed is an experimental air-propane burner operated by ONERA. It is dedicated to the study of the thermal degradation of composite materials. This project concerned the implementation of a three-solver coupling methodology to simulate the dynamics of the impinging flame. The methodology considered is based on the coupling between the variable density solver (VDS) and the radiative solver (RDS) of the massively parallel library YALES2 and the solver dedicated to the degradation of composite materials, MoDeTheC, developed by ONERA. Given the typical test times of the order of tens of seconds, a methodology based on 2D axisymmetric calculations was considered. Various tests were performed to determine the optimal coupling frequency between solvers. Cases dedicated to the injection of pyrolysis gasses were set up, with the aim of simulating the auto-ignition phenomenon. Comparisons with experimental data are presented.

==== C2 - Hydrodynamic and compressibility effects induced by NRP plasma discharges in reactive mixtures ====
Participants: S. Wang (EM2C), Y. Bechane (CORIA), B. Fiorina (EM2C)

To reduce the expensive computational cost of Plasma-Assisted Combustion (PAC) full 3D simulations, the EM2C laboratory has developed phenomenological approaches to model Nanosecond Repetitively Pulsed (NRP) plasma discharges in reacting flows: Castela et al. (2016) and Blanchard et al. (2023). From previous ECFDs, the above-mentioned models were implemented and validated in the Low-Mach number (YALES2-VDS) and Compressible (YALES-ECS) frameworks.
The main objective of ECFD8 was to assess the hydrodynamic and compressibility effects induced by the nanosecond plasma discharges with ECS solver. First, the hydrodynamic instabilities were successfully computed through 3D simulations of the pin-to-pin configuration. Then, the 3D flow tunnel configuration was successfully simulated with the plasma-assisted turbulent combustion modeling in compressible framework. In addition, during this workshop, HP-adaptation (hybrid schemes) was tested on the simulated configurations, and new features for the plasma discharge object were implemented.

==== C3 - Extending and validating a generalized formalism of virtual chemistry ====

==== C4 - Turbulent combustion model for NOx prediction ====

==== C5 - Towards 3D simulation of detonation combustion ====

==== C6 - Flame stabilitity of flame-holders within reheat conditions ====

Participants : N. Detomaso (Safran AE), R. Janodet (Safran AE), L. Carbajal (Safran AE)

The mechanisms of flame stabilisation in a flame-holder are still not completely understood. This is particularly true for reheat conditions : the hot viciated gases at inlet, highly compressible flows, and a strong liquid/gas coupling make the flame stability hard to predict. During the ECFD8, simplified configurations of flame-holders within reheat conditions were analysed. After some try and error simulations due to simplifications of the flame-holders, cases leading to flame stability (or not) were identified. Post-processing tools were developped in order to recover criteria relevant to the flame stability. This step marks the beginning of a systematic mapping of dimensionless characteristic times ratios, and a comparison with integral quantities.

==== C7 - Thermal radiation in oxyflames ====

==== C8 - A first step toward hybrid CPU / GPU for reactive flow in YALES2 ====

Participants: M. Laignel (CORIA), G. Lartigue (CORIA), K. Bioche (CORIA) and V. Moureau (CORIA)

In numerical simulations of reacting flows, one of the most computationally intensive tasks is the evaluation of source terms resulting from chemical reactions in the species transport equations. This step can account for up to 90% of the total simulation cost , depending on the complexity of the kinetic mechanism involved. To reduce this cost, various techniques such as mechanism reduction, virtual chemistry, etc. have been explored. However, the emergence of GPUs as powerful accelerators offers a promising alternative by providing massive parallelism. Despite their potential, GPUs often require significant adaptation of CPU-based codes. This project aims to address this challenge by taking a first step towards a hybrid CPU/GPU framework for reactive flow simulations. Specifically, the focus is on coupling Y2 with the updated version of the stiff time integration solver (CVODE), which is compatible with GPU (CUDA, HIP, OpenMP). The ultimate goal is to establish a foundation for hybrid computations by implementing and testing the updated solver on simplified test cases.

==== C9 - Soots numerical modeling ====

==== C10 - TECERACT : Tabulated chemistry generator for aeronautical combustion ====

==== C11 - Exploring efficient tabulation strategies for detailed chemistry ====

==== C12 - Dynamic sub-grid-scale modelling of multi-regime flame wrinkling ====

==== C13 - LES of a semi-industrial burner using a non-adiabatic virtual chemical scheme ====

=== User Experience & Data - L. Korzeczek, GDTECH ===

==== U1 - Low-fidelity (RANS) rotor/stator simulations, application to Kaplan Turbine - Y. Lakrifi, G. Balarac (LEGI), R. Mercier (SAFRAN), V. Moureau (CORIA) ====

==== U2 - Coupling PyTorch/YALES2, combustion cartesian look-up tables - J. Leparoux, N. Treleaven, S. Dillon (SAFRAN), K. Bioche, G. Lartigue (CORIA) ====

Participants: Julien Leparoux (Safran Tech), Kévin Bioche (CORIA), Ghislain Lartigue (CORIA), Nicholas Treleaven (Safran Tech)

Neural Networks offer a promising alternative to Cartesian look-up tables for combustion simulations, reducing memory footprint. In this project, we investigated how to integrate an NN model for real-time inference in the YALES2 platform, exploring two approaches: a Python interface and a Fortran Torch binding (using FTorch[https://github.com/Cambridge-ICCS/FTorch]). We validated that the model remains accurate when embedded online and identified improvements for robustness. Inference costs were evaluated on a Mac M3 and the Austral cluster, revealing a strong dependency on data volume. To optimize efficiency, we propose grouping cells at the processor level.

==== U3 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 - L. Korzeczek, S. Meynet (GDTECH), J. Leparoux, M. Cailler (SAFRAN) ====

Ecfd:ecfd 8th edition

2025-02-13T10:20:59Z

Benard: /* T6 - FSI model in Dorothy - Enzo MASCRIER (LOMC), Grégory PINON (LOMC) */

{{DISPLAYTITLE: ECFD workshop, 8th edition, 2025}}

== Description ==

* Event from '''27th of January to 7th of February 2025'''
* Location: [https://www.sport-normandie.fr/le-centre/le-site-de-houlgate Centre Sportif de Normandie], Houlgate, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* Participants from academics, HPC center/experts and industry are welcome
* The number of participants is limited to 68.

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes
* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Voivenel (CORIA).

[[File:ecfd8.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_8th_edition]]

[[File:Acknowledgments_ecfd8.png|text-bottom|600px]]

== News ==

* 23/10/2024: First announcement of the '''8th Extreme CFD Workshop & Hackathon''' !
* 22/11/2024: Deadline to submit your project

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

To come...

== Projects ==

=== Hackathon GENCI - P. Begou, LEGI ===
This ECFD8 GENCI Hackathon was a rich event, involving 4 differents CFD codes (AVBP, ParaDIGM, SONICS and YALES2) using various paradigms (C++/cuda/hip, Fortran/OpenMP/OpenACC) with several SDKs (AMD, Cray/HPE, Nvidia, Gnu) on a large range of GPU architectures (Nvidia A100, GH100, AMD instinct Mi210, Mi250, Mi300). This two-week event benefited from a high level support from three HPC mentors, two on-site from AMD (J. Noudohouenou and A. Tsetoglou) and one remote from CINES (M. Boudaoud).

==== H1 - ParaDIGM and SONICS on GPU, B. Maugars, G. Staffelbach, R.Cazalbou and B. Michel (ONERA)====

==== H2 - AVBP GPU offloading based on OpenMP, M.Ghenai, L. Legaux and A. Dauptain (CERFACS) ====

This hackathon provided a valuable opportunity to work on GPU offloading for AVBP. In the past, significant efforts were made to offload the entire AVBP code to GPUs. OpenACC was the primary strategy chosen, mainly due to access to NVIDIA's support, along with the availability of both software and hardware. This approach achieved good scalability performance.
Recently, with the deployment of new supercomputers like ADASTRA at CINES, some issues have emerged when running AVBP on AMD GPUs, including both MI250 and MI300. The closed-source nature of the Cray environment has also prevented CERFACS from deploying AVBP on local MI210 GPUs.
This hackathon was a great opportunity to address these challenges by exploring a new approach using OpenMP. An automatic translation tool was used to convert approximately 2,700 OpenACC directives to OpenMP, with each directive manually verified and fine-tuned afterward. AVBP with OpenMP had already been tested on NVIDIA GPUs, and during this hackathon, the focus was on extending support to AMD GPUs.
Two compilers were used: Cray and the newly released AFAR from AMD. With the support of AMD and CINES, a working environment for compiling AVBP was set up, and performance-related issues were identified. Additionally, two mini-apps were used for benchmarking. One of them achieved a 2.5× speedup when compiled with AFAR compared to Cray.
The next steps involve adapting the code to address necessary modifications, such as fixing issues related to Fortran indirections, and continuing performance evaluations with mini-apps. Further comparisons will be conducted using both compilers against results obtained with NVIDIA’s NVHPC.

==== H3 - YALES2 GPU from OpenACC to OpenMP, P. Bégou (LEGI), V. Moureau, G. Lartigue (CORIA) and R. Dubois (IMAG) ====
This Hackathon focuses on running Yales2 code on AMD Instinct Mi250 and Mi300 GPUs of the Adastra supercomputer (CINES).
Previously, a first solver in the Yales2 CFD code was successfully ported on the GPU accelerators of the Jean-Zay supercomputer (IDRIS) using Nvidia SDK but difficulties remain on Adastra AMD GPUs, mainly related to the available development tools. High compilation time and the impossibility to use debug flags at compile time as soon as OpenACC is enabled are a real challenge when tracking errors. The current project is to evaluate a freshly deployed version (at the begining of the workshop) of the AMD Fortran compiler. This requires moving to OpenMP paradigm, starting from scratch since the OpenACC branch has largely diverged from the master one while tracking spurious remaining bugs.
If the AMD compiler is able to build the cpu version of Yales2 "out of the box" (wich is not the case for Cray Fortran), the compilation time for each file is significantly higher. However, setting up a 2 stages dynamic compilation process allows for high parallelism that is not possible with Cray Fortran 18 and the library build time drops from nearly 2 hours (Cray Fortran 18) to 17 minutes (Amd Fortran compiler).
Large kernels have been ported from OpenACC to OpenMP, raising some difficulties when offloading intrinsics functions or using strutures attributes in kernels loops. These limitations were also known in the previous OpenACC work. The goal was mainly to check the correctness of the results. The offloading of the complex data structure of Yales2 code was then investigated. Here again some limitations of the "young" compiler were discovered and workarounds were implemented. Several reproducers were built during this ECFD8 and provided to developpers by the 2 on-site AMD engineers.
Preliminary tests on micro-applications show good performances of the generated binaries proving that this compiler could be a serious alternative on AMD GPUs and the goal is now to focus on this SDK in an OpenMP strategy while checking the portablility of this new implementation in Nvidia, Cray/HPE (and Gnu ?) environments.

=== Mesh adaptation - A. Grenouilloux, ONERA & G. Balarac, LEGI ===

==== M2 - Enhancement of mesh adaptation algorithms, B. Maugars (ONERA), B. Andrieu (ONERA), C. Benazet (ONERA), N. Dellinger (ONERA), G. Janodet (ONERA), G. Staffelbach (ONERA) ====
Mesh adaptation has become a crucial tool in order to automate industrial numerical simulations. ECDF7 allowed us to investigate the refine and EGADS libraries as tools for parallel mesh generation and adaptation using CAD as a geometric support. Since then, we fortified the workflow but some of our targeted industrial applications such as turbomachinery involve periodic boundary conditions. To manage these cases, the mesh generation and adaptation procedures must maintain matching periodic boundaries.
During ECFD8, we addressed multiple topics : periodic mesh generation from CAD model in EGADS, parallel and periodic metric gradation in ParaDiGM, making our parallel remeshing algorithm more generic to support non-manifold and periodic meshes. All these topics were covered, but there is still some work to be done to fully manage periodic mesh adaptation.
Non-manifold mesh adaptation was applied to changing mesh topologies. Here, an internal surface was described by a level-set and deformed up to hole creation to mimic ablation.

==== M3 - Development and definition of a new Automatic Mesh Convergence (AMC) driver for automating static mesh convergence in YALES2 (C. Papagiannis (LEGI/INRIA), G. Balarac (LEGI), O. LeMaître (INRIA), P.M. Congedo (INRIA)) ====
Static mesh adaptation requires re-adapting an existing (usually coarse) mesh, to conform to some quality metric that satisfies some optimality criterion. This involves statistics (Mean and RMS fields) that need to be adequately converged. We proposed and developed a novel AMC strategy that can converge an initial coarse mesh to an adequately refined one by taking into account the quality of the calculated statistics before each adaptation. Specifically, so far, the moment (in terms of integrating the LES equations) at which to launch the adaptation was unknown and had to be set apriori as a parameter of the AMC, dependent on the flow physics and therefore on the case at hand. Our method is automatically performing adaptations when the quality of the volume averaged mean field estimate (characterized by its RMSE) reaches the levels of the bias error that this field has, due to the mesh being far from the desired one (dictated by the quality criterion). This was achieved through drawing inspiration from a theoretical model that treats the AMC as a "conditionally" contractive mapping with a fixed point.

=== Numerics - M. Bernard, LEGI & G. Lartigue, CORIA ===

==== N1 - Traction open boundary condition ====

==== N2 - Treatment of Inlet conditions in High-Order solver. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====
In the context of node-centered Finite Volumes Method, spacial accuracy of a numerical scheme depends on ability to evaluate accurately fluxes through interface of each control volume (CV). Such accurate evaluation is not straightforward, especially when dealing with distorted grids. This project follows the work of [1] where fluxes use pointwise quantities, which are reconstructed from integrated quantities advanced in time. During the previous edition of the ECFD, a new data structure has been developed to store data at location of the boundary conditions facelets, with application to wall boundary conditions. During this 8th edition of the ECFD, we used the same data structure, but dedicated to the treatment of inlet conditions.
The inlet condition is then either imposed directly at facelets center, or at nodes position them extrapolated to facelets center by use of Taylor expansion. For this later solution, high-order treatment requires the successive derivatives to be computed in the plane of the boundary condition. This is not done yet, leading for the moment to low accuracy results but the framework is ready for upcoming implementation.

[1] ''A framework to perform high-order deconvolution for finite-volume method on simplicial meshes, , Bernard et. al., IJNMF 2020''

==== N3 - Conservative mesh-to-mesh interpolation. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====

Mesh to mesh interpolations occur quite often in CFD simulations : in the context of adaptative mesh convergence or in the case of dynamic mesh adaptation for for example.
Quality of the solution on the destination grid will depend on the characteristics of the interpolation method.
In this project, we did not focus on accuracy of the interpolation method but rather on conservativity characteristics.
A conservative interpolation ensures that the integral of the data on the source grid is exactly retrieved on the destination grid.
This property is highly interesting when dealing with scalar quantities or phase indicators, whose values should remained bounded.
In the context of nodes centered Finite Volume schemes, the methodology we used consists in (i) reconstructing element quantity from average nodal quantities on source grid.
Then, for a cell of the destination mesh, (ii) computing the geometrical intersection between cells of source and destination meshes to evaluate to evaluate the rate of quantities they.
Eventually, (iii) redistributing the solution from elements to control volumes of the destination mesh.
The overall process is fully conservative as it is based on geometrical intersection of locally integrated quantities.
The methodology as been implemented and tested on a few basic configurations and the conservativity is retrieved.

==== N4 - Determination of timestep in semi-implicit solver. T. Berthelon (LEGI), G. Balarac (LEGI), H. Lam (LEGI), M. El Moatamid (CORIA) ====
In order to reduce the computation time associated with incompressible LES simulations, an implicit time integration, based on BDF schemes, has been developed within the ICS solver. This integration eliminates the stability constraints associated with explicit schemes, and therefore opens up the question of the appropriate choice of time step.
In parallel, recent work has been carried out on meshing criteria in LES. The strategy in place consists of adapting the mesh by distinguishing two zones:
- "DNS" zones, where the meshing criterion is based on an estimate of the adimensioned spatial error.
- "LES" zones, where the meshing criterion is based on Kolmorogov theory.
During this project, the spatial criteria were extended to include temporal criteria. In the "DNS" zones, the time step is chosen using an estimate of the temporal error of the BDF scheme judiciously scaled to match the spatial error. In the "LES" zones, the time step is chosen using a scaling law associated with fully developed turbulence.
The new time step selection strategy has been tested on the case of a turbulent jet and leads to an accuracy equivalent to the explicit case while reducing the simulation return time by a factor of nearly 3.

Another aspect of this project was to integrate certain implicit temporal schemes (C-N and SDIRK) recently developed by Mr. El Moatamid into the incompressible solver.

==== N5 - Local timestep. T. Berthelon (LEGI), M. Bernard (LEGI), G. Balarac (LEGI) ====
RANS modelling has recently been developed within the YALES2 library. With this modeling strategy, the objective is to reach as quick as possible a steady state.
During this project, we investigate the use of a local time step to reduce the time to solution of steady computation in the incompressible solver.
This implies solving a variable-coefficient Poisson equation. Encouraging results were obtained in the simple case of "Couette plan" flow artificially constrained by a mesh variation. In fact, the use of local time-step reduce drastically the time to solution on this configuration. This method needs to be tested on real RANS case.

==== N6 - Distributed version of DOROTHY. M. Roperch, G. Pinon (LOMC) ====
Dorothy is a Lagrangian code using vortex particle method. This method is based on the discretisation of the fluid into vorticity carrying particles. Before ECFD7, all the processors knew all the particles. Since ECFD7, we have gradually moved to a fully distributed version. At ECFD8, the goal was to get a domain decomposition without knowing all the particles. This decomposition is done by dividing the domain by prime factors so that each subdomain has the same number of particles. Prior to this ECFD, a Python draft of this module was created. At ECFD8, this new algorithm was implemented and will soon be operational. The next step is to continue this work for the velocity calculation, as each particle affects the velocity at a given position, and calculating the velocity without knowing all the particles requires more MPI communication.

==== N7 - Implicit time advancement for low-Reynolds number flows with particles. S. Mendez, C. Raveleau (IMAG), M. El Moatamid, V. Moureau (CORIA) ====
IMAG runs numerous simulations of red blood cells under flow. Those simulations are at low Reynolds number (0.001 to 1.0, typically). Splitting of the time advancement is used to treat the diffusion terms implicitly, albeit with an important numerical cost: implicit diffusion is 50 to 60% of the computational cost. Recently, M. El Moatamid implemented a genral framework to deal with implicit time advancement for scalars. In this project, the general method has been transposed to the advancement of the velocity field in the ICS and RBC solvers of YALES2/YALES2BIO. This enables testing various linear solvers (GMRES based). However, such solvers do not decrease the CPU time compared to the existing implementation. However, while working on this, it was identified that residual recycling was not activated in the current implementation of the implicit diffusion. This sped up the implicit diffusion cost by 35%, for a total gain of 20% for the computation. In addition to this achievement, moving to the framework coded by Moncef will have other beneficial side effects: we anticipate simplifying the implementation, with an easier merging between YALES2BIO and YALES2. The method will also be implemented in the electrosatic solver, for which the Poisson problem should benefit from the new GMRES-based solvers. In addition, this project highlights the importance of improving the treatment of stiff source terms in the red blood cells simulations, to be able to overcome the current limitation in time step due to those term and have a chance to benefit from higher-order time schemes, efficient at high Fourier numbers.

==== N8 - Boundary Element Method in YALES2. B. Thibaud, S. Mendez (IMAG), G. Lartigue, P. Benard (CORIA), F. Nicoud (IMAG) ====
In the context of microfluidic systems for diagnosis, the Boundary Element Method alows to solve linear PDE such as electrostatic or Stokes. With well chosen kernel functions and the divergence theorem, this method allows to write on the boundary condition only the initial volumic problem. This project aimed at exploring the feasibility of the BEM in the context of massively parallel unstructured solver like YALES2 by developping a Julia demonstrator. The first step have been to implement and validate the method on simple configurations for the Laplace's equation. Only Neumann problems were considered (Dirichlet boundary conditions imposed). In a second time, the multi-domain approach has been identified to be the most suited in the framework of YALES. The inner domain is partitioned on each processor, each having a part of the physical boundary and interfaces between them. Every processor solve its own boundary problem and a parallel Dirichlet-Dirichlet fixed-point is used to converge the interface problem on the all domain. Applied to the ring case, with one interface, we managed to reproduce the linear convergence of the P-DD method.

=== Turbulence - L. Voivenel, CORIA & P. Bénard, CORIA ===

==== T1 - FSI-1D strategy for internal flows - Pierre BENEZ (SAFRAN Aerosystem), Renaud MERCIER (SafranTech), Yacine BECHANE (CORIA) ====

Many applications developed at Safran Aerosystem are based on internal turbulent flows coupled to a moving body. 2 cases were studied during this ECFD:

'''Case 1 (Incompressible flow)''': Translation of a piston subjected to a pressure difference in a pipe.

The challenges of this case are twofold: the small gap between the piston and the pipe and the large pressure gradient across the piston (>100bar). During the 1st week of ECFD, the CLIB (Conservative Lagrangian Immersed Boundary) solver was tested on this case. The study showed that the solver was unable to ensure the impermeability of the solid under these pressure conditions. In the rest of the study, a porous medium following Darcy's law will be added to the penalty force of the immersed solid to fully satisfy the impermeability of the piston.

'''Case 2 (Compressible flow)''': Rotation of a butterfly in a discharge vane.

The coupling between the ECS (Explicit Compressible Solver) and ALE (Arbitrary Lagrangian Solver) solvers having recently been implemented, this strategy was tested to model the opening of the valve by rotation of the butterfly. The challenge here lies in the small gap between the bottom of the butterfly and the vane casing. To limit the simulation cost, the gap is meshed with 1 element. In this case, MMG succeeded in adapting the mesh up to a critical angle at which the gap becomes too small (Work In Progress).

==== T2 - Dynamic Smagorinsky in Dorothy - Maëlenn ROPERCH (LOMC), Grégory PINON (LOMC) ====
Dorothy is a Lagrangian code using vortex particle method (VPM). This method is based on the discretisation of the fluid into vorticity carrying particles. In some cases, there is a perturbation in the wake that needs to be diffused. Two LES model for a turbulent viscosity exist currently in the code: a standard Smagorinsky model and a Mansour model. Both model apply the same LES constant everywhere. The aim of this ECFD is to add and compare two dynamic and local Smagorinsky models. The first version is a classical dynamic Smagorinsky model in VPM, quantities are filtered with a sum over all particles. Since the values of the sum terms are lower for more distant particles, a second version reduces the sum to neighbouring particles. Preliminary results shows some differences between the two method. The next step will be to validate and compare the results with the case of two vortex ring colliding.

==== T3 - Turbulence injection strategy for compressible flows - Patrick TENE HEDJE (UMONS), Laurent Bricteur (UMONS), Yacine BECHANE (CORIA), Pierre BENARD (CORIA) ====
Taking realistic turbulence into account in turbomachinery simulations for aeronautical applications remains a major challenge, particularly as regards the management of non-reflexive boundary conditions. The implementation of the actuator line method (ALM) in the YALES2 Library has enabled us to set up wind tunnel test reproduction setups, such as the modeling of turbulence grids. During the ECFD8, this project aimed to (WP1) finalize the implementation of this method in the Explicit Compressible Solver (ECS) of YALES 2 and (WP2) use this same method to model the interaction of the upstream stator wake on the downstream rotor wheels. The setups implemented have been successfully tested on simple channel flow cases. The aim is to continue the validation process on real turbomachinery geometry cases.

==== T4 - Improve wind farm modeling and simulation workflow - Pierre BENARD (CORIA), Ulysse VIGNY (UMONS), Etienne MULLER (CORIA & SGRE), Hakim HAMDANI (GDTech), Félix HOUTIN-MONGROLLE (SGRE), Anand PARINAM (CORIA & TUDelft), Hari MULAKALOORI (CORIA), Léa VOIVENEL (CORIA) ====
The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine. Still, applying this method to investigate a wind farm flow can be challenging, both in terms of computational cost and simulation setup. For instance, an inadequate management of the wind turbine individual modeling parts in a HPC context can end up being the main bottleneck of the simulation. From another perspective, a wind farm is usually composed of more than 50 wind turbines. For such a case, setting up all YALES2 required inputs manually can be very tedious and error-prone. This project thus mainly aimed to optimize the YALES2 ALM implementation and the user experience around it. Additionally, a cost-effective alternative to the ALM when modeling wind farm flows, namely the Rotating Actuator Disk Method (ADM-R), has been implemented for further investigations.

'''WP1''': Improve Actuator set rotor modelling
* Parallel processing of the ''actuator sets'' used to model the wind turbines
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified during the 5th iteration of the ECFD: (i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges. (ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications. During this workshop edition, the work focused on the re-synchronisation of the algorithm steps by allowing some packing an unpacking of the object to allow the transfert of the object inbetween workers. This is of major importance to enable load balancing and mesh adaptation during the temporal loop. This work required the refactoring of the involved oject structures.

* Rotating Actuator Disk Method (ADM-R):
According to the usual guidelines, the mesh requirements of the ALM, to profit entirely from its reachable accuracy, can be difficult to achieve or even unaffordable when simulating a wind farm flow, especially from the industrial point of view. Alternatives are available in the literature for this kind of application. Likely, the methods from the Actuator Disk family are the most prominent ones. Several kinds of implementation exist, which mostly differ by their capability to include the wake rotation. During the workshop, a new method from the Rotating Actuator Disk kind has been implemented and underwent an early validation on a single turbine setup. Applications to wind farm flows will follow.

'''WP2''': Improve tools User Experience

Three Python tools have been developed or improved :
*The first tool is the wind farm previsualisation tool, 'y2_wind_previsualization', which is used before the calculation run. This provides an interactive HTML interface for viewing global data for each turbine on the farm (position, hub height, yaw angle, etc.). The tool traces all of these via the parsing of the input file.
* The second tool is for duplicating rotor templates for a wind farm (`y2_wind_duplication`). This tool was developed in the previous ECFD, but this time it has been refactored and incorporated into the y2tools package.
* The third and final tool is a post-processing tool for the temporal processing of global wind turbine simulation metrics (Thrust, Power, etc.), `y2_post_wind`. This tool generates an interactive HTML plot of time-dependent global quantities.

==== T5 - Improve atmospheric inflow turbulence - Ulysse VIGNY (UMONS), Pierre BENARD (CORIA), Etienne MULLER (CORIA & SGRE), Hakim HAMDANI (GDTech), Félix HOUTIN-MONGROLLE (SGRE), Anand PARINAM (CORIA & TUDelft), Hari MULAKALOORI (CORIA), Léa VOIVENEL (CORIA) ====
Atmospheric inflow turbulence is generated using the precursor database method. A half-channel flow driven by a pressure gradient is used to obtain the inflow which is used as inlet boundary condition for the wind turbine simulation domain. This project aimed to improve the whole methodology, from generation to injection.

* WP1: Improve inflow generation

The inflow generation using a velocity controller implementation was initially unstructured and lacked robustness. This PID-based velocity controller allows users to impose a specified velocity and direction at a given height, primarily for atmospheric flow simulations in wind farms. During the workshop, the controller was integrated into the source code by applying velocity forcing as a source term in the flow field. The approach utilizes a target velocity, interior boundary and specification of the ground boundary. Additionally, when using the logarithmic law in atmospheric flows, inconsistencies between the computed and the intended wall shear stress for the target velocity sometimes led to diverging velocity profiles. This was due to mismatches in the source terms. To address this, the wall shear stress is now computed for the target velocity and imposed as a source term, ensuring consistency and stability.

* WP2: Improve injection methodology (method A)
The previous workflow used plane probes in the ASCII format to sample the flow. The COWIT2 toolbox was used to convert the file into turbulence box (.man format). While functioning, this methodology had two major flaws. First the probe files are heavy ~O(10Go). Second, the method requires a lot of human effort, allowing numerous sources of errors.
During this workshop, a new methodology has been developed. First, the probes are generated using the HDF5 format (now available for all probe types), leading to lighter file ~O(1Go). Second, Y2_tools is used to read HDF5 format (working for probes and temporals). HDF5 file is then converted into a Look-up Table. Finally, the Look-up Table is read directly by YALES2 as a boundary conditions.

* WP3: Improve injection methodology (method B)
Even though improvements achieved in WP2 prove to be very handy while removing many potential human errors, injecting a turbulent inflow through wind boxes ('offline' precursor approach) can sometimes remain cumbersome for several reasons: (1) no periodicity is enforced in the streamwise direction of those boxes, (2) potential high memory consumption, and (3) the boxes need to be moved to other cores whenever a mesh adaptation occurs. An alternative consists in co-simulating the precursor flow and the flow of interest (refered as the 'successor' simulation) at the same time ('online' precursor approach). The inlet boundary condition for the successor flow is then obtained by mapping the outflow of the precursor domain. During the workshop, some work has been initiated to implement this kind of coupling using the CWIPI library, for which YALES2 provides already an interface.

==== T6 - FSI model in Dorothy - Enzo MASCRIER (LOMC), Grégory PINON (LOMC) ====

The objective of the project was to enhance the numerical code Dorothy (developed at the LOMC) to perform fluid-structure interaction study. To achieve this goal, the blades must be capable of deforming in response to the incoming flow. The prediction of the blade deformation is modeled by a Timoshenko beam approach.

The code is presently written in a way that keeps the blades always in the rotor plane. Additionally, particles are currently emitted based on a local frame at each blade section. In a near future, when the blade will deform, this will change the orientation of the particles. Therefore, the particle emission must be updated accordingly to reflect these deformations.

During the first of ECFD8, a forced blade motion case was coded abd tested, allowing us to verify particle’s emission. This work will be further developed to deform the blade dynamically using the structural solver code at each time-step.

=== Two Phase Flow - J. Leparoux, SAFRAN & J. Carmona, CORIA ===

==== TP1 - Towards very small contact angles in Nucleate boiling ====

Participants: Henri Lam (LEGI), Mohammad Umair (LEGI), Manuel Bernard (LEGI), Robin Barbera (LEGI) and Giovanni Ghigliotti (LPSC)

The boiling solver (BOI) was not able to accurately impose a contact angle (angle formed by the two-phase interface on the wall) at values lower than 30°. This angle is needed when simulating nucleate boiling. A similar limitation in contact angle value was applying to the spray (SPS) solver. A modified version of the level set reinitialization has been implemented during ECFD8, based on a different normal vector in the blind spot region around the contact line, vector now chosen to be a zero-order extension from outside the blind spot. This modification, that implied other modifications to the level set reinitialization in the blind spot, has been tested successfully on the spray solver (where no phase change occurs). Then, this new reinitialisation has been tested in the boiling solver for nucleate boiling, with great improvements. Now simulations of nucleate boiling at very small contact angle (10°) can be accurately performed.
In the meanwhile, the level set reinitialization algorithm has been streamlined and the computational cost greatly reduced, resolving a computational cost issue that appeared when using the contact angle imposition both in the spray and boiling solvers, and that hampered its use in industrial configurations.

==== TP2 - Modeling spray-film interactions ====

Participants: Nicolas Gasnier (EM2C-SafranTech), Julien Leparoux (SafranTech), Mehdi Helal (CORIA-SafranTech) and Julien Carmona (CORIA)

The numerical simulation of sheared liquid films over walls with conventional high-fidelity methods requires fine meshes to depict accurately the dynamics of the fluids, as well as the gas-liquid interactions at the phase interface. In the context of thin film flows (with a thickness h ~ 1.0E-4 m), the spatial resolution required to ensure the validity of the simulations can be computationally prohibitive. To tackle this issue, a reduced-order model of thin film dynamics based on the Saint Venant equations has been implemented in the YALES2 platform over the last months. This numerical model is able to reproduce accurately the dynamics of strongly sheared film flows over partially wetted surfaces, and to take into account capillary effects. The objective of the ECFD8 was to couple the thin film framework with the pre-existing multiphase methods of the YALES2 platform: the Eulerian multiphase solver - based on the ACLS -, and the Lagrangian solver. This coupling was performed in three steps: first, a numerical method was designed to convert impinging Lagrangian droplets into source terms for the thin film model. Then, a phenomenological model was implemented to depict the atomization of films at sharp edges under the action of a high speed gas flow. The liquid film is converted into Lagragian droplets, whose dimensions are computed based on the PAMELA model. Finally, a numerical method has been designed to convert Eulerian droplets into source terms for the film model. This method is based on a transfer of the liquid properties from the nodes to the boundaries by using a set of fictitious particles. It ensures conservation of the liquid mass, and it is more robust than the first attempts that had been made during ECFD7.

==== TP3 - High-fidelity two-phase flow simulations of the purge of a fuel feed line ====

Participants: Thomas LAROCHE (Safran HE), Romain JANODET (Safran AE), Julien Leparoux (Safran Tech) and Melody Cailler (Safran Tech)

During the second week of the ECFD8, the fuel feed line purge process has been numerically investigated. In the context of aeronautical engines, the fuel feed line—carrying the fuel from the fuel tank to the injectors within the combustion chamber—needs to be purged at engine shutdown. This is intended to prevent fuel stagnation near hot metal parts, which could lead to coke formation and therefore decrease engine performance. Since this complex phenomenon is mainly driven by two-phase flow physics, the spray solver (SPS) of the YALES2 library has been considered in order to understand the physics of such process. The numerical setup was first converged on a simplified test case: the possibility of driving the flow dynamics with inlet and outlet pressure conditions was tested beforehand on a single-phase, incompressible case, and then on a two-phase flow problem. The setup has then been successfully applied to an industrial configuration: a pressure-swirl injector connected to a reduced portion of the fuel feed line. Due to the large scale of the domain, the interface resolution was set to 50μm, which is intentionally coarse for such problem. This initial computation successfully ran up to 3ms of physical time during the workshop, proving YALES2's capability to model the fuel purge. The computation is to be continued and analyzed further even after the workshop.

==== TP4 - Volume of Fluid solver in YALES2 ====

Participants: Léa Voivenel (CORIA), Julien Carmona (CORIA), Mehdi Helal (CORIA), Pierre Portais (CORIA), Julien Leparoux (Safran Tech), Mélody Cailler (Safran Tech) and Nicolas Gasnier (EM2C / Safran Tech)

The aim of this project was to develop an alternative strategy to the ACLS method already implemented in Yales2 to investigate incompressible two phase flows. The Volume of Fluid (VoF) methods were chosen as they show excellent performances, with interesting qualities such as inherent conservation of mass. Nevertheless, they remain challenging in an unstructured mesh framework.
The work conducted during ECFD8 can be divided into two parts.

First of all, we aimed to robustify and improve the new Volume of Fluid Solver (VFS) where a liquid volume fraction is transported with an imposed velocity field. In this simplified framework several resharpening strategies to counteract the liquid volume fraction numerical diffusion were explored. On one hand, a compressive velocity only acting at the interface was implemented as a source term in the Volume Fraction transport equation, <math> \frac{\partial \alpha}{\partial t}+\nabla \cdot(\alpha \boldsymbol{u})=\nabla .\left(\begin{array}{l}
\alpha(1-\alpha)^{\begin{array}{c}
\end{array}} C_\gamma|\boldsymbol{u}| \boldsymbol{n}_{\Gamma}
\end{array}\right)</math>. While satisfactory results were obtained, several improvements were identified: compute the source term at pairs instead of nodes and to investigate the influence of <math> C_\gamma </math> depending on the numerical scheme.
On the other hand, a strategy based on the solving of a "resharpening equation" at each timestep <math> \partial_\tau \alpha+\underline{\nabla} \cdot(\alpha(1-\alpha) \underline{n}-D(\underline{\nabla \alpha} \cdot \underline{n}) \underline{n})=0</math> was also explored. This second approach seemed more efficient but its cost must still be evaluated.

Finally, the second axis of this work aimed at coupling the liquid volume fraction transport with the momentum equation. A first working version of this new solver was proposed at the end of the 2 weeks. The next steps are the integration of the resharpening methods presented in the above section, the investigation of the problem of Poisson convergence for higher order schemes and the addition of the surface tension.

==== TP5 - Implement a local operator to distribute the solid volume of a particle over multiple cells ====

Participants: Théo Ndereyimana (Université de Sherbrooke), Stéphane Moreau (Université de Sherbrooke)

In the CFD-DEM, the cell size is required to be larger than the particle size for stability condition and keep feasible solid volume fraction. However, some applications require a cell size smaller than the particle. During this 8th edition of the ECFD, the use of operators to distribute the particle volume over multiple cells, ensuring a feasible solid volume particle has been tested on a fluidized bed configuration. The main operators tested (gather-scatter filter and gaussian filter) showed a tendency to blur the void structure interfaces. The equivalence of the gaussian filter of bandwidth <math>b</math> and the resolution of a diffusion equation over a pseudo-time <math>T=b^2/4</math> has been verified.
One anisotropic diffusion constant has been tested and shows a possibility to adress the sharpness requirements.

Another objective was to develop a post-processing tool to detect and track the void structures (bubbles) in a fluidized bed. Based on previous work from J. Carmona, a tool to track the bubbles has been initiated.

==== TP6 - Complex thermodynamics in sloshing tanks ====

Participants: C. Merlin (AGS), D. Fouquet (CORIA), V. Moureau (CORIA), J. Carmona (CORIA) and G. Lartigue (CORIA)

During this ECFD, the phase change solver was improved. After introduction of conservative transport for enthalpy and energy, a new framework for multi fluid two phase flow was used that relies on a new reinitialization of the conservative level set, the transport of discontinuous scalars like temperature, energy or enthalpy and the transport of weight to ensure consistency. The pressure loop was improved to ensure mass and energy conservation while meeting the low Mach hypothesis. Many 1D test cases were performed with only the gas phase and two phase flow to validate the pressure loop. The new framework for the transport of discontinuous scalar was first derived for temperature with constant properties in each phase. A first step was to extend it to sensible enthalpy with ideal gas behavior for the gas phase. Then, the framework was extended to NIST tabulated gas & liquid with ideal or real gas behavior. After some 2D cases, a tank pressurization test was investigated as well as a sloshing test without phase change (just the pressure drop due to the increase of the thermal exchange between the liquid and the gas phase).

=== Combustion - Y. Bechane, CORIA & S. Dillon, SAFRAN & K. Bioche, CORIA ===

==== C1 - LES of the thermal degradation of a composite material ====
Participants: A. Grenouilloux (ONERA), K. Bioche (CORIA), N. Dellinger (ONERA) and R. Letournel (SafranTech)

The FIRE test bed is an experimental air-propane burner operated by ONERA. It is dedicated to the study of the thermal degradation of composite materials. This project concerned the implementation of a three-solver coupling methodology to simulate the dynamics of the impinging flame. The methodology considered is based on the coupling between the variable density solver (VDS) and the radiative solver (RDS) of the massively parallel library YALES2 and the solver dedicated to the degradation of composite materials, MoDeTheC, developed by ONERA. Given the typical test times of the order of tens of seconds, a methodology based on 2D axisymmetric calculations was considered. Various tests were performed to determine the optimal coupling frequency between solvers. Cases dedicated to the injection of pyrolysis gasses were set up, with the aim of simulating the auto-ignition phenomenon. Comparisons with experimental data are presented.

==== C2 - Flame stabilization by NRP plasma discharge ====

==== C3 - Extending and validating a generalized formalism of virtual chemistry ====

==== C4 - Turbulent combustion model for NOx prediction ====

==== C5 - Towards 3D simulation of detonation combustion ====

==== C6 - Flame stabilitity of flame-holders within reheat conditions ====

Participants : N. Detomaso (Safran AE), R. Janodet (Safran AE), L. Carbajal (Safran AE)

The mechanisms of flame stabilisation in a flame-holder are still not completely understood. This is particularly true for reheat conditions : the hot viciated gases at inlet, highly compressible flows, and a strong liquid/gas coupling make the flame stability hard to predict. During the ECFD8, simplified configurations of flame-holders within reheat conditions were analysed. After some try and error simulations due to simplifications of the flame-holders, cases leading to flame stability (or not) were identified. Post-processing tools were developped in order to recover criteria relevant to the flame stability. This step marks the beginning of a systematic mapping of dimensionless characteristic times ratios, and a comparison with integral quantities.

==== C7 - Thermal radiation in oxyflames ====

==== C8 - A first step toward hybrid CPU / GPU for reactive flow in YALES2 ====

Participants: M. Laignel (CORIA), G. Lartigue (CORIA), K. Bioche (CORIA) and V. Moureau (CORIA)

In numerical simulations of reacting flows, one of the most computationally intensive tasks is the evaluation of source terms resulting from chemical reactions in the species transport equations. This step can account for up to 90% of the total simulation cost , depending on the complexity of the kinetic mechanism involved. To reduce this cost, various techniques such as mechanism reduction, virtual chemistry, etc. have been explored. However, the emergence of GPUs as powerful accelerators offers a promising alternative by providing massive parallelism. Despite their potential, GPUs often require significant adaptation of CPU-based codes. This project aims to address this challenge by taking a first step towards a hybrid CPU/GPU framework for reactive flow simulations. Specifically, the focus is on coupling Y2 with the updated version of the stiff time integration solver (CVODE), which is compatible with GPU (CUDA, HIP, OpenMP). The ultimate goal is to establish a foundation for hybrid computations by implementing and testing the updated solver on simplified test cases.

==== C9 - Soots numerical modeling ====

==== C10 - TECERACT : Tabulated chemistry generator for aeronautical combustion ====

==== C11 - Exploring efficient tabulation strategies for detailed chemistry ====

==== C12 - Dynamic sub-grid-scale modelling of multi-regime flame wrinkling ====

==== C13 - LES of a semi-industrial burner using a non-adiabatic virtual chemical scheme ====

=== User Experience & Data - L. Korzeczek, GDTECH ===

==== U1 - Low-fidelity (RANS) rotor/stator simulations, application to Kaplan Turbine - Y. Lakrifi, G. Balarac (LEGI), R. Mercier (SAFRAN), V. Moureau (CORIA) ====

==== U2 - Coupling PyTorch/YALES2, combustion cartesian look-up tables - J. Leparoux, N. Treleaven, S. Dillon (SAFRAN), K. Bioche, G. Lartigue (CORIA) ====

Participants: Julien Leparoux (Safran Tech), Kévin Bioche (CORIA), Ghislain Lartigue (CORIA), Nicholas Treleaven (Safran Tech)

Neural Networks offer a promising alternative to Cartesian look-up tables for combustion simulations, reducing memory footprint. In this project, we investigated how to integrate an NN model for real-time inference in the YALES2 platform, exploring two approaches: a Python interface and a Fortran Torch binding (using FTorch[https://github.com/Cambridge-ICCS/FTorch]). We validated that the model remains accurate when embedded online and identified improvements for robustness. Inference costs were evaluated on a Mac M3 and the Austral cluster, revealing a strong dependency on data volume. To optimize efficiency, we propose grouping cells at the processor level.

==== U3 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 - L. Korzeczek, S. Meynet (GDTECH), J. Leparoux, M. Cailler (SAFRAN) ====

Ecfd:ecfd 8th edition

2025-02-12T14:07:37Z

Benard: /* T1 - FSI-1D strategy for internal flows */

{{DISPLAYTITLE: ECFD workshop, 8th edition, 2025}}

== Description ==

* Event from '''27th of January to 7th of February 2025'''
* Location: [https://www.sport-normandie.fr/le-centre/le-site-de-houlgate Centre Sportif de Normandie], Houlgate, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* Participants from academics, HPC center/experts and industry are welcome
* The number of participants is limited to 68.

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes
* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Voivenel (CORIA).

[[File:ecfd8.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_8th_edition]]

[[File:Acknowledgments_ecfd8.png|text-bottom|600px]]

== News ==

* 23/10/2024: First announcement of the '''8th Extreme CFD Workshop & Hackathon''' !
* 22/11/2024: Deadline to submit your project

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

To come...

== Projects ==

=== Hackathon GENCI - P. Begou, LEGI ===
This ECFD8 GENCI Hackathon was a rich event, involving 4 differents CFD codes (AVBP, ParaDIGM, SONICS and YALES2) using various paradigms (C++/cuda/hip, Fortran/OpenMP/OpenACC) with several SDKs (AMD, Cray/HPE, Nvidia, Gnu) on a large range of GPU architectures (Nvidia A100, GH100, AMD instinct Mi210, Mi250, Mi300). This two-week event benefited from a high level support from three HPC mentors, two on-site from AMD (J. Noudohouenou and A. Tsetoglou) and one remote from CINES (M. Boudaoud).

==== H1 - ParaDIGM and SONICS on GPU, B. Maugars, G. Staffelbach, R.Cazalbou and B. Michel (ONERA)====

==== H2 - AVBP GPU offloading based on OpenMP, M.Ghenai, L. Legaux and A. Dauptain (CERFACS) ====

This hackathon provided a valuable opportunity to work on GPU offloading for AVBP. In the past, significant efforts were made to offload the entire AVBP code to GPUs. OpenACC was the primary strategy chosen, mainly due to access to NVIDIA's support, along with the availability of both software and hardware. This approach achieved good scalability performance.
Recently, with the deployment of new supercomputers like ADASTRA at CINES, some issues have emerged when running AVBP on AMD GPUs, including both MI250 and MI300. The closed-source nature of the Cray environment has also prevented CERFACS from deploying AVBP on local MI210 GPUs.
This hackathon was a great opportunity to address these challenges by exploring a new approach using OpenMP. An automatic translation tool was used to convert approximately 2,700 OpenACC directives to OpenMP, with each directive manually verified and fine-tuned afterward. AVBP with OpenMP had already been tested on NVIDIA GPUs, and during this hackathon, the focus was on extending support to AMD GPUs.
Two compilers were used: Cray and the newly released AFAR from AMD. With the support of AMD and CINES, a working environment for compiling AVBP was set up, and performance-related issues were identified. Additionally, two mini-apps were used for benchmarking. One of them achieved a 2.5× speedup when compiled with AFAR compared to Cray.
The next steps involve adapting the code to address necessary modifications, such as fixing issues related to Fortran indirections, and continuing performance evaluations with mini-apps. Further comparisons will be conducted using both compilers against results obtained with NVIDIA’s NVHPC.

==== H3 - YALES2 GPU from OpenACC to OpenMP, P. Bégou (LEGI), V. Moureau, G. Lartigue (CORIA) and R. Dubois (IMAG) ====
This Hackathon focuses on running Yales2 code on AMD Instinct Mi250 and Mi300 GPUs of the Adastra supercomputer (CINES).
Previously, a first solver in the Yales2 CFD code was successfully ported on the GPU accelerators of the Jean-Zay supercomputer (IDRIS) using Nvidia SDK but difficulties remain on Adastra AMD GPUs, mainly related to the available development tools. High compilation time and the impossibility to use debug flags at compile time as soon as OpenACC is enabled are a real challenge when tracking errors. The current project is to evaluate a freshly deployed version (at the begining of the workshop) of the AMD Fortran compiler. This requires moving to OpenMP paradigm, starting from scratch since the OpenACC branch has largely diverged from the master one while tracking spurious remaining bugs.
If the AMD compiler is able to build the cpu version of Yales2 "out of the box" (wich is not the case for Cray Fortran), the compilation time for each file is significantly higher. However, setting up a 2 stages dynamic compilation process allows for high parallelism that is not possible with Cray Fortran 18 and the library build time drops from nearly 2 hours (Cray Fortran 18) to 17 minutes (Amd Fortran compiler).
Large kernels have been ported from OpenACC to OpenMP, raising some difficulties when offloading intrinsics functions or using strutures attributes in kernels loops. These limitations were also known in the previous OpenACC work. The goal was mainly to check the correctness of the results. The offloading of the complex data structure of Yales2 code was then investigated. Here again some limitations of the "young" compiler were discovered and workarounds were implemented. Several reproducers were built during this ECFD8 and provided to developpers by the 2 on-site AMD engineers.
Preliminary tests on micro-applications show good performances of the generated binaries proving that this compiler could be a serious alternative on AMD GPUs and the goal is now to focus on this SDK in an OpenMP strategy while checking the portablility of this new implementation in Nvidia, Cray/HPE (and Gnu ?) environments.

=== Mesh adaptation - A. Grenouilloux, ONERA & G. Balarac, LEGI ===

==== M2 - Enhancement of mesh adaptation algorithms, B. Maugars (ONERA), B. Andrieu (ONERA), C. Benazet (ONERA), N. Dellinger (ONERA), G. Janodet (ONERA), G. Staffelbach (ONERA) ====
Mesh adaptation has become a crucial tool in order to automate industrial numerical simulations. ECDF7 allowed us to investigate the refine and EGADS libraries as tools for parallel mesh generation and adaptation using CAD as a geometric support. Since then, we fortified the workflow but some of our targeted industrial applications such as turbomachinery involve periodic boundary conditions. To manage these cases, the mesh generation and adaptation procedures must maintain matching periodic boundaries.
During ECFD8, we addressed multiple topics : periodic mesh generation from CAD model in EGADS, parallel and periodic metric gradation in ParaDiGM, making our parallel remeshing algorithm more generic to support non-manifold and periodic meshes. All these topics were covered, but there is still some work to be done to fully manage periodic mesh adaptation.
Non-manifold mesh adaptation was applied to changing mesh topologies. Here, an internal surface was described by a level-set and deformed up to hole creation to mimic ablation.

==== M3 - Development and definition of a new Automatic Mesh Convergence (AMC) driver for automating static mesh convergence in YALES2 (C. Papagiannis (LEGI/INRIA), G. Balarac (LEGI), O. LeMaître (INRIA), P.M. Congedo (INRIA)) ====
Static mesh adaptation requires re-adapting an existing (usually coarse) mesh, to conform to some quality metric that satisfies some optimality criterion. This involves statistics (Mean and RMS fields) that need to be adequately converged. We proposed and developed a novel AMC strategy that can converge an initial coarse mesh to an adequately refined one by taking into account the quality of the calculated statistics before each adaptation. Specifically, so far, the moment (in terms of integrating the LES equations) at which to launch the adaptation was unknown and had to be set apriori as a parameter of the AMC, dependent on the flow physics and therefore on the case at hand. Our method is automatically performing adaptations when the quality of the volume averaged mean field estimate (characterized by its RMSE) reaches the levels of the bias error that this field has, due to the mesh being far from the desired one (dictated by the quality criterion). This was achieved through drawing inspiration from a theoretical model that treats the AMC as a "conditionally" contractive mapping with a fixed point.

=== Numerics - M. Bernard, LEGI & G. Lartigue, CORIA ===

==== N1 - Traction open boundary condition ====

==== N2 - Treatment of Inlet conditions in High-Order solver. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====
In the context of node-centered Finite Volumes Method, spacial accuracy of a numerical scheme depends on ability to evaluate accurately fluxes through interface of each control volume (CV). Such accurate evaluation is not straightforward, especially when dealing with distorted grids. This project follows the work of [1] where fluxes use pointwise quantities, which are reconstructed from integrated quantities advanced in time. During the previous edition of the ECFD, a new data structure has been developed to store data at location of the boundary conditions facelets, with application to wall boundary conditions. During this 8th edition of the ECFD, we used the same data structure, but dedicated to the treatment of inlet conditions.
The inlet condition is then either imposed directly at facelets center, or at nodes position them extrapolated to facelets center by use of Taylor expansion. For this later solution, high-order treatment requires the successive derivatives to be computed in the plane of the boundary condition. This is not done yet, leading for the moment to low accuracy results but the framework is ready for upcoming implementation.

[1] ''A framework to perform high-order deconvolution for finite-volume method on simplicial meshes, , Bernard et. al., IJNMF 2020''

==== N3 - Conservative mesh-to-mesh interpolation. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====

Mesh to mesh interpolations occur quite often in CFD simulations : in the context of adaptative mesh convergence or in the case of dynamic mesh adaptation for for example.
Quality of the solution on the destination grid will depend on the characteristics of the interpolation method.
In this project, we did not focus on accuracy of the interpolation method but rather on conservativity characteristics.
A conservative interpolation ensures that the integral of the data on the source grid is exactly retrieved on the destination grid.
This property is highly interesting when dealing with scalar quantities or phase indicators, whose values should remained bounded.
In the context of nodes centered Finite Volume schemes, the methodology we used consists in (i) reconstructing element quantity from average nodal quantities on source grid.
Then, for a cell of the destination mesh, (ii) computing the geometrical intersection between cells of source and destination meshes to evaluate to evaluate the rate of quantities they.
Eventually, (iii) redistributing the solution from elements to control volumes of the destination mesh.
The overall process is fully conservative as it is based on geometrical intersection of locally integrated quantities.
The methodology as been implemented and tested on a few basic configurations and the conservativity is retrieved.

==== N4 - Determination of timestep in semi-implicit solver. T. Berthelon (LEGI), G. Balarac (LEGI), H. Lam (LEGI), M. El Moatamid (CORIA) ====
In order to reduce the computation time associated with incompressible LES simulations, an implicit time integration, based on BDF schemes, has been developed within the ICS solver. This integration eliminates the stability constraints associated with explicit schemes, and therefore opens up the question of the appropriate choice of time step.
In parallel, recent work has been carried out on meshing criteria in LES. The strategy in place consists of adapting the mesh by distinguishing two zones:
- "DNS" zones, where the meshing criterion is based on an estimate of the adimensioned spatial error.
- "LES" zones, where the meshing criterion is based on Kolmorogov theory.
During this project, the spatial criteria were extended to include temporal criteria. In the "DNS" zones, the time step is chosen using an estimate of the temporal error of the BDF scheme judiciously scaled to match the spatial error. In the "LES" zones, the time step is chosen using a scaling law associated with fully developed turbulence.
The new time step selection strategy has been tested on the case of a turbulent jet and leads to an accuracy equivalent to the explicit case while reducing the simulation return time by a factor of nearly 3.

Another aspect of this project was to integrate certain implicit temporal schemes (C-N and SDIRK) recently developed by Mr. El Moatamid into the incompressible solver.

==== N5 - Local timestep. T. Berthelon (LEGI), M. Bernard (LEGI), G. Balarac (LEGI) ====
RANS modelling has recently been developed within the YALES2 library. With this modeling strategy, the objective is to reach as quick as possible a steady state.
During this project, we investigate the use of a local time step to reduce the time to solution of steady computation in the incompressible solver.
This implies solving a variable-coefficient Poisson equation. Encouraging results were obtained in the simple case of "Couette plan" flow artificially constrained by a mesh variation. In fact, the use of local time-step reduce drastically the time to solution on this configuration. This method needs to be tested on real RANS case.

==== N6 - Distributed version of DOROTHY ====

==== N7 - Implicit time advancement for low-Reynolds number flows with particles. S. Mendez, C. Raveleau (IMAG), M. El Moatamid, V. Moureau (CORIA) ====
IMAG runs numerous simulations of red blood cells under flow. Those simulations are at low Reynolds number (0.001 to 1.0, typically). Splitting of the time advancement is used to treat the diffusion terms implicitly, albeit with an important numerical cost: implicit diffusion is 50 to 60% of the computational cost. Recently, M. El Moatamid implemented a genral framework to deal with implicit time advancement for scalars. In this project, the general method has been transposed to the advancement of the velocity field in the ICS and RBC solvers of YALES2/YALES2BIO. This enables testing various linear solvers (GMRES based). However, such solvers do not decrease the CPU time compared to the existing implementation. However, while working on this, it was identified that residual recycling was not activated in the current implementation of the implicit diffusion. This sped up the implicit diffusion cost by 35%, for a total gain of 20% for the computation. In addition to this achievement, moving to the framework coded by Moncef will have other beneficial side effects: we anticipate simplifying the implementation, with an easier merging between YALES2BIO and YALES2. The method will also be implemented in the electrosatic solver, for which the Poisson problem should benefit from the new GMRES-based solvers. In addition, this project highlights the importance of improving the treatment of stiff source terms in the red blood cells simulations, to be able to overcome the current limitation in time step due to those term and have a chance to benefit from higher-order time schemes, efficient at high Fourier numbers.

==== N8 - Boundary Element Method in YALES2. B. Thibaud, S. Mendez (IMAG), G. Lartigue, P. Benard (CORIA), F. Nicoud (IMAG) ====
In the context of microfluidic systems for diagnosis, the Boundary Element Method alows to solve linear PDE such as electrostatic or Stokes. With well chosen kernel functions and the divergence theorem, this method allows to write on the boundary condition only the initial volumic problem. This project aimed at exploring the feasibility of the BEM in the context of massively parallel unstructured solver like YALES2 by developping a Julia demonstrator. The first step have been to implement and validate the method on simple configurations for the Laplace's equation. Only Neumann problems were considered (Dirichlet boundary conditions imposed). In a second time, the multi-domain approach has been identified to be the most suited in the framework of YALES. The inner domain is partitioned on each processor, each having a part of the physical boundary and interfaces between them. Every processor solve its own boundary problem and a parallel Dirichlet-Dirichlet fixed-point is used to converge the interface problem on the all domain. Applied to the ring case, with one interface, we managed to reproduce the linear convergence of the P-DD method.

=== Turbulence - L. Voivenel, CORIA & P. Bénard, CORIA ===

==== T1 - FSI-1D strategy for internal flows - Pierre BENEZ (SAFRAN Aerosystem), Renaud MERCIER (SafranTech), Yacine BECHANE (CORIA) ====

Many applications developed at Safran Aerosystem are based on internal turbulent flows coupled to a moving body. 2 cases were studied during this ECFD:

'''Case 1 (Incompressible flow)''': Translation of a piston subjected to a pressure difference in a pipe.

The challenges of this case are twofold: the small gap between the piston and the pipe and the large pressure gradient across the piston (>100bar). During the 1st week of ECFD, the CLIB (Conservative Lagrangian Immersed Boundary) solver was tested on this case. The study showed that the solver was unable to ensure the impermeability of the solid under these pressure conditions. In the rest of the study, a porous medium following Darcy's law will be added to the penalty force of the immersed solid to fully satisfy the impermeability of the piston.

'''Case 2 (Compressible flow)''': Rotation of a butterfly in a discharge vane.

The coupling between the ECS (Explicit Compressible Solver) and ALE (Arbitrary Lagrangian Solver) solvers having recently been implemented, this strategy was tested to model the opening of the valve by rotation of the butterfly. The challenge here lies in the small gap between the bottom of the butterfly and the vane casing. To limit the simulation cost, the gap is meshed with 1 element. In this case, MMG succeeded in adapting the mesh up to a critical angle at which the gap becomes too small (Work In Progress).

==== T2 - Dynamic Smagorinsky in Dorothy - Maëlenn ROPERCH (LOMC), Grégory PINON (LOMC) ====

==== T3 - Turbulence injection strategy for compressible flows - Patrick TENE HEDJE (UMONS), Laurent Bricteur (UMONS), Yacine BECHANE (CORIA), Pierre BENARD (CORIA) ====

==== T4 - Improve wind farm modeling and simulation workflow - Pierre BENARD (CORIA), Ulysse VIGNY (UMONS), Etienne MULLER (CORIA & SGRE), Hakim HAMDANI (GDTech), Félix HOUTIN-MONGROLLE (SGRE), Anand PARINAM (CORIA & TUDelft), Hari MULAKALOORI (CORIA), Léa VOIVENEL (CORIA) ====
The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine. Still, applying this method to investigate a wind farm flow can be challenging, both in terms of computational cost and simulation setup. For instance, an inadequate management of the wind turbine individual modeling parts in a HPC context can end up being the main bottleneck of the simulation. From another perspective, a wind farm is usually composed of more than 50 wind turbines. For such a case, setting up all YALES2 required inputs manually can be very tedious and error-prone. This project thus mainly aimed to optimize the YALES2 ALM implementation and the user experience around it. Additionally, a cost-effective alternative to the ALM when modeling wind farm flows, namely the Rotating Actuator Disk Method (ADM-R), has been implemented for further investigations.

'''WP1''': Improve Actuator set rotor modelling
* Parallel processing of the ''actuator sets'' used to model the wind turbines
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified during the 5th iteration of the ECFD: (i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges. (ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications. During this workshop edition, the work focused on the re-synchronisation of the algorithm steps by allowing some packing an unpacking of the object to allow the transfert of the object inbetween workers. This is of major importance to enable load balancing and mesh adaptation during the temporal loop. This work required the refactoring of the involved oject structures.

* Rotating Actuator Disk Method (ADM-R):
According to the usual guidelines, the mesh requirements of the ALM, to profit entirely from its reachable accuracy, can be difficult to achieve or even unaffordable when simulating a wind farm flow, especially from the industrial point of view. Alternatives are available in the literature for this kind of application. Likely, the methods from the Actuator Disk family are the most prominent ones. Several kinds of implementation exist, which mostly differ by their capability to include the wake rotation. During the workshop, a new method from the Rotating Actuator Disk kind has been implemented and underwent an early validation on a single turbine setup. Applications to wind farm flows will follow.

'''WP2''': Improve tools User Experience

Three Python tools have been developed or improved :
*The first tool is the wind farm previsualisation tool, 'y2_wind_previsualization', which is used before the calculation run. This provides an interactive HTML interface for viewing global data for each turbine on the farm (position, hub height, yaw angle, etc.). The tool traces all of these via the parsing of the input file.
* The second tool is for duplicating rotor templates for a wind farm (`y2_wind_duplication`). This tool was developed in the previous ECFD, but this time it has been refactored and incorporated into the y2tools package.
* The third and final tool is a post-processing tool for the temporal processing of global wind turbine simulation metrics (Thrust, Power, etc.), `y2_post_wind`. This tool generates an interactive HTML plot of time-dependent global quantities.

==== T5 - Improve atmospheric inflow turbulence - Ulysse VIGNY (UMONS), Pierre BENARD (CORIA), Etienne MULLER (CORIA & SGRE), Hakim HAMDANI (GDTech), Félix HOUTIN-MONGROLLE (SGRE), Anand PARINAM (CORIA & TUDelft), Hari MULAKALOORI (CORIA), Léa VOIVENEL (CORIA) ====
Atmospheric inflow turbulence is generated using the precursor database method. A half-channel flow driven by a pressure gradient is used to obtain the inflow which is used as inlet boundary condition for the wind turbine simulation domain. This project aimed to improve the whole methodology, from generation to injection.

* WP1: Improve inflow generation
Anand: pressure controller

* WP2: Improve injection methodology (method A)
The previous workflow used plane probes in the ASCII format to sample the flow. The COWIT2 toolbox was used to convert the file into turbulence box (.man format). While functioning, this methodology had two major flaws. First the probe files are heavy ~O(10Go). Second, the method requires a lot of human effort, allowing numerous sources of errors.
During this workshop, a new methodology has been developed. First, the probes are generated using the HDF5 format (now available for all probe types), leading to lighter file ~O(1Go). Second, Y2_tools is used to read HDF5 format (working for probes and temporals). HDF5 file is then converted into a Look-up Table. Finally, the Look-up Table is read directly by YALES2 as a boundary conditions.

* WP3: Improve injection methodology (method B)
Even though improvements achieved in WP2 prove to be very handy while removing many potential human errors, injecting a turbulent inflow through wind boxes ('offline' precursor approach) can sometimes remain cumbersome for several reasons: (1) no periodicity is enforced in the streamwise direction of those boxes, (2) potential high memory consumption, and (3) the boxes need to be moved to other cores whenever a mesh adaptation occurs. An alternative consists in co-simulating the precursor flow and the flow of interest (refered as the 'successor' simulation) at the same time ('online' precursor approach). The inlet boundary condition for the successor flow is then obtained by mapping the outflow of the precursor domain. During the workshop, some work has been initiated to implement this kind of coupling using the CWIPI library, for which YALES2 provides already an interface.

==== T6 - FSI model in Dorothy - Enzo MASCRIER (LOMC), Grégory PINON (LOMC) ====

=== Two Phase Flow - J. Leparoux, SAFRAN & J. Carmona, CORIA ===

==== TP1 - Towards very small contact angles in Nucleate boiling ====

Participants: Henri Lam (LEGI), Mohammad Umair (LEGI), Manuel Bernard (LEGI), Robin Barbera (LEGI) and Giovanni Ghigliotti (LPSC)

The boiling solver (BOI) was not able to accurately impose a contact angle (angle formed by the two-phase interface on the wall) at values lower than 30°. This angle is needed when simulating nucleate boiling. A similar limitation in contact angle value was applying to the spray (SPS) solver. A modified version of the level set reinitialization has been implemented during ECFD8, based on a different normal vector in the blind spot region around the contact line, vector now chosen to be a zero-order extension from outside the blind spot. This modification, that implied other modifications to the level set reinitialization in the blind spot, has been tested successfully on the spray solver (where no phase change occurs). Then, this new reinitialisation has been tested in the boiling solver for nucleate boiling, with great improvements. Now simulations of nucleate boiling at very small contact angle (10°) can be accurately performed.
In the meanwhile, the level set reinitialization algorithm has been streamlined and the computational cost greatly reduced, resolving a computational cost issue that appeared when using the contact angle imposition both in the spray and boiling solvers, and that hampered its use in industrial configurations.

==== TP2 - Modeling spray-film interactions ====

Participants: Nicolas Gasnier (EM2C-SafranTech), Julien Leparoux (SafranTech), Mehdi Helal (CORIA-SafranTech) and Julien Carmona (CORIA)

The numerical simulation of sheared liquid films over walls with conventional high-fidelity methods requires fine meshes to depict accurately the dynamics of the fluids, as well as the gas-liquid interactions at the phase interface. In the context of thin film flows (with a thickness h ~ 1.0E-4 m), the spatial resolution required to ensure the validity of the simulations can be computationally prohibitive. To tackle this issue, a reduced-order model of thin film dynamics based on the Saint Venant equations has been implemented in the YALES2 platform over the last months. This numerical model is able to reproduce accurately the dynamics of strongly sheared film flows over partially wetted surfaces, and to take into account capillary effects. The objective of the ECFD8 was to couple the thin film framework with the pre-existing multiphase methods of the YALES2 platform: the Eulerian multiphase solver - based on the ACLS -, and the Lagrangian solver. This coupling was performed in three steps: first, a numerical method was designed to convert impinging Lagrangian droplets into source terms for the thin film model. Then, a phenomenological model was implemented to depict the atomization of films at sharp edges under the action of a high speed gas flow. The liquid film is converted into Lagragian droplets, whose dimensions are computed based on the PAMELA model. Finally, a numerical method has been designed to convert Eulerian droplets into source terms for the film model. This method is based on a transfer of the liquid properties from the nodes to the boundaries by using a set of fictitious particles. It ensures conservation of the liquid mass, and it is more robust than the first attempts that had been made during ECFD7.

==== TP3 - High-fidelity two-phase flow simulations of the purge of a fuel feed line ====

Participants: Thomas LAROCHE (Safran HE), Romain JANODET (Safran AE), Julien Leparoux (Safran Tech) and Melody Cailler (Safran Tech)

During the second week of the ECFD8, the fuel feed line purge process has been numerically investigated. In the context of aeronautical engines, the fuel feed line—carrying the fuel from the fuel tank to the injectors within the combustion chamber—needs to be purged at engine shutdown. This is intended to prevent fuel stagnation near hot metal parts, which could lead to coke formation and therefore decrease engine performance. Since this complex phenomenon is mainly driven by two-phase flow physics, the spray solver (SPS) of the YALES2 library has been considered in order to understand the physics of such process. The numerical setup was first converged on a simplified test case: the possibility of driving the flow dynamics with inlet and outlet pressure conditions was tested beforehand on a single-phase, incompressible case, and then on a two-phase flow problem. The setup has then been successfully applied to an industrial configuration: a pressure-swirl injector connected to a reduced portion of the fuel feed line. Due to the large scale of the domain, the interface resolution was set to 50μm, which is intentionally coarse for such problem. This initial computation successfully ran up to 3ms of physical time during the workshop, proving YALES2's capability to model the fuel purge. The computation is to be continued and analyzed further even after the workshop.

==== TP4 - Volume of Fluid solver in YALES2 ====

Participants: Léa Voivenel (CORIA), Julien Carmona (CORIA), Mehdi Helal (CORIA), Pierre Portais (CORIA), Julien Leparoux (Safran Tech), Mélody Cailler (Safran Tech) and Nicolas Gasnier (EM2C / Safran Tech)

==== TP5 - Implement a local operator to distribute the solid volume of a particle over multiple cells ====

Participants: Théo Ndereyimana (Université de Sherbrooke), Stéphane Moreau (Université de Sherbrooke)

In the CFD-DEM, the cell size is required to be larger than the particle size for stability condition and keep feasible solid volume fraction. However, some applications require a cell size smaller than the particle. During this 8th edition of the ECFD, the use of operators to distribute the particle volume over multiple cells, ensuring a feasible solid volume particle has been tested on a fluidized bed configuration. The main operators tested (gather-scatter filter and gaussian filter) showed a tendency to blur the void structure interfaces. The equivalence of the gaussian filter of bandwidth <math>b</math> and the resolution of a diffusion equation over a pseudo-time <math>T=b^2/4</math> has been verified.
One anisotropic diffusion constant has been tested and shows a possibility to adress the sharpness requirements.

Another objective was to develop a post-processing tool to detect and track the void structures (bubbles) in a fluidized bed. Based on previous work from J. Carmona, a tool to track the bubbles has been initiated.

==== TP6 - Complex thermodynamics in sloshing tanks ====

Participants: C. Merlin (AGS), D. Fouquet (CORIA), V. Moureau (CORIA), J. Carmona (CORIA) and G. Lartigue (CORIA)

During this ECFD, the phase change solver was improved. After introduction of conservative transport for enthalpy and energy, a new framework for multi fluid two phase flow was used that relies on a new reinitialization of the conservative level set, the transport of discontinuous scalars like temperature, energy or enthalpy and the transport of weight to ensure consistency. The pressure loop was improved to ensure mass and energy conservation while meeting the low Mach hypothesis. Many 1D test cases were performed with only the gas phase and two phase flow to validate the pressure loop. The new framework for the transport of discontinuous scalar was first derived for temperature with constant properties in each phase. A first step was to extend it to sensible enthalpy with ideal gas behavior for the gas phase. Then, the framework was extended to NIST tabulated gas & liquid with ideal or real gas behavior. After some 2D cases, a tank pressurization test was investigated as well as a sloshing test without phase change (just the pressure drop due to the increase of the thermal exchange between the liquid and the gas phase).

=== Combustion - Y. Bechane, CORIA & S. Dillon, SAFRAN & K. Bioche, CORIA ===

==== C1 - LES of the thermal degradation of a composite material ====
Participants: A. Grenouilloux (ONERA), K. Bioche (CORIA), N. Dellinger (ONERA) and R. Letournel (SafranTech)

The FIRE test bed is an experimental air-propane burner operated by ONERA. It is dedicated to the study of the thermal degradation of composite materials. This project concerned the implementation of a three-solver coupling methodology to simulate the dynamics of the impinging flame. The methodology considered is based on the coupling between the variable density solver (VDS) and the radiative solver (RDS) of the massively parallel library YALES2 and the solver dedicated to the degradation of composite materials, MoDeTheC, developed by ONERA. Given the typical test times of the order of tens of seconds, a methodology based on 2D axisymmetric calculations was considered. Various tests were performed to determine the optimal coupling frequency between solvers. Cases dedicated to the injection of pyrolysis gasses were set up, with the aim of simulating the auto-ignition phenomenon. Comparisons with experimental data are presented.

==== C2 - Flame stabilization by NRP plasma discharge ====

==== C3 - Extending and validating a generalized formalism of virtual chemistry ====

==== C4 - Turbulent combustion model for NOx prediction ====

==== C5 - Towards 3D simulation of detonation combustion ====

==== C6 - Flame stabilitity of flame-holders within reheat conditions ====

Participants : N. Detomaso (Safran AE), R. Janodet (Safran AE), L. Carbajal (Safran AE)

The mechanisms of flame stabilisation in a flame-holder are still not completely understood. This is particularly true for reheat conditions : the hot viciated gases at inlet, highly compressible flows, and a strong liquid/gas coupling make the flame stability hard to predict. During the ECFD8, simplified configurations of flame-holders within reheat conditions were analysed. After some try and error simulations due to simplifications of the flame-holders, cases leading to flame stability (or not) were identified. Post-processing tools were developped in order to recover criteria relevant to the flame stability. This step marks the beginning of a systematic mapping of dimensionless characteristic times ratios, and a comparison with integral quantities.

==== C7 - Thermal radiation in oxyflames ====

==== C8 - A first step toward hybrid CPU / GPU for reactive flow in YALES2 ====

Participants: M. Laignel (CORIA), G. Lartigue (CORIA), K. Bioche (CORIA) and V. Moureau (CORIA)

In numerical simulations of reacting flows, one of the most computationally intensive tasks is the evaluation of source terms resulting from chemical reactions in the species transport equations. This step can account for up to 90% of the total simulation cost , depending on the complexity of the kinetic mechanism involved. To reduce this cost, various techniques such as mechanism reduction, virtual chemistry, etc. have been explored. However, the emergence of GPUs as powerful accelerators offers a promising alternative by providing massive parallelism. Despite their potential, GPUs often require significant adaptation of CPU-based codes. This project aims to address this challenge by taking a first step towards a hybrid CPU/GPU framework for reactive flow simulations. Specifically, the focus is on coupling Y2 with the updated version of the stiff time integration solver (CVODE), which is compatible with GPU (CUDA, HIP, OpenMP). The ultimate goal is to establish a foundation for hybrid computations by implementing and testing the updated solver on simplified test cases.

==== C9 - Soots numerical modeling ====

==== C10 - TECERACT : Tabulated chemistry generator for aeronautical combustion ====

==== C11 - Exploring efficient tabulation strategies for detailed chemistry ====

==== C12 - Dynamic sub-grid-scale modelling of multi-regime flame wrinkling ====

==== C13 - LES of a semi-industrial burner using a non-adiabatic virtual chemical scheme ====

=== User Experience & Data - L. Korzeczek, GDTECH ===

==== U1 - Low-fidelity (RANS) rotor/stator simulations, application to Kaplan Turbine - Y. Lakrifi, G. Balarac (LEGI), R. Mercier (SAFRAN), V. Moureau (CORIA) ====

==== U2 - Coupling PyTorch/YALES2, combustion cartesian look-up tables - J. Leparoux, N. Treleaven, S. Dillon (SAFRAN), K. Bioche, G. Lartigue (CORIA) ====

Participants: Julien Leparoux (Safran Tech), Kévin Bioche (CORIA), Ghislain Lartigue (CORIA), Nicholas Treleaven (Safran Tech)

Neural Networks offer a promising alternative to Cartesian look-up tables for combustion simulations, reducing memory footprint. In this project, we investigated how to integrate an NN model for real-time inference in the YALES2 platform, exploring two approaches: a Python interface and a Fortran Torch binding (using FTorch[https://github.com/Cambridge-ICCS/FTorch]). We validated that the model remains accurate when embedded online and identified improvements for robustness. Inference costs were evaluated on a Mac M3 and the Austral cluster, revealing a strong dependency on data volume. To optimize efficiency, we propose grouping cells at the processor level.

==== U3 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 - L. Korzeczek, S. Meynet (GDTECH), J. Leparoux, M. Cailler (SAFRAN) ====

Ecfd:ecfd 8th edition

2025-02-12T14:06:43Z

Benard: /* T5 - Improve atmospheric inflow turbulence */

{{DISPLAYTITLE: ECFD workshop, 8th edition, 2025}}

== Description ==

* Event from '''27th of January to 7th of February 2025'''
* Location: [https://www.sport-normandie.fr/le-centre/le-site-de-houlgate Centre Sportif de Normandie], Houlgate, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* Participants from academics, HPC center/experts and industry are welcome
* The number of participants is limited to 68.

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes
* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Voivenel (CORIA).

[[File:ecfd8.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_8th_edition]]

[[File:Acknowledgments_ecfd8.png|text-bottom|600px]]

== News ==

* 23/10/2024: First announcement of the '''8th Extreme CFD Workshop & Hackathon''' !
* 22/11/2024: Deadline to submit your project

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

To come...

== Projects ==

=== Hackathon GENCI - P. Begou, LEGI ===
This ECFD8 GENCI Hackathon was a rich event, involving 4 differents CFD codes (AVBP, ParaDIGM, SONICS and YALES2) using various paradigms (C++/cuda/hip, Fortran/OpenMP/OpenACC) with several SDKs (AMD, Cray/HPE, Nvidia, Gnu) on a large range of GPU architectures (Nvidia A100, GH100, AMD instinct Mi210, Mi250, Mi300). This two-week event benefited from a high level support from three HPC mentors, two on-site from AMD (J. Noudohouenou and A. Tsetoglou) and one remote from CINES (M. Boudaoud).

==== H1 - ParaDIGM and SONICS on GPU, B. Maugars, G. Staffelbach, R.Cazalbou and B. Michel (ONERA)====

==== H2 - AVBP GPU offloading based on OpenMP, M.Ghenai, L. Legaux and A. Dauptain (CERFACS) ====

This hackathon provided a valuable opportunity to work on GPU offloading for AVBP. In the past, significant efforts were made to offload the entire AVBP code to GPUs. OpenACC was the primary strategy chosen, mainly due to access to NVIDIA's support, along with the availability of both software and hardware. This approach achieved good scalability performance.
Recently, with the deployment of new supercomputers like ADASTRA at CINES, some issues have emerged when running AVBP on AMD GPUs, including both MI250 and MI300. The closed-source nature of the Cray environment has also prevented CERFACS from deploying AVBP on local MI210 GPUs.
This hackathon was a great opportunity to address these challenges by exploring a new approach using OpenMP. An automatic translation tool was used to convert approximately 2,700 OpenACC directives to OpenMP, with each directive manually verified and fine-tuned afterward. AVBP with OpenMP had already been tested on NVIDIA GPUs, and during this hackathon, the focus was on extending support to AMD GPUs.
Two compilers were used: Cray and the newly released AFAR from AMD. With the support of AMD and CINES, a working environment for compiling AVBP was set up, and performance-related issues were identified. Additionally, two mini-apps were used for benchmarking. One of them achieved a 2.5× speedup when compiled with AFAR compared to Cray.
The next steps involve adapting the code to address necessary modifications, such as fixing issues related to Fortran indirections, and continuing performance evaluations with mini-apps. Further comparisons will be conducted using both compilers against results obtained with NVIDIA’s NVHPC.

==== H3 - YALES2 GPU from OpenACC to OpenMP, P. Bégou (LEGI), V. Moureau, G. Lartigue (CORIA) and R. Dubois (IMAG) ====
This Hackathon focuses on running Yales2 code on AMD Instinct Mi250 and Mi300 GPUs of the Adastra supercomputer (CINES).
Previously, a first solver in the Yales2 CFD code was successfully ported on the GPU accelerators of the Jean-Zay supercomputer (IDRIS) using Nvidia SDK but difficulties remain on Adastra AMD GPUs, mainly related to the available development tools. High compilation time and the impossibility to use debug flags at compile time as soon as OpenACC is enabled are a real challenge when tracking errors. The current project is to evaluate a freshly deployed version (at the begining of the workshop) of the AMD Fortran compiler. This requires moving to OpenMP paradigm, starting from scratch since the OpenACC branch has largely diverged from the master one while tracking spurious remaining bugs.
If the AMD compiler is able to build the cpu version of Yales2 "out of the box" (wich is not the case for Cray Fortran), the compilation time for each file is significantly higher. However, setting up a 2 stages dynamic compilation process allows for high parallelism that is not possible with Cray Fortran 18 and the library build time drops from nearly 2 hours (Cray Fortran 18) to 17 minutes (Amd Fortran compiler).
Large kernels have been ported from OpenACC to OpenMP, raising some difficulties when offloading intrinsics functions or using strutures attributes in kernels loops. These limitations were also known in the previous OpenACC work. The goal was mainly to check the correctness of the results. The offloading of the complex data structure of Yales2 code was then investigated. Here again some limitations of the "young" compiler were discovered and workarounds were implemented. Several reproducers were built during this ECFD8 and provided to developpers by the 2 on-site AMD engineers.
Preliminary tests on micro-applications show good performances of the generated binaries proving that this compiler could be a serious alternative on AMD GPUs and the goal is now to focus on this SDK in an OpenMP strategy while checking the portablility of this new implementation in Nvidia, Cray/HPE (and Gnu ?) environments.

=== Mesh adaptation - A. Grenouilloux, ONERA & G. Balarac, LEGI ===

==== M2 - Enhancement of mesh adaptation algorithms, B. Maugars (ONERA), B. Andrieu (ONERA), C. Benazet (ONERA), N. Dellinger (ONERA), G. Janodet (ONERA), G. Staffelbach (ONERA) ====
Mesh adaptation has become a crucial tool in order to automate industrial numerical simulations. ECDF7 allowed us to investigate the refine and EGADS libraries as tools for parallel mesh generation and adaptation using CAD as a geometric support. Since then, we fortified the workflow but some of our targeted industrial applications such as turbomachinery involve periodic boundary conditions. To manage these cases, the mesh generation and adaptation procedures must maintain matching periodic boundaries.
During ECFD8, we addressed multiple topics : periodic mesh generation from CAD model in EGADS, parallel and periodic metric gradation in ParaDiGM, making our parallel remeshing algorithm more generic to support non-manifold and periodic meshes. All these topics were covered, but there is still some work to be done to fully manage periodic mesh adaptation.
Non-manifold mesh adaptation was applied to changing mesh topologies. Here, an internal surface was described by a level-set and deformed up to hole creation to mimic ablation.

==== M3 - Development and definition of a new Automatic Mesh Convergence (AMC) driver for automating static mesh convergence in YALES2 (C. Papagiannis (LEGI/INRIA), G. Balarac (LEGI), O. LeMaître (INRIA), P.M. Congedo (INRIA)) ====
Static mesh adaptation requires re-adapting an existing (usually coarse) mesh, to conform to some quality metric that satisfies some optimality criterion. This involves statistics (Mean and RMS fields) that need to be adequately converged. We proposed and developed a novel AMC strategy that can converge an initial coarse mesh to an adequately refined one by taking into account the quality of the calculated statistics before each adaptation. Specifically, so far, the moment (in terms of integrating the LES equations) at which to launch the adaptation was unknown and had to be set apriori as a parameter of the AMC, dependent on the flow physics and therefore on the case at hand. Our method is automatically performing adaptations when the quality of the volume averaged mean field estimate (characterized by its RMSE) reaches the levels of the bias error that this field has, due to the mesh being far from the desired one (dictated by the quality criterion). This was achieved through drawing inspiration from a theoretical model that treats the AMC as a "conditionally" contractive mapping with a fixed point.

=== Numerics - M. Bernard, LEGI & G. Lartigue, CORIA ===

==== N1 - Traction open boundary condition ====

==== N2 - Treatment of Inlet conditions in High-Order solver. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====
In the context of node-centered Finite Volumes Method, spacial accuracy of a numerical scheme depends on ability to evaluate accurately fluxes through interface of each control volume (CV). Such accurate evaluation is not straightforward, especially when dealing with distorted grids. This project follows the work of [1] where fluxes use pointwise quantities, which are reconstructed from integrated quantities advanced in time. During the previous edition of the ECFD, a new data structure has been developed to store data at location of the boundary conditions facelets, with application to wall boundary conditions. During this 8th edition of the ECFD, we used the same data structure, but dedicated to the treatment of inlet conditions.
The inlet condition is then either imposed directly at facelets center, or at nodes position them extrapolated to facelets center by use of Taylor expansion. For this later solution, high-order treatment requires the successive derivatives to be computed in the plane of the boundary condition. This is not done yet, leading for the moment to low accuracy results but the framework is ready for upcoming implementation.

[1] ''A framework to perform high-order deconvolution for finite-volume method on simplicial meshes, , Bernard et. al., IJNMF 2020''

==== N3 - Conservative mesh-to-mesh interpolation. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====

Mesh to mesh interpolations occur quite often in CFD simulations : in the context of adaptative mesh convergence or in the case of dynamic mesh adaptation for for example.
Quality of the solution on the destination grid will depend on the characteristics of the interpolation method.
In this project, we did not focus on accuracy of the interpolation method but rather on conservativity characteristics.
A conservative interpolation ensures that the integral of the data on the source grid is exactly retrieved on the destination grid.
This property is highly interesting when dealing with scalar quantities or phase indicators, whose values should remained bounded.
In the context of nodes centered Finite Volume schemes, the methodology we used consists in (i) reconstructing element quantity from average nodal quantities on source grid.
Then, for a cell of the destination mesh, (ii) computing the geometrical intersection between cells of source and destination meshes to evaluate to evaluate the rate of quantities they.
Eventually, (iii) redistributing the solution from elements to control volumes of the destination mesh.
The overall process is fully conservative as it is based on geometrical intersection of locally integrated quantities.
The methodology as been implemented and tested on a few basic configurations and the conservativity is retrieved.

==== N4 - Determination of timestep in semi-implicit solver. T. Berthelon (LEGI), G. Balarac (LEGI), H. Lam (LEGI), M. El Moatamid (CORIA) ====
In order to reduce the computation time associated with incompressible LES simulations, an implicit time integration, based on BDF schemes, has been developed within the ICS solver. This integration eliminates the stability constraints associated with explicit schemes, and therefore opens up the question of the appropriate choice of time step.
In parallel, recent work has been carried out on meshing criteria in LES. The strategy in place consists of adapting the mesh by distinguishing two zones:
- "DNS" zones, where the meshing criterion is based on an estimate of the adimensioned spatial error.
- "LES" zones, where the meshing criterion is based on Kolmorogov theory.
During this project, the spatial criteria were extended to include temporal criteria. In the "DNS" zones, the time step is chosen using an estimate of the temporal error of the BDF scheme judiciously scaled to match the spatial error. In the "LES" zones, the time step is chosen using a scaling law associated with fully developed turbulence.
The new time step selection strategy has been tested on the case of a turbulent jet and leads to an accuracy equivalent to the explicit case while reducing the simulation return time by a factor of nearly 3.

Another aspect of this project was to integrate certain implicit temporal schemes (C-N and SDIRK) recently developed by Mr. El Moatamid into the incompressible solver.

==== N5 - Local timestep. T. Berthelon (LEGI), M. Bernard (LEGI), G. Balarac (LEGI) ====
RANS modelling has recently been developed within the YALES2 library. With this modeling strategy, the objective is to reach as quick as possible a steady state.
During this project, we investigate the use of a local time step to reduce the time to solution of steady computation in the incompressible solver.
This implies solving a variable-coefficient Poisson equation. Encouraging results were obtained in the simple case of "Couette plan" flow artificially constrained by a mesh variation. In fact, the use of local time-step reduce drastically the time to solution on this configuration. This method needs to be tested on real RANS case.

==== N6 - Distributed version of DOROTHY ====

==== N7 - Implicit time advancement for low-Reynolds number flows with particles. S. Mendez, C. Raveleau (IMAG), M. El Moatamid, V. Moureau (CORIA) ====
IMAG runs numerous simulations of red blood cells under flow. Those simulations are at low Reynolds number (0.001 to 1.0, typically). Splitting of the time advancement is used to treat the diffusion terms implicitly, albeit with an important numerical cost: implicit diffusion is 50 to 60% of the computational cost. Recently, M. El Moatamid implemented a genral framework to deal with implicit time advancement for scalars. In this project, the general method has been transposed to the advancement of the velocity field in the ICS and RBC solvers of YALES2/YALES2BIO. This enables testing various linear solvers (GMRES based). However, such solvers do not decrease the CPU time compared to the existing implementation. However, while working on this, it was identified that residual recycling was not activated in the current implementation of the implicit diffusion. This sped up the implicit diffusion cost by 35%, for a total gain of 20% for the computation. In addition to this achievement, moving to the framework coded by Moncef will have other beneficial side effects: we anticipate simplifying the implementation, with an easier merging between YALES2BIO and YALES2. The method will also be implemented in the electrosatic solver, for which the Poisson problem should benefit from the new GMRES-based solvers. In addition, this project highlights the importance of improving the treatment of stiff source terms in the red blood cells simulations, to be able to overcome the current limitation in time step due to those term and have a chance to benefit from higher-order time schemes, efficient at high Fourier numbers.

==== N8 - Boundary Element Method in YALES2. B. Thibaud, S. Mendez (IMAG), G. Lartigue, P. Benard (CORIA), F. Nicoud (IMAG) ====
In the context of microfluidic systems for diagnosis, the Boundary Element Method alows to solve linear PDE such as electrostatic or Stokes. With well chosen kernel functions and the divergence theorem, this method allows to write on the boundary condition only the initial volumic problem. This project aimed at exploring the feasibility of the BEM in the context of massively parallel unstructured solver like YALES2 by developping a Julia demonstrator. The first step have been to implement and validate the method on simple configurations for the Laplace's equation. Only Neumann problems were considered (Dirichlet boundary conditions imposed). In a second time, the multi-domain approach has been identified to be the most suited in the framework of YALES. The inner domain is partitioned on each processor, each having a part of the physical boundary and interfaces between them. Every processor solve its own boundary problem and a parallel Dirichlet-Dirichlet fixed-point is used to converge the interface problem on the all domain. Applied to the ring case, with one interface, we managed to reproduce the linear convergence of the P-DD method.

=== Turbulence - L. Voivenel, CORIA & P. Bénard, CORIA ===

==== T1 - FSI-1D strategy for internal flows====

Many applications developed at Safran Aerosystem are based on internal turbulent flows coupled to a moving body. 2 cases were studied during this ECFD:

'''Case 1 (Incompressible flow)''': Translation of a piston subjected to a pressure difference in a pipe.

The challenges of this case are twofold: the small gap between the piston and the pipe and the large pressure gradient across the piston (>100bar). During the 1st week of ECFD, the CLIB (Conservative Lagrangian Immersed Boundary) solver was tested on this case. The study showed that the solver was unable to ensure the impermeability of the solid under these pressure conditions. In the rest of the study, a porous medium following Darcy's law will be added to the penalty force of the immersed solid to fully satisfy the impermeability of the piston.

'''Case 2 (Compressible flow)''': Rotation of a butterfly in a discharge vane.

The coupling between the ECS (Explicit Compressible Solver) and ALE (Arbitrary Lagrangian Solver) solvers having recently been implemented, this strategy was tested to model the opening of the valve by rotation of the butterfly. The challenge here lies in the small gap between the bottom of the butterfly and the vane casing. To limit the simulation cost, the gap is meshed with 1 element. In this case, MMG succeeded in adapting the mesh up to a critical angle at which the gap becomes too small (Work In Progress).

==== T2 - Dynamic Smagorinsky in Dorothy - Maëlenn ROPERCH (LOMC), Grégory PINON (LOMC) ====

==== T3 - Turbulence injection strategy for compressible flows - Patrick TENE HEDJE (UMONS), Laurent Bricteur (UMONS), Yacine BECHANE (CORIA), Pierre BENARD (CORIA) ====

==== T4 - Improve wind farm modeling and simulation workflow - Pierre BENARD (CORIA), Ulysse VIGNY (UMONS), Etienne MULLER (CORIA & SGRE), Hakim HAMDANI (GDTech), Félix HOUTIN-MONGROLLE (SGRE), Anand PARINAM (CORIA & TUDelft), Hari MULAKALOORI (CORIA), Léa VOIVENEL (CORIA) ====
The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine. Still, applying this method to investigate a wind farm flow can be challenging, both in terms of computational cost and simulation setup. For instance, an inadequate management of the wind turbine individual modeling parts in a HPC context can end up being the main bottleneck of the simulation. From another perspective, a wind farm is usually composed of more than 50 wind turbines. For such a case, setting up all YALES2 required inputs manually can be very tedious and error-prone. This project thus mainly aimed to optimize the YALES2 ALM implementation and the user experience around it. Additionally, a cost-effective alternative to the ALM when modeling wind farm flows, namely the Rotating Actuator Disk Method (ADM-R), has been implemented for further investigations.

'''WP1''': Improve Actuator set rotor modelling
* Parallel processing of the ''actuator sets'' used to model the wind turbines
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified during the 5th iteration of the ECFD: (i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges. (ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications. During this workshop edition, the work focused on the re-synchronisation of the algorithm steps by allowing some packing an unpacking of the object to allow the transfert of the object inbetween workers. This is of major importance to enable load balancing and mesh adaptation during the temporal loop. This work required the refactoring of the involved oject structures.

* Rotating Actuator Disk Method (ADM-R):
According to the usual guidelines, the mesh requirements of the ALM, to profit entirely from its reachable accuracy, can be difficult to achieve or even unaffordable when simulating a wind farm flow, especially from the industrial point of view. Alternatives are available in the literature for this kind of application. Likely, the methods from the Actuator Disk family are the most prominent ones. Several kinds of implementation exist, which mostly differ by their capability to include the wake rotation. During the workshop, a new method from the Rotating Actuator Disk kind has been implemented and underwent an early validation on a single turbine setup. Applications to wind farm flows will follow.

'''WP2''': Improve tools User Experience

Three Python tools have been developed or improved :
*The first tool is the wind farm previsualisation tool, 'y2_wind_previsualization', which is used before the calculation run. This provides an interactive HTML interface for viewing global data for each turbine on the farm (position, hub height, yaw angle, etc.). The tool traces all of these via the parsing of the input file.
* The second tool is for duplicating rotor templates for a wind farm (`y2_wind_duplication`). This tool was developed in the previous ECFD, but this time it has been refactored and incorporated into the y2tools package.
* The third and final tool is a post-processing tool for the temporal processing of global wind turbine simulation metrics (Thrust, Power, etc.), `y2_post_wind`. This tool generates an interactive HTML plot of time-dependent global quantities.

==== T5 - Improve atmospheric inflow turbulence - Ulysse VIGNY (UMONS), Pierre BENARD (CORIA), Etienne MULLER (CORIA & SGRE), Hakim HAMDANI (GDTech), Félix HOUTIN-MONGROLLE (SGRE), Anand PARINAM (CORIA & TUDelft), Hari MULAKALOORI (CORIA), Léa VOIVENEL (CORIA) ====
Atmospheric inflow turbulence is generated using the precursor database method. A half-channel flow driven by a pressure gradient is used to obtain the inflow which is used as inlet boundary condition for the wind turbine simulation domain. This project aimed to improve the whole methodology, from generation to injection.

* WP1: Improve inflow generation
Anand: pressure controller

* WP2: Improve injection methodology (method A)
The previous workflow used plane probes in the ASCII format to sample the flow. The COWIT2 toolbox was used to convert the file into turbulence box (.man format). While functioning, this methodology had two major flaws. First the probe files are heavy ~O(10Go). Second, the method requires a lot of human effort, allowing numerous sources of errors.
During this workshop, a new methodology has been developed. First, the probes are generated using the HDF5 format (now available for all probe types), leading to lighter file ~O(1Go). Second, Y2_tools is used to read HDF5 format (working for probes and temporals). HDF5 file is then converted into a Look-up Table. Finally, the Look-up Table is read directly by YALES2 as a boundary conditions.

* WP3: Improve injection methodology (method B)
Even though improvements achieved in WP2 prove to be very handy while removing many potential human errors, injecting a turbulent inflow through wind boxes ('offline' precursor approach) can sometimes remain cumbersome for several reasons: (1) no periodicity is enforced in the streamwise direction of those boxes, (2) potential high memory consumption, and (3) the boxes need to be moved to other cores whenever a mesh adaptation occurs. An alternative consists in co-simulating the precursor flow and the flow of interest (refered as the 'successor' simulation) at the same time ('online' precursor approach). The inlet boundary condition for the successor flow is then obtained by mapping the outflow of the precursor domain. During the workshop, some work has been initiated to implement this kind of coupling using the CWIPI library, for which YALES2 provides already an interface.

==== T6 - FSI model in Dorothy - Enzo MASCRIER (LOMC), Grégory PINON (LOMC) ====

=== Two Phase Flow - J. Leparoux, SAFRAN & J. Carmona, CORIA ===

==== TP1 - Towards very small contact angles in Nucleate boiling ====

Participants: Henri Lam (LEGI), Mohammad Umair (LEGI), Manuel Bernard (LEGI), Robin Barbera (LEGI) and Giovanni Ghigliotti (LPSC)

The boiling solver (BOI) was not able to accurately impose a contact angle (angle formed by the two-phase interface on the wall) at values lower than 30°. This angle is needed when simulating nucleate boiling. A similar limitation in contact angle value was applying to the spray (SPS) solver. A modified version of the level set reinitialization has been implemented during ECFD8, based on a different normal vector in the blind spot region around the contact line, vector now chosen to be a zero-order extension from outside the blind spot. This modification, that implied other modifications to the level set reinitialization in the blind spot, has been tested successfully on the spray solver (where no phase change occurs). Then, this new reinitialisation has been tested in the boiling solver for nucleate boiling, with great improvements. Now simulations of nucleate boiling at very small contact angle (10°) can be accurately performed.
In the meanwhile, the level set reinitialization algorithm has been streamlined and the computational cost greatly reduced, resolving a computational cost issue that appeared when using the contact angle imposition both in the spray and boiling solvers, and that hampered its use in industrial configurations.

==== TP2 - Modeling spray-film interactions ====

Participants: Nicolas Gasnier (EM2C-SafranTech), Julien Leparoux (SafranTech), Mehdi Helal (CORIA-SafranTech) and Julien Carmona (CORIA)

The numerical simulation of sheared liquid films over walls with conventional high-fidelity methods requires fine meshes to depict accurately the dynamics of the fluids, as well as the gas-liquid interactions at the phase interface. In the context of thin film flows (with a thickness h ~ 1.0E-4 m), the spatial resolution required to ensure the validity of the simulations can be computationally prohibitive. To tackle this issue, a reduced-order model of thin film dynamics based on the Saint Venant equations has been implemented in the YALES2 platform over the last months. This numerical model is able to reproduce accurately the dynamics of strongly sheared film flows over partially wetted surfaces, and to take into account capillary effects. The objective of the ECFD8 was to couple the thin film framework with the pre-existing multiphase methods of the YALES2 platform: the Eulerian multiphase solver - based on the ACLS -, and the Lagrangian solver. This coupling was performed in three steps: first, a numerical method was designed to convert impinging Lagrangian droplets into source terms for the thin film model. Then, a phenomenological model was implemented to depict the atomization of films at sharp edges under the action of a high speed gas flow. The liquid film is converted into Lagragian droplets, whose dimensions are computed based on the PAMELA model. Finally, a numerical method has been designed to convert Eulerian droplets into source terms for the film model. This method is based on a transfer of the liquid properties from the nodes to the boundaries by using a set of fictitious particles. It ensures conservation of the liquid mass, and it is more robust than the first attempts that had been made during ECFD7.

==== TP3 - High-fidelity two-phase flow simulations of the purge of a fuel feed line ====

Participants: Thomas LAROCHE (Safran HE), Romain JANODET (Safran AE), Julien Leparoux (Safran Tech) and Melody Cailler (Safran Tech)

During the second week of the ECFD8, the fuel feed line purge process has been numerically investigated. In the context of aeronautical engines, the fuel feed line—carrying the fuel from the fuel tank to the injectors within the combustion chamber—needs to be purged at engine shutdown. This is intended to prevent fuel stagnation near hot metal parts, which could lead to coke formation and therefore decrease engine performance. Since this complex phenomenon is mainly driven by two-phase flow physics, the spray solver (SPS) of the YALES2 library has been considered in order to understand the physics of such process. The numerical setup was first converged on a simplified test case: the possibility of driving the flow dynamics with inlet and outlet pressure conditions was tested beforehand on a single-phase, incompressible case, and then on a two-phase flow problem. The setup has then been successfully applied to an industrial configuration: a pressure-swirl injector connected to a reduced portion of the fuel feed line. Due to the large scale of the domain, the interface resolution was set to 50μm, which is intentionally coarse for such problem. This initial computation successfully ran up to 3ms of physical time during the workshop, proving YALES2's capability to model the fuel purge. The computation is to be continued and analyzed further even after the workshop.

==== TP4 - Volume of Fluid solver in YALES2 ====

Participants: Léa Voivenel (CORIA), Julien Carmona (CORIA), Mehdi Helal (CORIA), Pierre Portais (CORIA), Julien Leparoux (Safran Tech), Mélody Cailler (Safran Tech) and Nicolas Gasnier (EM2C / Safran Tech)

==== TP5 - Implement a local operator to distribute the solid volume of a particle over multiple cells ====

Participants: Théo Ndereyimana (Université de Sherbrooke), Stéphane Moreau (Université de Sherbrooke)

In the CFD-DEM, the cell size is required to be larger than the particle size for stability condition and keep feasible solid volume fraction. However, some applications require a cell size smaller than the particle. During this 8th edition of the ECFD, the use of operators to distribute the particle volume over multiple cells, ensuring a feasible solid volume particle has been tested on a fluidized bed configuration. The main operators tested (gather-scatter filter and gaussian filter) showed a tendency to blur the void structure interfaces. The equivalence of the gaussian filter of bandwidth <math>b</math> and the resolution of a diffusion equation over a pseudo-time <math>T=b^2/4</math> has been verified.
One anisotropic diffusion constant has been tested and shows a possibility to adress the sharpness requirements.

Another objective was to develop a post-processing tool to detect and track the void structures (bubbles) in a fluidized bed. Based on previous work from J. Carmona, a tool to track the bubbles has been initiated.

==== TP6 - Complex thermodynamics in sloshing tanks ====

Participants: C. Merlin (AGS), D. Fouquet (CORIA), V. Moureau (CORIA), J. Carmona (CORIA) and G. Lartigue (CORIA)

During this ECFD, the phase change solver was improved. After introduction of conservative transport for enthalpy and energy, a new framework for multi fluid two phase flow was used that relies on a new reinitialization of the conservative level set, the transport of discontinuous scalars like temperature, energy or enthalpy and the transport of weight to ensure consistency. The pressure loop was improved to ensure mass and energy conservation while meeting the low Mach hypothesis. Many 1D test cases were performed with only the gas phase and two phase flow to validate the pressure loop. The new framework for the transport of discontinuous scalar was first derived for temperature with constant properties in each phase. A first step was to extend it to sensible enthalpy with ideal gas behavior for the gas phase. Then, the framework was extended to NIST tabulated gas & liquid with ideal or real gas behavior. After some 2D cases, a tank pressurization test was investigated as well as a sloshing test without phase change (just the pressure drop due to the increase of the thermal exchange between the liquid and the gas phase).

=== Combustion - Y. Bechane, CORIA & S. Dillon, SAFRAN & K. Bioche, CORIA ===

==== C1 - LES of the thermal degradation of a composite material ====
Participants: A. Grenouilloux (ONERA), K. Bioche (CORIA), N. Dellinger (ONERA) and R. Letournel (SafranTech)

The FIRE test bed is an experimental air-propane burner operated by ONERA. It is dedicated to the study of the thermal degradation of composite materials. This project concerned the implementation of a three-solver coupling methodology to simulate the dynamics of the impinging flame. The methodology considered is based on the coupling between the variable density solver (VDS) and the radiative solver (RDS) of the massively parallel library YALES2 and the solver dedicated to the degradation of composite materials, MoDeTheC, developed by ONERA. Given the typical test times of the order of tens of seconds, a methodology based on 2D axisymmetric calculations was considered. Various tests were performed to determine the optimal coupling frequency between solvers. Cases dedicated to the injection of pyrolysis gasses were set up, with the aim of simulating the auto-ignition phenomenon. Comparisons with experimental data are presented.

==== C2 - Flame stabilization by NRP plasma discharge ====

==== C3 - Extending and validating a generalized formalism of virtual chemistry ====

==== C4 - Turbulent combustion model for NOx prediction ====

==== C5 - Towards 3D simulation of detonation combustion ====

==== C6 - Flame stabilitity of flame-holders within reheat conditions ====

Participants : N. Detomaso (Safran AE), R. Janodet (Safran AE), L. Carbajal (Safran AE)

The mechanisms of flame stabilisation in a flame-holder are still not completely understood. This is particularly true for reheat conditions : the hot viciated gases at inlet, highly compressible flows, and a strong liquid/gas coupling make the flame stability hard to predict. During the ECFD8, simplified configurations of flame-holders within reheat conditions were analysed. After some try and error simulations due to simplifications of the flame-holders, cases leading to flame stability (or not) were identified. Post-processing tools were developped in order to recover criteria relevant to the flame stability. This step marks the beginning of a systematic mapping of dimensionless characteristic times ratios, and a comparison with integral quantities.

==== C7 - Thermal radiation in oxyflames ====

==== C8 - A first step toward hybrid CPU / GPU for reactive flow in YALES2 ====

Participants: M. Laignel (CORIA), G. Lartigue (CORIA), K. Bioche (CORIA) and V. Moureau (CORIA)

In numerical simulations of reacting flows, one of the most computationally intensive tasks is the evaluation of source terms resulting from chemical reactions in the species transport equations. This step can account for up to 90% of the total simulation cost , depending on the complexity of the kinetic mechanism involved. To reduce this cost, various techniques such as mechanism reduction, virtual chemistry, etc. have been explored. However, the emergence of GPUs as powerful accelerators offers a promising alternative by providing massive parallelism. Despite their potential, GPUs often require significant adaptation of CPU-based codes. This project aims to address this challenge by taking a first step towards a hybrid CPU/GPU framework for reactive flow simulations. Specifically, the focus is on coupling Y2 with the updated version of the stiff time integration solver (CVODE), which is compatible with GPU (CUDA, HIP, OpenMP). The ultimate goal is to establish a foundation for hybrid computations by implementing and testing the updated solver on simplified test cases.

==== C9 - Soots numerical modeling ====

==== C10 - TECERACT : Tabulated chemistry generator for aeronautical combustion ====

==== C11 - Exploring efficient tabulation strategies for detailed chemistry ====

==== C12 - Dynamic sub-grid-scale modelling of multi-regime flame wrinkling ====

==== C13 - LES of a semi-industrial burner using a non-adiabatic virtual chemical scheme ====

=== User Experience & Data - L. Korzeczek, GDTECH ===

==== U1 - Low-fidelity (RANS) rotor/stator simulations, application to Kaplan Turbine - Y. Lakrifi, G. Balarac (LEGI), R. Mercier (SAFRAN), V. Moureau (CORIA) ====

==== U2 - Coupling PyTorch/YALES2, combustion cartesian look-up tables - J. Leparoux, N. Treleaven, S. Dillon (SAFRAN), K. Bioche, G. Lartigue (CORIA) ====

Participants: Julien Leparoux (Safran Tech), Kévin Bioche (CORIA), Ghislain Lartigue (CORIA), Nicholas Treleaven (Safran Tech)

Neural Networks offer a promising alternative to Cartesian look-up tables for combustion simulations, reducing memory footprint. In this project, we investigated how to integrate an NN model for real-time inference in the YALES2 platform, exploring two approaches: a Python interface and a Fortran Torch binding (using FTorch[https://github.com/Cambridge-ICCS/FTorch]). We validated that the model remains accurate when embedded online and identified improvements for robustness. Inference costs were evaluated on a Mac M3 and the Austral cluster, revealing a strong dependency on data volume. To optimize efficiency, we propose grouping cells at the processor level.

==== U3 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 - L. Korzeczek, S. Meynet (GDTECH), J. Leparoux, M. Cailler (SAFRAN) ====

Ecfd:ecfd 8th edition

2025-02-12T14:06:16Z

Benard: /* T4 - Improve wind farm modeling and simulation workflow */

{{DISPLAYTITLE: ECFD workshop, 8th edition, 2025}}

== Description ==

* Event from '''27th of January to 7th of February 2025'''
* Location: [https://www.sport-normandie.fr/le-centre/le-site-de-houlgate Centre Sportif de Normandie], Houlgate, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* Participants from academics, HPC center/experts and industry are welcome
* The number of participants is limited to 68.

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes
* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Voivenel (CORIA).

[[File:ecfd8.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_8th_edition]]

[[File:Acknowledgments_ecfd8.png|text-bottom|600px]]

== News ==

* 23/10/2024: First announcement of the '''8th Extreme CFD Workshop & Hackathon''' !
* 22/11/2024: Deadline to submit your project

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

To come...

== Projects ==

=== Hackathon GENCI - P. Begou, LEGI ===
This ECFD8 GENCI Hackathon was a rich event, involving 4 differents CFD codes (AVBP, ParaDIGM, SONICS and YALES2) using various paradigms (C++/cuda/hip, Fortran/OpenMP/OpenACC) with several SDKs (AMD, Cray/HPE, Nvidia, Gnu) on a large range of GPU architectures (Nvidia A100, GH100, AMD instinct Mi210, Mi250, Mi300). This two-week event benefited from a high level support from three HPC mentors, two on-site from AMD (J. Noudohouenou and A. Tsetoglou) and one remote from CINES (M. Boudaoud).

==== H1 - ParaDIGM and SONICS on GPU, B. Maugars, G. Staffelbach, R.Cazalbou and B. Michel (ONERA)====

==== H2 - AVBP GPU offloading based on OpenMP, M.Ghenai, L. Legaux and A. Dauptain (CERFACS) ====

This hackathon provided a valuable opportunity to work on GPU offloading for AVBP. In the past, significant efforts were made to offload the entire AVBP code to GPUs. OpenACC was the primary strategy chosen, mainly due to access to NVIDIA's support, along with the availability of both software and hardware. This approach achieved good scalability performance.
Recently, with the deployment of new supercomputers like ADASTRA at CINES, some issues have emerged when running AVBP on AMD GPUs, including both MI250 and MI300. The closed-source nature of the Cray environment has also prevented CERFACS from deploying AVBP on local MI210 GPUs.
This hackathon was a great opportunity to address these challenges by exploring a new approach using OpenMP. An automatic translation tool was used to convert approximately 2,700 OpenACC directives to OpenMP, with each directive manually verified and fine-tuned afterward. AVBP with OpenMP had already been tested on NVIDIA GPUs, and during this hackathon, the focus was on extending support to AMD GPUs.
Two compilers were used: Cray and the newly released AFAR from AMD. With the support of AMD and CINES, a working environment for compiling AVBP was set up, and performance-related issues were identified. Additionally, two mini-apps were used for benchmarking. One of them achieved a 2.5× speedup when compiled with AFAR compared to Cray.
The next steps involve adapting the code to address necessary modifications, such as fixing issues related to Fortran indirections, and continuing performance evaluations with mini-apps. Further comparisons will be conducted using both compilers against results obtained with NVIDIA’s NVHPC.

==== H3 - YALES2 GPU from OpenACC to OpenMP, P. Bégou (LEGI), V. Moureau, G. Lartigue (CORIA) and R. Dubois (IMAG) ====
This Hackathon focuses on running Yales2 code on AMD Instinct Mi250 and Mi300 GPUs of the Adastra supercomputer (CINES).
Previously, a first solver in the Yales2 CFD code was successfully ported on the GPU accelerators of the Jean-Zay supercomputer (IDRIS) using Nvidia SDK but difficulties remain on Adastra AMD GPUs, mainly related to the available development tools. High compilation time and the impossibility to use debug flags at compile time as soon as OpenACC is enabled are a real challenge when tracking errors. The current project is to evaluate a freshly deployed version (at the begining of the workshop) of the AMD Fortran compiler. This requires moving to OpenMP paradigm, starting from scratch since the OpenACC branch has largely diverged from the master one while tracking spurious remaining bugs.
If the AMD compiler is able to build the cpu version of Yales2 "out of the box" (wich is not the case for Cray Fortran), the compilation time for each file is significantly higher. However, setting up a 2 stages dynamic compilation process allows for high parallelism that is not possible with Cray Fortran 18 and the library build time drops from nearly 2 hours (Cray Fortran 18) to 17 minutes (Amd Fortran compiler).
Large kernels have been ported from OpenACC to OpenMP, raising some difficulties when offloading intrinsics functions or using strutures attributes in kernels loops. These limitations were also known in the previous OpenACC work. The goal was mainly to check the correctness of the results. The offloading of the complex data structure of Yales2 code was then investigated. Here again some limitations of the "young" compiler were discovered and workarounds were implemented. Several reproducers were built during this ECFD8 and provided to developpers by the 2 on-site AMD engineers.
Preliminary tests on micro-applications show good performances of the generated binaries proving that this compiler could be a serious alternative on AMD GPUs and the goal is now to focus on this SDK in an OpenMP strategy while checking the portablility of this new implementation in Nvidia, Cray/HPE (and Gnu ?) environments.

=== Mesh adaptation - A. Grenouilloux, ONERA & G. Balarac, LEGI ===

==== M2 - Enhancement of mesh adaptation algorithms, B. Maugars (ONERA), B. Andrieu (ONERA), C. Benazet (ONERA), N. Dellinger (ONERA), G. Janodet (ONERA), G. Staffelbach (ONERA) ====
Mesh adaptation has become a crucial tool in order to automate industrial numerical simulations. ECDF7 allowed us to investigate the refine and EGADS libraries as tools for parallel mesh generation and adaptation using CAD as a geometric support. Since then, we fortified the workflow but some of our targeted industrial applications such as turbomachinery involve periodic boundary conditions. To manage these cases, the mesh generation and adaptation procedures must maintain matching periodic boundaries.
During ECFD8, we addressed multiple topics : periodic mesh generation from CAD model in EGADS, parallel and periodic metric gradation in ParaDiGM, making our parallel remeshing algorithm more generic to support non-manifold and periodic meshes. All these topics were covered, but there is still some work to be done to fully manage periodic mesh adaptation.
Non-manifold mesh adaptation was applied to changing mesh topologies. Here, an internal surface was described by a level-set and deformed up to hole creation to mimic ablation.

==== M3 - Development and definition of a new Automatic Mesh Convergence (AMC) driver for automating static mesh convergence in YALES2 (C. Papagiannis (LEGI/INRIA), G. Balarac (LEGI), O. LeMaître (INRIA), P.M. Congedo (INRIA)) ====
Static mesh adaptation requires re-adapting an existing (usually coarse) mesh, to conform to some quality metric that satisfies some optimality criterion. This involves statistics (Mean and RMS fields) that need to be adequately converged. We proposed and developed a novel AMC strategy that can converge an initial coarse mesh to an adequately refined one by taking into account the quality of the calculated statistics before each adaptation. Specifically, so far, the moment (in terms of integrating the LES equations) at which to launch the adaptation was unknown and had to be set apriori as a parameter of the AMC, dependent on the flow physics and therefore on the case at hand. Our method is automatically performing adaptations when the quality of the volume averaged mean field estimate (characterized by its RMSE) reaches the levels of the bias error that this field has, due to the mesh being far from the desired one (dictated by the quality criterion). This was achieved through drawing inspiration from a theoretical model that treats the AMC as a "conditionally" contractive mapping with a fixed point.

=== Numerics - M. Bernard, LEGI & G. Lartigue, CORIA ===

==== N1 - Traction open boundary condition ====

==== N2 - Treatment of Inlet conditions in High-Order solver. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====
In the context of node-centered Finite Volumes Method, spacial accuracy of a numerical scheme depends on ability to evaluate accurately fluxes through interface of each control volume (CV). Such accurate evaluation is not straightforward, especially when dealing with distorted grids. This project follows the work of [1] where fluxes use pointwise quantities, which are reconstructed from integrated quantities advanced in time. During the previous edition of the ECFD, a new data structure has been developed to store data at location of the boundary conditions facelets, with application to wall boundary conditions. During this 8th edition of the ECFD, we used the same data structure, but dedicated to the treatment of inlet conditions.
The inlet condition is then either imposed directly at facelets center, or at nodes position them extrapolated to facelets center by use of Taylor expansion. For this later solution, high-order treatment requires the successive derivatives to be computed in the plane of the boundary condition. This is not done yet, leading for the moment to low accuracy results but the framework is ready for upcoming implementation.

[1] ''A framework to perform high-order deconvolution for finite-volume method on simplicial meshes, , Bernard et. al., IJNMF 2020''

==== N3 - Conservative mesh-to-mesh interpolation. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====

Mesh to mesh interpolations occur quite often in CFD simulations : in the context of adaptative mesh convergence or in the case of dynamic mesh adaptation for for example.
Quality of the solution on the destination grid will depend on the characteristics of the interpolation method.
In this project, we did not focus on accuracy of the interpolation method but rather on conservativity characteristics.
A conservative interpolation ensures that the integral of the data on the source grid is exactly retrieved on the destination grid.
This property is highly interesting when dealing with scalar quantities or phase indicators, whose values should remained bounded.
In the context of nodes centered Finite Volume schemes, the methodology we used consists in (i) reconstructing element quantity from average nodal quantities on source grid.
Then, for a cell of the destination mesh, (ii) computing the geometrical intersection between cells of source and destination meshes to evaluate to evaluate the rate of quantities they.
Eventually, (iii) redistributing the solution from elements to control volumes of the destination mesh.
The overall process is fully conservative as it is based on geometrical intersection of locally integrated quantities.
The methodology as been implemented and tested on a few basic configurations and the conservativity is retrieved.

==== N4 - Determination of timestep in semi-implicit solver. T. Berthelon (LEGI), G. Balarac (LEGI), H. Lam (LEGI), M. El Moatamid (CORIA) ====
In order to reduce the computation time associated with incompressible LES simulations, an implicit time integration, based on BDF schemes, has been developed within the ICS solver. This integration eliminates the stability constraints associated with explicit schemes, and therefore opens up the question of the appropriate choice of time step.
In parallel, recent work has been carried out on meshing criteria in LES. The strategy in place consists of adapting the mesh by distinguishing two zones:
- "DNS" zones, where the meshing criterion is based on an estimate of the adimensioned spatial error.
- "LES" zones, where the meshing criterion is based on Kolmorogov theory.
During this project, the spatial criteria were extended to include temporal criteria. In the "DNS" zones, the time step is chosen using an estimate of the temporal error of the BDF scheme judiciously scaled to match the spatial error. In the "LES" zones, the time step is chosen using a scaling law associated with fully developed turbulence.
The new time step selection strategy has been tested on the case of a turbulent jet and leads to an accuracy equivalent to the explicit case while reducing the simulation return time by a factor of nearly 3.

Another aspect of this project was to integrate certain implicit temporal schemes (C-N and SDIRK) recently developed by Mr. El Moatamid into the incompressible solver.

==== N5 - Local timestep. T. Berthelon (LEGI), M. Bernard (LEGI), G. Balarac (LEGI) ====
RANS modelling has recently been developed within the YALES2 library. With this modeling strategy, the objective is to reach as quick as possible a steady state.
During this project, we investigate the use of a local time step to reduce the time to solution of steady computation in the incompressible solver.
This implies solving a variable-coefficient Poisson equation. Encouraging results were obtained in the simple case of "Couette plan" flow artificially constrained by a mesh variation. In fact, the use of local time-step reduce drastically the time to solution on this configuration. This method needs to be tested on real RANS case.

==== N6 - Distributed version of DOROTHY ====

==== N7 - Implicit time advancement for low-Reynolds number flows with particles. S. Mendez, C. Raveleau (IMAG), M. El Moatamid, V. Moureau (CORIA) ====
IMAG runs numerous simulations of red blood cells under flow. Those simulations are at low Reynolds number (0.001 to 1.0, typically). Splitting of the time advancement is used to treat the diffusion terms implicitly, albeit with an important numerical cost: implicit diffusion is 50 to 60% of the computational cost. Recently, M. El Moatamid implemented a genral framework to deal with implicit time advancement for scalars. In this project, the general method has been transposed to the advancement of the velocity field in the ICS and RBC solvers of YALES2/YALES2BIO. This enables testing various linear solvers (GMRES based). However, such solvers do not decrease the CPU time compared to the existing implementation. However, while working on this, it was identified that residual recycling was not activated in the current implementation of the implicit diffusion. This sped up the implicit diffusion cost by 35%, for a total gain of 20% for the computation. In addition to this achievement, moving to the framework coded by Moncef will have other beneficial side effects: we anticipate simplifying the implementation, with an easier merging between YALES2BIO and YALES2. The method will also be implemented in the electrosatic solver, for which the Poisson problem should benefit from the new GMRES-based solvers. In addition, this project highlights the importance of improving the treatment of stiff source terms in the red blood cells simulations, to be able to overcome the current limitation in time step due to those term and have a chance to benefit from higher-order time schemes, efficient at high Fourier numbers.

==== N8 - Boundary Element Method in YALES2. B. Thibaud, S. Mendez (IMAG), G. Lartigue, P. Benard (CORIA), F. Nicoud (IMAG) ====
In the context of microfluidic systems for diagnosis, the Boundary Element Method alows to solve linear PDE such as electrostatic or Stokes. With well chosen kernel functions and the divergence theorem, this method allows to write on the boundary condition only the initial volumic problem. This project aimed at exploring the feasibility of the BEM in the context of massively parallel unstructured solver like YALES2 by developping a Julia demonstrator. The first step have been to implement and validate the method on simple configurations for the Laplace's equation. Only Neumann problems were considered (Dirichlet boundary conditions imposed). In a second time, the multi-domain approach has been identified to be the most suited in the framework of YALES. The inner domain is partitioned on each processor, each having a part of the physical boundary and interfaces between them. Every processor solve its own boundary problem and a parallel Dirichlet-Dirichlet fixed-point is used to converge the interface problem on the all domain. Applied to the ring case, with one interface, we managed to reproduce the linear convergence of the P-DD method.

=== Turbulence - L. Voivenel, CORIA & P. Bénard, CORIA ===

==== T1 - FSI-1D strategy for internal flows====

Many applications developed at Safran Aerosystem are based on internal turbulent flows coupled to a moving body. 2 cases were studied during this ECFD:

'''Case 1 (Incompressible flow)''': Translation of a piston subjected to a pressure difference in a pipe.

The challenges of this case are twofold: the small gap between the piston and the pipe and the large pressure gradient across the piston (>100bar). During the 1st week of ECFD, the CLIB (Conservative Lagrangian Immersed Boundary) solver was tested on this case. The study showed that the solver was unable to ensure the impermeability of the solid under these pressure conditions. In the rest of the study, a porous medium following Darcy's law will be added to the penalty force of the immersed solid to fully satisfy the impermeability of the piston.

'''Case 2 (Compressible flow)''': Rotation of a butterfly in a discharge vane.

The coupling between the ECS (Explicit Compressible Solver) and ALE (Arbitrary Lagrangian Solver) solvers having recently been implemented, this strategy was tested to model the opening of the valve by rotation of the butterfly. The challenge here lies in the small gap between the bottom of the butterfly and the vane casing. To limit the simulation cost, the gap is meshed with 1 element. In this case, MMG succeeded in adapting the mesh up to a critical angle at which the gap becomes too small (Work In Progress).

==== T2 - Dynamic Smagorinsky in Dorothy - Maëlenn ROPERCH (LOMC), Grégory PINON (LOMC) ====

==== T3 - Turbulence injection strategy for compressible flows - Patrick TENE HEDJE (UMONS), Laurent Bricteur (UMONS), Yacine BECHANE (CORIA), Pierre BENARD (CORIA) ====

==== T4 - Improve wind farm modeling and simulation workflow - Pierre BENARD (CORIA), Ulysse VIGNY (UMONS), Etienne MULLER (CORIA & SGRE), Hakim HAMDANI (GDTech), Félix HOUTIN-MONGROLLE (SGRE), Anand PARINAM (CORIA & TUDelft), Hari MULAKALOORI (CORIA), Léa VOIVENEL (CORIA) ====
The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine. Still, applying this method to investigate a wind farm flow can be challenging, both in terms of computational cost and simulation setup. For instance, an inadequate management of the wind turbine individual modeling parts in a HPC context can end up being the main bottleneck of the simulation. From another perspective, a wind farm is usually composed of more than 50 wind turbines. For such a case, setting up all YALES2 required inputs manually can be very tedious and error-prone. This project thus mainly aimed to optimize the YALES2 ALM implementation and the user experience around it. Additionally, a cost-effective alternative to the ALM when modeling wind farm flows, namely the Rotating Actuator Disk Method (ADM-R), has been implemented for further investigations.

'''WP1''': Improve Actuator set rotor modelling
* Parallel processing of the ''actuator sets'' used to model the wind turbines
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified during the 5th iteration of the ECFD: (i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges. (ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications. During this workshop edition, the work focused on the re-synchronisation of the algorithm steps by allowing some packing an unpacking of the object to allow the transfert of the object inbetween workers. This is of major importance to enable load balancing and mesh adaptation during the temporal loop. This work required the refactoring of the involved oject structures.

* Rotating Actuator Disk Method (ADM-R):
According to the usual guidelines, the mesh requirements of the ALM, to profit entirely from its reachable accuracy, can be difficult to achieve or even unaffordable when simulating a wind farm flow, especially from the industrial point of view. Alternatives are available in the literature for this kind of application. Likely, the methods from the Actuator Disk family are the most prominent ones. Several kinds of implementation exist, which mostly differ by their capability to include the wake rotation. During the workshop, a new method from the Rotating Actuator Disk kind has been implemented and underwent an early validation on a single turbine setup. Applications to wind farm flows will follow.

'''WP2''': Improve tools User Experience

Three Python tools have been developed or improved :
*The first tool is the wind farm previsualisation tool, 'y2_wind_previsualization', which is used before the calculation run. This provides an interactive HTML interface for viewing global data for each turbine on the farm (position, hub height, yaw angle, etc.). The tool traces all of these via the parsing of the input file.
* The second tool is for duplicating rotor templates for a wind farm (`y2_wind_duplication`). This tool was developed in the previous ECFD, but this time it has been refactored and incorporated into the y2tools package.
* The third and final tool is a post-processing tool for the temporal processing of global wind turbine simulation metrics (Thrust, Power, etc.), `y2_post_wind`. This tool generates an interactive HTML plot of time-dependent global quantities.

==== T5 - Improve atmospheric inflow turbulence ====
Atmospheric inflow turbulence is generated using the precursor database method. A half-channel flow driven by a pressure gradient is used to obtain the inflow which is used as inlet boundary condition for the wind turbine simulation domain. This project aimed to improve the whole methodology, from generation to injection.

* WP1: Improve inflow generation
Anand: pressure controller

* WP2: Improve injection methodology (method A)
The previous workflow used plane probes in the ASCII format to sample the flow. The COWIT2 toolbox was used to convert the file into turbulence box (.man format). While functioning, this methodology had two major flaws. First the probe files are heavy ~O(10Go). Second, the method requires a lot of human effort, allowing numerous sources of errors.
During this workshop, a new methodology has been developed. First, the probes are generated using the HDF5 format (now available for all probe types), leading to lighter file ~O(1Go). Second, Y2_tools is used to read HDF5 format (working for probes and temporals). HDF5 file is then converted into a Look-up Table. Finally, the Look-up Table is read directly by YALES2 as a boundary conditions.

* WP3: Improve injection methodology (method B)
Even though improvements achieved in WP2 prove to be very handy while removing many potential human errors, injecting a turbulent inflow through wind boxes ('offline' precursor approach) can sometimes remain cumbersome for several reasons: (1) no periodicity is enforced in the streamwise direction of those boxes, (2) potential high memory consumption, and (3) the boxes need to be moved to other cores whenever a mesh adaptation occurs. An alternative consists in co-simulating the precursor flow and the flow of interest (refered as the 'successor' simulation) at the same time ('online' precursor approach). The inlet boundary condition for the successor flow is then obtained by mapping the outflow of the precursor domain. During the workshop, some work has been initiated to implement this kind of coupling using the CWIPI library, for which YALES2 provides already an interface.

==== T6 - FSI model in Dorothy - Enzo MASCRIER (LOMC), Grégory PINON (LOMC) ====

=== Two Phase Flow - J. Leparoux, SAFRAN & J. Carmona, CORIA ===

==== TP1 - Towards very small contact angles in Nucleate boiling ====

Participants: Henri Lam (LEGI), Mohammad Umair (LEGI), Manuel Bernard (LEGI), Robin Barbera (LEGI) and Giovanni Ghigliotti (LPSC)

The boiling solver (BOI) was not able to accurately impose a contact angle (angle formed by the two-phase interface on the wall) at values lower than 30°. This angle is needed when simulating nucleate boiling. A similar limitation in contact angle value was applying to the spray (SPS) solver. A modified version of the level set reinitialization has been implemented during ECFD8, based on a different normal vector in the blind spot region around the contact line, vector now chosen to be a zero-order extension from outside the blind spot. This modification, that implied other modifications to the level set reinitialization in the blind spot, has been tested successfully on the spray solver (where no phase change occurs). Then, this new reinitialisation has been tested in the boiling solver for nucleate boiling, with great improvements. Now simulations of nucleate boiling at very small contact angle (10°) can be accurately performed.
In the meanwhile, the level set reinitialization algorithm has been streamlined and the computational cost greatly reduced, resolving a computational cost issue that appeared when using the contact angle imposition both in the spray and boiling solvers, and that hampered its use in industrial configurations.

==== TP2 - Modeling spray-film interactions ====

Participants: Nicolas Gasnier (EM2C-SafranTech), Julien Leparoux (SafranTech), Mehdi Helal (CORIA-SafranTech) and Julien Carmona (CORIA)

The numerical simulation of sheared liquid films over walls with conventional high-fidelity methods requires fine meshes to depict accurately the dynamics of the fluids, as well as the gas-liquid interactions at the phase interface. In the context of thin film flows (with a thickness h ~ 1.0E-4 m), the spatial resolution required to ensure the validity of the simulations can be computationally prohibitive. To tackle this issue, a reduced-order model of thin film dynamics based on the Saint Venant equations has been implemented in the YALES2 platform over the last months. This numerical model is able to reproduce accurately the dynamics of strongly sheared film flows over partially wetted surfaces, and to take into account capillary effects. The objective of the ECFD8 was to couple the thin film framework with the pre-existing multiphase methods of the YALES2 platform: the Eulerian multiphase solver - based on the ACLS -, and the Lagrangian solver. This coupling was performed in three steps: first, a numerical method was designed to convert impinging Lagrangian droplets into source terms for the thin film model. Then, a phenomenological model was implemented to depict the atomization of films at sharp edges under the action of a high speed gas flow. The liquid film is converted into Lagragian droplets, whose dimensions are computed based on the PAMELA model. Finally, a numerical method has been designed to convert Eulerian droplets into source terms for the film model. This method is based on a transfer of the liquid properties from the nodes to the boundaries by using a set of fictitious particles. It ensures conservation of the liquid mass, and it is more robust than the first attempts that had been made during ECFD7.

==== TP3 - High-fidelity two-phase flow simulations of the purge of a fuel feed line ====

Participants: Thomas LAROCHE (Safran HE), Romain JANODET (Safran AE), Julien Leparoux (Safran Tech) and Melody Cailler (Safran Tech)

During the second week of the ECFD8, the fuel feed line purge process has been numerically investigated. In the context of aeronautical engines, the fuel feed line—carrying the fuel from the fuel tank to the injectors within the combustion chamber—needs to be purged at engine shutdown. This is intended to prevent fuel stagnation near hot metal parts, which could lead to coke formation and therefore decrease engine performance. Since this complex phenomenon is mainly driven by two-phase flow physics, the spray solver (SPS) of the YALES2 library has been considered in order to understand the physics of such process. The numerical setup was first converged on a simplified test case: the possibility of driving the flow dynamics with inlet and outlet pressure conditions was tested beforehand on a single-phase, incompressible case, and then on a two-phase flow problem. The setup has then been successfully applied to an industrial configuration: a pressure-swirl injector connected to a reduced portion of the fuel feed line. Due to the large scale of the domain, the interface resolution was set to 50μm, which is intentionally coarse for such problem. This initial computation successfully ran up to 3ms of physical time during the workshop, proving YALES2's capability to model the fuel purge. The computation is to be continued and analyzed further even after the workshop.

==== TP4 - Volume of Fluid solver in YALES2 ====

Participants: Léa Voivenel (CORIA), Julien Carmona (CORIA), Mehdi Helal (CORIA), Pierre Portais (CORIA), Julien Leparoux (Safran Tech), Mélody Cailler (Safran Tech) and Nicolas Gasnier (EM2C / Safran Tech)

==== TP5 - Implement a local operator to distribute the solid volume of a particle over multiple cells ====

Participants: Théo Ndereyimana (Université de Sherbrooke), Stéphane Moreau (Université de Sherbrooke)

In the CFD-DEM, the cell size is required to be larger than the particle size for stability condition and keep feasible solid volume fraction. However, some applications require a cell size smaller than the particle. During this 8th edition of the ECFD, the use of operators to distribute the particle volume over multiple cells, ensuring a feasible solid volume particle has been tested on a fluidized bed configuration. The main operators tested (gather-scatter filter and gaussian filter) showed a tendency to blur the void structure interfaces. The equivalence of the gaussian filter of bandwidth <math>b</math> and the resolution of a diffusion equation over a pseudo-time <math>T=b^2/4</math> has been verified.
One anisotropic diffusion constant has been tested and shows a possibility to adress the sharpness requirements.

Another objective was to develop a post-processing tool to detect and track the void structures (bubbles) in a fluidized bed. Based on previous work from J. Carmona, a tool to track the bubbles has been initiated.

==== TP6 - Complex thermodynamics in sloshing tanks ====

Participants: C. Merlin (AGS), D. Fouquet (CORIA), V. Moureau (CORIA), J. Carmona (CORIA) and G. Lartigue (CORIA)

During this ECFD, the phase change solver was improved. After introduction of conservative transport for enthalpy and energy, a new framework for multi fluid two phase flow was used that relies on a new reinitialization of the conservative level set, the transport of discontinuous scalars like temperature, energy or enthalpy and the transport of weight to ensure consistency. The pressure loop was improved to ensure mass and energy conservation while meeting the low Mach hypothesis. Many 1D test cases were performed with only the gas phase and two phase flow to validate the pressure loop. The new framework for the transport of discontinuous scalar was first derived for temperature with constant properties in each phase. A first step was to extend it to sensible enthalpy with ideal gas behavior for the gas phase. Then, the framework was extended to NIST tabulated gas & liquid with ideal or real gas behavior. After some 2D cases, a tank pressurization test was investigated as well as a sloshing test without phase change (just the pressure drop due to the increase of the thermal exchange between the liquid and the gas phase).

=== Combustion - Y. Bechane, CORIA & S. Dillon, SAFRAN & K. Bioche, CORIA ===

==== C1 - LES of the thermal degradation of a composite material ====
Participants: A. Grenouilloux (ONERA), K. Bioche (CORIA), N. Dellinger (ONERA) and R. Letournel (SafranTech)

The FIRE test bed is an experimental air-propane burner operated by ONERA. It is dedicated to the study of the thermal degradation of composite materials. This project concerned the implementation of a three-solver coupling methodology to simulate the dynamics of the impinging flame. The methodology considered is based on the coupling between the variable density solver (VDS) and the radiative solver (RDS) of the massively parallel library YALES2 and the solver dedicated to the degradation of composite materials, MoDeTheC, developed by ONERA. Given the typical test times of the order of tens of seconds, a methodology based on 2D axisymmetric calculations was considered. Various tests were performed to determine the optimal coupling frequency between solvers. Cases dedicated to the injection of pyrolysis gasses were set up, with the aim of simulating the auto-ignition phenomenon. Comparisons with experimental data are presented.

==== C2 - Flame stabilization by NRP plasma discharge ====

==== C3 - Extending and validating a generalized formalism of virtual chemistry ====

==== C4 - Turbulent combustion model for NOx prediction ====

==== C5 - Towards 3D simulation of detonation combustion ====

==== C6 - Flame stabilitity of flame-holders within reheat conditions ====

Participants : N. Detomaso (Safran AE), R. Janodet (Safran AE), L. Carbajal (Safran AE)

The mechanisms of flame stabilisation in a flame-holder are still not completely understood. This is particularly true for reheat conditions : the hot viciated gases at inlet, highly compressible flows, and a strong liquid/gas coupling make the flame stability hard to predict. During the ECFD8, simplified configurations of flame-holders within reheat conditions were analysed. After some try and error simulations due to simplifications of the flame-holders, cases leading to flame stability (or not) were identified. Post-processing tools were developped in order to recover criteria relevant to the flame stability. This step marks the beginning of a systematic mapping of dimensionless characteristic times ratios, and a comparison with integral quantities.

==== C7 - Thermal radiation in oxyflames ====

==== C8 - A first step toward hybrid CPU / GPU for reactive flow in YALES2 ====

Participants: M. Laignel (CORIA), G. Lartigue (CORIA), K. Bioche (CORIA) and V. Moureau (CORIA)

In numerical simulations of reacting flows, one of the most computationally intensive tasks is the evaluation of source terms resulting from chemical reactions in the species transport equations. This step can account for up to 90% of the total simulation cost , depending on the complexity of the kinetic mechanism involved. To reduce this cost, various techniques such as mechanism reduction, virtual chemistry, etc. have been explored. However, the emergence of GPUs as powerful accelerators offers a promising alternative by providing massive parallelism. Despite their potential, GPUs often require significant adaptation of CPU-based codes. This project aims to address this challenge by taking a first step towards a hybrid CPU/GPU framework for reactive flow simulations. Specifically, the focus is on coupling Y2 with the updated version of the stiff time integration solver (CVODE), which is compatible with GPU (CUDA, HIP, OpenMP). The ultimate goal is to establish a foundation for hybrid computations by implementing and testing the updated solver on simplified test cases.

==== C9 - Soots numerical modeling ====

==== C10 - TECERACT : Tabulated chemistry generator for aeronautical combustion ====

==== C11 - Exploring efficient tabulation strategies for detailed chemistry ====

==== C12 - Dynamic sub-grid-scale modelling of multi-regime flame wrinkling ====

==== C13 - LES of a semi-industrial burner using a non-adiabatic virtual chemical scheme ====

=== User Experience & Data - L. Korzeczek, GDTECH ===

==== U1 - Low-fidelity (RANS) rotor/stator simulations, application to Kaplan Turbine - Y. Lakrifi, G. Balarac (LEGI), R. Mercier (SAFRAN), V. Moureau (CORIA) ====

==== U2 - Coupling PyTorch/YALES2, combustion cartesian look-up tables - J. Leparoux, N. Treleaven, S. Dillon (SAFRAN), K. Bioche, G. Lartigue (CORIA) ====

Participants: Julien Leparoux (Safran Tech), Kévin Bioche (CORIA), Ghislain Lartigue (CORIA), Nicholas Treleaven (Safran Tech)

Neural Networks offer a promising alternative to Cartesian look-up tables for combustion simulations, reducing memory footprint. In this project, we investigated how to integrate an NN model for real-time inference in the YALES2 platform, exploring two approaches: a Python interface and a Fortran Torch binding (using FTorch[https://github.com/Cambridge-ICCS/FTorch]). We validated that the model remains accurate when embedded online and identified improvements for robustness. Inference costs were evaluated on a Mac M3 and the Austral cluster, revealing a strong dependency on data volume. To optimize efficiency, we propose grouping cells at the processor level.

==== U3 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 - L. Korzeczek, S. Meynet (GDTECH), J. Leparoux, M. Cailler (SAFRAN) ====

Ecfd:ecfd 8th edition

2025-02-12T14:04:47Z

Benard: /* T6 - FSI model in Dorothy */

{{DISPLAYTITLE: ECFD workshop, 8th edition, 2025}}

== Description ==

* Event from '''27th of January to 7th of February 2025'''
* Location: [https://www.sport-normandie.fr/le-centre/le-site-de-houlgate Centre Sportif de Normandie], Houlgate, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* Participants from academics, HPC center/experts and industry are welcome
* The number of participants is limited to 68.

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes
* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Voivenel (CORIA).

[[File:ecfd8.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_8th_edition]]

[[File:Acknowledgments_ecfd8.png|text-bottom|600px]]

== News ==

* 23/10/2024: First announcement of the '''8th Extreme CFD Workshop & Hackathon''' !
* 22/11/2024: Deadline to submit your project

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

To come...

== Projects ==

=== Hackathon GENCI - P. Begou, LEGI ===
This ECFD8 GENCI Hackathon was a rich event, involving 4 differents CFD codes (AVBP, ParaDIGM, SONICS and YALES2) using various paradigms (C++/cuda/hip, Fortran/OpenMP/OpenACC) with several SDKs (AMD, Cray/HPE, Nvidia, Gnu) on a large range of GPU architectures (Nvidia A100, GH100, AMD instinct Mi210, Mi250, Mi300). This two-week event benefited from a high level support from three HPC mentors, two on-site from AMD (J. Noudohouenou and A. Tsetoglou) and one remote from CINES (M. Boudaoud).

==== H1 - ParaDIGM and SONICS on GPU, B. Maugars, G. Staffelbach, R.Cazalbou and B. Michel (ONERA)====

==== H2 - AVBP GPU offloading based on OpenMP, M.Ghenai, L. Legaux and A. Dauptain (CERFACS) ====

This hackathon provided a valuable opportunity to work on GPU offloading for AVBP. In the past, significant efforts were made to offload the entire AVBP code to GPUs. OpenACC was the primary strategy chosen, mainly due to access to NVIDIA's support, along with the availability of both software and hardware. This approach achieved good scalability performance.
Recently, with the deployment of new supercomputers like ADASTRA at CINES, some issues have emerged when running AVBP on AMD GPUs, including both MI250 and MI300. The closed-source nature of the Cray environment has also prevented CERFACS from deploying AVBP on local MI210 GPUs.
This hackathon was a great opportunity to address these challenges by exploring a new approach using OpenMP. An automatic translation tool was used to convert approximately 2,700 OpenACC directives to OpenMP, with each directive manually verified and fine-tuned afterward. AVBP with OpenMP had already been tested on NVIDIA GPUs, and during this hackathon, the focus was on extending support to AMD GPUs.
Two compilers were used: Cray and the newly released AFAR from AMD. With the support of AMD and CINES, a working environment for compiling AVBP was set up, and performance-related issues were identified. Additionally, two mini-apps were used for benchmarking. One of them achieved a 2.5× speedup when compiled with AFAR compared to Cray.
The next steps involve adapting the code to address necessary modifications, such as fixing issues related to Fortran indirections, and continuing performance evaluations with mini-apps. Further comparisons will be conducted using both compilers against results obtained with NVIDIA’s NVHPC.

==== H3 - YALES2 GPU from OpenACC to OpenMP, P. Bégou (LEGI), V. Moureau, G. Lartigue (CORIA) and R. Dubois (IMAG) ====
This Hackathon focuses on running Yales2 code on AMD Instinct Mi250 and Mi300 GPUs of the Adastra supercomputer (CINES).
Previously, a first solver in the Yales2 CFD code was successfully ported on the GPU accelerators of the Jean-Zay supercomputer (IDRIS) using Nvidia SDK but difficulties remain on Adastra AMD GPUs, mainly related to the available development tools. High compilation time and the impossibility to use debug flags at compile time as soon as OpenACC is enabled are a real challenge when tracking errors. The current project is to evaluate a freshly deployed version (at the begining of the workshop) of the AMD Fortran compiler. This requires moving to OpenMP paradigm, starting from scratch since the OpenACC branch has largely diverged from the master one while tracking spurious remaining bugs.
If the AMD compiler is able to build the cpu version of Yales2 "out of the box" (wich is not the case for Cray Fortran), the compilation time for each file is significantly higher. However, setting up a 2 stages dynamic compilation process allows for high parallelism that is not possible with Cray Fortran 18 and the library build time drops from nearly 2 hours (Cray Fortran 18) to 17 minutes (Amd Fortran compiler).
Large kernels have been ported from OpenACC to OpenMP, raising some difficulties when offloading intrinsics functions or using strutures attributes in kernels loops. These limitations were also known in the previous OpenACC work. The goal was mainly to check the correctness of the results. The offloading of the complex data structure of Yales2 code was then investigated. Here again some limitations of the "young" compiler were discovered and workarounds were implemented. Several reproducers were built during this ECFD8 and provided to developpers by the 2 on-site AMD engineers.
Preliminary tests on micro-applications show good performances of the generated binaries proving that this compiler could be a serious alternative on AMD GPUs and the goal is now to focus on this SDK in an OpenMP strategy while checking the portablility of this new implementation in Nvidia, Cray/HPE (and Gnu ?) environments.

=== Mesh adaptation - A. Grenouilloux, ONERA & G. Balarac, LEGI ===

==== M2 - Enhancement of mesh adaptation algorithms, B. Maugars (ONERA), B. Andrieu (ONERA), C. Benazet (ONERA), N. Dellinger (ONERA), G. Janodet (ONERA), G. Staffelbach (ONERA) ====
Mesh adaptation has become a crucial tool in order to automate industrial numerical simulations. ECDF7 allowed us to investigate the refine and EGADS libraries as tools for parallel mesh generation and adaptation using CAD as a geometric support. Since then, we fortified the workflow but some of our targeted industrial applications such as turbomachinery involve periodic boundary conditions. To manage these cases, the mesh generation and adaptation procedures must maintain matching periodic boundaries.
During ECFD8, we addressed multiple topics : periodic mesh generation from CAD model in EGADS, parallel and periodic metric gradation in ParaDiGM, making our parallel remeshing algorithm more generic to support non-manifold and periodic meshes. All these topics were covered, but there is still some work to be done to fully manage periodic mesh adaptation.
Non-manifold mesh adaptation was applied to changing mesh topologies. Here, an internal surface was described by a level-set and deformed up to hole creation to mimic ablation.

==== M3 - Development and definition of a new Automatic Mesh Convergence (AMC) driver for automating static mesh convergence in YALES2 (C. Papagiannis (LEGI/INRIA), G. Balarac (LEGI), O. LeMaître (INRIA), P.M. Congedo (INRIA)) ====
Static mesh adaptation requires re-adapting an existing (usually coarse) mesh, to conform to some quality metric that satisfies some optimality criterion. This involves statistics (Mean and RMS fields) that need to be adequately converged. We proposed and developed a novel AMC strategy that can converge an initial coarse mesh to an adequately refined one by taking into account the quality of the calculated statistics before each adaptation. Specifically, so far, the moment (in terms of integrating the LES equations) at which to launch the adaptation was unknown and had to be set apriori as a parameter of the AMC, dependent on the flow physics and therefore on the case at hand. Our method is automatically performing adaptations when the quality of the volume averaged mean field estimate (characterized by its RMSE) reaches the levels of the bias error that this field has, due to the mesh being far from the desired one (dictated by the quality criterion). This was achieved through drawing inspiration from a theoretical model that treats the AMC as a "conditionally" contractive mapping with a fixed point.

=== Numerics - M. Bernard, LEGI & G. Lartigue, CORIA ===

==== N1 - Traction open boundary condition ====

==== N2 - Treatment of Inlet conditions in High-Order solver. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====
In the context of node-centered Finite Volumes Method, spacial accuracy of a numerical scheme depends on ability to evaluate accurately fluxes through interface of each control volume (CV). Such accurate evaluation is not straightforward, especially when dealing with distorted grids. This project follows the work of [1] where fluxes use pointwise quantities, which are reconstructed from integrated quantities advanced in time. During the previous edition of the ECFD, a new data structure has been developed to store data at location of the boundary conditions facelets, with application to wall boundary conditions. During this 8th edition of the ECFD, we used the same data structure, but dedicated to the treatment of inlet conditions.
The inlet condition is then either imposed directly at facelets center, or at nodes position them extrapolated to facelets center by use of Taylor expansion. For this later solution, high-order treatment requires the successive derivatives to be computed in the plane of the boundary condition. This is not done yet, leading for the moment to low accuracy results but the framework is ready for upcoming implementation.

[1] ''A framework to perform high-order deconvolution for finite-volume method on simplicial meshes, , Bernard et. al., IJNMF 2020''

==== N3 - Conservative mesh-to-mesh interpolation. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====

Mesh to mesh interpolations occur quite often in CFD simulations : in the context of adaptative mesh convergence or in the case of dynamic mesh adaptation for for example.
Quality of the solution on the destination grid will depend on the characteristics of the interpolation method.
In this project, we did not focus on accuracy of the interpolation method but rather on conservativity characteristics.
A conservative interpolation ensures that the integral of the data on the source grid is exactly retrieved on the destination grid.
This property is highly interesting when dealing with scalar quantities or phase indicators, whose values should remained bounded.
In the context of nodes centered Finite Volume schemes, the methodology we used consists in (i) reconstructing element quantity from average nodal quantities on source grid.
Then, for a cell of the destination mesh, (ii) computing the geometrical intersection between cells of source and destination meshes to evaluate to evaluate the rate of quantities they.
Eventually, (iii) redistributing the solution from elements to control volumes of the destination mesh.
The overall process is fully conservative as it is based on geometrical intersection of locally integrated quantities.
The methodology as been implemented and tested on a few basic configurations and the conservativity is retrieved.

==== N4 - Determination of timestep in semi-implicit solver. T. Berthelon (LEGI), G. Balarac (LEGI), H. Lam (LEGI), M. El Moatamid (CORIA) ====
In order to reduce the computation time associated with incompressible LES simulations, an implicit time integration, based on BDF schemes, has been developed within the ICS solver. This integration eliminates the stability constraints associated with explicit schemes, and therefore opens up the question of the appropriate choice of time step.
In parallel, recent work has been carried out on meshing criteria in LES. The strategy in place consists of adapting the mesh by distinguishing two zones:
- "DNS" zones, where the meshing criterion is based on an estimate of the adimensioned spatial error.
- "LES" zones, where the meshing criterion is based on Kolmorogov theory.
During this project, the spatial criteria were extended to include temporal criteria. In the "DNS" zones, the time step is chosen using an estimate of the temporal error of the BDF scheme judiciously scaled to match the spatial error. In the "LES" zones, the time step is chosen using a scaling law associated with fully developed turbulence.
The new time step selection strategy has been tested on the case of a turbulent jet and leads to an accuracy equivalent to the explicit case while reducing the simulation return time by a factor of nearly 3.

Another aspect of this project was to integrate certain implicit temporal schemes (C-N and SDIRK) recently developed by Mr. El Moatamid into the incompressible solver.

==== N5 - Local timestep. T. Berthelon (LEGI), M. Bernard (LEGI), G. Balarac (LEGI) ====
RANS modelling has recently been developed within the YALES2 library. With this modeling strategy, the objective is to reach as quick as possible a steady state.
During this project, we investigate the use of a local time step to reduce the time to solution of steady computation in the incompressible solver.
This implies solving a variable-coefficient Poisson equation. Encouraging results were obtained in the simple case of "Couette plan" flow artificially constrained by a mesh variation. In fact, the use of local time-step reduce drastically the time to solution on this configuration. This method needs to be tested on real RANS case.

==== N6 - Distributed version of DOROTHY ====

==== N7 - Implicit time advancement for low-Reynolds number flows with particles. S. Mendez, C. Raveleau (IMAG), M. El Moatamid, V. Moureau (CORIA) ====
IMAG runs numerous simulations of red blood cells under flow. Those simulations are at low Reynolds number (0.001 to 1.0, typically). Splitting of the time advancement is used to treat the diffusion terms implicitly, albeit with an important numerical cost: implicit diffusion is 50 to 60% of the computational cost. Recently, M. El Moatamid implemented a genral framework to deal with implicit time advancement for scalars. In this project, the general method has been transposed to the advancement of the velocity field in the ICS and RBC solvers of YALES2/YALES2BIO. This enables testing various linear solvers (GMRES based). However, such solvers do not decrease the CPU time compared to the existing implementation. However, while working on this, it was identified that residual recycling was not activated in the current implementation of the implicit diffusion. This sped up the implicit diffusion cost by 35%, for a total gain of 20% for the computation. In addition to this achievement, moving to the framework coded by Moncef will have other beneficial side effects: we anticipate simplifying the implementation, with an easier merging between YALES2BIO and YALES2. The method will also be implemented in the electrosatic solver, for which the Poisson problem should benefit from the new GMRES-based solvers. In addition, this project highlights the importance of improving the treatment of stiff source terms in the red blood cells simulations, to be able to overcome the current limitation in time step due to those term and have a chance to benefit from higher-order time schemes, efficient at high Fourier numbers.

==== N8 - Boundary Element Method in YALES2. B. Thibaud, S. Mendez (IMAG), G. Lartigue, P. Benard (CORIA), F. Nicoud (IMAG) ====
In the context of microfluidic systems for diagnosis, the Boundary Element Method alows to solve linear PDE such as electrostatic or Stokes. With well chosen kernel functions and the divergence theorem, this method allows to write on the boundary condition only the initial volumic problem. This project aimed at exploring the feasibility of the BEM in the context of massively parallel unstructured solver like YALES2 by developping a Julia demonstrator. The first step have been to implement and validate the method on simple configurations for the Laplace's equation. Only Neumann problems were considered (Dirichlet boundary conditions imposed). In a second time, the multi-domain approach has been identified to be the most suited in the framework of YALES. The inner domain is partitioned on each processor, each having a part of the physical boundary and interfaces between them. Every processor solve its own boundary problem and a parallel Dirichlet-Dirichlet fixed-point is used to converge the interface problem on the all domain. Applied to the ring case, with one interface, we managed to reproduce the linear convergence of the P-DD method.

=== Turbulence - L. Voivenel, CORIA & P. Bénard, CORIA ===

==== T1 - FSI-1D strategy for internal flows====

Many applications developed at Safran Aerosystem are based on internal turbulent flows coupled to a moving body. 2 cases were studied during this ECFD:

'''Case 1 (Incompressible flow)''': Translation of a piston subjected to a pressure difference in a pipe.

The challenges of this case are twofold: the small gap between the piston and the pipe and the large pressure gradient across the piston (>100bar). During the 1st week of ECFD, the CLIB (Conservative Lagrangian Immersed Boundary) solver was tested on this case. The study showed that the solver was unable to ensure the impermeability of the solid under these pressure conditions. In the rest of the study, a porous medium following Darcy's law will be added to the penalty force of the immersed solid to fully satisfy the impermeability of the piston.

'''Case 2 (Compressible flow)''': Rotation of a butterfly in a discharge vane.

The coupling between the ECS (Explicit Compressible Solver) and ALE (Arbitrary Lagrangian Solver) solvers having recently been implemented, this strategy was tested to model the opening of the valve by rotation of the butterfly. The challenge here lies in the small gap between the bottom of the butterfly and the vane casing. To limit the simulation cost, the gap is meshed with 1 element. In this case, MMG succeeded in adapting the mesh up to a critical angle at which the gap becomes too small (Work In Progress).

==== T2 - Dynamic Smagorinsky in Dorothy - Maëlenn ROPERCH (LOMC), Grégory PINON (LOMC) ====

==== T3 - Turbulence injection strategy for compressible flows - Patrick TENE HEDJE (UMONS), Laurent Bricteur (UMONS), Yacine BECHANE (CORIA), Pierre BENARD (CORIA) ====

==== T4 - Improve wind farm modeling and simulation workflow ====
The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine. Still, applying this method to investigate a wind farm flow can be challenging, both in terms of computational cost and simulation setup. For instance, an inadequate management of the wind turbine individual modeling parts in a HPC context can end up being the main bottleneck of the simulation. From another perspective, a wind farm is usually composed of more than 50 wind turbines. For such a case, setting up all YALES2 required inputs manually can be very tedious and error-prone. This project thus mainly aimed to optimize the YALES2 ALM implementation and the user experience around it. Additionally, a cost-effective alternative to the ALM when modeling wind farm flows, namely the Rotating Actuator Disk Method (ADM-R), has been implemented for further investigations.

'''WP1''': Improve Actuator set rotor modelling
* Parallel processing of the ''actuator sets'' used to model the wind turbines
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified during the 5th iteration of the ECFD: (i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges. (ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications. During this workshop edition, the work focused on the re-synchronisation of the algorithm steps by allowing some packing an unpacking of the object to allow the transfert of the object inbetween workers. This is of major importance to enable load balancing and mesh adaptation during the temporal loop. This work required the refactoring of the involved oject structures.

* Rotating Actuator Disk Method (ADM-R):
According to the usual guidelines, the mesh requirements of the ALM, to profit entirely from its reachable accuracy, can be difficult to achieve or even unaffordable when simulating a wind farm flow, especially from the industrial point of view. Alternatives are available in the literature for this kind of application. Likely, the methods from the Actuator Disk family are the most prominent ones. Several kinds of implementation exist, which mostly differ by their capability to include the wake rotation. During the workshop, a new method from the Rotating Actuator Disk kind has been implemented and underwent an early validation on a single turbine setup. Applications to wind farm flows will follow.

'''WP2''': Improve tools User Experience

Three Python tools have been developed or improved :
*The first tool is the wind farm previsualisation tool, 'y2_wind_previsualization', which is used before the calculation run. This provides an interactive HTML interface for viewing global data for each turbine on the farm (position, hub height, yaw angle, etc.). The tool traces all of these via the parsing of the input file.
* The second tool is for duplicating rotor templates for a wind farm (`y2_wind_duplication`). This tool was developed in the previous ECFD, but this time it has been refactored and incorporated into the y2tools package.
* The third and final tool is a post-processing tool for the temporal processing of global wind turbine simulation metrics (Thrust, Power, etc.), `y2_post_wind`. This tool generates an interactive HTML plot of time-dependent global quantities.

==== T5 - Improve atmospheric inflow turbulence ====
Atmospheric inflow turbulence is generated using the precursor database method. A half-channel flow driven by a pressure gradient is used to obtain the inflow which is used as inlet boundary condition for the wind turbine simulation domain. This project aimed to improve the whole methodology, from generation to injection.

* WP1: Improve inflow generation
Anand: pressure controller

* WP2: Improve injection methodology (method A)
The previous workflow used plane probes in the ASCII format to sample the flow. The COWIT2 toolbox was used to convert the file into turbulence box (.man format). While functioning, this methodology had two major flaws. First the probe files are heavy ~O(10Go). Second, the method requires a lot of human effort, allowing numerous sources of errors.
During this workshop, a new methodology has been developed. First, the probes are generated using the HDF5 format (now available for all probe types), leading to lighter file ~O(1Go). Second, Y2_tools is used to read HDF5 format (working for probes and temporals). HDF5 file is then converted into a Look-up Table. Finally, the Look-up Table is read directly by YALES2 as a boundary conditions.

* WP3: Improve injection methodology (method B)
Even though improvements achieved in WP2 prove to be very handy while removing many potential human errors, injecting a turbulent inflow through wind boxes ('offline' precursor approach) can sometimes remain cumbersome for several reasons: (1) no periodicity is enforced in the streamwise direction of those boxes, (2) potential high memory consumption, and (3) the boxes need to be moved to other cores whenever a mesh adaptation occurs. An alternative consists in co-simulating the precursor flow and the flow of interest (refered as the 'successor' simulation) at the same time ('online' precursor approach). The inlet boundary condition for the successor flow is then obtained by mapping the outflow of the precursor domain. During the workshop, some work has been initiated to implement this kind of coupling using the CWIPI library, for which YALES2 provides already an interface.

==== T6 - FSI model in Dorothy - Enzo MASCRIER (LOMC), Grégory PINON (LOMC) ====

=== Two Phase Flow - J. Leparoux, SAFRAN & J. Carmona, CORIA ===

==== TP1 - Towards very small contact angles in Nucleate boiling ====

Participants: Henri Lam (LEGI), Mohammad Umair (LEGI), Manuel Bernard (LEGI), Robin Barbera (LEGI) and Giovanni Ghigliotti (LPSC)

The boiling solver (BOI) was not able to accurately impose a contact angle (angle formed by the two-phase interface on the wall) at values lower than 30°. This angle is needed when simulating nucleate boiling. A similar limitation in contact angle value was applying to the spray (SPS) solver. A modified version of the level set reinitialization has been implemented during ECFD8, based on a different normal vector in the blind spot region around the contact line, vector now chosen to be a zero-order extension from outside the blind spot. This modification, that implied other modifications to the level set reinitialization in the blind spot, has been tested successfully on the spray solver (where no phase change occurs). Then, this new reinitialisation has been tested in the boiling solver for nucleate boiling, with great improvements. Now simulations of nucleate boiling at very small contact angle (10°) can be accurately performed.
In the meanwhile, the level set reinitialization algorithm has been streamlined and the computational cost greatly reduced, resolving a computational cost issue that appeared when using the contact angle imposition both in the spray and boiling solvers, and that hampered its use in industrial configurations.

==== TP2 - Modeling spray-film interactions ====

Participants: Nicolas Gasnier (EM2C-SafranTech), Julien Leparoux (SafranTech), Mehdi Helal (CORIA-SafranTech) and Julien Carmona (CORIA)

The numerical simulation of sheared liquid films over walls with conventional high-fidelity methods requires fine meshes to depict accurately the dynamics of the fluids, as well as the gas-liquid interactions at the phase interface. In the context of thin film flows (with a thickness h ~ 1.0E-4 m), the spatial resolution required to ensure the validity of the simulations can be computationally prohibitive. To tackle this issue, a reduced-order model of thin film dynamics based on the Saint Venant equations has been implemented in the YALES2 platform over the last months. This numerical model is able to reproduce accurately the dynamics of strongly sheared film flows over partially wetted surfaces, and to take into account capillary effects. The objective of the ECFD8 was to couple the thin film framework with the pre-existing multiphase methods of the YALES2 platform: the Eulerian multiphase solver - based on the ACLS -, and the Lagrangian solver. This coupling was performed in three steps: first, a numerical method was designed to convert impinging Lagrangian droplets into source terms for the thin film model. Then, a phenomenological model was implemented to depict the atomization of films at sharp edges under the action of a high speed gas flow. The liquid film is converted into Lagragian droplets, whose dimensions are computed based on the PAMELA model. Finally, a numerical method has been designed to convert Eulerian droplets into source terms for the film model. This method is based on a transfer of the liquid properties from the nodes to the boundaries by using a set of fictitious particles. It ensures conservation of the liquid mass, and it is more robust than the first attempts that had been made during ECFD7.

==== TP3 - High-fidelity two-phase flow simulations of the purge of a fuel feed line ====

Participants: Thomas LAROCHE (Safran HE), Romain JANODET (Safran AE), Julien Leparoux (Safran Tech) and Melody Cailler (Safran Tech)

During the second week of the ECFD8, the fuel feed line purge process has been numerically investigated. In the context of aeronautical engines, the fuel feed line—carrying the fuel from the fuel tank to the injectors within the combustion chamber—needs to be purged at engine shutdown. This is intended to prevent fuel stagnation near hot metal parts, which could lead to coke formation and therefore decrease engine performance. Since this complex phenomenon is mainly driven by two-phase flow physics, the spray solver (SPS) of the YALES2 library has been considered in order to understand the physics of such process. The numerical setup was first converged on a simplified test case: the possibility of driving the flow dynamics with inlet and outlet pressure conditions was tested beforehand on a single-phase, incompressible case, and then on a two-phase flow problem. The setup has then been successfully applied to an industrial configuration: a pressure-swirl injector connected to a reduced portion of the fuel feed line. Due to the large scale of the domain, the interface resolution was set to 50μm, which is intentionally coarse for such problem. This initial computation successfully ran up to 3ms of physical time during the workshop, proving YALES2's capability to model the fuel purge. The computation is to be continued and analyzed further even after the workshop.

==== TP4 - Volume of Fluid solver in YALES2 ====

Participants: Léa Voivenel (CORIA), Julien Carmona (CORIA), Mehdi Helal (CORIA), Pierre Portais (CORIA), Julien Leparoux (Safran Tech), Mélody Cailler (Safran Tech) and Nicolas Gasnier (EM2C / Safran Tech)

==== TP5 - Implement a local operator to distribute the solid volume of a particle over multiple cells ====

Participants: Théo Ndereyimana (Université de Sherbrooke), Stéphane Moreau (Université de Sherbrooke)

In the CFD-DEM, the cell size is required to be larger than the particle size for stability condition and keep feasible solid volume fraction. However, some applications require a cell size smaller than the particle. During this 8th edition of the ECFD, the use of operators to distribute the particle volume over multiple cells, ensuring a feasible solid volume particle has been tested on a fluidized bed configuration. The main operators tested (gather-scatter filter and gaussian filter) showed a tendency to blur the void structure interfaces. The equivalence of the gaussian filter of bandwidth <math>b</math> and the resolution of a diffusion equation over a pseudo-time <math>T=b^2/4</math> has been verified.
One anisotropic diffusion constant has been tested and shows a possibility to adress the sharpness requirements.

Another objective was to develop a post-processing tool to detect and track the void structures (bubbles) in a fluidized bed. Based on previous work from J. Carmona, a tool to track the bubbles has been initiated.

==== TP6 - Complex thermodynamics in sloshing tanks ====

Participants: C. Merlin (AGS), D. Fouquet (CORIA), V. Moureau (CORIA), J. Carmona (CORIA) and G. Lartigue (CORIA)

During this ECFD, the phase change solver was improved. After introduction of conservative transport for enthalpy and energy, a new framework for multi fluid two phase flow was used that relies on a new reinitialization of the conservative level set, the transport of discontinuous scalars like temperature, energy or enthalpy and the transport of weight to ensure consistency. The pressure loop was improved to ensure mass and energy conservation while meeting the low Mach hypothesis. Many 1D test cases were performed with only the gas phase and two phase flow to validate the pressure loop. The new framework for the transport of discontinuous scalar was first derived for temperature with constant properties in each phase. A first step was to extend it to sensible enthalpy with ideal gas behavior for the gas phase. Then, the framework was extended to NIST tabulated gas & liquid with ideal or real gas behavior. After some 2D cases, a tank pressurization test was investigated as well as a sloshing test without phase change (just the pressure drop due to the increase of the thermal exchange between the liquid and the gas phase).

=== Combustion - Y. Bechane, CORIA & S. Dillon, SAFRAN & K. Bioche, CORIA ===

==== C1 - LES of the thermal degradation of a composite material ====
Participants: A. Grenouilloux (ONERA), K. Bioche (CORIA), N. Dellinger (ONERA) and R. Letournel (SafranTech)

The FIRE test bed is an experimental air-propane burner operated by ONERA. It is dedicated to the study of the thermal degradation of composite materials. This project concerned the implementation of a three-solver coupling methodology to simulate the dynamics of the impinging flame. The methodology considered is based on the coupling between the variable density solver (VDS) and the radiative solver (RDS) of the massively parallel library YALES2 and the solver dedicated to the degradation of composite materials, MoDeTheC, developed by ONERA. Given the typical test times of the order of tens of seconds, a methodology based on 2D axisymmetric calculations was considered. Various tests were performed to determine the optimal coupling frequency between solvers. Cases dedicated to the injection of pyrolysis gasses were set up, with the aim of simulating the auto-ignition phenomenon. Comparisons with experimental data are presented.

==== C2 - Flame stabilization by NRP plasma discharge ====

==== C3 - Extending and validating a generalized formalism of virtual chemistry ====

==== C4 - Turbulent combustion model for NOx prediction ====

==== C5 - Towards 3D simulation of detonation combustion ====

==== C6 - Flame stabilitity of flame-holders within reheat conditions ====

Participants : N. Detomaso (Safran AE), R. Janodet (Safran AE), L. Carbajal (Safran AE)

The mechanisms of flame stabilisation in a flame-holder are still not completely understood. This is particularly true for reheat conditions : the hot viciated gases at inlet, highly compressible flows, and a strong liquid/gas coupling make the flame stability hard to predict. During the ECFD8, simplified configurations of flame-holders within reheat conditions were analysed. After some try and error simulations due to simplifications of the flame-holders, cases leading to flame stability (or not) were identified. Post-processing tools were developped in order to recover criteria relevant to the flame stability. This step marks the beginning of a systematic mapping of dimensionless characteristic times ratios, and a comparison with integral quantities.

==== C7 - Thermal radiation in oxyflames ====

==== C8 - A first step toward hybrid CPU / GPU for reactive flow in YALES2 ====

Participants: M. Laignel (CORIA), G. Lartigue (CORIA), K. Bioche (CORIA) and V. Moureau (CORIA)

In numerical simulations of reacting flows, one of the most computationally intensive tasks is the evaluation of source terms resulting from chemical reactions in the species transport equations. This step can account for up to 90% of the total simulation cost , depending on the complexity of the kinetic mechanism involved. To reduce this cost, various techniques such as mechanism reduction, virtual chemistry, etc. have been explored. However, the emergence of GPUs as powerful accelerators offers a promising alternative by providing massive parallelism. Despite their potential, GPUs often require significant adaptation of CPU-based codes. This project aims to address this challenge by taking a first step towards a hybrid CPU/GPU framework for reactive flow simulations. Specifically, the focus is on coupling Y2 with the updated version of the stiff time integration solver (CVODE), which is compatible with GPU (CUDA, HIP, OpenMP). The ultimate goal is to establish a foundation for hybrid computations by implementing and testing the updated solver on simplified test cases.

==== C9 - Soots numerical modeling ====

==== C10 - TECERACT : Tabulated chemistry generator for aeronautical combustion ====

==== C11 - Exploring efficient tabulation strategies for detailed chemistry ====

==== C12 - Dynamic sub-grid-scale modelling of multi-regime flame wrinkling ====

==== C13 - LES of a semi-industrial burner using a non-adiabatic virtual chemical scheme ====

=== User Experience & Data - L. Korzeczek, GDTECH ===

==== U1 - Low-fidelity (RANS) rotor/stator simulations, application to Kaplan Turbine - Y. Lakrifi, G. Balarac (LEGI), R. Mercier (SAFRAN), V. Moureau (CORIA) ====

==== U2 - Coupling PyTorch/YALES2, combustion cartesian look-up tables - J. Leparoux, N. Treleaven, S. Dillon (SAFRAN), K. Bioche, G. Lartigue (CORIA) ====

Participants: Julien Leparoux (Safran Tech), Kévin Bioche (CORIA), Ghislain Lartigue (CORIA), Nicholas Treleaven (Safran Tech)

Neural Networks offer a promising alternative to Cartesian look-up tables for combustion simulations, reducing memory footprint. In this project, we investigated how to integrate an NN model for real-time inference in the YALES2 platform, exploring two approaches: a Python interface and a Fortran Torch binding (using FTorch[https://github.com/Cambridge-ICCS/FTorch]). We validated that the model remains accurate when embedded online and identified improvements for robustness. Inference costs were evaluated on a Mac M3 and the Austral cluster, revealing a strong dependency on data volume. To optimize efficiency, we propose grouping cells at the processor level.

==== U3 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 - L. Korzeczek, S. Meynet (GDTECH), J. Leparoux, M. Cailler (SAFRAN) ====

Ecfd:ecfd 8th edition

2025-02-12T14:04:14Z

Benard: /* T2 - Dynamic Smagorinsky in Dorothy */

{{DISPLAYTITLE: ECFD workshop, 8th edition, 2025}}

== Description ==

* Event from '''27th of January to 7th of February 2025'''
* Location: [https://www.sport-normandie.fr/le-centre/le-site-de-houlgate Centre Sportif de Normandie], Houlgate, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* Participants from academics, HPC center/experts and industry are welcome
* The number of participants is limited to 68.

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes
* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Voivenel (CORIA).

[[File:ecfd8.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_8th_edition]]

[[File:Acknowledgments_ecfd8.png|text-bottom|600px]]

== News ==

* 23/10/2024: First announcement of the '''8th Extreme CFD Workshop & Hackathon''' !
* 22/11/2024: Deadline to submit your project

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

To come...

== Projects ==

=== Hackathon GENCI - P. Begou, LEGI ===
This ECFD8 GENCI Hackathon was a rich event, involving 4 differents CFD codes (AVBP, ParaDIGM, SONICS and YALES2) using various paradigms (C++/cuda/hip, Fortran/OpenMP/OpenACC) with several SDKs (AMD, Cray/HPE, Nvidia, Gnu) on a large range of GPU architectures (Nvidia A100, GH100, AMD instinct Mi210, Mi250, Mi300). This two-week event benefited from a high level support from three HPC mentors, two on-site from AMD (J. Noudohouenou and A. Tsetoglou) and one remote from CINES (M. Boudaoud).

==== H1 - ParaDIGM and SONICS on GPU, B. Maugars, G. Staffelbach, R.Cazalbou and B. Michel (ONERA)====

==== H2 - AVBP GPU offloading based on OpenMP, M.Ghenai, L. Legaux and A. Dauptain (CERFACS) ====

This hackathon provided a valuable opportunity to work on GPU offloading for AVBP. In the past, significant efforts were made to offload the entire AVBP code to GPUs. OpenACC was the primary strategy chosen, mainly due to access to NVIDIA's support, along with the availability of both software and hardware. This approach achieved good scalability performance.
Recently, with the deployment of new supercomputers like ADASTRA at CINES, some issues have emerged when running AVBP on AMD GPUs, including both MI250 and MI300. The closed-source nature of the Cray environment has also prevented CERFACS from deploying AVBP on local MI210 GPUs.
This hackathon was a great opportunity to address these challenges by exploring a new approach using OpenMP. An automatic translation tool was used to convert approximately 2,700 OpenACC directives to OpenMP, with each directive manually verified and fine-tuned afterward. AVBP with OpenMP had already been tested on NVIDIA GPUs, and during this hackathon, the focus was on extending support to AMD GPUs.
Two compilers were used: Cray and the newly released AFAR from AMD. With the support of AMD and CINES, a working environment for compiling AVBP was set up, and performance-related issues were identified. Additionally, two mini-apps were used for benchmarking. One of them achieved a 2.5× speedup when compiled with AFAR compared to Cray.
The next steps involve adapting the code to address necessary modifications, such as fixing issues related to Fortran indirections, and continuing performance evaluations with mini-apps. Further comparisons will be conducted using both compilers against results obtained with NVIDIA’s NVHPC.

==== H3 - YALES2 GPU from OpenACC to OpenMP, P. Bégou (LEGI), V. Moureau, G. Lartigue (CORIA) and R. Dubois (IMAG) ====
This Hackathon focuses on running Yales2 code on AMD Instinct Mi250 and Mi300 GPUs of the Adastra supercomputer (CINES).
Previously, a first solver in the Yales2 CFD code was successfully ported on the GPU accelerators of the Jean-Zay supercomputer (IDRIS) using Nvidia SDK but difficulties remain on Adastra AMD GPUs, mainly related to the available development tools. High compilation time and the impossibility to use debug flags at compile time as soon as OpenACC is enabled are a real challenge when tracking errors. The current project is to evaluate a freshly deployed version (at the begining of the workshop) of the AMD Fortran compiler. This requires moving to OpenMP paradigm, starting from scratch since the OpenACC branch has largely diverged from the master one while tracking spurious remaining bugs.
If the AMD compiler is able to build the cpu version of Yales2 "out of the box" (wich is not the case for Cray Fortran), the compilation time for each file is significantly higher. However, setting up a 2 stages dynamic compilation process allows for high parallelism that is not possible with Cray Fortran 18 and the library build time drops from nearly 2 hours (Cray Fortran 18) to 17 minutes (Amd Fortran compiler).
Large kernels have been ported from OpenACC to OpenMP, raising some difficulties when offloading intrinsics functions or using strutures attributes in kernels loops. These limitations were also known in the previous OpenACC work. The goal was mainly to check the correctness of the results. The offloading of the complex data structure of Yales2 code was then investigated. Here again some limitations of the "young" compiler were discovered and workarounds were implemented. Several reproducers were built during this ECFD8 and provided to developpers by the 2 on-site AMD engineers.
Preliminary tests on micro-applications show good performances of the generated binaries proving that this compiler could be a serious alternative on AMD GPUs and the goal is now to focus on this SDK in an OpenMP strategy while checking the portablility of this new implementation in Nvidia, Cray/HPE (and Gnu ?) environments.

=== Mesh adaptation - A. Grenouilloux, ONERA & G. Balarac, LEGI ===

==== M2 - Enhancement of mesh adaptation algorithms, B. Maugars (ONERA), B. Andrieu (ONERA), C. Benazet (ONERA), N. Dellinger (ONERA), G. Janodet (ONERA), G. Staffelbach (ONERA) ====
Mesh adaptation has become a crucial tool in order to automate industrial numerical simulations. ECDF7 allowed us to investigate the refine and EGADS libraries as tools for parallel mesh generation and adaptation using CAD as a geometric support. Since then, we fortified the workflow but some of our targeted industrial applications such as turbomachinery involve periodic boundary conditions. To manage these cases, the mesh generation and adaptation procedures must maintain matching periodic boundaries.
During ECFD8, we addressed multiple topics : periodic mesh generation from CAD model in EGADS, parallel and periodic metric gradation in ParaDiGM, making our parallel remeshing algorithm more generic to support non-manifold and periodic meshes. All these topics were covered, but there is still some work to be done to fully manage periodic mesh adaptation.
Non-manifold mesh adaptation was applied to changing mesh topologies. Here, an internal surface was described by a level-set and deformed up to hole creation to mimic ablation.

==== M3 - Development and definition of a new Automatic Mesh Convergence (AMC) driver for automating static mesh convergence in YALES2 (C. Papagiannis (LEGI/INRIA), G. Balarac (LEGI), O. LeMaître (INRIA), P.M. Congedo (INRIA)) ====
Static mesh adaptation requires re-adapting an existing (usually coarse) mesh, to conform to some quality metric that satisfies some optimality criterion. This involves statistics (Mean and RMS fields) that need to be adequately converged. We proposed and developed a novel AMC strategy that can converge an initial coarse mesh to an adequately refined one by taking into account the quality of the calculated statistics before each adaptation. Specifically, so far, the moment (in terms of integrating the LES equations) at which to launch the adaptation was unknown and had to be set apriori as a parameter of the AMC, dependent on the flow physics and therefore on the case at hand. Our method is automatically performing adaptations when the quality of the volume averaged mean field estimate (characterized by its RMSE) reaches the levels of the bias error that this field has, due to the mesh being far from the desired one (dictated by the quality criterion). This was achieved through drawing inspiration from a theoretical model that treats the AMC as a "conditionally" contractive mapping with a fixed point.

=== Numerics - M. Bernard, LEGI & G. Lartigue, CORIA ===

==== N1 - Traction open boundary condition ====

==== N2 - Treatment of Inlet conditions in High-Order solver. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====
In the context of node-centered Finite Volumes Method, spacial accuracy of a numerical scheme depends on ability to evaluate accurately fluxes through interface of each control volume (CV). Such accurate evaluation is not straightforward, especially when dealing with distorted grids. This project follows the work of [1] where fluxes use pointwise quantities, which are reconstructed from integrated quantities advanced in time. During the previous edition of the ECFD, a new data structure has been developed to store data at location of the boundary conditions facelets, with application to wall boundary conditions. During this 8th edition of the ECFD, we used the same data structure, but dedicated to the treatment of inlet conditions.
The inlet condition is then either imposed directly at facelets center, or at nodes position them extrapolated to facelets center by use of Taylor expansion. For this later solution, high-order treatment requires the successive derivatives to be computed in the plane of the boundary condition. This is not done yet, leading for the moment to low accuracy results but the framework is ready for upcoming implementation.

[1] ''A framework to perform high-order deconvolution for finite-volume method on simplicial meshes, , Bernard et. al., IJNMF 2020''

==== N3 - Conservative mesh-to-mesh interpolation. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====

Mesh to mesh interpolations occur quite often in CFD simulations : in the context of adaptative mesh convergence or in the case of dynamic mesh adaptation for for example.
Quality of the solution on the destination grid will depend on the characteristics of the interpolation method.
In this project, we did not focus on accuracy of the interpolation method but rather on conservativity characteristics.
A conservative interpolation ensures that the integral of the data on the source grid is exactly retrieved on the destination grid.
This property is highly interesting when dealing with scalar quantities or phase indicators, whose values should remained bounded.
In the context of nodes centered Finite Volume schemes, the methodology we used consists in (i) reconstructing element quantity from average nodal quantities on source grid.
Then, for a cell of the destination mesh, (ii) computing the geometrical intersection between cells of source and destination meshes to evaluate to evaluate the rate of quantities they.
Eventually, (iii) redistributing the solution from elements to control volumes of the destination mesh.
The overall process is fully conservative as it is based on geometrical intersection of locally integrated quantities.
The methodology as been implemented and tested on a few basic configurations and the conservativity is retrieved.

==== N4 - Determination of timestep in semi-implicit solver. T. Berthelon (LEGI), G. Balarac (LEGI), H. Lam (LEGI), M. El Moatamid (CORIA) ====
In order to reduce the computation time associated with incompressible LES simulations, an implicit time integration, based on BDF schemes, has been developed within the ICS solver. This integration eliminates the stability constraints associated with explicit schemes, and therefore opens up the question of the appropriate choice of time step.
In parallel, recent work has been carried out on meshing criteria in LES. The strategy in place consists of adapting the mesh by distinguishing two zones:
- "DNS" zones, where the meshing criterion is based on an estimate of the adimensioned spatial error.
- "LES" zones, where the meshing criterion is based on Kolmorogov theory.
During this project, the spatial criteria were extended to include temporal criteria. In the "DNS" zones, the time step is chosen using an estimate of the temporal error of the BDF scheme judiciously scaled to match the spatial error. In the "LES" zones, the time step is chosen using a scaling law associated with fully developed turbulence.
The new time step selection strategy has been tested on the case of a turbulent jet and leads to an accuracy equivalent to the explicit case while reducing the simulation return time by a factor of nearly 3.

Another aspect of this project was to integrate certain implicit temporal schemes (C-N and SDIRK) recently developed by Mr. El Moatamid into the incompressible solver.

==== N5 - Local timestep. T. Berthelon (LEGI), M. Bernard (LEGI), G. Balarac (LEGI) ====
RANS modelling has recently been developed within the YALES2 library. With this modeling strategy, the objective is to reach as quick as possible a steady state.
During this project, we investigate the use of a local time step to reduce the time to solution of steady computation in the incompressible solver.
This implies solving a variable-coefficient Poisson equation. Encouraging results were obtained in the simple case of "Couette plan" flow artificially constrained by a mesh variation. In fact, the use of local time-step reduce drastically the time to solution on this configuration. This method needs to be tested on real RANS case.

==== N6 - Distributed version of DOROTHY ====

==== N7 - Implicit time advancement for low-Reynolds number flows with particles. S. Mendez, C. Raveleau (IMAG), M. El Moatamid, V. Moureau (CORIA) ====
IMAG runs numerous simulations of red blood cells under flow. Those simulations are at low Reynolds number (0.001 to 1.0, typically). Splitting of the time advancement is used to treat the diffusion terms implicitly, albeit with an important numerical cost: implicit diffusion is 50 to 60% of the computational cost. Recently, M. El Moatamid implemented a genral framework to deal with implicit time advancement for scalars. In this project, the general method has been transposed to the advancement of the velocity field in the ICS and RBC solvers of YALES2/YALES2BIO. This enables testing various linear solvers (GMRES based). However, such solvers do not decrease the CPU time compared to the existing implementation. However, while working on this, it was identified that residual recycling was not activated in the current implementation of the implicit diffusion. This sped up the implicit diffusion cost by 35%, for a total gain of 20% for the computation. In addition to this achievement, moving to the framework coded by Moncef will have other beneficial side effects: we anticipate simplifying the implementation, with an easier merging between YALES2BIO and YALES2. The method will also be implemented in the electrosatic solver, for which the Poisson problem should benefit from the new GMRES-based solvers. In addition, this project highlights the importance of improving the treatment of stiff source terms in the red blood cells simulations, to be able to overcome the current limitation in time step due to those term and have a chance to benefit from higher-order time schemes, efficient at high Fourier numbers.

==== N8 - Boundary Element Method in YALES2. B. Thibaud, S. Mendez (IMAG), G. Lartigue, P. Benard (CORIA), F. Nicoud (IMAG) ====
In the context of microfluidic systems for diagnosis, the Boundary Element Method alows to solve linear PDE such as electrostatic or Stokes. With well chosen kernel functions and the divergence theorem, this method allows to write on the boundary condition only the initial volumic problem. This project aimed at exploring the feasibility of the BEM in the context of massively parallel unstructured solver like YALES2 by developping a Julia demonstrator. The first step have been to implement and validate the method on simple configurations for the Laplace's equation. Only Neumann problems were considered (Dirichlet boundary conditions imposed). In a second time, the multi-domain approach has been identified to be the most suited in the framework of YALES. The inner domain is partitioned on each processor, each having a part of the physical boundary and interfaces between them. Every processor solve its own boundary problem and a parallel Dirichlet-Dirichlet fixed-point is used to converge the interface problem on the all domain. Applied to the ring case, with one interface, we managed to reproduce the linear convergence of the P-DD method.

=== Turbulence - L. Voivenel, CORIA & P. Bénard, CORIA ===

==== T1 - FSI-1D strategy for internal flows====

Many applications developed at Safran Aerosystem are based on internal turbulent flows coupled to a moving body. 2 cases were studied during this ECFD:

'''Case 1 (Incompressible flow)''': Translation of a piston subjected to a pressure difference in a pipe.

The challenges of this case are twofold: the small gap between the piston and the pipe and the large pressure gradient across the piston (>100bar). During the 1st week of ECFD, the CLIB (Conservative Lagrangian Immersed Boundary) solver was tested on this case. The study showed that the solver was unable to ensure the impermeability of the solid under these pressure conditions. In the rest of the study, a porous medium following Darcy's law will be added to the penalty force of the immersed solid to fully satisfy the impermeability of the piston.

'''Case 2 (Compressible flow)''': Rotation of a butterfly in a discharge vane.

The coupling between the ECS (Explicit Compressible Solver) and ALE (Arbitrary Lagrangian Solver) solvers having recently been implemented, this strategy was tested to model the opening of the valve by rotation of the butterfly. The challenge here lies in the small gap between the bottom of the butterfly and the vane casing. To limit the simulation cost, the gap is meshed with 1 element. In this case, MMG succeeded in adapting the mesh up to a critical angle at which the gap becomes too small (Work In Progress).

==== T2 - Dynamic Smagorinsky in Dorothy - Maëlenn ROPERCH (LOMC), Grégory PINON (LOMC) ====

==== T3 - Turbulence injection strategy for compressible flows - Patrick TENE HEDJE (UMONS), Laurent Bricteur (UMONS), Yacine BECHANE (CORIA), Pierre BENARD (CORIA) ====

==== T4 - Improve wind farm modeling and simulation workflow ====
The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine. Still, applying this method to investigate a wind farm flow can be challenging, both in terms of computational cost and simulation setup. For instance, an inadequate management of the wind turbine individual modeling parts in a HPC context can end up being the main bottleneck of the simulation. From another perspective, a wind farm is usually composed of more than 50 wind turbines. For such a case, setting up all YALES2 required inputs manually can be very tedious and error-prone. This project thus mainly aimed to optimize the YALES2 ALM implementation and the user experience around it. Additionally, a cost-effective alternative to the ALM when modeling wind farm flows, namely the Rotating Actuator Disk Method (ADM-R), has been implemented for further investigations.

'''WP1''': Improve Actuator set rotor modelling
* Parallel processing of the ''actuator sets'' used to model the wind turbines
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified during the 5th iteration of the ECFD: (i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges. (ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications. During this workshop edition, the work focused on the re-synchronisation of the algorithm steps by allowing some packing an unpacking of the object to allow the transfert of the object inbetween workers. This is of major importance to enable load balancing and mesh adaptation during the temporal loop. This work required the refactoring of the involved oject structures.

* Rotating Actuator Disk Method (ADM-R):
According to the usual guidelines, the mesh requirements of the ALM, to profit entirely from its reachable accuracy, can be difficult to achieve or even unaffordable when simulating a wind farm flow, especially from the industrial point of view. Alternatives are available in the literature for this kind of application. Likely, the methods from the Actuator Disk family are the most prominent ones. Several kinds of implementation exist, which mostly differ by their capability to include the wake rotation. During the workshop, a new method from the Rotating Actuator Disk kind has been implemented and underwent an early validation on a single turbine setup. Applications to wind farm flows will follow.

'''WP2''': Improve tools User Experience

Three Python tools have been developed or improved :
*The first tool is the wind farm previsualisation tool, 'y2_wind_previsualization', which is used before the calculation run. This provides an interactive HTML interface for viewing global data for each turbine on the farm (position, hub height, yaw angle, etc.). The tool traces all of these via the parsing of the input file.
* The second tool is for duplicating rotor templates for a wind farm (`y2_wind_duplication`). This tool was developed in the previous ECFD, but this time it has been refactored and incorporated into the y2tools package.
* The third and final tool is a post-processing tool for the temporal processing of global wind turbine simulation metrics (Thrust, Power, etc.), `y2_post_wind`. This tool generates an interactive HTML plot of time-dependent global quantities.

==== T5 - Improve atmospheric inflow turbulence ====
Atmospheric inflow turbulence is generated using the precursor database method. A half-channel flow driven by a pressure gradient is used to obtain the inflow which is used as inlet boundary condition for the wind turbine simulation domain. This project aimed to improve the whole methodology, from generation to injection.

* WP1: Improve inflow generation
Anand: pressure controller

* WP2: Improve injection methodology (method A)
The previous workflow used plane probes in the ASCII format to sample the flow. The COWIT2 toolbox was used to convert the file into turbulence box (.man format). While functioning, this methodology had two major flaws. First the probe files are heavy ~O(10Go). Second, the method requires a lot of human effort, allowing numerous sources of errors.
During this workshop, a new methodology has been developed. First, the probes are generated using the HDF5 format (now available for all probe types), leading to lighter file ~O(1Go). Second, Y2_tools is used to read HDF5 format (working for probes and temporals). HDF5 file is then converted into a Look-up Table. Finally, the Look-up Table is read directly by YALES2 as a boundary conditions.

* WP3: Improve injection methodology (method B)
Even though improvements achieved in WP2 prove to be very handy while removing many potential human errors, injecting a turbulent inflow through wind boxes ('offline' precursor approach) can sometimes remain cumbersome for several reasons: (1) no periodicity is enforced in the streamwise direction of those boxes, (2) potential high memory consumption, and (3) the boxes need to be moved to other cores whenever a mesh adaptation occurs. An alternative consists in co-simulating the precursor flow and the flow of interest (refered as the 'successor' simulation) at the same time ('online' precursor approach). The inlet boundary condition for the successor flow is then obtained by mapping the outflow of the precursor domain. During the workshop, some work has been initiated to implement this kind of coupling using the CWIPI library, for which YALES2 provides already an interface.

==== T6 - FSI model in Dorothy ====

=== Two Phase Flow - J. Leparoux, SAFRAN & J. Carmona, CORIA ===

==== TP1 - Towards very small contact angles in Nucleate boiling ====

Participants: Henri Lam (LEGI), Mohammad Umair (LEGI), Manuel Bernard (LEGI), Robin Barbera (LEGI) and Giovanni Ghigliotti (LPSC)

The boiling solver (BOI) was not able to accurately impose a contact angle (angle formed by the two-phase interface on the wall) at values lower than 30°. This angle is needed when simulating nucleate boiling. A similar limitation in contact angle value was applying to the spray (SPS) solver. A modified version of the level set reinitialization has been implemented during ECFD8, based on a different normal vector in the blind spot region around the contact line, vector now chosen to be a zero-order extension from outside the blind spot. This modification, that implied other modifications to the level set reinitialization in the blind spot, has been tested successfully on the spray solver (where no phase change occurs). Then, this new reinitialisation has been tested in the boiling solver for nucleate boiling, with great improvements. Now simulations of nucleate boiling at very small contact angle (10°) can be accurately performed.
In the meanwhile, the level set reinitialization algorithm has been streamlined and the computational cost greatly reduced, resolving a computational cost issue that appeared when using the contact angle imposition both in the spray and boiling solvers, and that hampered its use in industrial configurations.

==== TP2 - Modeling spray-film interactions ====

Participants: Nicolas Gasnier (EM2C-SafranTech), Julien Leparoux (SafranTech), Mehdi Helal (CORIA-SafranTech) and Julien Carmona (CORIA)

The numerical simulation of sheared liquid films over walls with conventional high-fidelity methods requires fine meshes to depict accurately the dynamics of the fluids, as well as the gas-liquid interactions at the phase interface. In the context of thin film flows (with a thickness h ~ 1.0E-4 m), the spatial resolution required to ensure the validity of the simulations can be computationally prohibitive. To tackle this issue, a reduced-order model of thin film dynamics based on the Saint Venant equations has been implemented in the YALES2 platform over the last months. This numerical model is able to reproduce accurately the dynamics of strongly sheared film flows over partially wetted surfaces, and to take into account capillary effects. The objective of the ECFD8 was to couple the thin film framework with the pre-existing multiphase methods of the YALES2 platform: the Eulerian multiphase solver - based on the ACLS -, and the Lagrangian solver. This coupling was performed in three steps: first, a numerical method was designed to convert impinging Lagrangian droplets into source terms for the thin film model. Then, a phenomenological model was implemented to depict the atomization of films at sharp edges under the action of a high speed gas flow. The liquid film is converted into Lagragian droplets, whose dimensions are computed based on the PAMELA model. Finally, a numerical method has been designed to convert Eulerian droplets into source terms for the film model. This method is based on a transfer of the liquid properties from the nodes to the boundaries by using a set of fictitious particles. It ensures conservation of the liquid mass, and it is more robust than the first attempts that had been made during ECFD7.

==== TP3 - High-fidelity two-phase flow simulations of the purge of a fuel feed line ====

Participants: Thomas LAROCHE (Safran HE), Romain JANODET (Safran AE), Julien Leparoux (Safran Tech) and Melody Cailler (Safran Tech)

During the second week of the ECFD8, the fuel feed line purge process has been numerically investigated. In the context of aeronautical engines, the fuel feed line—carrying the fuel from the fuel tank to the injectors within the combustion chamber—needs to be purged at engine shutdown. This is intended to prevent fuel stagnation near hot metal parts, which could lead to coke formation and therefore decrease engine performance. Since this complex phenomenon is mainly driven by two-phase flow physics, the spray solver (SPS) of the YALES2 library has been considered in order to understand the physics of such process. The numerical setup was first converged on a simplified test case: the possibility of driving the flow dynamics with inlet and outlet pressure conditions was tested beforehand on a single-phase, incompressible case, and then on a two-phase flow problem. The setup has then been successfully applied to an industrial configuration: a pressure-swirl injector connected to a reduced portion of the fuel feed line. Due to the large scale of the domain, the interface resolution was set to 50μm, which is intentionally coarse for such problem. This initial computation successfully ran up to 3ms of physical time during the workshop, proving YALES2's capability to model the fuel purge. The computation is to be continued and analyzed further even after the workshop.

==== TP4 - Volume of Fluid solver in YALES2 ====

Participants: Léa Voivenel (CORIA), Julien Carmona (CORIA), Mehdi Helal (CORIA), Pierre Portais (CORIA), Julien Leparoux (Safran Tech), Mélody Cailler (Safran Tech) and Nicolas Gasnier (EM2C / Safran Tech)

==== TP5 - Implement a local operator to distribute the solid volume of a particle over multiple cells ====

Participants: Théo Ndereyimana (Université de Sherbrooke), Stéphane Moreau (Université de Sherbrooke)

In the CFD-DEM, the cell size is required to be larger than the particle size for stability condition and keep feasible solid volume fraction. However, some applications require a cell size smaller than the particle. During this 8th edition of the ECFD, the use of operators to distribute the particle volume over multiple cells, ensuring a feasible solid volume particle has been tested on a fluidized bed configuration. The main operators tested (gather-scatter filter and gaussian filter) showed a tendency to blur the void structure interfaces. The equivalence of the gaussian filter of bandwidth <math>b</math> and the resolution of a diffusion equation over a pseudo-time <math>T=b^2/4</math> has been verified.
One anisotropic diffusion constant has been tested and shows a possibility to adress the sharpness requirements.

Another objective was to develop a post-processing tool to detect and track the void structures (bubbles) in a fluidized bed. Based on previous work from J. Carmona, a tool to track the bubbles has been initiated.

==== TP6 - Complex thermodynamics in sloshing tanks ====

Participants: C. Merlin (AGS), D. Fouquet (CORIA), V. Moureau (CORIA), J. Carmona (CORIA) and G. Lartigue (CORIA)

During this ECFD, the phase change solver was improved. After introduction of conservative transport for enthalpy and energy, a new framework for multi fluid two phase flow was used that relies on a new reinitialization of the conservative level set, the transport of discontinuous scalars like temperature, energy or enthalpy and the transport of weight to ensure consistency. The pressure loop was improved to ensure mass and energy conservation while meeting the low Mach hypothesis. Many 1D test cases were performed with only the gas phase and two phase flow to validate the pressure loop. The new framework for the transport of discontinuous scalar was first derived for temperature with constant properties in each phase. A first step was to extend it to sensible enthalpy with ideal gas behavior for the gas phase. Then, the framework was extended to NIST tabulated gas & liquid with ideal or real gas behavior. After some 2D cases, a tank pressurization test was investigated as well as a sloshing test without phase change (just the pressure drop due to the increase of the thermal exchange between the liquid and the gas phase).

=== Combustion - Y. Bechane, CORIA & S. Dillon, SAFRAN & K. Bioche, CORIA ===

==== C1 - LES of the thermal degradation of a composite material ====
Participants: A. Grenouilloux (ONERA), K. Bioche (CORIA), N. Dellinger (ONERA) and R. Letournel (SafranTech)

The FIRE test bed is an experimental air-propane burner operated by ONERA. It is dedicated to the study of the thermal degradation of composite materials. This project concerned the implementation of a three-solver coupling methodology to simulate the dynamics of the impinging flame. The methodology considered is based on the coupling between the variable density solver (VDS) and the radiative solver (RDS) of the massively parallel library YALES2 and the solver dedicated to the degradation of composite materials, MoDeTheC, developed by ONERA. Given the typical test times of the order of tens of seconds, a methodology based on 2D axisymmetric calculations was considered. Various tests were performed to determine the optimal coupling frequency between solvers. Cases dedicated to the injection of pyrolysis gasses were set up, with the aim of simulating the auto-ignition phenomenon. Comparisons with experimental data are presented.

==== C2 - Flame stabilization by NRP plasma discharge ====

==== C3 - Extending and validating a generalized formalism of virtual chemistry ====

==== C4 - Turbulent combustion model for NOx prediction ====

==== C5 - Towards 3D simulation of detonation combustion ====

==== C6 - Flame stabilitity of flame-holders within reheat conditions ====

Participants : N. Detomaso (Safran AE), R. Janodet (Safran AE), L. Carbajal (Safran AE)

The mechanisms of flame stabilisation in a flame-holder are still not completely understood. This is particularly true for reheat conditions : the hot viciated gases at inlet, highly compressible flows, and a strong liquid/gas coupling make the flame stability hard to predict. During the ECFD8, simplified configurations of flame-holders within reheat conditions were analysed. After some try and error simulations due to simplifications of the flame-holders, cases leading to flame stability (or not) were identified. Post-processing tools were developped in order to recover criteria relevant to the flame stability. This step marks the beginning of a systematic mapping of dimensionless characteristic times ratios, and a comparison with integral quantities.

==== C7 - Thermal radiation in oxyflames ====

==== C8 - A first step toward hybrid CPU / GPU for reactive flow in YALES2 ====

Participants: M. Laignel (CORIA), G. Lartigue (CORIA), K. Bioche (CORIA) and V. Moureau (CORIA)

In numerical simulations of reacting flows, one of the most computationally intensive tasks is the evaluation of source terms resulting from chemical reactions in the species transport equations. This step can account for up to 90% of the total simulation cost , depending on the complexity of the kinetic mechanism involved. To reduce this cost, various techniques such as mechanism reduction, virtual chemistry, etc. have been explored. However, the emergence of GPUs as powerful accelerators offers a promising alternative by providing massive parallelism. Despite their potential, GPUs often require significant adaptation of CPU-based codes. This project aims to address this challenge by taking a first step towards a hybrid CPU/GPU framework for reactive flow simulations. Specifically, the focus is on coupling Y2 with the updated version of the stiff time integration solver (CVODE), which is compatible with GPU (CUDA, HIP, OpenMP). The ultimate goal is to establish a foundation for hybrid computations by implementing and testing the updated solver on simplified test cases.

==== C9 - Soots numerical modeling ====

==== C10 - TECERACT : Tabulated chemistry generator for aeronautical combustion ====

==== C11 - Exploring efficient tabulation strategies for detailed chemistry ====

==== C12 - Dynamic sub-grid-scale modelling of multi-regime flame wrinkling ====

==== C13 - LES of a semi-industrial burner using a non-adiabatic virtual chemical scheme ====

=== User Experience & Data - L. Korzeczek, GDTECH ===

==== U1 - Low-fidelity (RANS) rotor/stator simulations, application to Kaplan Turbine - Y. Lakrifi, G. Balarac (LEGI), R. Mercier (SAFRAN), V. Moureau (CORIA) ====

==== U2 - Coupling PyTorch/YALES2, combustion cartesian look-up tables - J. Leparoux, N. Treleaven, S. Dillon (SAFRAN), K. Bioche, G. Lartigue (CORIA) ====

Participants: Julien Leparoux (Safran Tech), Kévin Bioche (CORIA), Ghislain Lartigue (CORIA), Nicholas Treleaven (Safran Tech)

Neural Networks offer a promising alternative to Cartesian look-up tables for combustion simulations, reducing memory footprint. In this project, we investigated how to integrate an NN model for real-time inference in the YALES2 platform, exploring two approaches: a Python interface and a Fortran Torch binding (using FTorch[https://github.com/Cambridge-ICCS/FTorch]). We validated that the model remains accurate when embedded online and identified improvements for robustness. Inference costs were evaluated on a Mac M3 and the Austral cluster, revealing a strong dependency on data volume. To optimize efficiency, we propose grouping cells at the processor level.

==== U3 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 - L. Korzeczek, S. Meynet (GDTECH), J. Leparoux, M. Cailler (SAFRAN) ====

Ecfd:ecfd 8th edition

2025-02-12T14:03:40Z

Benard: /* Turbulence - L. Voivenel, CORIA & P. Bénard, CORIA */

{{DISPLAYTITLE: ECFD workshop, 8th edition, 2025}}

== Description ==

* Event from '''27th of January to 7th of February 2025'''
* Location: [https://www.sport-normandie.fr/le-centre/le-site-de-houlgate Centre Sportif de Normandie], Houlgate, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* Participants from academics, HPC center/experts and industry are welcome
* The number of participants is limited to 68.

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes
* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Voivenel (CORIA).

[[File:ecfd8.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_8th_edition]]

[[File:Acknowledgments_ecfd8.png|text-bottom|600px]]

== News ==

* 23/10/2024: First announcement of the '''8th Extreme CFD Workshop & Hackathon''' !
* 22/11/2024: Deadline to submit your project

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

To come...

== Projects ==

=== Hackathon GENCI - P. Begou, LEGI ===
This ECFD8 GENCI Hackathon was a rich event, involving 4 differents CFD codes (AVBP, ParaDIGM, SONICS and YALES2) using various paradigms (C++/cuda/hip, Fortran/OpenMP/OpenACC) with several SDKs (AMD, Cray/HPE, Nvidia, Gnu) on a large range of GPU architectures (Nvidia A100, GH100, AMD instinct Mi210, Mi250, Mi300). This two-week event benefited from a high level support from three HPC mentors, two on-site from AMD (J. Noudohouenou and A. Tsetoglou) and one remote from CINES (M. Boudaoud).

==== H1 - ParaDIGM and SONICS on GPU, B. Maugars, G. Staffelbach, R.Cazalbou and B. Michel (ONERA)====

==== H2 - AVBP GPU offloading based on OpenMP, M.Ghenai, L. Legaux and A. Dauptain (CERFACS) ====

This hackathon provided a valuable opportunity to work on GPU offloading for AVBP. In the past, significant efforts were made to offload the entire AVBP code to GPUs. OpenACC was the primary strategy chosen, mainly due to access to NVIDIA's support, along with the availability of both software and hardware. This approach achieved good scalability performance.
Recently, with the deployment of new supercomputers like ADASTRA at CINES, some issues have emerged when running AVBP on AMD GPUs, including both MI250 and MI300. The closed-source nature of the Cray environment has also prevented CERFACS from deploying AVBP on local MI210 GPUs.
This hackathon was a great opportunity to address these challenges by exploring a new approach using OpenMP. An automatic translation tool was used to convert approximately 2,700 OpenACC directives to OpenMP, with each directive manually verified and fine-tuned afterward. AVBP with OpenMP had already been tested on NVIDIA GPUs, and during this hackathon, the focus was on extending support to AMD GPUs.
Two compilers were used: Cray and the newly released AFAR from AMD. With the support of AMD and CINES, a working environment for compiling AVBP was set up, and performance-related issues were identified. Additionally, two mini-apps were used for benchmarking. One of them achieved a 2.5× speedup when compiled with AFAR compared to Cray.
The next steps involve adapting the code to address necessary modifications, such as fixing issues related to Fortran indirections, and continuing performance evaluations with mini-apps. Further comparisons will be conducted using both compilers against results obtained with NVIDIA’s NVHPC.

==== H3 - YALES2 GPU from OpenACC to OpenMP, P. Bégou (LEGI), V. Moureau, G. Lartigue (CORIA) and R. Dubois (IMAG) ====
This Hackathon focuses on running Yales2 code on AMD Instinct Mi250 and Mi300 GPUs of the Adastra supercomputer (CINES).
Previously, a first solver in the Yales2 CFD code was successfully ported on the GPU accelerators of the Jean-Zay supercomputer (IDRIS) using Nvidia SDK but difficulties remain on Adastra AMD GPUs, mainly related to the available development tools. High compilation time and the impossibility to use debug flags at compile time as soon as OpenACC is enabled are a real challenge when tracking errors. The current project is to evaluate a freshly deployed version (at the begining of the workshop) of the AMD Fortran compiler. This requires moving to OpenMP paradigm, starting from scratch since the OpenACC branch has largely diverged from the master one while tracking spurious remaining bugs.
If the AMD compiler is able to build the cpu version of Yales2 "out of the box" (wich is not the case for Cray Fortran), the compilation time for each file is significantly higher. However, setting up a 2 stages dynamic compilation process allows for high parallelism that is not possible with Cray Fortran 18 and the library build time drops from nearly 2 hours (Cray Fortran 18) to 17 minutes (Amd Fortran compiler).
Large kernels have been ported from OpenACC to OpenMP, raising some difficulties when offloading intrinsics functions or using strutures attributes in kernels loops. These limitations were also known in the previous OpenACC work. The goal was mainly to check the correctness of the results. The offloading of the complex data structure of Yales2 code was then investigated. Here again some limitations of the "young" compiler were discovered and workarounds were implemented. Several reproducers were built during this ECFD8 and provided to developpers by the 2 on-site AMD engineers.
Preliminary tests on micro-applications show good performances of the generated binaries proving that this compiler could be a serious alternative on AMD GPUs and the goal is now to focus on this SDK in an OpenMP strategy while checking the portablility of this new implementation in Nvidia, Cray/HPE (and Gnu ?) environments.

=== Mesh adaptation - A. Grenouilloux, ONERA & G. Balarac, LEGI ===

==== M2 - Enhancement of mesh adaptation algorithms, B. Maugars (ONERA), B. Andrieu (ONERA), C. Benazet (ONERA), N. Dellinger (ONERA), G. Janodet (ONERA), G. Staffelbach (ONERA) ====
Mesh adaptation has become a crucial tool in order to automate industrial numerical simulations. ECDF7 allowed us to investigate the refine and EGADS libraries as tools for parallel mesh generation and adaptation using CAD as a geometric support. Since then, we fortified the workflow but some of our targeted industrial applications such as turbomachinery involve periodic boundary conditions. To manage these cases, the mesh generation and adaptation procedures must maintain matching periodic boundaries.
During ECFD8, we addressed multiple topics : periodic mesh generation from CAD model in EGADS, parallel and periodic metric gradation in ParaDiGM, making our parallel remeshing algorithm more generic to support non-manifold and periodic meshes. All these topics were covered, but there is still some work to be done to fully manage periodic mesh adaptation.
Non-manifold mesh adaptation was applied to changing mesh topologies. Here, an internal surface was described by a level-set and deformed up to hole creation to mimic ablation.

==== M3 - Development and definition of a new Automatic Mesh Convergence (AMC) driver for automating static mesh convergence in YALES2 (C. Papagiannis (LEGI/INRIA), G. Balarac (LEGI), O. LeMaître (INRIA), P.M. Congedo (INRIA)) ====
Static mesh adaptation requires re-adapting an existing (usually coarse) mesh, to conform to some quality metric that satisfies some optimality criterion. This involves statistics (Mean and RMS fields) that need to be adequately converged. We proposed and developed a novel AMC strategy that can converge an initial coarse mesh to an adequately refined one by taking into account the quality of the calculated statistics before each adaptation. Specifically, so far, the moment (in terms of integrating the LES equations) at which to launch the adaptation was unknown and had to be set apriori as a parameter of the AMC, dependent on the flow physics and therefore on the case at hand. Our method is automatically performing adaptations when the quality of the volume averaged mean field estimate (characterized by its RMSE) reaches the levels of the bias error that this field has, due to the mesh being far from the desired one (dictated by the quality criterion). This was achieved through drawing inspiration from a theoretical model that treats the AMC as a "conditionally" contractive mapping with a fixed point.

=== Numerics - M. Bernard, LEGI & G. Lartigue, CORIA ===

==== N1 - Traction open boundary condition ====

==== N2 - Treatment of Inlet conditions in High-Order solver. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====
In the context of node-centered Finite Volumes Method, spacial accuracy of a numerical scheme depends on ability to evaluate accurately fluxes through interface of each control volume (CV). Such accurate evaluation is not straightforward, especially when dealing with distorted grids. This project follows the work of [1] where fluxes use pointwise quantities, which are reconstructed from integrated quantities advanced in time. During the previous edition of the ECFD, a new data structure has been developed to store data at location of the boundary conditions facelets, with application to wall boundary conditions. During this 8th edition of the ECFD, we used the same data structure, but dedicated to the treatment of inlet conditions.
The inlet condition is then either imposed directly at facelets center, or at nodes position them extrapolated to facelets center by use of Taylor expansion. For this later solution, high-order treatment requires the successive derivatives to be computed in the plane of the boundary condition. This is not done yet, leading for the moment to low accuracy results but the framework is ready for upcoming implementation.

[1] ''A framework to perform high-order deconvolution for finite-volume method on simplicial meshes, , Bernard et. al., IJNMF 2020''

==== N3 - Conservative mesh-to-mesh interpolation. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====

Mesh to mesh interpolations occur quite often in CFD simulations : in the context of adaptative mesh convergence or in the case of dynamic mesh adaptation for for example.
Quality of the solution on the destination grid will depend on the characteristics of the interpolation method.
In this project, we did not focus on accuracy of the interpolation method but rather on conservativity characteristics.
A conservative interpolation ensures that the integral of the data on the source grid is exactly retrieved on the destination grid.
This property is highly interesting when dealing with scalar quantities or phase indicators, whose values should remained bounded.
In the context of nodes centered Finite Volume schemes, the methodology we used consists in (i) reconstructing element quantity from average nodal quantities on source grid.
Then, for a cell of the destination mesh, (ii) computing the geometrical intersection between cells of source and destination meshes to evaluate to evaluate the rate of quantities they.
Eventually, (iii) redistributing the solution from elements to control volumes of the destination mesh.
The overall process is fully conservative as it is based on geometrical intersection of locally integrated quantities.
The methodology as been implemented and tested on a few basic configurations and the conservativity is retrieved.

==== N4 - Determination of timestep in semi-implicit solver. T. Berthelon (LEGI), G. Balarac (LEGI), H. Lam (LEGI), M. El Moatamid (CORIA) ====
In order to reduce the computation time associated with incompressible LES simulations, an implicit time integration, based on BDF schemes, has been developed within the ICS solver. This integration eliminates the stability constraints associated with explicit schemes, and therefore opens up the question of the appropriate choice of time step.
In parallel, recent work has been carried out on meshing criteria in LES. The strategy in place consists of adapting the mesh by distinguishing two zones:
- "DNS" zones, where the meshing criterion is based on an estimate of the adimensioned spatial error.
- "LES" zones, where the meshing criterion is based on Kolmorogov theory.
During this project, the spatial criteria were extended to include temporal criteria. In the "DNS" zones, the time step is chosen using an estimate of the temporal error of the BDF scheme judiciously scaled to match the spatial error. In the "LES" zones, the time step is chosen using a scaling law associated with fully developed turbulence.
The new time step selection strategy has been tested on the case of a turbulent jet and leads to an accuracy equivalent to the explicit case while reducing the simulation return time by a factor of nearly 3.

Another aspect of this project was to integrate certain implicit temporal schemes (C-N and SDIRK) recently developed by Mr. El Moatamid into the incompressible solver.

==== N5 - Local timestep. T. Berthelon (LEGI), M. Bernard (LEGI), G. Balarac (LEGI) ====
RANS modelling has recently been developed within the YALES2 library. With this modeling strategy, the objective is to reach as quick as possible a steady state.
During this project, we investigate the use of a local time step to reduce the time to solution of steady computation in the incompressible solver.
This implies solving a variable-coefficient Poisson equation. Encouraging results were obtained in the simple case of "Couette plan" flow artificially constrained by a mesh variation. In fact, the use of local time-step reduce drastically the time to solution on this configuration. This method needs to be tested on real RANS case.

==== N6 - Distributed version of DOROTHY ====

==== N7 - Implicit time advancement for low-Reynolds number flows with particles. S. Mendez, C. Raveleau (IMAG), M. El Moatamid, V. Moureau (CORIA) ====
IMAG runs numerous simulations of red blood cells under flow. Those simulations are at low Reynolds number (0.001 to 1.0, typically). Splitting of the time advancement is used to treat the diffusion terms implicitly, albeit with an important numerical cost: implicit diffusion is 50 to 60% of the computational cost. Recently, M. El Moatamid implemented a genral framework to deal with implicit time advancement for scalars. In this project, the general method has been transposed to the advancement of the velocity field in the ICS and RBC solvers of YALES2/YALES2BIO. This enables testing various linear solvers (GMRES based). However, such solvers do not decrease the CPU time compared to the existing implementation. However, while working on this, it was identified that residual recycling was not activated in the current implementation of the implicit diffusion. This sped up the implicit diffusion cost by 35%, for a total gain of 20% for the computation. In addition to this achievement, moving to the framework coded by Moncef will have other beneficial side effects: we anticipate simplifying the implementation, with an easier merging between YALES2BIO and YALES2. The method will also be implemented in the electrosatic solver, for which the Poisson problem should benefit from the new GMRES-based solvers. In addition, this project highlights the importance of improving the treatment of stiff source terms in the red blood cells simulations, to be able to overcome the current limitation in time step due to those term and have a chance to benefit from higher-order time schemes, efficient at high Fourier numbers.

==== N8 - Boundary Element Method in YALES2. B. Thibaud, S. Mendez (IMAG), G. Lartigue, P. Benard (CORIA), F. Nicoud (IMAG) ====
In the context of microfluidic systems for diagnosis, the Boundary Element Method alows to solve linear PDE such as electrostatic or Stokes. With well chosen kernel functions and the divergence theorem, this method allows to write on the boundary condition only the initial volumic problem. This project aimed at exploring the feasibility of the BEM in the context of massively parallel unstructured solver like YALES2 by developping a Julia demonstrator. The first step have been to implement and validate the method on simple configurations for the Laplace's equation. Only Neumann problems were considered (Dirichlet boundary conditions imposed). In a second time, the multi-domain approach has been identified to be the most suited in the framework of YALES. The inner domain is partitioned on each processor, each having a part of the physical boundary and interfaces between them. Every processor solve its own boundary problem and a parallel Dirichlet-Dirichlet fixed-point is used to converge the interface problem on the all domain. Applied to the ring case, with one interface, we managed to reproduce the linear convergence of the P-DD method.

=== Turbulence - L. Voivenel, CORIA & P. Bénard, CORIA ===

==== T1 - FSI-1D strategy for internal flows====

Many applications developed at Safran Aerosystem are based on internal turbulent flows coupled to a moving body. 2 cases were studied during this ECFD:

'''Case 1 (Incompressible flow)''': Translation of a piston subjected to a pressure difference in a pipe.

The challenges of this case are twofold: the small gap between the piston and the pipe and the large pressure gradient across the piston (>100bar). During the 1st week of ECFD, the CLIB (Conservative Lagrangian Immersed Boundary) solver was tested on this case. The study showed that the solver was unable to ensure the impermeability of the solid under these pressure conditions. In the rest of the study, a porous medium following Darcy's law will be added to the penalty force of the immersed solid to fully satisfy the impermeability of the piston.

'''Case 2 (Compressible flow)''': Rotation of a butterfly in a discharge vane.

The coupling between the ECS (Explicit Compressible Solver) and ALE (Arbitrary Lagrangian Solver) solvers having recently been implemented, this strategy was tested to model the opening of the valve by rotation of the butterfly. The challenge here lies in the small gap between the bottom of the butterfly and the vane casing. To limit the simulation cost, the gap is meshed with 1 element. In this case, MMG succeeded in adapting the mesh up to a critical angle at which the gap becomes too small (Work In Progress).

==== T2 - Dynamic Smagorinsky in Dorothy ====

==== T3 - Turbulence injection strategy for compressible flows - Patrick TENE HEDJE (UMONS), Laurent Bricteur (UMONS), Yacine BECHANE (CORIA), Pierre BENARD (CORIA) ====

==== T4 - Improve wind farm modeling and simulation workflow ====
The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine. Still, applying this method to investigate a wind farm flow can be challenging, both in terms of computational cost and simulation setup. For instance, an inadequate management of the wind turbine individual modeling parts in a HPC context can end up being the main bottleneck of the simulation. From another perspective, a wind farm is usually composed of more than 50 wind turbines. For such a case, setting up all YALES2 required inputs manually can be very tedious and error-prone. This project thus mainly aimed to optimize the YALES2 ALM implementation and the user experience around it. Additionally, a cost-effective alternative to the ALM when modeling wind farm flows, namely the Rotating Actuator Disk Method (ADM-R), has been implemented for further investigations.

'''WP1''': Improve Actuator set rotor modelling
* Parallel processing of the ''actuator sets'' used to model the wind turbines
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified during the 5th iteration of the ECFD: (i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges. (ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications. During this workshop edition, the work focused on the re-synchronisation of the algorithm steps by allowing some packing an unpacking of the object to allow the transfert of the object inbetween workers. This is of major importance to enable load balancing and mesh adaptation during the temporal loop. This work required the refactoring of the involved oject structures.

* Rotating Actuator Disk Method (ADM-R):
According to the usual guidelines, the mesh requirements of the ALM, to profit entirely from its reachable accuracy, can be difficult to achieve or even unaffordable when simulating a wind farm flow, especially from the industrial point of view. Alternatives are available in the literature for this kind of application. Likely, the methods from the Actuator Disk family are the most prominent ones. Several kinds of implementation exist, which mostly differ by their capability to include the wake rotation. During the workshop, a new method from the Rotating Actuator Disk kind has been implemented and underwent an early validation on a single turbine setup. Applications to wind farm flows will follow.

'''WP2''': Improve tools User Experience

Three Python tools have been developed or improved :
*The first tool is the wind farm previsualisation tool, 'y2_wind_previsualization', which is used before the calculation run. This provides an interactive HTML interface for viewing global data for each turbine on the farm (position, hub height, yaw angle, etc.). The tool traces all of these via the parsing of the input file.
* The second tool is for duplicating rotor templates for a wind farm (`y2_wind_duplication`). This tool was developed in the previous ECFD, but this time it has been refactored and incorporated into the y2tools package.
* The third and final tool is a post-processing tool for the temporal processing of global wind turbine simulation metrics (Thrust, Power, etc.), `y2_post_wind`. This tool generates an interactive HTML plot of time-dependent global quantities.

==== T5 - Improve atmospheric inflow turbulence ====
Atmospheric inflow turbulence is generated using the precursor database method. A half-channel flow driven by a pressure gradient is used to obtain the inflow which is used as inlet boundary condition for the wind turbine simulation domain. This project aimed to improve the whole methodology, from generation to injection.

* WP1: Improve inflow generation
Anand: pressure controller

* WP2: Improve injection methodology (method A)
The previous workflow used plane probes in the ASCII format to sample the flow. The COWIT2 toolbox was used to convert the file into turbulence box (.man format). While functioning, this methodology had two major flaws. First the probe files are heavy ~O(10Go). Second, the method requires a lot of human effort, allowing numerous sources of errors.
During this workshop, a new methodology has been developed. First, the probes are generated using the HDF5 format (now available for all probe types), leading to lighter file ~O(1Go). Second, Y2_tools is used to read HDF5 format (working for probes and temporals). HDF5 file is then converted into a Look-up Table. Finally, the Look-up Table is read directly by YALES2 as a boundary conditions.

* WP3: Improve injection methodology (method B)
Even though improvements achieved in WP2 prove to be very handy while removing many potential human errors, injecting a turbulent inflow through wind boxes ('offline' precursor approach) can sometimes remain cumbersome for several reasons: (1) no periodicity is enforced in the streamwise direction of those boxes, (2) potential high memory consumption, and (3) the boxes need to be moved to other cores whenever a mesh adaptation occurs. An alternative consists in co-simulating the precursor flow and the flow of interest (refered as the 'successor' simulation) at the same time ('online' precursor approach). The inlet boundary condition for the successor flow is then obtained by mapping the outflow of the precursor domain. During the workshop, some work has been initiated to implement this kind of coupling using the CWIPI library, for which YALES2 provides already an interface.

==== T6 - FSI model in Dorothy ====

=== Two Phase Flow - J. Leparoux, SAFRAN & J. Carmona, CORIA ===

==== TP1 - Towards very small contact angles in Nucleate boiling ====

Participants: Henri Lam (LEGI), Mohammad Umair (LEGI), Manuel Bernard (LEGI), Robin Barbera (LEGI) and Giovanni Ghigliotti (LPSC)

The boiling solver (BOI) was not able to accurately impose a contact angle (angle formed by the two-phase interface on the wall) at values lower than 30°. This angle is needed when simulating nucleate boiling. A similar limitation in contact angle value was applying to the spray (SPS) solver. A modified version of the level set reinitialization has been implemented during ECFD8, based on a different normal vector in the blind spot region around the contact line, vector now chosen to be a zero-order extension from outside the blind spot. This modification, that implied other modifications to the level set reinitialization in the blind spot, has been tested successfully on the spray solver (where no phase change occurs). Then, this new reinitialisation has been tested in the boiling solver for nucleate boiling, with great improvements. Now simulations of nucleate boiling at very small contact angle (10°) can be accurately performed.
In the meanwhile, the level set reinitialization algorithm has been streamlined and the computational cost greatly reduced, resolving a computational cost issue that appeared when using the contact angle imposition both in the spray and boiling solvers, and that hampered its use in industrial configurations.

==== TP2 - Modeling spray-film interactions ====

Participants: Nicolas Gasnier (EM2C-SafranTech), Julien Leparoux (SafranTech), Mehdi Helal (CORIA-SafranTech) and Julien Carmona (CORIA)

The numerical simulation of sheared liquid films over walls with conventional high-fidelity methods requires fine meshes to depict accurately the dynamics of the fluids, as well as the gas-liquid interactions at the phase interface. In the context of thin film flows (with a thickness h ~ 1.0E-4 m), the spatial resolution required to ensure the validity of the simulations can be computationally prohibitive. To tackle this issue, a reduced-order model of thin film dynamics based on the Saint Venant equations has been implemented in the YALES2 platform over the last months. This numerical model is able to reproduce accurately the dynamics of strongly sheared film flows over partially wetted surfaces, and to take into account capillary effects. The objective of the ECFD8 was to couple the thin film framework with the pre-existing multiphase methods of the YALES2 platform: the Eulerian multiphase solver - based on the ACLS -, and the Lagrangian solver. This coupling was performed in three steps: first, a numerical method was designed to convert impinging Lagrangian droplets into source terms for the thin film model. Then, a phenomenological model was implemented to depict the atomization of films at sharp edges under the action of a high speed gas flow. The liquid film is converted into Lagragian droplets, whose dimensions are computed based on the PAMELA model. Finally, a numerical method has been designed to convert Eulerian droplets into source terms for the film model. This method is based on a transfer of the liquid properties from the nodes to the boundaries by using a set of fictitious particles. It ensures conservation of the liquid mass, and it is more robust than the first attempts that had been made during ECFD7.

==== TP3 - High-fidelity two-phase flow simulations of the purge of a fuel feed line ====

Participants: Thomas LAROCHE (Safran HE), Romain JANODET (Safran AE), Julien Leparoux (Safran Tech) and Melody Cailler (Safran Tech)

During the second week of the ECFD8, the fuel feed line purge process has been numerically investigated. In the context of aeronautical engines, the fuel feed line—carrying the fuel from the fuel tank to the injectors within the combustion chamber—needs to be purged at engine shutdown. This is intended to prevent fuel stagnation near hot metal parts, which could lead to coke formation and therefore decrease engine performance. Since this complex phenomenon is mainly driven by two-phase flow physics, the spray solver (SPS) of the YALES2 library has been considered in order to understand the physics of such process. The numerical setup was first converged on a simplified test case: the possibility of driving the flow dynamics with inlet and outlet pressure conditions was tested beforehand on a single-phase, incompressible case, and then on a two-phase flow problem. The setup has then been successfully applied to an industrial configuration: a pressure-swirl injector connected to a reduced portion of the fuel feed line. Due to the large scale of the domain, the interface resolution was set to 50μm, which is intentionally coarse for such problem. This initial computation successfully ran up to 3ms of physical time during the workshop, proving YALES2's capability to model the fuel purge. The computation is to be continued and analyzed further even after the workshop.

==== TP4 - Volume of Fluid solver in YALES2 ====

Participants: Léa Voivenel (CORIA), Julien Carmona (CORIA), Mehdi Helal (CORIA), Pierre Portais (CORIA), Julien Leparoux (Safran Tech), Mélody Cailler (Safran Tech) and Nicolas Gasnier (EM2C / Safran Tech)

==== TP5 - Implement a local operator to distribute the solid volume of a particle over multiple cells ====

Participants: Théo Ndereyimana (Université de Sherbrooke), Stéphane Moreau (Université de Sherbrooke)

In the CFD-DEM, the cell size is required to be larger than the particle size for stability condition and keep feasible solid volume fraction. However, some applications require a cell size smaller than the particle. During this 8th edition of the ECFD, the use of operators to distribute the particle volume over multiple cells, ensuring a feasible solid volume particle has been tested on a fluidized bed configuration. The main operators tested (gather-scatter filter and gaussian filter) showed a tendency to blur the void structure interfaces. The equivalence of the gaussian filter of bandwidth <math>b</math> and the resolution of a diffusion equation over a pseudo-time <math>T=b^2/4</math> has been verified.
One anisotropic diffusion constant has been tested and shows a possibility to adress the sharpness requirements.

Another objective was to develop a post-processing tool to detect and track the void structures (bubbles) in a fluidized bed. Based on previous work from J. Carmona, a tool to track the bubbles has been initiated.

==== TP6 - Complex thermodynamics in sloshing tanks ====

Participants: C. Merlin (AGS), D. Fouquet (CORIA), V. Moureau (CORIA), J. Carmona (CORIA) and G. Lartigue (CORIA)

During this ECFD, the phase change solver was improved. After introduction of conservative transport for enthalpy and energy, a new framework for multi fluid two phase flow was used that relies on a new reinitialization of the conservative level set, the transport of discontinuous scalars like temperature, energy or enthalpy and the transport of weight to ensure consistency. The pressure loop was improved to ensure mass and energy conservation while meeting the low Mach hypothesis. Many 1D test cases were performed with only the gas phase and two phase flow to validate the pressure loop. The new framework for the transport of discontinuous scalar was first derived for temperature with constant properties in each phase. A first step was to extend it to sensible enthalpy with ideal gas behavior for the gas phase. Then, the framework was extended to NIST tabulated gas & liquid with ideal or real gas behavior. After some 2D cases, a tank pressurization test was investigated as well as a sloshing test without phase change (just the pressure drop due to the increase of the thermal exchange between the liquid and the gas phase).

=== Combustion - Y. Bechane, CORIA & S. Dillon, SAFRAN & K. Bioche, CORIA ===

==== C1 - LES of the thermal degradation of a composite material ====
Participants: A. Grenouilloux (ONERA), K. Bioche (CORIA), N. Dellinger (ONERA) and R. Letournel (SafranTech)

The FIRE test bed is an experimental air-propane burner operated by ONERA. It is dedicated to the study of the thermal degradation of composite materials. This project concerned the implementation of a three-solver coupling methodology to simulate the dynamics of the impinging flame. The methodology considered is based on the coupling between the variable density solver (VDS) and the radiative solver (RDS) of the massively parallel library YALES2 and the solver dedicated to the degradation of composite materials, MoDeTheC, developed by ONERA. Given the typical test times of the order of tens of seconds, a methodology based on 2D axisymmetric calculations was considered. Various tests were performed to determine the optimal coupling frequency between solvers. Cases dedicated to the injection of pyrolysis gasses were set up, with the aim of simulating the auto-ignition phenomenon. Comparisons with experimental data are presented.

==== C2 - Flame stabilization by NRP plasma discharge ====

==== C3 - Extending and validating a generalized formalism of virtual chemistry ====

==== C4 - Turbulent combustion model for NOx prediction ====

==== C5 - Towards 3D simulation of detonation combustion ====

==== C6 - Flame stabilitity of flame-holders within reheat conditions ====

Participants : N. Detomaso (Safran AE), R. Janodet (Safran AE), L. Carbajal (Safran AE)

The mechanisms of flame stabilisation in a flame-holder are still not completely understood. This is particularly true for reheat conditions : the hot viciated gases at inlet, highly compressible flows, and a strong liquid/gas coupling make the flame stability hard to predict. During the ECFD8, simplified configurations of flame-holders within reheat conditions were analysed. After some try and error simulations due to simplifications of the flame-holders, cases leading to flame stability (or not) were identified. Post-processing tools were developped in order to recover criteria relevant to the flame stability. This step marks the beginning of a systematic mapping of dimensionless characteristic times ratios, and a comparison with integral quantities.

==== C7 - Thermal radiation in oxyflames ====

==== C8 - A first step toward hybrid CPU / GPU for reactive flow in YALES2 ====

Participants: M. Laignel (CORIA), G. Lartigue (CORIA), K. Bioche (CORIA) and V. Moureau (CORIA)

In numerical simulations of reacting flows, one of the most computationally intensive tasks is the evaluation of source terms resulting from chemical reactions in the species transport equations. This step can account for up to 90% of the total simulation cost , depending on the complexity of the kinetic mechanism involved. To reduce this cost, various techniques such as mechanism reduction, virtual chemistry, etc. have been explored. However, the emergence of GPUs as powerful accelerators offers a promising alternative by providing massive parallelism. Despite their potential, GPUs often require significant adaptation of CPU-based codes. This project aims to address this challenge by taking a first step towards a hybrid CPU/GPU framework for reactive flow simulations. Specifically, the focus is on coupling Y2 with the updated version of the stiff time integration solver (CVODE), which is compatible with GPU (CUDA, HIP, OpenMP). The ultimate goal is to establish a foundation for hybrid computations by implementing and testing the updated solver on simplified test cases.

==== C9 - Soots numerical modeling ====

==== C10 - TECERACT : Tabulated chemistry generator for aeronautical combustion ====

==== C11 - Exploring efficient tabulation strategies for detailed chemistry ====

==== C12 - Dynamic sub-grid-scale modelling of multi-regime flame wrinkling ====

==== C13 - LES of a semi-industrial burner using a non-adiabatic virtual chemical scheme ====

=== User Experience & Data - L. Korzeczek, GDTECH ===

==== U1 - Low-fidelity (RANS) rotor/stator simulations, application to Kaplan Turbine - Y. Lakrifi, G. Balarac (LEGI), R. Mercier (SAFRAN), V. Moureau (CORIA) ====

==== U2 - Coupling PyTorch/YALES2, combustion cartesian look-up tables - J. Leparoux, N. Treleaven, S. Dillon (SAFRAN), K. Bioche, G. Lartigue (CORIA) ====

Participants: Julien Leparoux (Safran Tech), Kévin Bioche (CORIA), Ghislain Lartigue (CORIA), Nicholas Treleaven (Safran Tech)

Neural Networks offer a promising alternative to Cartesian look-up tables for combustion simulations, reducing memory footprint. In this project, we investigated how to integrate an NN model for real-time inference in the YALES2 platform, exploring two approaches: a Python interface and a Fortran Torch binding (using FTorch[https://github.com/Cambridge-ICCS/FTorch]). We validated that the model remains accurate when embedded online and identified improvements for robustness. Inference costs were evaluated on a Mac M3 and the Austral cluster, revealing a strong dependency on data volume. To optimize efficiency, we propose grouping cells at the processor level.

==== U3 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 - L. Korzeczek, S. Meynet (GDTECH), J. Leparoux, M. Cailler (SAFRAN) ====

Ecfd:ecfd 8th edition

2025-02-12T13:56:07Z

Benard: /* T2 - Dynamic Smagorinsky in Dorothy */

{{DISPLAYTITLE: ECFD workshop, 8th edition, 2025}}

== Description ==

* Event from '''27th of January to 7th of February 2025'''
* Location: [https://www.sport-normandie.fr/le-centre/le-site-de-houlgate Centre Sportif de Normandie], Houlgate, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* Participants from academics, HPC center/experts and industry are welcome
* The number of participants is limited to 68.

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes
* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Voivenel (CORIA).

[[File:ecfd8.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_8th_edition]]

[[File:Acknowledgments_ecfd8.png|text-bottom|600px]]

== News ==

* 23/10/2024: First announcement of the '''8th Extreme CFD Workshop & Hackathon''' !
* 22/11/2024: Deadline to submit your project

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

To come...

== Projects ==

=== Hackathon GENCI - P. Begou, LEGI ===
This ECFD8 GENCI Hackathon was a rich event, involving 4 differents CFD codes (AVBP, ParaDIGM, SONICS and YALES2) using various paradigms (C++/cuda/hip, Fortran/OpenMP/OpenACC) with several SDKs (AMD, Cray/HPE, Nvidia, Gnu) on a large range of GPU architectures (Nvidia A100, GH100, AMD instinct Mi210, Mi250, Mi300). This two-week event benefited from a high level support from three HPC mentors, two on-site from AMD (J. Noudohouenou and A. Tsetoglou) and one remote from CINES (M. Boudaoud).

==== H1 - ParaDIGM and SONICS on GPU, B. Maugars, G. Staffelbach, R.Cazalbou and B. Michel (ONERA)====

==== H2 - AVBP GPU offloading based on OpenMP, M.Ghenai, L. Legaux and A. Dauptain (CERFACS) ====

This hackathon provided a valuable opportunity to work on GPU offloading for AVBP. In the past, significant efforts were made to offload the entire AVBP code to GPUs. OpenACC was the primary strategy chosen, mainly due to access to NVIDIA's support, along with the availability of both software and hardware. This approach achieved good scalability performance.
Recently, with the deployment of new supercomputers like ADASTRA at CINES, some issues have emerged when running AVBP on AMD GPUs, including both MI250 and MI300. The closed-source nature of the Cray environment has also prevented CERFACS from deploying AVBP on local MI210 GPUs.
This hackathon was a great opportunity to address these challenges by exploring a new approach using OpenMP. An automatic translation tool was used to convert approximately 2,700 OpenACC directives to OpenMP, with each directive manually verified and fine-tuned afterward. AVBP with OpenMP had already been tested on NVIDIA GPUs, and during this hackathon, the focus was on extending support to AMD GPUs.
Two compilers were used: Cray and the newly released AFAR from AMD. With the support of AMD and CINES, a working environment for compiling AVBP was set up, and performance-related issues were identified. Additionally, two mini-apps were used for benchmarking. One of them achieved a 2.5× speedup when compiled with AFAR compared to Cray.
The next steps involve adapting the code to address necessary modifications, such as fixing issues related to Fortran indirections, and continuing performance evaluations with mini-apps. Further comparisons will be conducted using both compilers against results obtained with NVIDIA’s NVHPC.

==== H3 - YALES2 GPU from OpenACC to OpenMP, P. Bégou (LEGI), V. Moureau, G. Lartigue (CORIA) and R. Dubois (IMAG) ====
This Hackathon focuses on running Yales2 code on AMD Instinct Mi250 and Mi300 GPUs of the Adastra supercomputer (CINES).
Previously, a first solver in the Yales2 CFD code was successfully ported on the GPU accelerators of the Jean-Zay supercomputer (IDRIS) using Nvidia SDK but difficulties remain on Adastra AMD GPUs, mainly related to the available development tools. High compilation time and the impossibility to use debug flags at compile time as soon as OpenACC is enabled are a real challenge when tracking errors. The current project is to evaluate a freshly deployed version (at the begining of the workshop) of the AMD Fortran compiler. This requires moving to OpenMP paradigm, starting from scratch since the OpenACC branch has largely diverged from the master one while tracking spurious remaining bugs.
If the AMD compiler is able to build the cpu version of Yales2 "out of the box" (wich is not the case for Cray Fortran), the compilation time for each file is significantly higher. However, setting up a 2 stages dynamic compilation process allows for high parallelism that is not possible with Cray Fortran 18 and the library build time drops from nearly 2 hours (Cray Fortran 18) to 17 minutes (Amd Fortran compiler).
Large kernels have been ported from OpenACC to OpenMP, raising some difficulties when offloading intrinsics functions or using strutures attributes in kernels loops. These limitations were also known in the previous OpenACC work. The goal was mainly to check the correctness of the results. The offloading of the complex data structure of Yales2 code was then investigated. Here again some limitations of the "young" compiler were discovered and workarounds were implemented. Several reproducers were built during this ECFD8 and provided to developpers by the 2 on-site AMD engineers.
Preliminary tests on micro-applications show good performances of the generated binaries proving that this compiler could be a serious alternative on AMD GPUs and the goal is now to focus on this SDK in an OpenMP strategy while checking the portablility of this new implementation in Nvidia, Cray/HPE (and Gnu ?) environments.

=== Mesh adaptation - A. Grenouilloux, ONERA & G. Balarac, LEGI ===

==== M2 - Enhancement of mesh adaptation algorithms, B. Maugars (ONERA), B. Andrieu (ONERA), C. Benazet (ONERA), N. Dellinger (ONERA), G. Janodet (ONERA), G. Staffelbach (ONERA) ====
Mesh adaptation has become a crucial tool in order to automate industrial numerical simulations. ECDF7 allowed us to investigate the refine and EGADS libraries as tools for parallel mesh generation and adaptation using CAD as a geometric support. Since then, we fortified the workflow but some of our targeted industrial applications such as turbomachinery involve periodic boundary conditions. To manage these cases, the mesh generation and adaptation procedures must maintain matching periodic boundaries.
During ECFD8, we addressed multiple topics : periodic mesh generation from CAD model in EGADS, parallel and periodic metric gradation in ParaDiGM, making our parallel remeshing algorithm more generic to support non-manifold and periodic meshes. All these topics were covered, but there is still some work to be done to fully manage periodic mesh adaptation.
Non-manifold mesh adaptation was applied to changing mesh topologies. Here, an internal surface was described by a level-set and deformed up to hole creation to mimic ablation.

==== M3 - Development and definition of a new Automatic Mesh Convergence (AMC) driver for automating static mesh convergence in YALES2 (C. Papagiannis (LEGI/INRIA), G. Balarac (LEGI), O. LeMaître (INRIA), P.M. Congedo (INRIA)) ====
Static mesh adaptation requires re-adapting an existing (usually coarse) mesh, to conform to some quality metric that satisfies some optimality criterion. This involves statistics (Mean and RMS fields) that need to be adequately converged. We proposed and developed a novel AMC strategy that can converge an initial coarse mesh to an adequately refined one by taking into account the quality of the calculated statistics before each adaptation. Specifically, so far, the moment (in terms of integrating the LES equations) at which to launch the adaptation was unknown and had to be set apriori as a parameter of the AMC, dependent on the flow physics and therefore on the case at hand. Our method is automatically performing adaptations when the quality of the volume averaged mean field estimate (characterized by its RMSE) reaches the levels of the bias error that this field has, due to the mesh being far from the desired one (dictated by the quality criterion). This was achieved through drawing inspiration from a theoretical model that treats the AMC as a "conditionally" contractive mapping with a fixed point.

=== Numerics - M. Bernard, LEGI & G. Lartigue, CORIA ===

==== N1 - Traction open boundary condition ====

==== N2 - Treatment of Inlet conditions in High-Order solver. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====
In the context of node-centered Finite Volumes Method, spacial accuracy of a numerical scheme depends on ability to evaluate accurately fluxes through interface of each control volume (CV). Such accurate evaluation is not straightforward, especially when dealing with distorted grids. This project follows the work of [1] where fluxes use pointwise quantities, which are reconstructed from integrated quantities advanced in time. During the previous edition of the ECFD, a new data structure has been developed to store data at location of the boundary conditions facelets, with application to wall boundary conditions. During this 8th edition of the ECFD, we used the same data structure, but dedicated to the treatment of inlet conditions.
The inlet condition is then either imposed directly at facelets center, or at nodes position them extrapolated to facelets center by use of Taylor expansion. For this later solution, high-order treatment requires the successive derivatives to be computed in the plane of the boundary condition. This is not done yet, leading for the moment to low accuracy results but the framework is ready for upcoming implementation.

[1] ''A framework to perform high-order deconvolution for finite-volume method on simplicial meshes, , Bernard et. al., IJNMF 2020''

==== N3 - Conservative mesh-to-mesh interpolation. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) ====

Mesh to mesh interpolations occur quite often in CFD simulations : in the context of adaptative mesh convergence or in the case of dynamic mesh adaptation for for example.
Quality of the solution on the destination grid will depend on the characteristics of the interpolation method.
In this project, we did not focus on accuracy of the interpolation method but rather on conservativity characteristics.
A conservative interpolation ensures that the integral of the data on the source grid is exactly retrieved on the destination grid.
This property is highly interesting when dealing with scalar quantities or phase indicators, whose values should remained bounded.
In the context of nodes centered Finite Volume schemes, the methodology we used consists in (i) reconstructing element quantity from average nodal quantities on source grid.
Then, for a cell of the destination mesh, (ii) computing the geometrical intersection between cells of source and destination meshes to evaluate to evaluate the rate of quantities they.
Eventually, (iii) redistributing the solution from elements to control volumes of the destination mesh.
The overall process is fully conservative as it is based on geometrical intersection of locally integrated quantities.
The methodology as been implemented and tested on a few basic configurations and the conservativity is retrieved.

==== N4 - Determination of timestep in semi-implicit solver. T. Berthelon (LEGI), G. Balarac (LEGI), H. Lam (LEGI), M. El Moatamid (CORIA) ====
In order to reduce the computation time associated with incompressible LES simulations, an implicit time integration, based on BDF schemes, has been developed within the ICS solver. This integration eliminates the stability constraints associated with explicit schemes, and therefore opens up the question of the appropriate choice of time step.
In parallel, recent work has been carried out on meshing criteria in LES. The strategy in place consists of adapting the mesh by distinguishing two zones:
- "DNS" zones, where the meshing criterion is based on an estimate of the adimensioned spatial error.
- "LES" zones, where the meshing criterion is based on Kolmorogov theory.
During this project, the spatial criteria were extended to include temporal criteria. In the "DNS" zones, the time step is chosen using an estimate of the temporal error of the BDF scheme judiciously scaled to match the spatial error. In the "LES" zones, the time step is chosen using a scaling law associated with fully developed turbulence.
The new time step selection strategy has been tested on the case of a turbulent jet and leads to an accuracy equivalent to the explicit case while reducing the simulation return time by a factor of nearly 3.

Another aspect of this project was to integrate certain implicit temporal schemes (C-N and SDIRK) recently developed by Mr. El Moatamid into the incompressible solver.

==== N5 - Local timestep. T. Berthelon (LEGI), M. Bernard (LEGI), G. Balarac (LEGI) ====
RANS modelling has recently been developed within the YALES2 library. With this modeling strategy, the objective is to reach as quick as possible a steady state.
During this project, we investigate the use of a local time step to reduce the time to solution of steady computation in the incompressible solver.
This implies solving a variable-coefficient Poisson equation. Encouraging results were obtained in the simple case of "Couette plan" flow artificially constrained by a mesh variation. In fact, the use of local time-step reduce drastically the time to solution on this configuration. This method needs to be tested on real RANS case.

==== N6 - Distributed version of DOROTHY ====

==== N7 - Implicit time advancement for low-Reynolds number flows with particles. S. Mendez, C. Raveleau (IMAG), M. El Moatamid, V. Moureau (CORIA) ====
IMAG runs numerous simulations of red blood cells under flow. Those simulations are at low Reynolds number (0.001 to 1.0, typically). Splitting of the time advancement is used to treat the diffusion terms implicitly, albeit with an important numerical cost: implicit diffusion is 50 to 60% of the computational cost. Recently, M. El Moatamid implemented a genral framework to deal with implicit time advancement for scalars. In this project, the general method has been transposed to the advancement of the velocity field in the ICS and RBC solvers of YALES2/YALES2BIO. This enables testing various linear solvers (GMRES based). However, such solvers do not decrease the CPU time compared to the existing implementation. However, while working on this, it was identified that residual recycling was not activated in the current implementation of the implicit diffusion. This sped up the implicit diffusion cost by 35%, for a total gain of 20% for the computation. In addition to this achievement, moving to the framework coded by Moncef will have other beneficial side effects: we anticipate simplifying the implementation, with an easier merging between YALES2BIO and YALES2. The method will also be implemented in the electrosatic solver, for which the Poisson problem should benefit from the new GMRES-based solvers. In addition, this project highlights the importance of improving the treatment of stiff source terms in the red blood cells simulations, to be able to overcome the current limitation in time step due to those term and have a chance to benefit from higher-order time schemes, efficient at high Fourier numbers.

==== N8 - Boundary Element Method in YALES2. B. Thibaud, S. Mendez (IMAG), G. Lartigue, P. Benard (CORIA), F. Nicoud (IMAG) ====
In the context of microfluidic systems for diagnosis, the Boundary Element Method alows to solve linear PDE such as electrostatic or Stokes. With well chosen kernel functions and the divergence theorem, this method allows to write on the boundary condition only the initial volumic problem. This project aimed at exploring the feasibility of the BEM in the context of massively parallel unstructured solver like YALES2 by developping a Julia demonstrator. The first step have been to implement and validate the method on simple configurations for the Laplace's equation. Only Neumann problems were considered (Dirichlet boundary conditions imposed). In a second time, the multi-domain approach has been identified to be the most suited in the framework of YALES. The inner domain is partitioned on each processor, each having a part of the physical boundary and interfaces between them. Every processor solve its own boundary problem and a parallel Dirichlet-Dirichlet fixed-point is used to converge the interface problem on the all domain. Applied to the ring case, with one interface, we managed to reproduce the linear convergence of the P-DD method.

=== Turbulence - L. Voivenel, CORIA & P. Bénard, CORIA ===

==== T1 - FSI-1D strategy for internal flows====

Many applications developed at Safran Aerosystem are based on internal turbulent flows coupled to a moving body. 2 cases were studied during this ECFD:

'''Case 1 (Incompressible flow)''': Translation of a piston subjected to a pressure difference in a pipe.

The challenges of this case are twofold: the small gap between the piston and the pipe and the large pressure gradient across the piston (>100bar). During the 1st week of ECFD, the CLIB (Conservative Lagrangian Immersed Boundary) solver was tested on this case. The study showed that the solver was unable to ensure the impermeability of the solid under these pressure conditions. In the rest of the study, a porous medium following Darcy's law will be added to the penalty force of the immersed solid to fully satisfy the impermeability of the piston.

'''Case 2 (Compressible flow)''': Rotation of a butterfly in a discharge vane.

The coupling between the ECS (Explicit Compressible Solver) and ALE (Arbitrary Lagrangian Solver) solvers having recently been implemented, this strategy was tested to model the opening of the valve by rotation of the butterfly. The challenge here lies in the small gap between the bottom of the butterfly and the vane casing. To limit the simulation cost, the gap is meshed with 1 element. In this case, MMG succeeded in adapting the mesh up to a critical angle at which the gap becomes too small (Work In Progress).

==== T2 - Dynamic Smagorinsky in Dorothy - Patrick TENE HEDJE (UMONS), Laurent Bricteur (UMONS), Yacine BECHANE (CORIA), Pierre BENARD (CORIA) ====

==== T3 - Turbulence injection strategy for compressible flows ====

==== T4 - Improve wind farm modeling and simulation workflow ====
The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine. Still, applying this method to investigate a wind farm flow can be challenging, both in terms of computational cost and simulation setup. For instance, an inadequate management of the wind turbine individual modeling parts in a HPC context can end up being the main bottleneck of the simulation. From another perspective, a wind farm is usually composed of more than 50 wind turbines. For such a case, setting up all YALES2 required inputs manually can be very tedious and error-prone. This project thus mainly aimed to optimize the YALES2 ALM implementation and the user experience around it. Additionally, a cost-effective alternative to the ALM when modeling wind farm flows, namely the Rotating Actuator Disk Method (ADM-R), has been implemented for further investigations.

'''WP1''': Improve Actuator set rotor modelling
* Parallel processing of the ''actuator sets'' used to model the wind turbines
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified during the 5th iteration of the ECFD: (i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges. (ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications. During this workshop edition, the work focused on the re-synchronisation of the algorithm steps by allowing some packing an unpacking of the object to allow the transfert of the object inbetween workers. This is of major importance to enable load balancing and mesh adaptation during the temporal loop. This work required the refactoring of the involved oject structures.

* Rotating Actuator Disk Method (ADM-R):
According to the usual guidelines, the mesh requirements of the ALM, to profit entirely from its reachable accuracy, can be difficult to achieve or even unaffordable when simulating a wind farm flow, especially from the industrial point of view. Alternatives are available in the literature for this kind of application. Likely, the methods from the Actuator Disk family are the most prominent ones. Several kinds of implementation exist, which mostly differ by their capability to include the wake rotation. During the workshop, a new method from the Rotating Actuator Disk kind has been implemented and underwent an early validation on a single turbine setup. Applications to wind farm flows will follow.

'''WP2''': Improve tools User Experience

Three Python tools have been developed or improved :
*The first tool is the wind farm previsualisation tool, 'y2_wind_previsualization', which is used before the calculation run. This provides an interactive HTML interface for viewing global data for each turbine on the farm (position, hub height, yaw angle, etc.). The tool traces all of these via the parsing of the input file.
* The second tool is for duplicating rotor templates for a wind farm (`y2_wind_duplication`). This tool was developed in the previous ECFD, but this time it has been refactored and incorporated into the y2tools package.
* The third and final tool is a post-processing tool for the temporal processing of global wind turbine simulation metrics (Thrust, Power, etc.), `y2_post_wind`. This tool generates an interactive HTML plot of time-dependent global quantities.

==== T5 - Improve atmospheric inflow turbulence ====
Atmospheric inflow turbulence is generated using the precursor database method. A half-channel flow driven by a pressure gradient is used to obtain the inflow which is used as inlet boundary condition for the wind turbine simulation domain. This project aimed to improve the whole methodology, from generation to injection.

* WP1: Improve inflow generation
Anand: pressure controller

* WP2: Improve injection methodology (method A)
The previous workflow used plane probes in the ASCII format to sample the flow. The COWIT2 toolbox was used to convert the file into turbulence box (.man format). While functioning, this methodology had two major flaws. First the probe files are heavy ~O(10Go). Second, the method requires a lot of human effort, allowing numerous sources of errors.
During this workshop, a new methodology has been developed. First, the probes are generated using the HDF5 format (now available for all probe types), leading to lighter file ~O(1Go). Second, Y2_tools is used to read HDF5 format (working for probes and temporals). HDF5 file is then converted into a Look-up Table. Finally, the Look-up Table is read directly by YALES2 as a boundary conditions.

* WP3: Improve injection methodology (method B)
Even though improvements achieved in WP2 prove to be very handy while removing many potential human errors, injecting a turbulent inflow through wind boxes ('offline' precursor approach) can sometimes remain cumbersome for several reasons: (1) no periodicity is enforced in the streamwise direction of those boxes, (2) potential high memory consumption, and (3) the boxes need to be moved to other cores whenever a mesh adaptation occurs. An alternative consists in co-simulating the precursor flow and the flow of interest (refered as the 'successor' simulation) at the same time ('online' precursor approach). The inlet boundary condition for the successor flow is then obtained by mapping the outflow of the precursor domain. During the workshop, some work has been initiated to implement this kind of coupling using the CWIPI library, for which YALES2 provides already an interface.

==== T6 - FSI model in Dorothy ====

=== Two Phase Flow - J. Leparoux, SAFRAN & J. Carmona, CORIA ===

==== TP1 - Towards very small contact angles in Nucleate boiling ====

Participants: Henri Lam (LEGI), Mohammad Umair (LEGI), Manuel Bernard (LEGI), Robin Barbera (LEGI) and Giovanni Ghigliotti (LPSC)

The boiling solver (BOI) was not able to accurately impose a contact angle (angle formed by the two-phase interface on the wall) at values lower than 30°. This angle is needed when simulating nucleate boiling. A similar limitation in contact angle value was applying to the spray (SPS) solver. A modified version of the level set reinitialization has been implemented during ECFD8, based on a different normal vector in the blind spot region around the contact line, vector now chosen to be a zero-order extension from outside the blind spot. This modification, that implied other modifications to the level set reinitialization in the blind spot, has been tested successfully on the spray solver (where no phase change occurs). Then, this new reinitialisation has been tested in the boiling solver for nucleate boiling, with great improvements. Now simulations of nucleate boiling at very small contact angle (10°) can be accurately performed.
In the meanwhile, the level set reinitialization algorithm has been streamlined and the computational cost greatly reduced, resolving a computational cost issue that appeared when using the contact angle imposition both in the spray and boiling solvers, and that hampered its use in industrial configurations.

==== TP2 - Modeling spray-film interactions ====

Participants: Nicolas Gasnier (EM2C-SafranTech), Julien Leparoux (SafranTech), Mehdi Helal (CORIA-SafranTech) and Julien Carmona (CORIA)

The numerical simulation of sheared liquid films over walls with conventional high-fidelity methods requires fine meshes to depict accurately the dynamics of the fluids, as well as the gas-liquid interactions at the phase interface. In the context of thin film flows (with a thickness h ~ 1.0E-4 m), the spatial resolution required to ensure the validity of the simulations can be computationally prohibitive. To tackle this issue, a reduced-order model of thin film dynamics based on the Saint Venant equations has been implemented in the YALES2 platform over the last months. This numerical model is able to reproduce accurately the dynamics of strongly sheared film flows over partially wetted surfaces, and to take into account capillary effects. The objective of the ECFD8 was to couple the thin film framework with the pre-existing multiphase methods of the YALES2 platform: the Eulerian multiphase solver - based on the ACLS -, and the Lagrangian solver. This coupling was performed in three steps: first, a numerical method was designed to convert impinging Lagrangian droplets into source terms for the thin film model. Then, a phenomenological model was implemented to depict the atomization of films at sharp edges under the action of a high speed gas flow. The liquid film is converted into Lagragian droplets, whose dimensions are computed based on the PAMELA model. Finally, a numerical method has been designed to convert Eulerian droplets into source terms for the film model. This method is based on a transfer of the liquid properties from the nodes to the boundaries by using a set of fictitious particles. It ensures conservation of the liquid mass, and it is more robust than the first attempts that had been made during ECFD7.

==== TP3 - High-fidelity two-phase flow simulations of the purge of a fuel feed line ====

Participants: Thomas LAROCHE (Safran HE), Romain JANODET (Safran AE), Julien Leparoux (Safran Tech) and Melody Cailler (Safran Tech)

During the second week of the ECFD8, the fuel feed line purge process has been numerically investigated. In the context of aeronautical engines, the fuel feed line—carrying the fuel from the fuel tank to the injectors within the combustion chamber—needs to be purged at engine shutdown. This is intended to prevent fuel stagnation near hot metal parts, which could lead to coke formation and therefore decrease engine performance. Since this complex phenomenon is mainly driven by two-phase flow physics, the spray solver (SPS) of the YALES2 library has been considered in order to understand the physics of such process. The numerical setup was first converged on a simplified test case: the possibility of driving the flow dynamics with inlet and outlet pressure conditions was tested beforehand on a single-phase, incompressible case, and then on a two-phase flow problem. The setup has then been successfully applied to an industrial configuration: a pressure-swirl injector connected to a reduced portion of the fuel feed line. Due to the large scale of the domain, the interface resolution was set to 50μm, which is intentionally coarse for such problem. This initial computation successfully ran up to 3ms of physical time during the workshop, proving YALES2's capability to model the fuel purge. The computation is to be continued and analyzed further even after the workshop.

==== TP4 - Volume of Fluid solver in YALES2 ====

Participants: Léa Voivenel (CORIA), Julien Carmona (CORIA), Mehdi Helal (CORIA), Pierre Portais (CORIA), Julien Leparoux (Safran Tech), Mélody Cailler (Safran Tech) and Nicolas Gasnier (EM2C / Safran Tech)

==== TP5 - Implement a local operator to distribute the solid volume of a particle over multiple cells ====

Participants: Théo Ndereyimana (Université de Sherbrooke), Stéphane Moreau (Université de Sherbrooke)

In the CFD-DEM, the cell size is required to be larger than the particle size for stability condition and keep feasible solid volume fraction. However, some applications require a cell size smaller than the particle. During this 8th edition of the ECFD, the use of operators to distribute the particle volume over multiple cells, ensuring a feasible solid volume particle has been tested on a fluidized bed configuration. The main operators tested (gather-scatter filter and gaussian filter) showed a tendency to blur the void structure interfaces. The equivalence of the gaussian filter of bandwidth <math>b</math> and the resolution of a diffusion equation over a pseudo-time <math>T=b^2/4</math> has been verified.
One anisotropic diffusion constant has been tested and shows a possibility to adress the sharpness requirements.

Another objective was to develop a post-processing tool to detect and track the void structures (bubbles) in a fluidized bed. Based on previous work from J. Carmona, a tool to track the bubbles has been initiated.

==== TP6 - Complex thermodynamics in sloshing tanks ====

Participants: C. Merlin (AGS), D. Fouquet (CORIA), V. Moureau (CORIA), J. Carmona (CORIA) and G. Lartigue (CORIA)

During this ECFD, the phase change solver was improved. After introduction of conservative transport for enthalpy and energy, a new framework for multi fluid two phase flow was used that relies on a new reinitialization of the conservative level set, the transport of discontinuous scalars like temperature, energy or enthalpy and the transport of weight to ensure consistency. The pressure loop was improved to ensure mass and energy conservation while meeting the low Mach hypothesis. Many 1D test cases were performed with only the gas phase and two phase flow to validate the pressure loop. The new framework for the transport of discontinuous scalar was first derived for temperature with constant properties in each phase. A first step was to extend it to sensible enthalpy with ideal gas behavior for the gas phase. Then, the framework was extended to NIST tabulated gas & liquid with ideal or real gas behavior. After some 2D cases, a tank pressurization test was investigated as well as a sloshing test without phase change (just the pressure drop due to the increase of the thermal exchange between the liquid and the gas phase).

=== Combustion - Y. Bechane, CORIA & S. Dillon, SAFRAN & K. Bioche, CORIA ===

==== C1 - LES of the thermal degradation of a composite material ====
Participants: A. Grenouilloux (ONERA), K. Bioche (CORIA), N. Dellinger (ONERA) and R. Letournel (SafranTech)

The FIRE test bed is an experimental air-propane burner operated by ONERA. It is dedicated to the study of the thermal degradation of composite materials. This project concerned the implementation of a three-solver coupling methodology to simulate the dynamics of the impinging flame. The methodology considered is based on the coupling between the variable density solver (VDS) and the radiative solver (RDS) of the massively parallel library YALES2 and the solver dedicated to the degradation of composite materials, MoDeTheC, developed by ONERA. Given the typical test times of the order of tens of seconds, a methodology based on 2D axisymmetric calculations was considered. Various tests were performed to determine the optimal coupling frequency between solvers. Cases dedicated to the injection of pyrolysis gasses were set up, with the aim of simulating the auto-ignition phenomenon. Comparisons with experimental data are presented.

==== C2 - Flame stabilization by NRP plasma discharge ====

==== C3 - Extending and validating a generalized formalism of virtual chemistry ====

==== C4 - Turbulent combustion model for NOx prediction ====

==== C5 - Towards 3D simulation of detonation combustion ====

==== C6 - Flame stabilitity of flame-holders within reheat conditions ====

Participants : N. Detomaso (Safran AE), R. Janodet (Safran AE), L. Carbajal (Safran AE)

The mechanisms of flame stabilisation in a flame-holder are still not completely understood. This is particularly true for reheat conditions : the hot viciated gases at inlet, highly compressible flows, and a strong liquid/gas coupling make the flame stability hard to predict. During the ECFD8, simplified configurations of flame-holders within reheat conditions were analysed. After some try and error simulations due to simplifications of the flame-holders, cases leading to flame stability (or not) were identified. Post-processing tools were developped in order to recover criteria relevant to the flame stability. This step marks the beginning of a systematic mapping of dimensionless characteristic times ratios, and a comparison with integral quantities.

==== C7 - Thermal radiation in oxyflames ====

==== C8 - A first step toward hybrid CPU / GPU for reactive flow in YALES2 ====

Participants: M. Laignel (CORIA), G. Lartigue (CORIA), K. Bioche (CORIA) and V. Moureau (CORIA)

In numerical simulations of reacting flows, one of the most computationally intensive tasks is the evaluation of source terms resulting from chemical reactions in the species transport equations. This step can account for up to 90% of the total simulation cost , depending on the complexity of the kinetic mechanism involved. To reduce this cost, various techniques such as mechanism reduction, virtual chemistry, etc. have been explored. However, the emergence of GPUs as powerful accelerators offers a promising alternative by providing massive parallelism. Despite their potential, GPUs often require significant adaptation of CPU-based codes. This project aims to address this challenge by taking a first step towards a hybrid CPU/GPU framework for reactive flow simulations. Specifically, the focus is on coupling Y2 with the updated version of the stiff time integration solver (CVODE), which is compatible with GPU (CUDA, HIP, OpenMP). The ultimate goal is to establish a foundation for hybrid computations by implementing and testing the updated solver on simplified test cases.

==== C9 - Soots numerical modeling ====

==== C10 - TECERACT : Tabulated chemistry generator for aeronautical combustion ====

==== C11 - Exploring efficient tabulation strategies for detailed chemistry ====

==== C12 - Dynamic sub-grid-scale modelling of multi-regime flame wrinkling ====

==== C13 - LES of a semi-industrial burner using a non-adiabatic virtual chemical scheme ====

=== User Experience & Data - L. Korzeczek, GDTECH ===

==== U1 - Low-fidelity (RANS) rotor/stator simulations, application to Kaplan Turbine - Y. Lakrifi, G. Balarac (LEGI), R. Mercier (SAFRAN), V. Moureau (CORIA) ====

==== U2 - Coupling PyTorch/YALES2, combustion cartesian look-up tables - J. Leparoux, N. Treleaven, S. Dillon (SAFRAN), K. Bioche, G. Lartigue (CORIA) ====

Participants: Julien Leparoux (Safran Tech), Kévin Bioche (CORIA), Ghislain Lartigue (CORIA), Nicholas Treleaven (Safran Tech)

Neural Networks offer a promising alternative to Cartesian look-up tables for combustion simulations, reducing memory footprint. In this project, we investigated how to integrate an NN model for real-time inference in the YALES2 platform, exploring two approaches: a Python interface and a Fortran Torch binding (using FTorch[https://github.com/Cambridge-ICCS/FTorch]). We validated that the model remains accurate when embedded online and identified improvements for robustness. Inference costs were evaluated on a Mac M3 and the Austral cluster, revealing a strong dependency on data volume. To optimize efficiency, we propose grouping cells at the processor level.

==== U3 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 - L. Korzeczek, S. Meynet (GDTECH), J. Leparoux, M. Cailler (SAFRAN) ====

Ecfd:ecfd 8th edition

2025-02-07T08:47:10Z

Benard: /* T3 - Turbulence injection strategy for �compressible flows */

Ecfd:ecfd 8th edition

2025-02-07T08:46:59Z

Benard: /* T5 */

Ecfd:ecfd 8th edition

2025-02-07T08:46:34Z

Benard: /* T3 */

Ecfd:ecfd 8th edition

2025-02-07T08:46:15Z

Benard: /* T6 */

Ecfd:ecfd 8th edition

2025-02-07T08:45:56Z

Benard: /* T1 */

Ecfd:ecfd 8th edition

2025-02-07T08:45:42Z

Benard: /* T2 */

Ecfd:ecfd 8th edition

2025-02-07T08:43:55Z

Benard: /* T4 */

Ecfd:ecfd 8th edition

2025-02-07T08:08:52Z

Benard: /* Turbulence - L. Voivenel, CORIA & P. Bénard, CORIA */

Ecfd:ecfd 7th edition

2024-03-06T09:26:01Z

Benard: /* Turbulence - P. Benard, CORIA & L. Bricteux, UMONS */

{{DISPLAYTITLE: ECFD workshop, 7th edition, 2024}}

== Description ==

* Event from '''22th of January to 2nd of February 2024'''
* Location: [https://www.hotelclubdelaplage.com Hôtel Club de la Plage], Merville-Franceville, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* More than 70 participants from academics, HPC center/experts and industry.

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes
* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Viovenel (CORIA).
[[File:ecfd7.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_6th_edition]]

[[File:sponsor_ecfd7.png|text-bottom|600px]]



== Agenda ==

[[File:agenda_ecfd7.png|text-bottom|600px]]

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

To come...

== Projects ==

=== Hackathon GENCI - P. Begou, LEGI ===
The '''GENCI Hackathon''' will be devoted to porting two CFD codes to the Mi250 GPUs of the Adastra supercomputer deployed by GENCI at CINES.

For the '''YALES2''' code the goal is to obtain a first reference version giving the expected results then, if possible, to start its optimization to gain performance. The approach is OpenACC based with the objective of an implementation as least intrusive as possible in the existing code and which remains portable with the work done on the Nvidia GPUs of the Jean-Zay supercomputer at IDRIS.

The porting of the '''AVBP''' code is more advanced with a prototype already functional on Adastra but "hard-coded". The objective is to rationalize this first implementation, to integrate the latest developments in the code, to centralize memory management (host and device), to work on porting the Lagrangian part of the code and, of course, to improve the global performance.

This Hackathon is supported by GENCI, HPE, AMD and CINES with the presence on site of several development experts on AMD GPUS.

=== Mesh adaptation - R. Letournel, Safran ===

==== M1: ASMR for reheat chamber applications - Paul Pouech (CERFACS), Thibault Duranton, Luis Carbajal Carrasco (Safran) ====

Combustion in reheat chambers feature a wide range of lenght scales. Mesh refinement is thus mandatory to capture the flow characteristics within a reasonnable CPU cost for LES computations using the AVBP code. The purpose of this project is to consolidate mesh refinement criteria and strategy in an academic reference case. The retained workflow is supported by the [https://lemmings.readthedocs.io/en/latest/readme_copy.html Lemmings] code that calls the Tékigô wrapper for the mesh adaptations. During the ECFD7, the convergence time needed to have significant distribution of quantities of interest was analysed. An optimum runtime, based on a characteristic flow time-scale, was thus identified and led to a reduced running time for each adaptation step. As a second step, discussions with the ECFD7 participants led to the identification of interesting refinement criteria, namely the flame sensor or the mach rms for instance. Parametric analysis showed the robustness of the workflow based on a ponderation of different criteria. Finally, in order to facilitate the use of the workflow, efforts were made to improve the user experience by making it more human readable.

==== M2: Parallel remeshing - B. Andrieu, C. Benazet, K. Hoogveld, B. Maugars, E. Quémerais (ONERA) ====

Mesh adaptation is a crucial tool in order to automate industrial RANS numerical simulations. To meet this need, we need to carry out mesh adaptation as quickly as possible by setting up an efficient, parallel solution. To this end, we have explored two avenues: a parallel edge-splitting algorithm that has recently been initiated in the ParaDiGM library, and a solution based on [https://github.com/nasa/refine the refine library] for adapting meshes with MPI implementation. On the one hand, we fixed several bugs in our split operator, and validated it on test cases of increasing complexity with a node-centered solver. In addition, we've added interfaces to refine so as to avoid using files, and call directly in library mode. We also investigated geometric projection issues during the mesh adaptation procedure, notably by looking at solutions such as EGADS, which offers a simplified API for CAD interrogation. We finally implemented metric gradation (in serial), metric intersection and complexity computations. All the ingredients we've tested give us a clearer picture of the entire mesh adaptation process.

==== M3: Anisotropic mesh refinement - R. Barbera (LEGI/Safran), G. Ghigliotti, G. Balarac (LEGI), R. Letournel (Safran) ====

Mesh adaptation is now a key feature for simulations of complex industrial flows. For transient flows such as multiphase and/or reactive flows, where regions of interest are strongly moving in space, dynamic mesh adaptation appears as the most suitable strategy. This strategy is now widely used in YALES2 based on isotropic mesh definition. The purpose of this project is to adapt this strategy to an anisotropic framework to reduce the overall simulation costs (in term of memory consumption, cpu cost and time to solution). In order to be able to handle multiphase flows, the main objective of the project is to study the conditions for accurately describing the dynamics of the level-set function with an anisotropic mesh. Accuracy is mainly assessed in terms of interface position and mass conservation. The inaccuracy of mass conservation is mainly due to interpolation errors after the adaptation step. Furthermore, inaccuracy in interface position may be due to misalignment between the anisotropic mesh elements and the interface normal. The first methodological corrections have been proposed, as an adaptation of the level-set reinitialization algorithm to the anisotropic mesh.

=== Numerics - S. Mendez, IMAG & G. Balarac, LEGI ===

==== N1: Treatment of boundary conditions for high-order schemes - M. Bernard & G. Balarac (LEGI), G. Lartigue (Total Energies) ====

In the context of Finite Volumes Method, spacial accuracy of a numerical scheme depends on ability to evaluate accurately fluxes through interface of each control volume (CV).
Such accurate evaluation is not straightforward, especially when dealing with distorted grids.
This project follows the work of [1] where fluxes use pointwise quantities, which are reconstructed from integrated quantities advanced in time.
During the workshop, task force was dedicated to the treatment of **inlet** boundary conditions (BC) and **non-planar walls**.
For inlet BC, the key resides in the spatial integration of convective flux over discrete faces of the CV touching the boundary.
Such treatment lead to exact integration for linear inlet profile and large error reduction on other profiles.
Concerning non-planar walls, the strategy adopted consists in the enforcement of the BC on each discrete face, by modifying the normal component of the wall gradient in order to evaluate accurately the diffusive flux.
Again, a large reduction of this error has been observed.

[1] ''A framework to perform high-order deconvolution for finite-volume method on simplicial meshes, , Bernard et. al., IJNMF 2020''

==== N2: Implementation of linearised implicit time integration in ALE solver - T. Berthelon, G. Balarac (LEGI) ====

An linearised implicit time integration has recently been developed in the incompressible solver of YALES2. This new integration scheme allows to use larger time-step that the ones constraints by classic stability criteria inherent to explicit time integration method. This allows to reduce the restitution time of Large Eddy Simulations [1].
The objective of this project was to implement this new time integration in the ale solver in order to be able to reduce restitution time of moving mesh configuration.

The developments were validated on a scalar advection case and on a rotor-stator interaction case. Although the results seem to be in line with the explicit integration methods, the validation of the temporal convergence to 2nd order remains to be shown.

[1] ''Toward the use of LES for industrial complex geometries. Part II: Reduce the time-to-solution by using a linearised implicit time advancement, Berthelon et al., JoT, 2023''

==== N3: Parallelisation of Actuator Line Method - H. Mulakaloori (CORIA), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE) ====

The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified: (i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges. (ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications.

==== N4: Non-uniform outlet pressure and coupling with CWIPI - J. B. Lagaert (LMO), Y. Lakrifi, T. Berthelon, G.Balarac (LEGI) , R. Letournel (Safran) ====

In simulations, artificial boundaries need to be introduced due to the limited size of computational domains. At these boundaries, flow variables need to be calculated in a way that will not induce any perturbation of the interior solution. During ECFD#7, a generic outlet boundary condition defined from non-uniform pressure has been implemented in Yales2. This non-uniform pressure can de determined from a traction model (null or advected from the interior domain, for example). This non-uniform pressure can also be deducted through a coupling between two simulations. In this case a coupling via CWIPI is performed where the velocity and the pressure are exchanged at the common boundary to define the inlet and outlet conditions, respectively.

==== N5: Optimization of the RBC solver - F. Rojas, S. Mendez (IMAG) ====

In the study of blood diseases, the mechanical behaviour of Red Blood Cells (RBCs) is one of the most relevant effects to take into account in the numerical models but also in experimental setups. Our system of interest is the thin gap of a rheometer where RBC suspensions are placed to explore their properties. To interpret the experimental data, the simulations of large suspensions of RBC are required to determine the blood’s microstructure (spatial arrangement of cells) and its rheological properties.

Currently, YALES2BIO’s RBC solver is capable to manage thousands of cells, but in order to approach closer to the experimental scales, we propose the characterisation and optimisation of its performance to reduce the computational requirements and increase the RBC’s number and domain sizes in our simulations. During the workshop a parametric study was carried out to obtain the strong and weak scaling. Studying the increase in the volume fraction allowed us to quantify how the cost of the simulation increases rapidly with the RBC’s number and identify which routines have the biggest impact on the performance. One conclusion is that the cost is spread of several routines, which makes code optimization more cumbersome. However, the amount of RBCs and RBC nodes duplicated over processors is identified as a key factor for performance. Indeed, as RBCs may interact with several partitions, it is duplicated as much as needed based on criteria of boundaing box intersections. However, the current criteria have been shown to be too loose.

In order to limit the amount of work during the RBC processing, stricter criteria were introduced to avoid unnecessary calculations at the level of the nodes with a small gain in performance. On the other hand, much better results were obtained using cartesian partitioning to optimise the bounding box of each processor, reducing the involved RBC operations: this demonstrates that the performances of the RBC solver may be optimized by a stricter selection of RBC duplicates over processors.

We thank Ghislain Lartigue and Renaud Mercier for helpful discussions.

==== N6: Electrodeformation of red blood cells, extension to 3D and improved accuracy at membrane - A. Spadotto, S. Mendez (IMAG), M. Bernard (LEGI) ====

The Leaky Dielectric Model is a popular framework to describe electric stresses over micro-scale membranes. We have adopted it to simulate the effect of a DC electric field on a red blood cell using the YALES2BIO solver. The goal of the project is to reproduce the electric charging process of the membrane, as well as the resulting stresses, which may yield to electrodeformation of the cell. From the point of view of the implementation, the grid is represented by a 2D surface mesh embedded in a 3D eulerian grid. The need to make variables stored on the surface interact with quantities stored on the Eulerian grid calls for a proper bidirectional 2D-membrane/3D-grid dynamic connectivity. The advancement of theis task during this ECFD has led to the first 3D simulation of a charging fixed spherical shell. Moreover, the estimation of grid variables on elements cut by the membrane has been improved thanks to a High-Order extrapolation. The latter has been successfully tested on 2D configurations. The project opens the way for a series of validation tests. In particular, future work will demand treatment of instabilities emerging in symmetrical configurations.

==== N7: Optimisation Dorothy - M. Roperch, G. Pinon (LOMC), B. Gaston (CRIANN), P. Benard (CORIA) ====

Dorothy is a Lagrangian code using the particle vortex method. This method must have a homogeneous distribution of particles in space. To achieve this, at regular intervals during the simulation a Cartesian grid with new particles is created. The weights of the old particles are interpolated for each of the new particles. Before ECFD7, all the processors knew the general grid and the new particles. The aim of ECFD was to parallelize this module to avoid memory problem. To do this, each processor creates a grid corresponding to the particles it knows. They then exchange data on the supperposition zones. This solves the issue because the quantity of new particles known is smaller. During ECFD7, a trial on a ring vortex case was successfully carried out to test domain communications and supperposition. The next step will be to implement this new method in the Dorothy code.

=== Turbulence - P. Benard, CORIA & L. Bricteux, UMONS ===

==== T1: Wall Law for immersed boundaries – P. Bénez (CORIA), M. Cailler (Safran), S. Meynet (GDTech), J. Carmona (CORIA), Y. Bechane (CORIA) ====
Conservative Lagrangian Immersed Boundaries (CLIB) are now a useful way to take into account complex geometries in YALES2. In order to study highly turbulent configurations, it appears necessary to implement wall law models adapted to this method. If we consider a non-moving immersed body, developing wall-law models in a conservative immersed boundary formalism presents numerous challenges related to the diffuse interface property of the solid and the continuous formulation of the penalty force. During the ECFD, a new formulation of the penalty force has been established to ensure the imposition of the wall shear stress across the immersed solid interface. A strategy based on the use of two near-wall level sets was implemented to estimate the wall shear stress from the LES fluid velocity field at a distance D from the solid interface. At the end of the ECFD, turbulent flat plate cases were set up to start the validation of the strategy implemented for a logarithmic wall law. Future works will focus on validating this strategy for fixed solids.

==== T2: Turbulence injection Compressible flows – P. Tene Hedje (UMONS), J. Carmona (CORIA), Y. Bechane (CORIA), L. Bricteux (UMONS) ====

Turbulence injection for compressible flows remains a real challenge. Indeed, In these types of flow, the acoustic waves must also be controlled on boundaries. In addition, the non-reflective formulation of the Navier-Stokes characteristic Boundary Conditions (NSCBC) generally used in compressible solvers produce spurious pressure oscillations when applied to turbulent flows, making turbulence injection difficult for such applications. During the ECFD, two turbulence injection approaches were investigated and applied within the framework of the Explicit compressible solver (ECS) of YALES2. The first involved modifying the NSCBC formulation to inject turbulence from the inlet of the domain. To this end, the vortical-flow characteristic boundary condition [1] was implemented in ECS and the first validations were performed. The second was to use AL to generate a turbulence grid in the flow [2]. Future works will focus on further validating these approaches.

[1] ''Guézennec et al., Acoustically nonreflecting and reflecting boundary conditions for vortcity injection in compressible solvers, AIAA journal, 47(7), 1709-1722, 2009.''

[2] ''Houtin-Mongrolle et al., Actuator line method applied to grid turbulence generation for large-Eddy simulations, Journal of Turbulence, 21(8), 407-433, (2020).''

==== T3: Aero-servo-elastic simulations of wind turbines including atmospheric effects – E. Muller (SGRE), U. Vigny (UMONS), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE) ====
Aero-servo-elastic engineering solvers used in the industry (i.e., BHawC) for structural response and power assessments are unsuited for wake simulations, as aerodynamic loads are usually derived from a BEM-like method. To tackle this, the YALES2 library was coupled (P11-ECFD3) to BHawC, the Siemens Gamesa Renewable Energy (SGRE) in-house certification code for wind turbines. This allowed the investigation of neutral atmospheric conditions. This project aims to include stable and unstable atmospheric conditions into this coupling based on the development done in T4-ECFD7. Therefore, this project is divided into three work packages:
Work package 1: Adjustment and refactoring of the existing coupling library between YALES2 and BHawC.
Work package 2: Rethink how turbulence is injected into the simulation (recycling in SGRE setup) to consider thermal and Coriolis effects.
Work package 3: Adapt how the blade forces are computed in the coupling to consider the resulting density fluctuations.

==== T4: Atmospheric solver – U. Vigny (UMONS), L. Voivenel (CORIA), S. Zeoli (UMONS), P. Benard (CORIA) ====
Wind turbines, bigger and bigger, are now influenced by atmospheric flows. An atmospheric solver has already been developed in YALES2 to represents some of its effects (Coriolis, veer, thermal stratification). In this continuum, the project has been divided into two work-packages.
- Work-package 1: The use of the Variable density solver (VDS).
Before ECFD7, thermal stratification was taken into account using the Boussinesq buoyancy approximation within the incompressible solver framework. Now, VDS can be used, taking into account all thermal effect. Results are promissing.
- Work-package 2: Wall law velocity filtering.
Wall law are using velocity at the first grid node to compute wall shear stress. Before ECFD7, atmospheric wall law were using the local velocity, leading sometimes to convergence errors. Now a gather-scatter filter can be used to average velocity (and temperature) at first grid node.

==== T5: Implementation of the RVMs-WALE model in YALES2 – L. Bricteux (UMONS), P. Benard (CORIA), Y. Bechane (CORIA) ====
This study focused on the implementation of an advanced multiscale variational subgrid-scale model, incorporating scaling based on the WALE (Wall-Adapting Local Eddy-viscosity) model within YALES2. This model has demonstrated efficiency across various flow configurations, and it is anticipated that its multiscale nature can enhance the spectral selectivity. The aim is to ensure that its dissipative effects specifically target the smallest scales near the cut-off point.

Additionally, collaborative work with G. Balarac aimed to enhance the mesh adaptation strategy for wall-bounded flows with massive boundary layer detachment and vortical wake.
This new strategy based on vortex detection was developed during the ECFD6 and ECFD4 workshops. We have now shown that this strategy is effective.
Flow simulations around a hemisphere at Reynolds number Re=55K have been conducted, and we anticipate publishing the results soon."

==== T6: Development of coupling between YALES2-OpenFAST – A. Parinam (TUDelft/CORIA), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE) ====
Aero-servo-elastic engineering solvers, commonly used in the wind energy industry for structural response and power assessments, are unsuited for wake simulations, as aerodynamic loads are usually derived from a BEM-like method. To tackle this, the target is to couple the YALES2 library to OpenFAST, an NREL code for wind turbines, in the same way as the already existing YALES2-BHawC coupling.
An external coupling library has been created, linking the YALES2 and OpenFAST libraries and enabling the exchange of information between the data structures of each code. This data exchange has been tested and validated during the ECFD7. The next steps rely on exchanging the proper data during the actuator line tilmestep and further validate the coupling.

==== T7: Confidence intervals for estimators – C. Papagiannis, G.Balarac (LEGI), R. Letournel (Safran) ====

The inference of statistics for the results of LES requires quantifying the quality of the estimates from the available sample. In the case of CFD the sample is the number of measurements of a QoI and is very closely connected to the simulation's time step. Since our computational resources are finite, the sampling error of the estimates will never vanish. The purpose of this project is to provide Confidence Intervals (CI) to the inferred statistics so the user can have an indicator of the quality level of the simulation 'on-the-fly'. This requires the calculation of the autocorrelation of the collected samples, to correct the estimated sample variance used for the CI. This was achieved through a ''selective autocorrelation function estimator'' that also takes into account the non-constant time steps. With this we calculated sample-size independent confidence intervals that provide the corrected variance, compared to a naive estimation of the sample variance that assumed the samples as fully uncorrelated. With this we pave the way for having a universal estimator for the autocorrelation of some QoI, that incorporates autocorrelations and cross-correlations with the time-step.

=== Two Phase Flow - M. Cailler, Safran & V. Moureau, CORIA ===

==== P1: Level set reinitialization at the contact line for boiling flows - H. Lam, M. Benard, G. Ghigliotti (LEGI) ====

In Savinien Pertant's PhD thesis (2022), DNS of nucleate boiling at the bubble scale were performed, but suffered some lack of accuracy in the imposition of the contact angle.
Indeed, the contact angle was not well respected, with a difference of around 10 degrees between the desired angle and the angle measured on the solution.
This lack of accuracy, that contrast with the accurate imposition obtained in the spray solver (SPS), is due to fluctuations of the contact line. This behavior that was traced back to the modifications of the level set reinitialisation needed to take correctly into account the triple line, and for which the solution applied in 2022 was to revert to the standard Janodet reinitialisation.
S. Pertant tested, at the very end of its PhD, a correction which nullifies the temperature transport term at the first node from the wall of the contact line. This correction was introduced to overcome an instability of the code when the contact line velocity on the substrate changes direction, from receding to advancing.
It turned out at the ECFD7 that this correction proves to be very efficient to stabilise the contact line for contact angles between 50 to 90 degrees even in the case of the use of the level set reinitialization. We were able to simulate nucleate boiling with a smooth contact line at the triple line and a precision in the angle of the contact angle of around +-0.5 degrees. More work remains to be able to run DNS of nucleate boiling for extreme contact angles (<50° and >100°). Moreover, longer runs will be needed to further confirm these results.

==== P2: Compatibility of Boiling solver with PCS and MPH structure - H. Lam, M. Benard, G. Ghigliotti (LEGI) ====

The boiling solver does not work since the introduction of MPH data structure and PCS solver in March 2022. An investigation work was carried out to understand the changes made between the previous and the new version of the different solvers. A simple test case was created to show potential differences between the working version of the code and the new one. Several problems were spotted: the order of level set declaration became important as it is the first one declared which is advected. Sign convention was chosen differently for the mass transfer rate. The temporal discretization of the level set was different.
A test case with no flow and at an imposed mass transfer rate (i.e., no coupling of the level set with the temperature field) was run successfully and the results of the commit prior to the March 2022 modifications were retrieved. More work is needed to find the origin of the differences between the two solvers when the temperature field is solved and coupled with the level set and the velocity field. New common test cases for the two solvers will have to be implemented in order to cross-validate the results and avoid such cases happening again (i.e., cross-fertilization).

==== P3: Blood platelets adhesion model - C. Raveleau, S. Mendez, F. Nicoud (IMAG) ====

Medical devices in contact with blood (e.g. artificial valves) are used to treat various cardiovascular diseases, but their thrombogenicity remains the main unresolved issue in their development. A numerical model of blood platelets is being constructed to help to understand the effect of microstructuration on the thrombogenicity of artificial surface. The Force Coupling Method (FCM) was previously implemented and allows the modelisation of ellipsoidal particle and their interaction with the surrounding fluid. During the workshop, the particle model was extended to include adhesive and repulsive interactions with walls or with other particles. The adhesive bonds are modeled with springs forming when the distance between a node of a particle surface and a node of the wall or another particle is smaller than a given threshold. The stiffness of the bond is increased after a given formation time to mimic the 2-step adhesion process of platelets to von Willebrand Factor. A Lennard-Jones potential was used to model the collision of particles. Future work will aim at generalizing these implementations for an arbitrary number of particles (currently only working for 2 particles) and ensuring the interactions are unaltered by the crossing of a periodic boundary.

==== P4: vWF Unfolding - C. Raveleau, S. Mendez, F. Nicoud (IMAG) ====

Under certain circumstances, platelets do not bind to the surface but to a specific protein called Willebrand factor, which has the unique property of unfolding once subjected to a sufficiently high shear flow. The aim of this project was to investigate how to represent this mechanism within the YALES2 framework. During ECFD7, the immersed-boundary methodology already used to treat thin membranes (such as the red blood cell membrane) was extended to cover 1D elements evolving in a 3D flow. Preliminary tests have been successfully carried out, notably on a embedded beam immersed in shear flow, showing the potential of this approach to include another ingredient relevant to thrombosis modeling. Future work will include adding a repulsive force to avoid non-physical binding, as well as carrying out simulations involving Willebrand factor, platelets and red blood cells.

==== P5: Towards even more efficient particle algorithms - M. Helal (CORIA/Safran), M. Cailler (Safran) ====

Lagrangian particles are widely used in the YALES2 plateform to model: liquid spray, granular flow, two-phase flows with SPH approach or solids in IB method.
Though important developments to handle efficiently high number of particles in massively parallel simulations, the growing use of particles in Yales2 make necessary to re-evaluate and optimize the performances of Lagrangian particles algorithms handling.
Objective of this project was twofold: analyze and improve the performance and robustness of the newly developed SPH solver of YALES2 and improve the performance of the Lagrangian particle relocation (identification of connectivity between Lagrangian and Eulerian grid) during the Dynamic Mesh Adaptation.
Regarding the first subject, profiling tools have been used to identify the hot-spots and bottle-necks in the SPH solver. Optimizations including code factorization, removal of string comparison allows to reduce the computational cost by a factor 3. Moreover, robustification of the solver was achieved.
In the second sub-project, a new implicit 4th-level decomposition has been introduced. This implicit decomposition consists in contiguous coloring of sub-el_grp in element group. The availability of smallest group of elements has been used to improve the local particle relocation algorithm that mainly relies on bounding-box comparison. This new relocation algorithm has been tested for various number of sub-el_grp on a representative case of gear lubrication showing a decrease by a factor 3 to 5 of the relocation algorithm. Perspective is to extend the use of sub-el_grp to the interpolation algorithm.

==== P6: Two fluid and phase change in PCS - C. Merlin (Ariane Group), J. Carmona (CORIA), V. Moureau (CORIA) ====

==== P8: Wall liquid film numerical model - N. Gasnier (EM2C/Safran), J. Leparoux (Safran), J. Carmona (CORIA) ====
Wall liquid films are likely to be formed when fuel sprays impact the walls of aeronautical fuel injection systems. Such phenomenon may have a significant influence on the whole combustion process, however the small scales involved prevent from performing high fidelity simulations of film flows in the context of industrial geometries. Therefore, a low order model is required to model the dynamics of thin liquid flows under the action of spray droplets and of a turbulent gas shear. During ECFD7, a liquid film numerical model accounting for the influence of surface tension as well as gas shear, and based on the 2-dimensional Shallow Water Equations was implemented in Yales2. This model was then coupled to an algorithm ensuring a proper transition between fully resolved liquid structures (levelset) and film model during liquid droplet impacts on a solid wall.

==== P9: Casting simulation for the study of ceramic core displacement - S. Sirot, R. Mercier, M. Cailler (Safran), S. Meynet (GDTech) ====

Ceramic core displacement and deformation during the casting process is a major source of cooled blades manufacturing scrap. A possible source of core deformation may be the fluidic forces due to the filling of the mold with the liquid alloy. Predictive numerical simulations of the casting process would be an essential asset to increase the efficiency of the conception and industrial processes. During the workshop, a numerical methodology to simulate the filling process was drawn, with several modelling levels (with or without surface tension and slipping-wall conditions), in order to estimate the relevance of each of these models. Numerical results were then compared to available experimental results. Numerical deformation of the core was approximated as a beam flexion. Despite this post-processing approximation, the correlation between experimental measurements and numerical simulations is satisfying. The evolution of the core displacement with the inlet velocity of the fluid also has the same behaviour in the experiments and in the simulation. Future work will aim at including the dynamic contact angles in the simulations, in order to evaluate the relevance of this finer modelling, as well as correlating simulations with experiments on cases more representative of the industrial process.

==== P10: Velocity regularization for Euler-Lagrange conversion - I. El Yamani (CORIA/Safran), M. Cailler (Safran), L. Voivenel, J. Carmona (CORIA) ====

The Euler Lagrange multi scale approach aims to reduce the computational costs when simulating two phase flow. To reduce the cost even more, more droplets have to be converted in the Lagrangian formalism where droplets are seen as point forces. It implies that droplets can not always check the hypothesis of the LPP (Lagrangian Particle Point) formalism which is that the diameter of the particle has to be much smaller than the cell size. This hypothesis allows to have a good approximation of the undisturbed velocity for the Lagrangian particle. If the hypothesis is not checked when a Eulerian droplet is converted into a Lagrangian particle a residual velocity field can exists and therefore the velocity given to the particle is impacted by itself. This project aims to filter the gaseous velocity field through a gaussian filtering to remove the contribution of the Eulerian droplet to better approximate the undisturbed velocity.

=== Combustion - K. Bioche, CORIA & R. Mercier, Safran ===

==== C1: Plasma discharge models for reacting system - S. Wang, B. Kruljevic, B. Fiorina (EM2C), Y. Bechane (CORIA) ====

To reduce the expensive computational cost of Plasma-Assisted Combustion (PAC) full 3D simulations, the EM2C laboratory has developed phenomenological approaches to model Nanosecond Repetitively Pulsed (NRP) plasma discharges in reacting flows (Castela 2016 & Blanchard 2023). As part of previous works and ECFDs, both models were implemented and validated in the Low-Mach number framework (YALES2-VDS). While they were also implemented in the Compressible framework (YALES2-ECS), the validation against existing measurements or computations remained. During the workshop, numerical simulations of pin-to-pin configurations were performed with different numerical schemes and reactive mixtures to validate both models in ECS. The energy deposition was relatively well-validated through 2D simulations in the conditions of Castela et al. CNF 2016 and Rusterholtz et al. JPhysD 2013. A glimpse of baroclinic instabilities was observed through 3D simulations in the conditions of Castela et al. PROCI 2017.

==== C3: Dynamic sub-grid-scale wrinkling model for diffusion flames - S. Dillon (EM2C/Safran), R. Mercier (Safran), E. Espada, B. Fiorina, D. Veynante (EM2C) ====

Large-eddy-simulation (LES) of reactive flows is widely used in both academic and industrial applications. Combustion phenomena occur at a scale often smaller than the LES mesh size, therefore, turbulent combustion models are required to account for unresolved turbulent flame interactions. The modeling of sub-grid-scale (SGS) flame turbulence interactions can be described with a flame surface wrinkling factor which measures the ratio of the total flame surface area to the resolved flame surface area. Flame surface wrinkling models are often expressed by assuming equilibrium between turbulent motions and flame surface wrinkling, however, in realistic burners non-equilibrium is present and dynamic models are needed to adapt model parameters. Fractal-like models require information about the outer and inner cut-off length scales along with a fractal exponent, which is determined dynamically from resolved scales in the LES. The dynamic formalism can be coupled with the Filtered Tabulated Chemistry for LES (F-TACLES) model, where the required cut-off length scales are tabulated in the F-TACLES table along with other filtered thermochemical variables. The coupling of the F-TACLES model with the dynamic formalism has been previously applied to premixed flames in the past, however, the formal extension to non-premixed flames has never been investigated. The objective of this project is to investigate the performance of the dynamic SGS flame surface wrinkling model coupled with the F-TACLES model for non-premixed flames. A priori tests are conducted on a 2D H2/Air reactive mixing layer and HYLON, a 3D turbulent dual-swirl coaxial H2/Air injector. In both 2D and 3D cases, the modelled flame surface density shows good agreement with the filtered flame surface density extracted from the DNS. Moreover, the variation of the fractal model exponent in the HYLON test case is significant, highlighting the importance of the dynamic procedure. A posteriori tests were also conducted, and modelled chemical reaction rates show promising results.

==== C4: Developement of an automated virtual scheme generator for CFD - T. Luu, M. Hustache, N. Darabiha, B. Fiorina (EM2C) ====

In reactive CFD simulations, a non-negligible part of the time cost is spent in the resolution of the chemical system. Simplified chemistry models aim to reduce the number of transported species while still ensuring a correct representation of the phenomena of interest. Among them, the virtual chemistry method consists of using “virtual” species and reactions to reproduce detailed chemistry results through a mechanism of drastically smaller size. These “virtual” species and reactions are optimized to target quantities of interest such as temperature, laminar flame speed or pollutants. In practice, the optimization is done using a learning database composed of representative canonical reactive configurations computed with detailed chemistry. The objective of this project was to develop a tool to easily generate virtual schemes. The tool, named VISION (Virtual Scheme optimizatION), is currently able to both generate a user-defined database of wide reactive configurations and optimize a given scheme structure using either CANTERA or REGATH.

==== C5: Partially-Stirred reactor model for MILD combustion - E. Stendardo, L. Bricteux (UMONS), M. Laignel, K. Bioche (CORIA), J. Blondeau (VUB) ====

MILD combustion produces intense turbulence and extensive reaction zones, necessitating costly mesh refinement over large areas. Practical mesh lacks precision, leading to sub-grid heterogeneity and turbulent fluctuations. A Partially Stirred Reactor model was implemented to address turbulence-combustion interaction. This model multiplies the source term by a limiter factor, allowing modelling of residence time in the inner cell reactive structure. Testing various limiter formulations based on mixing and chemical timescales revealed increased computational costs. Future work aims to reduce costs by utilizing the model only where necessary. This ongoing research seeks to optimize performance while minimizing computational overhead for efficient application in engineering scenarios.

==== C6: Static Mesh Adaptation for Hydrogen High pressure combustion using GPUs - G. Hexilar, C. Brunet, R. Mari, S. Richard (Safran), P. Pouech, Q. Douasbin, G. Staffelbach (Cerfacs) ====

This research project focuses on advancing the understanding of hydrogen combustion under high-pressure conditions (up to 10bars), employing an automated workflow coupled with static mesh adaptation to tailor computational simulations to specific requirements. The study aims to enhance the accuracy and efficiency of combustion models by utilizing both Central Processing Units (CPUs) and Graphics Processing Units (GPUs). The automated workflow streamlines the simulation process, optimizing resource utilization and minimizing manual intervention. Static mesh adaptation further refines the computational mesh based on evolving combustion dynamics, ensuring accurate representation of high-pressure hydrogen combustion phenomena. By leveraging the parallel processing capabilities of GPUs alongside traditional CPUs, the research team aims to achieve significant computational speedup. This innovative approach not only contributes to fundamental insights into high-pressure hydrogen combustion but also establishes a robust framework for scalable and efficient simulations in complex reactive flow scenarios.

==== C7: High fidelity simulation of a cone calorimeter - A. Grenouilloux, K. Bioche (CORIA), N. Dellinger (ONERA), R. Letournel (Safran) ====

A methodology to simulate the decomposition of a composite sample in a calorimeter cone has been proposed. The configuration consists in the imposition of an incident radiative flux that heats the test coupon until it degrades. During test campaigns, the composite degradation can lead to the auto-ignition of the outgassed species, a phenomenon that needs to be predicted by the simulation. The variety of physical phenomena encountered, as well as the different characteristic time-scales, require the implementation of a coupled simulation. Hence, the proposed methodology relies on the coupling between two solvers of the massively parallel library YALES2 (fluid, radiation) and the MoDeThec solver developed at ONERA (solid degradation). First, a set of elementary validation tests to characterize the composite’s properties has been performed, showing good agreement with experimental data. A reduced three-equation kinetic scheme for the ignition delay has been derived, which aligns with experimental observations. Additionally, the framework for the coupled simulation involving the three solvers has been implemented.

=== User Experience & Data - L. Korzeczek, GDTech ===

==== U1: Refactoring the YALES2 tools - J. Leparoux, M. Cailler (Safran), L. Voivenel, J. Carmona, I. El Yamani (CORIA), S. Meynet, L. Korzeczek (GDTech) ====

The YALES2 distribution "tools" are becoming difficult to read and are a mixture of several types of tools. This leaves developers and end users unaware of what exists and how to use it (duplicated functions or tools) and makes it impossible to propose generic data analysis tools (FFT, confidence intervals, ...) that can be easily applied to YALES2 data structures. The main efforts have been concentrated on promoting a new architecture for YALES2 distribution tools with an object-oriented structure, including a refactored version of the main readers of YALES2 data. Several tutorials using Jupyter notebooks have been published for demonstration and explanation. A new CLI is now available under the name 'y2tools'. More work is needed before this structure can be pushed to the master trunk.

==== U2: Improved USEX for Multi-Scale Eulerian-Lagrangian simulation - L. Voivenel, J. Carmona, I. El Yamani (CORIA) J. Leparoux, M. Cailler (Safran) ====

The multi-scale Eulerian-Lagrangian approach has now reached a certain maturity and is being used to simulate fuel spray atomization. Post-treatments of these multi-scale simulations require the development of specific tools that track liquid structures either described in an Eulerian or Lagrangian way. In this project, we implemented a strategy to register in a post-treatment particle-set all Eulerian droplets crossing an arbitrarily shaped surface (described with an interior-boundary). The strategy is based on artificial Eulerian droplet advancement (using a Lagrangian representation) and verification of the new Eulerian droplet position compared to the surface of interest. We used this strategy to build a new post-treatment that allows to track both Eulerian and Lagrangian structures and build particle size or velocity distributions.

==== U3: Evaluate technological debt - P. Pouech, T. Marzlin, A. Dauptain (CERFACS) ====

The tool dedicated to cleaning and extracting meta data from the codebase (Tucan) is now able to interpret the most common #Ifdef statements for C and Fortran. Is has be tested successfully on the codebases of Yales2 , Neko and some elements of the Chemkin II package, for Fortran language. It has also included more tests, especially on C++ samples and large files. The call-graph aggregator “Marauder’s map” also evolved a lot. It was used as a refactoring monitoring tool for the UX project U1 “Refactoring Yales2 tools”, on a codebase mixing Python and Fortran files, providing feedback to both U3 and U1 teams. With this experience, we plan to add at least two additional complexity metrics in the months to come : the single component #Ifdef footprint , and the Custom Structures footprint : declarations vs usage in the code.

==== U4: CWIPI 1.0 porting - N. Dellinger, B. Andrieu, K. Hoogveld, E. Quémerais (ONERA), A. Grenouilloux (CORIA), R. Letournel (Safran) ====

Coupling is a cornerstone of numerical simulation, especially for addressing multi-physics problems using highly-specialized solvers for each phenomenon. The CWIPI library, developed at ONERA for coupling codes in a massively parallel environment, has been used in YALES2 for many years for internal and external coupling.
Significant developments have been carried out in recent years to improve the performance and usability of CWIPI, resulting in the release of version 1 in july 2023. This version features a completely revised API to overcome the limitations of version 0.12 and offer more possibilities to users.
The goal of this project was to support users in their transition to version 1. A training course based on Jupyter Notebooks was first organized. Assistance was then provided to successfully port MoDeTheC's and YALES2's internal couplings to the new version. Some fixes were made in CWIPI along the way, and will be reported in a new patched version.

==== U5: Integration of YALES2 in PRESTO supervisor - A. Pushkarev (GE Vernova), G. Balarac (LEGI) ====

A Graphical User Interface (GUI) exists at GE Vernova (Hydro) provides a user-friendly solution to perform numerical simulations for typical geometries of hydroelectric turbine in varied operation regimes. Previously, we implemented an interface for YALES2 code as alternative of CFX solver for this GUI client. Actual project is dedicated to implementation of the automatic mesh generation process for the runner section of the turbine using only section profile files of geometry such as blade profiles, meridional channel section, guide vane profile, etc... The algorithm should be able to generate a new *.msh mesh file once geometry profiles are updated as well as to setup standard named sections of the numerical domain.

==== U6: Optimization of YALES2 compilation time - R. Mercier (Safran), G. Lartigue (Total Energy) ====

YALES2 Compilation time are very different depending on the compilation options but also depending on the machine where the code is built. In particular, compiling on cluster always takes more time than on local station because of the slower filesystems. On these machines, the compilation time scales with the number of .f90 files to compile. 1) During this project we proposed a detailed compilation timing system activated with Y2_COMPILE_TIMER = TRUE and plotted using the tool y2_compilation_gantt.py. 2) This new profiling tool allowed us to spot some very large modules including a lot a dependencies. To enhance the tasks parallelization, the expl_comp_numerics module has been spliced in several modules. An important gain in compilation time has been obtained. 3) Another optimization of the compilation time have been developed using an on-the-fly modification of the .f90 tree. The use of _h.f90 is dynamically removed and replaced by including equivalent .defs files allowing the divide by 2 the number of .f90 files to compile. Some demonstrations have been performed but this compilation mode still have to be cross-check with different compilation option to ensure its robustness.
At the end of the project, the sum of all optimization allowed us to divide by 2 the compilation time.

Ecfd:ecfd 7th edition

2024-03-06T09:17:25Z

Benard: /* Numerics - S. Mendez, IMAG & G. Balarac, LEGI */

{{DISPLAYTITLE: ECFD workshop, 7th edition, 2024}}

== Description ==

* Event from '''22th of January to 2nd of February 2024'''
* Location: [https://www.hotelclubdelaplage.com Hôtel Club de la Plage], Merville-Franceville, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* More than 70 participants from academics, HPC center/experts and industry.

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes
* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Viovenel (CORIA).
[[File:ecfd7.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_6th_edition]]

[[File:sponsor_ecfd7.png|text-bottom|600px]]



== Agenda ==

[[File:agenda_ecfd7.png|text-bottom|600px]]

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

To come...

== Projects ==

=== Hackathon GENCI - P. Begou, LEGI ===
The '''GENCI Hackathon''' will be devoted to porting two CFD codes to the Mi250 GPUs of the Adastra supercomputer deployed by GENCI at CINES.

For the '''YALES2''' code the goal is to obtain a first reference version giving the expected results then, if possible, to start its optimization to gain performance. The approach is OpenACC based with the objective of an implementation as least intrusive as possible in the existing code and which remains portable with the work done on the Nvidia GPUs of the Jean-Zay supercomputer at IDRIS.

The porting of the '''AVBP''' code is more advanced with a prototype already functional on Adastra but "hard-coded". The objective is to rationalize this first implementation, to integrate the latest developments in the code, to centralize memory management (host and device), to work on porting the Lagrangian part of the code and, of course, to improve the global performance.

This Hackathon is supported by GENCI, HPE, AMD and CINES with the presence on site of several development experts on AMD GPUS.

=== Mesh adaptation - R. Letournel, Safran ===

==== M1: ASMR for reheat chamber applications - Paul Pouech (CERFACS), Thibault Duranton, Luis Carbajal Carrasco (Safran) ====

Combustion in reheat chambers feature a wide range of lenght scales. Mesh refinement is thus mandatory to capture the flow characteristics within a reasonnable CPU cost for LES computations using the AVBP code. The purpose of this project is to consolidate mesh refinement criteria and strategy in an academic reference case. The retained workflow is supported by the [https://lemmings.readthedocs.io/en/latest/readme_copy.html Lemmings] code that calls the Tékigô wrapper for the mesh adaptations. During the ECFD7, the convergence time needed to have significant distribution of quantities of interest was analysed. An optimum runtime, based on a characteristic flow time-scale, was thus identified and led to a reduced running time for each adaptation step. As a second step, discussions with the ECFD7 participants led to the identification of interesting refinement criteria, namely the flame sensor or the mach rms for instance. Parametric analysis showed the robustness of the workflow based on a ponderation of different criteria. Finally, in order to facilitate the use of the workflow, efforts were made to improve the user experience by making it more human readable.

==== M2: Parallel remeshing - B. Andrieu, C. Benazet, K. Hoogveld, B. Maugars, E. Quémerais (ONERA) ====

Mesh adaptation is a crucial tool in order to automate industrial RANS numerical simulations. To meet this need, we need to carry out mesh adaptation as quickly as possible by setting up an efficient, parallel solution. To this end, we have explored two avenues: a parallel edge-splitting algorithm that has recently been initiated in the ParaDiGM library, and a solution based on [https://github.com/nasa/refine the refine library] for adapting meshes with MPI implementation. On the one hand, we fixed several bugs in our split operator, and validated it on test cases of increasing complexity with a node-centered solver. In addition, we've added interfaces to refine so as to avoid using files, and call directly in library mode. We also investigated geometric projection issues during the mesh adaptation procedure, notably by looking at solutions such as EGADS, which offers a simplified API for CAD interrogation. We finally implemented metric gradation (in serial), metric intersection and complexity computations. All the ingredients we've tested give us a clearer picture of the entire mesh adaptation process.

==== M3: Anisotropic mesh refinement - R. Barbera (LEGI/Safran), G. Ghigliotti, G. Balarac (LEGI), R. Letournel (Safran) ====

Mesh adaptation is now a key feature for simulations of complex industrial flows. For transient flows such as multiphase and/or reactive flows, where regions of interest are strongly moving in space, dynamic mesh adaptation appears as the most suitable strategy. This strategy is now widely used in YALES2 based on isotropic mesh definition. The purpose of this project is to adapt this strategy to an anisotropic framework to reduce the overall simulation costs (in term of memory consumption, cpu cost and time to solution). In order to be able to handle multiphase flows, the main objective of the project is to study the conditions for accurately describing the dynamics of the level-set function with an anisotropic mesh. Accuracy is mainly assessed in terms of interface position and mass conservation. The inaccuracy of mass conservation is mainly due to interpolation errors after the adaptation step. Furthermore, inaccuracy in interface position may be due to misalignment between the anisotropic mesh elements and the interface normal. The first methodological corrections have been proposed, as an adaptation of the level-set reinitialization algorithm to the anisotropic mesh.

=== Numerics - S. Mendez, IMAG & G. Balarac, LEGI ===

==== N1: Treatment of boundary conditions for high-order schemes - M. Bernard & G. Balarac (LEGI), G. Lartigue (Total Energies) ====

In the context of Finite Volumes Method, spacial accuracy of a numerical scheme depends on ability to evaluate accurately fluxes through interface of each control volume (CV).
Such accurate evaluation is not straightforward, especially when dealing with distorted grids.
This project follows the work of [1] where fluxes use pointwise quantities, which are reconstructed from integrated quantities advanced in time.
During the workshop, task force was dedicated to the treatment of **inlet** boundary conditions (BC) and **non-planar walls**.
For inlet BC, the key resides in the spatial integration of convective flux over discrete faces of the CV touching the boundary.
Such treatment lead to exact integration for linear inlet profile and large error reduction on other profiles.
Concerning non-planar walls, the strategy adopted consists in the enforcement of the BC on each discrete face, by modifying the normal component of the wall gradient in order to evaluate accurately the diffusive flux.
Again, a large reduction of this error has been observed.

[1] ''A framework to perform high-order deconvolution for finite-volume method on simplicial meshes, , Bernard et. al., IJNMF 2020''

==== N2: Implementation of linearised implicit time integration in ALE solver - T. Berthelon, G. Balarac (LEGI) ====

An linearised implicit time integration has recently been developed in the incompressible solver of YALES2. This new integration scheme allows to use larger time-step that the ones constraints by classic stability criteria inherent to explicit time integration method. This allows to reduce the restitution time of Large Eddy Simulations [1].
The objective of this project was to implement this new time integration in the ale solver in order to be able to reduce restitution time of moving mesh configuration.

The developments were validated on a scalar advection case and on a rotor-stator interaction case. Although the results seem to be in line with the explicit integration methods, the validation of the temporal convergence to 2nd order remains to be shown.

[1] ''Toward the use of LES for industrial complex geometries. Part II: Reduce the time-to-solution by using a linearised implicit time advancement, Berthelon et al., JoT, 2023''

==== N3: Parallelisation of Actuator Line Method - H. Mulakaloori (CORIA), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE) ====

The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified: (i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges. (ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications.

==== N4: Non-uniform outlet pressure and coupling with CWIPI - J. B. Lagaert (LMO), Y. Lakrifi, T. Berthelon, G.Balarac (LEGI) , R. Letournel (Safran) ====

In simulations, artificial boundaries need to be introduced due to the limited size of computational domains. At these boundaries, flow variables need to be calculated in a way that will not induce any perturbation of the interior solution. During ECFD#7, a generic outlet boundary condition defined from non-uniform pressure has been implemented in Yales2. This non-uniform pressure can de determined from a traction model (null or advected from the interior domain, for example). This non-uniform pressure can also be deducted through a coupling between two simulations. In this case a coupling via CWIPI is performed where the velocity and the pressure are exchanged at the common boundary to define the inlet and outlet conditions, respectively.

==== N5: Optimization of the RBC solver - F. Rojas, S. Mendez (IMAG) ====

In the study of blood diseases, the mechanical behaviour of Red Blood Cells (RBCs) is one of the most relevant effects to take into account in the numerical models but also in experimental setups. Our system of interest is the thin gap of a rheometer where RBC suspensions are placed to explore their properties. To interpret the experimental data, the simulations of large suspensions of RBC are required to determine the blood’s microstructure (spatial arrangement of cells) and its rheological properties.

Currently, YALES2BIO’s RBC solver is capable to manage thousands of cells, but in order to approach closer to the experimental scales, we propose the characterisation and optimisation of its performance to reduce the computational requirements and increase the RBC’s number and domain sizes in our simulations. During the workshop a parametric study was carried out to obtain the strong and weak scaling. Studying the increase in the volume fraction allowed us to quantify how the cost of the simulation increases rapidly with the RBC’s number and identify which routines have the biggest impact on the performance. One conclusion is that the cost is spread of several routines, which makes code optimization more cumbersome. However, the amount of RBCs and RBC nodes duplicated over processors is identified as a key factor for performance. Indeed, as RBCs may interact with several partitions, it is duplicated as much as needed based on criteria of boundaing box intersections. However, the current criteria have been shown to be too loose.

In order to limit the amount of work during the RBC processing, stricter criteria were introduced to avoid unnecessary calculations at the level of the nodes with a small gain in performance. On the other hand, much better results were obtained using cartesian partitioning to optimise the bounding box of each processor, reducing the involved RBC operations: this demonstrates that the performances of the RBC solver may be optimized by a stricter selection of RBC duplicates over processors.

We thank Ghislain Lartigue and Renaud Mercier for helpful discussions.

==== N6: Electrodeformation of red blood cells, extension to 3D and improved accuracy at membrane - A. Spadotto, S. Mendez (IMAG), M. Bernard (LEGI) ====

The Leaky Dielectric Model is a popular framework to describe electric stresses over micro-scale membranes. We have adopted it to simulate the effect of a DC electric field on a red blood cell using the YALES2BIO solver. The goal of the project is to reproduce the electric charging process of the membrane, as well as the resulting stresses, which may yield to electrodeformation of the cell. From the point of view of the implementation, the grid is represented by a 2D surface mesh embedded in a 3D eulerian grid. The need to make variables stored on the surface interact with quantities stored on the Eulerian grid calls for a proper bidirectional 2D-membrane/3D-grid dynamic connectivity. The advancement of theis task during this ECFD has led to the first 3D simulation of a charging fixed spherical shell. Moreover, the estimation of grid variables on elements cut by the membrane has been improved thanks to a High-Order extrapolation. The latter has been successfully tested on 2D configurations. The project opens the way for a series of validation tests. In particular, future work will demand treatment of instabilities emerging in symmetrical configurations.

==== N7: Optimisation Dorothy - M. Roperch, G. Pinon (LOMC), B. Gaston (CRIANN), P. Benard (CORIA) ====

Dorothy is a Lagrangian code using the particle vortex method. This method must have a homogeneous distribution of particles in space. To achieve this, at regular intervals during the simulation a Cartesian grid with new particles is created. The weights of the old particles are interpolated for each of the new particles. Before ECFD7, all the processors knew the general grid and the new particles. The aim of ECFD was to parallelize this module to avoid memory problem. To do this, each processor creates a grid corresponding to the particles it knows. They then exchange data on the supperposition zones. This solves the issue because the quantity of new particles known is smaller. During ECFD7, a trial on a ring vortex case was successfully carried out to test domain communications and supperposition. The next step will be to implement this new method in the Dorothy code.

=== Turbulence - P. Benard, CORIA & L. Bricteux, UMONS ===

==== T1: Wall Law for immersed boundaries – P. Bénez (CORIA), M. Cailler (Safran), S. Meynet (GDTech), J. Carmona (CORIA), Y. Bechane (CORIA) ====
Conservative Lagrangian Immersed Boundaries (CLIB) are now a useful way to take into account complex geometries in YALES2. In order to study highly turbulent configurations, it appears necessary to implement wall law models adapted to this method. If we consider a non-moving immersed body, developing wall-law models in a conservative immersed boundary formalism presents numerous challenges related to the diffuse interface property of the solid and the continuous formulation of the penalty force. During the ECFD, a new formulation of the penalty force has been established to ensure the imposition of the wall shear stress across the immersed solid interface. A strategy based on the use of two near-wall level sets was implemented to estimate the wall shear stress from the LES fluid velocity field at a distance D from the solid interface. At the end of the ECFD, turbulent flat plate cases were set up to start the validation of the strategy implemented for a logarithmic wall law. Future works will focus on validating this strategy for fixed solids.

==== T2: Turbulence injection Compressible flows – P. Tene Hedje (UMONS), J. Carmona (CORIA), Y. Bechane (CORIA), L. Bricteux (UMONS) ====

Turbulence injection for compressible flows remains a real challenge. Indeed, In these types of flow, the acoustic waves must also be controlled on boundaries. In addition, the non-reflective formulation of the Navier-Stokes characteristic Boundary Conditions (NSCBC) generally used in compressible solvers produce spurious pressure oscillations when applied to turbulent flows, making turbulence injection difficult for such applications. During the ECFD, two turbulence injection approaches were investigated and applied within the framework of the Explicit compressible solver (ECS) of YALES2. The first involved modifying the NSCBC formulation to inject turbulence from the inlet of the domain. To this end, the vortical-flow characteristic boundary condition [1] was implemented in ECS and the first validations were performed. The second was to use AL to generate a turbulence grid in the flow [2]. Future works will focus on further validating these approaches.

[1] ''Guézennec et al., Acoustically nonreflecting and reflecting boundary conditions for vortcity injection in compressible solvers, AIAA journal, 47(7), 1709-1722, 2009.''

[2] ''Houtin-Mongrolle et al., Actuator line method applied to grid turbulence generation for large-Eddy simulations, Journal of Turbulence, 21(8), 407-433, (2020).''

==== T3: Aero-servo-elastic simulations of wind turbines including atmospheric effects – E. Muller (SGRE), U. Vigny (UMONS), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE) ====
Aero-servo-elastic engineering solvers used in the industry (i.e., BHawC) for structural response and power assessments are unsuited for wake simulations, as aerodynamic loads are usually derived from a BEM-like method. To tackle this, the YALES2 library was coupled (P11-ECFD3) to BHawC, the Siemens Gamesa Renewable Energy (SGRE) in-house certification code for wind turbines. This allowed the investigation of neutral atmospheric conditions. This project aims to include stable and unstable atmospheric conditions into this coupling based on the development done in T4-ECFD7. Therefore, this project is divided into three work packages:
Work package 1: Adjustment and refactoring of the existing coupling library between YALES2 and BHawC.
Work package 2: Rethink how turbulence is injected into the simulation (recycling in SGRE setup) to consider thermal and Coriolis effects.
Work package 3: Adapt how the blade forces are computed in the coupling to consider the resulting density fluctuations.

==== T4: Atmospheric solver – U. Vigny (UMONS), L. Voivenel (CORIA), S. Zeoli (UMONS), P. Benard (CORIA) ====
Wind turbines, bigger and bigger, are now influenced by atmospheric flows. An atmospheric solver has already been developed in YALES2 to represents some of its effects (Coriolis, veer, thermal stratification). In this continuum, the project has been divided into two work-packages.
- Work-package 1: The use of the Variable density solver (VDS).
Before ECFD7, thermal stratification was taken into account using the Boussinesq buoyancy approximation within the incompressible solver framework. Now, VDS can be used, taking into account all thermal effect. Results are promissing.
- Work-package 2: Wall law velocity filtering.
Wall law are using velocity at the first grid node to compute wall shear stress. Before ECFD7, atmospheric wall law were using the local velocity, leading sometimes to convergence errors. Now a gather-scatter filter can be used to average velocity (and temperature) at first grid node.

==== T5: Implementation of the RVMs-WALE model in YALES2 – L. Bricteux (UMONS), P. Benard (CORIA), Y. Bechane (CORIA) ====
This study focused on the implementation of an advanced multiscale variational subgrid-scale model, incorporating scaling based on the WALE (Wall-Adapting Local Eddy-viscosity) model within YALES2. This model has demonstrated efficiency across various flow configurations, and it is anticipated that its multiscale nature can enhance the spectral selectivity. The aim is to ensure that its dissipative effects specifically target the smallest scales near the cut-off point.

Additionally, collaborative work with G. Balarac aimed to enhance the mesh adaptation strategy for wall-bounded flows with massive boundary layer detachment and vortical wake.
This new strategy based on vortex detection was developed during the ECFD6 and ECFD4 workshops. We have now shown that this strategy is effective.
Flow simulations around a hemisphere at Reynolds number Re=55K have been conducted, and we anticipate publishing the results soon."

==== T6: Development of coupling between YALES2-OpenFAST – A. Parinam (TUDelft/CORIA), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE) ====

==== T7: Confidence intervals for estimators – C. Papagiannis, G.Balarac (LEGI), R. Letournel (Safran) ====

The inference of statistics for the results of LES requires quantifying the quality of the estimates from the available sample. In the case of CFD the sample is the number of measurements of a QoI and is very closely connected to the simulation's time step. Since our computational resources are finite, the sampling error of the estimates will never vanish. The purpose of this project is to provide Confidence Intervals (CI) to the inferred statistics so the user can have an indicator of the quality level of the simulation 'on-the-fly'. This requires the calculation of the autocorrelation of the collected samples, to correct the estimated sample variance used for the CI. This was achieved through a ''selective autocorrelation function estimator'' that also takes into account the non-constant time steps. With this we calculated sample-size independent confidence intervals that provide the corrected variance, compared to a naive estimation of the sample variance that assumed the samples as fully uncorrelated. With this we pave the way for having a universal estimator for the autocorrelation of some QoI, that incorporates autocorrelations and cross-correlations with the time-step.

=== Two Phase Flow - M. Cailler, Safran & V. Moureau, CORIA ===

==== P1: Level set reinitialization at the contact line for boiling flows - H. Lam, M. Benard, G. Ghigliotti (LEGI) ====

In Savinien Pertant's PhD thesis (2022), DNS of nucleate boiling at the bubble scale were performed, but suffered some lack of accuracy in the imposition of the contact angle.
Indeed, the contact angle was not well respected, with a difference of around 10 degrees between the desired angle and the angle measured on the solution.
This lack of accuracy, that contrast with the accurate imposition obtained in the spray solver (SPS), is due to fluctuations of the contact line. This behavior that was traced back to the modifications of the level set reinitialisation needed to take correctly into account the triple line, and for which the solution applied in 2022 was to revert to the standard Janodet reinitialisation.
S. Pertant tested, at the very end of its PhD, a correction which nullifies the temperature transport term at the first node from the wall of the contact line. This correction was introduced to overcome an instability of the code when the contact line velocity on the substrate changes direction, from receding to advancing.
It turned out at the ECFD7 that this correction proves to be very efficient to stabilise the contact line for contact angles between 50 to 90 degrees even in the case of the use of the level set reinitialization. We were able to simulate nucleate boiling with a smooth contact line at the triple line and a precision in the angle of the contact angle of around +-0.5 degrees. More work remains to be able to run DNS of nucleate boiling for extreme contact angles (<50° and >100°). Moreover, longer runs will be needed to further confirm these results.

==== P2: Compatibility of Boiling solver with PCS and MPH structure - H. Lam, M. Benard, G. Ghigliotti (LEGI) ====

The boiling solver does not work since the introduction of MPH data structure and PCS solver in March 2022. An investigation work was carried out to understand the changes made between the previous and the new version of the different solvers. A simple test case was created to show potential differences between the working version of the code and the new one. Several problems were spotted: the order of level set declaration became important as it is the first one declared which is advected. Sign convention was chosen differently for the mass transfer rate. The temporal discretization of the level set was different.
A test case with no flow and at an imposed mass transfer rate (i.e., no coupling of the level set with the temperature field) was run successfully and the results of the commit prior to the March 2022 modifications were retrieved. More work is needed to find the origin of the differences between the two solvers when the temperature field is solved and coupled with the level set and the velocity field. New common test cases for the two solvers will have to be implemented in order to cross-validate the results and avoid such cases happening again (i.e., cross-fertilization).

==== P3: Blood platelets adhesion model - C. Raveleau, S. Mendez, F. Nicoud (IMAG) ====

Medical devices in contact with blood (e.g. artificial valves) are used to treat various cardiovascular diseases, but their thrombogenicity remains the main unresolved issue in their development. A numerical model of blood platelets is being constructed to help to understand the effect of microstructuration on the thrombogenicity of artificial surface. The Force Coupling Method (FCM) was previously implemented and allows the modelisation of ellipsoidal particle and their interaction with the surrounding fluid. During the workshop, the particle model was extended to include adhesive and repulsive interactions with walls or with other particles. The adhesive bonds are modeled with springs forming when the distance between a node of a particle surface and a node of the wall or another particle is smaller than a given threshold. The stiffness of the bond is increased after a given formation time to mimic the 2-step adhesion process of platelets to von Willebrand Factor. A Lennard-Jones potential was used to model the collision of particles. Future work will aim at generalizing these implementations for an arbitrary number of particles (currently only working for 2 particles) and ensuring the interactions are unaltered by the crossing of a periodic boundary.

==== P4: vWF Unfolding - C. Raveleau, S. Mendez, F. Nicoud (IMAG) ====

Under certain circumstances, platelets do not bind to the surface but to a specific protein called Willebrand factor, which has the unique property of unfolding once subjected to a sufficiently high shear flow. The aim of this project was to investigate how to represent this mechanism within the YALES2 framework. During ECFD7, the immersed-boundary methodology already used to treat thin membranes (such as the red blood cell membrane) was extended to cover 1D elements evolving in a 3D flow. Preliminary tests have been successfully carried out, notably on a embedded beam immersed in shear flow, showing the potential of this approach to include another ingredient relevant to thrombosis modeling. Future work will include adding a repulsive force to avoid non-physical binding, as well as carrying out simulations involving Willebrand factor, platelets and red blood cells.

==== P5: Towards even more efficient particle algorithms - M. Helal (CORIA/Safran), M. Cailler (Safran) ====

Lagrangian particles are widely used in the YALES2 plateform to model: liquid spray, granular flow, two-phase flows with SPH approach or solids in IB method.
Though important developments to handle efficiently high number of particles in massively parallel simulations, the growing use of particles in Yales2 make necessary to re-evaluate and optimize the performances of Lagrangian particles algorithms handling.
Objective of this project was twofold: analyze and improve the performance and robustness of the newly developed SPH solver of YALES2 and improve the performance of the Lagrangian particle relocation (identification of connectivity between Lagrangian and Eulerian grid) during the Dynamic Mesh Adaptation.
Regarding the first subject, profiling tools have been used to identify the hot-spots and bottle-necks in the SPH solver. Optimizations including code factorization, removal of string comparison allows to reduce the computational cost by a factor 3. Moreover, robustification of the solver was achieved.
In the second sub-project, a new implicit 4th-level decomposition has been introduced. This implicit decomposition consists in contiguous coloring of sub-el_grp in element group. The availability of smallest group of elements has been used to improve the local particle relocation algorithm that mainly relies on bounding-box comparison. This new relocation algorithm has been tested for various number of sub-el_grp on a representative case of gear lubrication showing a decrease by a factor 3 to 5 of the relocation algorithm. Perspective is to extend the use of sub-el_grp to the interpolation algorithm.

==== P6: Two fluid and phase change in PCS - C. Merlin (Ariane Group), J. Carmona (CORIA), V. Moureau (CORIA) ====

==== P8: Wall liquid film numerical model - N. Gasnier (EM2C/Safran), J. Leparoux (Safran), J. Carmona (CORIA) ====
Wall liquid films are likely to be formed when fuel sprays impact the walls of aeronautical fuel injection systems. Such phenomenon may have a significant influence on the whole combustion process, however the small scales involved prevent from performing high fidelity simulations of film flows in the context of industrial geometries. Therefore, a low order model is required to model the dynamics of thin liquid flows under the action of spray droplets and of a turbulent gas shear. During ECFD7, a liquid film numerical model accounting for the influence of surface tension as well as gas shear, and based on the 2-dimensional Shallow Water Equations was implemented in Yales2. This model was then coupled to an algorithm ensuring a proper transition between fully resolved liquid structures (levelset) and film model during liquid droplet impacts on a solid wall.

==== P9: Casting simulation for the study of ceramic core displacement - S. Sirot, R. Mercier, M. Cailler (Safran), S. Meynet (GDTech) ====

Ceramic core displacement and deformation during the casting process is a major source of cooled blades manufacturing scrap. A possible source of core deformation may be the fluidic forces due to the filling of the mold with the liquid alloy. Predictive numerical simulations of the casting process would be an essential asset to increase the efficiency of the conception and industrial processes. During the workshop, a numerical methodology to simulate the filling process was drawn, with several modelling levels (with or without surface tension and slipping-wall conditions), in order to estimate the relevance of each of these models. Numerical results were then compared to available experimental results. Numerical deformation of the core was approximated as a beam flexion. Despite this post-processing approximation, the correlation between experimental measurements and numerical simulations is satisfying. The evolution of the core displacement with the inlet velocity of the fluid also has the same behaviour in the experiments and in the simulation. Future work will aim at including the dynamic contact angles in the simulations, in order to evaluate the relevance of this finer modelling, as well as correlating simulations with experiments on cases more representative of the industrial process.

==== P10: Velocity regularization for Euler-Lagrange conversion - I. El Yamani (CORIA/Safran), M. Cailler (Safran), L. Voivenel, J. Carmona (CORIA) ====

The Euler Lagrange multi scale approach aims to reduce the computational costs when simulating two phase flow. To reduce the cost even more, more droplets have to be converted in the Lagrangian formalism where droplets are seen as point forces. It implies that droplets can not always check the hypothesis of the LPP (Lagrangian Particle Point) formalism which is that the diameter of the particle has to be much smaller than the cell size. This hypothesis allows to have a good approximation of the undisturbed velocity for the Lagrangian particle. If the hypothesis is not checked when a Eulerian droplet is converted into a Lagrangian particle a residual velocity field can exists and therefore the velocity given to the particle is impacted by itself. This project aims to filter the gaseous velocity field through a gaussian filtering to remove the contribution of the Eulerian droplet to better approximate the undisturbed velocity.

=== Combustion - K. Bioche, CORIA & R. Mercier, Safran ===

==== C1: Plasma discharge models for reacting system - S. Wang, B. Kruljevic, B. Fiorina (EM2C), Y. Bechane (CORIA) ====

To reduce the expensive computational cost of Plasma-Assisted Combustion (PAC) full 3D simulations, the EM2C laboratory has developed phenomenological approaches to model Nanosecond Repetitively Pulsed (NRP) plasma discharges in reacting flows (Castela 2016 & Blanchard 2023). As part of previous works and ECFDs, both models were implemented and validated in the Low-Mach number framework (YALES2-VDS). While they were also implemented in the Compressible framework (YALES2-ECS), the validation against existing measurements or computations remained. During the workshop, numerical simulations of pin-to-pin configurations were performed with different numerical schemes and reactive mixtures to validate both models in ECS. The energy deposition was relatively well-validated through 2D simulations in the conditions of Castela et al. CNF 2016 and Rusterholtz et al. JPhysD 2013. A glimpse of baroclinic instabilities was observed through 3D simulations in the conditions of Castela et al. PROCI 2017.

==== C3: Dynamic sub-grid-scale wrinkling model for diffusion flames - S. Dillon (EM2C/Safran), R. Mercier (Safran), E. Espada, B. Fiorina, D. Veynante (EM2C) ====

Large-eddy-simulation (LES) of reactive flows is widely used in both academic and industrial applications. Combustion phenomena occur at a scale often smaller than the LES mesh size, therefore, turbulent combustion models are required to account for unresolved turbulent flame interactions. The modeling of sub-grid-scale (SGS) flame turbulence interactions can be described with a flame surface wrinkling factor which measures the ratio of the total flame surface area to the resolved flame surface area. Flame surface wrinkling models are often expressed by assuming equilibrium between turbulent motions and flame surface wrinkling, however, in realistic burners non-equilibrium is present and dynamic models are needed to adapt model parameters. Fractal-like models require information about the outer and inner cut-off length scales along with a fractal exponent, which is determined dynamically from resolved scales in the LES. The dynamic formalism can be coupled with the Filtered Tabulated Chemistry for LES (F-TACLES) model, where the required cut-off length scales are tabulated in the F-TACLES table along with other filtered thermochemical variables. The coupling of the F-TACLES model with the dynamic formalism has been previously applied to premixed flames in the past, however, the formal extension to non-premixed flames has never been investigated. The objective of this project is to investigate the performance of the dynamic SGS flame surface wrinkling model coupled with the F-TACLES model for non-premixed flames. A priori tests are conducted on a 2D H2/Air reactive mixing layer and HYLON, a 3D turbulent dual-swirl coaxial H2/Air injector. In both 2D and 3D cases, the modelled flame surface density shows good agreement with the filtered flame surface density extracted from the DNS. Moreover, the variation of the fractal model exponent in the HYLON test case is significant, highlighting the importance of the dynamic procedure. A posteriori tests were also conducted, and modelled chemical reaction rates show promising results.

==== C4: Developement of an automated virtual scheme generator for CFD - T. Luu, M. Hustache, N. Darabiha, B. Fiorina (EM2C) ====

In reactive CFD simulations, a non-negligible part of the time cost is spent in the resolution of the chemical system. Simplified chemistry models aim to reduce the number of transported species while still ensuring a correct representation of the phenomena of interest. Among them, the virtual chemistry method consists of using “virtual” species and reactions to reproduce detailed chemistry results through a mechanism of drastically smaller size. These “virtual” species and reactions are optimized to target quantities of interest such as temperature, laminar flame speed or pollutants. In practice, the optimization is done using a learning database composed of representative canonical reactive configurations computed with detailed chemistry. The objective of this project was to develop a tool to easily generate virtual schemes. The tool, named VISION (Virtual Scheme optimizatION), is currently able to both generate a user-defined database of wide reactive configurations and optimize a given scheme structure using either CANTERA or REGATH.

==== C5: Partially-Stirred reactor model for MILD combustion - E. Stendardo, L. Bricteux (UMONS), M. Laignel, K. Bioche (CORIA), J. Blondeau (VUB) ====

MILD combustion produces intense turbulence and extensive reaction zones, necessitating costly mesh refinement over large areas. Practical mesh lacks precision, leading to sub-grid heterogeneity and turbulent fluctuations. A Partially Stirred Reactor model was implemented to address turbulence-combustion interaction. This model multiplies the source term by a limiter factor, allowing modelling of residence time in the inner cell reactive structure. Testing various limiter formulations based on mixing and chemical timescales revealed increased computational costs. Future work aims to reduce costs by utilizing the model only where necessary. This ongoing research seeks to optimize performance while minimizing computational overhead for efficient application in engineering scenarios.

==== C6: Static Mesh Adaptation for Hydrogen High pressure combustion using GPUs - G. Hexilar, C. Brunet, R. Mari, S. Richard (Safran), P. Pouech, Q. Douasbin, G. Staffelbach (Cerfacs) ====

This research project focuses on advancing the understanding of hydrogen combustion under high-pressure conditions (up to 10bars), employing an automated workflow coupled with static mesh adaptation to tailor computational simulations to specific requirements. The study aims to enhance the accuracy and efficiency of combustion models by utilizing both Central Processing Units (CPUs) and Graphics Processing Units (GPUs). The automated workflow streamlines the simulation process, optimizing resource utilization and minimizing manual intervention. Static mesh adaptation further refines the computational mesh based on evolving combustion dynamics, ensuring accurate representation of high-pressure hydrogen combustion phenomena. By leveraging the parallel processing capabilities of GPUs alongside traditional CPUs, the research team aims to achieve significant computational speedup. This innovative approach not only contributes to fundamental insights into high-pressure hydrogen combustion but also establishes a robust framework for scalable and efficient simulations in complex reactive flow scenarios.

==== C7: High fidelity simulation of a cone calorimeter - A. Grenouilloux, K. Bioche (CORIA), N. Dellinger (ONERA), R. Letournel (Safran) ====

A methodology to simulate the decomposition of a composite sample in a calorimeter cone has been proposed. The configuration consists in the imposition of an incident radiative flux that heats the test coupon until it degrades. During test campaigns, the composite degradation can lead to the auto-ignition of the outgassed species, a phenomenon that needs to be predicted by the simulation. The variety of physical phenomena encountered, as well as the different characteristic time-scales, require the implementation of a coupled simulation. Hence, the proposed methodology relies on the coupling between two solvers of the massively parallel library YALES2 (fluid, radiation) and the MoDeThec solver developed at ONERA (solid degradation). First, a set of elementary validation tests to characterize the composite’s properties has been performed, showing good agreement with experimental data. A reduced three-equation kinetic scheme for the ignition delay has been derived, which aligns with experimental observations. Additionally, the framework for the coupled simulation involving the three solvers has been implemented.

=== User Experience & Data - L. Korzeczek, GDTech ===

==== U1: Refactoring the YALES2 tools - J. Leparoux, M. Cailler (Safran), L. Voivenel, J. Carmona, I. El Yamani (CORIA), S. Meynet, L. Korzeczek (GDTech) ====

The YALES2 distribution "tools" are becoming difficult to read and are a mixture of several types of tools. This leaves developers and end users unaware of what exists and how to use it (duplicated functions or tools) and makes it impossible to propose generic data analysis tools (FFT, confidence intervals, ...) that can be easily applied to YALES2 data structures. The main efforts have been concentrated on promoting a new architecture for YALES2 distribution tools with an object-oriented structure, including a refactored version of the main readers of YALES2 data. Several tutorials using Jupyter notebooks have been published for demonstration and explanation. A new CLI is now available under the name 'y2tools'. More work is needed before this structure can be pushed to the master trunk.

==== U2: Improved USEX for Multi-Scale Eulerian-Lagrangian simulation - L. Voivenel, J. Carmona, I. El Yamani (CORIA) J. Leparoux, M. Cailler (Safran) ====

The multi-scale Eulerian-Lagrangian approach has now reached a certain maturity and is being used to simulate fuel spray atomization. Post-treatments of these multi-scale simulations require the development of specific tools that track liquid structures either described in an Eulerian or Lagrangian way. In this project, we implemented a strategy to register in a post-treatment particle-set all Eulerian droplets crossing an arbitrarily shaped surface (described with an interior-boundary). The strategy is based on artificial Eulerian droplet advancement (using a Lagrangian representation) and verification of the new Eulerian droplet position compared to the surface of interest. We used this strategy to build a new post-treatment that allows to track both Eulerian and Lagrangian structures and build particle size or velocity distributions.

==== U3: Evaluate technological debt - P. Pouech, T. Marzlin, A. Dauptain (CERFACS) ====

The tool dedicated to cleaning and extracting meta data from the codebase (Tucan) is now able to interpret the most common #Ifdef statements for C and Fortran. Is has be tested successfully on the codebases of Yales2 , Neko and some elements of the Chemkin II package, for Fortran language. It has also included more tests, especially on C++ samples and large files. The call-graph aggregator “Marauder’s map” also evolved a lot. It was used as a refactoring monitoring tool for the UX project U1 “Refactoring Yales2 tools”, on a codebase mixing Python and Fortran files, providing feedback to both U3 and U1 teams. With this experience, we plan to add at least two additional complexity metrics in the months to come : the single component #Ifdef footprint , and the Custom Structures footprint : declarations vs usage in the code.

==== U4: CWIPI 1.0 porting - N. Dellinger, B. Andrieu, K. Hoogveld, E. Quémerais (ONERA), A. Grenouilloux (CORIA), R. Letournel (Safran) ====

Coupling is a cornerstone of numerical simulation, especially for addressing multi-physics problems using highly-specialized solvers for each phenomenon. The CWIPI library, developed at ONERA for coupling codes in a massively parallel environment, has been used in YALES2 for many years for internal and external coupling.
Significant developments have been carried out in recent years to improve the performance and usability of CWIPI, resulting in the release of version 1 in july 2023. This version features a completely revised API to overcome the limitations of version 0.12 and offer more possibilities to users.
The goal of this project was to support users in their transition to version 1. A training course based on Jupyter Notebooks was first organized. Assistance was then provided to successfully port MoDeTheC's and YALES2's internal couplings to the new version. Some fixes were made in CWIPI along the way, and will be reported in a new patched version.

==== U5: Integration of YALES2 in PRESTO supervisor - A. Pushkarev (GE Vernova), G. Balarac (LEGI) ====

A Graphical User Interface (GUI) exists at GE Vernova (Hydro) provides a user-friendly solution to perform numerical simulations for typical geometries of hydroelectric turbine in varied operation regimes. Previously, we implemented an interface for YALES2 code as alternative of CFX solver for this GUI client. Actual project is dedicated to implementation of the automatic mesh generation process for the runner section of the turbine using only section profile files of geometry such as blade profiles, meridional channel section, guide vane profile, etc... The algorithm should be able to generate a new *.msh mesh file once geometry profiles are updated as well as to setup standard named sections of the numerical domain.

==== U6: Optimization of YALES2 compilation time - R. Mercier (Safran), G. Lartigue (Total Energy) ====

YALES2 Compilation time are very different depending on the compilation options but also depending on the machine where the code is built. In particular, compiling on cluster always takes more time than on local station because of the slower filesystems. On these machines, the compilation time scales with the number of .f90 files to compile. 1) During this project we proposed a detailed compilation timing system activated with Y2_COMPILE_TIMER = TRUE and plotted using the tool y2_compilation_gantt.py. 2) This new profiling tool allowed us to spot some very large modules including a lot a dependencies. To enhance the tasks parallelization, the expl_comp_numerics module has been spliced in several modules. An important gain in compilation time has been obtained. 3) Another optimization of the compilation time have been developed using an on-the-fly modification of the .f90 tree. The use of _h.f90 is dynamically removed and replaced by including equivalent .defs files allowing the divide by 2 the number of .f90 files to compile. Some demonstrations have been performed but this compilation mode still have to be cross-check with different compilation option to ensure its robustness.
At the end of the project, the sum of all optimization allowed us to divide by 2 the compilation time.

Ecfd:ecfd 7th edition

2024-02-06T13:21:21Z

Benard: /* Turbulence - P. Benard, CORIA & L. Bricteux, UMONS */

Ecfd:ecfd 7th edition

2024-02-06T13:18:12Z

Benard: /* Turbulence - P. Benard, CORIA & L. Bricteux, UMONS */

Ecfd:ecfd 6th edition

2023-02-06T17:13:43Z

Benard: /* Turbulence - P. Benard, CORIA & G. Balarac, LEGI */

{{DISPLAYTITLE: ECFD workshop, 6th edition, 2022}}

== Description ==
{| align="right" style="text-align:center;" cellpadding="2"
| [[File:Logo_ECFD6.png | center | thumb | 350px | ECFD6 workshop logo.]]
|}
* Event from '''23th of January to 3rd of February 2023'''
* Location: [https://www.hotelclubdelaplage.com Hôtel Club de la Plage], Merville-Franceville, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* More than 60 participants from academics, HPC center/experts and industry.

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes

[[File:Banniere_ECFD6.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_6th_edition]]

[[File:Banniere_ECFD6_sponso.png|text-bottom|600px]]



== Agenda ==

[[File:ECFD6_program.png|text-bottom|600px]]

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

== Projects ==

=== Hackathon - G. Staffelbach, CERFACS & P. Begou, LEGI ===

=== Mesh adaptation - R. Letournel, Safran & V. Moureau, CORIA ===

=== Numerics - S. Mendez, IMAG & M. Bernard, LEGI ===

* '''Sub-project 1: Multi-level domain decomposition method (DDM) for coupled systems of differential-algebraic equations (A. Quirós Rodrígues, V. Le Chenadec)'''
The numerical approximation of multi-physics problems gives rise to complex linear systems, the solution of which leverages preconditioning techniques such as multi-grid or domain decomposition methods. This project aimed at coupling two Julia packages that being actively developed: a two-dimensional Navier-Stokes solver for free-surface and two-phase flows (Flower.jl) on the other, and a Domain Decomposition package for Cartesian grids (DDM.jl). The decomposed matrix-vector product was optimised to reduce the overhead associated with halo exchanges. The implementation of a deflated Conjugate Gradient as well as one- and two-level Additive Schwartz Method were also completed and shown to significant reduce the number of iterations for inverting monolithic systems (i.e. without resorting to operator splitting), shown to be independent of the number of subdomains for constant property flows. Future work will focus on a further optimisation of the implementation for vectorisation and multi-threading, and extension of the deflation to generalised coarse spaces to support highly discontinuous transport properties (GenEO).

* '''Sub-project 2: Ghost fluid method (GFM) for Electrodeformation (A. Spadotto , S. Mendez)'''
According to the Leaky Dielectric Model, red blood cells (RBCs) are subject to a force which is proportional to the jump of Maxwell tensor. This latter is a quantity scaling as the square of the electric field, which under the quasi-static hypothesis is defined as the gradient of the electrostatic potential. To work out the potential, an elliptic interface problem must be solved, taking into account the presence of the RBC membrane. The aim of the project was implementing the Ghost Fluid Method (GFM) to face the interface problem. Good results were obtained on unstructured meshes. Secondly, a gradient calculation was performed applying the Green-Gauss scheme, modified in the style of GFM. Future work will focus on interpolation of the gradient field onto the membrane to get an estimation of the effort. Possibly, high-order schemes for the gradient calculation will be explored. In a second time, the effort calculation will be merged into an Immersed Boundary solver for the RBC dynamics.

* '''Sub-project 3: Optimization of the high order framework (HOF) for Navier-Stokes incompressible (M. Bernard, P. Bégou, G. Lartigue, G. Balarac)'''
Over the past years, a framework has been developed to improve the spatial accuracy of numerical schemes on distorted meshes.
However, even if the solution is more precise, the computational cost of the overall resolution of Navier-Stokes equations is large.
As a consequence, HOF becomes profitable only on thin meshes thanks to a better spatial convergence order.
The code has been analized with different analysis tools (MAQAO, Gprof, Scalasca).
The main time consuming routines have been identified and improved.
Moreover, some algorithms have been refactors such that the resolution of Navier-Stokes equations has been speed-up by a factor 2.5.

* '''Sub-project 4: Force coupling method (FCM) for particulate flows (C. Raveleau, S. Mendez)'''

* '''Sub-project 5: Breaking limitations of the linearized implicit time advancement (T. Berthelon, G. Lartigue, G. Balarac)'''
The explicit time advancement classically used in ics solver is limited by CFL constraint. In order to get ride of this constraint, an implicit time advancement method, based on the linearization of the convective term, has been recently developed.
However, the method is limited by difficulties to solve linear system, with the BiCGSTAB2 algorithm, during the prediction step. The objective of this project was to understand these limitations. The correction of a bug on the boundary conditions (viscosity imposed at zero) was identified. In addition, the spatial scheme and the presence of a buffer zone at the end of the domain showed a great influence on the convergence of the prediction. The perspectives for a more robust and efficient use of this temporal integration consist in working on the spatial schemes and on the pre-conditioning.

* '''Sub-project 6: Development of a traction open boundary condition (TOBC) in Yales2 (J.B. Lagaert, Guillaume Balarac)'''

* '''Sub-project 7: Development of new spatial differential operators in Yales2 (M. Bernard, G. Lartigue)
It exists different philosophies for computing differential operators on distorted meshes.
In a HPC context, the 2 main approaches are the Green-Gauss operators and the Least-Squares operators.
During ECFD#6, 2 new types of "non-compact" Hessian operators have been implemented by computing successively the gradient operator, eather with Green-Gauss gradient, or with Least-Squares gradient.
Those operators lead to good convergence order, even on distorted mehes.
However, their application on low-resolution signals lead to large error magnitude due to their extended stencil.
Another pair of gradient & hessian Least-Squares operators have been implemented, leading to 2nd and 1st order accuracy for the gradient and hessian respectively.
Those operators have very interesting characteristics as their stencil is restricted to the direct neighbors only and their computational cost remains low.

'''

* '''Sub-project 8: DOROTHY optimization (M. Roperch, H. Mulakaloori, G. Pinon, P. Bénard)'''

* '''Sub-project 9: Anamika, a tool to improve programming productivity (K. Mohana Muraly, G. Staffelbach)'''

=== Turbulence - P. Benard, CORIA & G. Balarac, LEGI ===

* '''Sub-project 1: Explore hybrid RANS/LES strategies (T. Berthelon, G. Balarac)'''

For complex industrial applications, LES can still lead to a too long restitution time. In other hand, statistical approaches can lead too a lack of accuracy. In this project, the potentiality of hybrid approaches combining both have been explored. Conventional hybrid RANS/LES approaches consider a unique solution field, with an unique transport equation and a clusre terme modeled using RANS or LES models depending of the regions. The main idea is to evaluate a strategy based on a separation between mean fields and fluctuations with distinct coupled transport equations. First elements of validation using YALES2 code are shown that it was possible to correct the prediction of a RANS models, by performing LES of the fluctuations. Next steps should be to consider disctinct meshes, or even computational domains for RANS and LES with this strategy.

* '''Sub-project 2: Flow Instabilities over Rotating curved Surfaces (S. Sawaf, M. Shadloo, A. Hadjadj, S. Moreau, S. Poncet)'''

For evaluating the effect of the clearance between the blade tip and the casing of axial ducted fans on noise emissions, LES offers excellent tool to capture the consitricted flow around the blade tip especially for small clearances where RANS fails because of unsteady flow conditions. LES simulation of the aerodynamics is the first step toward extracting accoustics data that helps to improve the design of axial ducted fans so they comply with the noise emission regulations in admistrative buildings. noise emmisions are estimated using analytical aeroacoustic models informed by data that are extracted from the LES simulations.

* '''Sub-project 3: Automatic statistical convergence metric (C. Papagiannis, G. Balarac, O. Le Maitre, P. Congedo)'''

Statistics accumulation can be an important part of the restitution time in unsteady simulations (DNS/LES). In this project, the goal was to estimate uncertainties on the "finite time statistics". For time correlated data, it can be shown that the variance of the mean estimator (i.e. the fluctuation of the estimation of the mean) is dependent of the correlation time. Modeling this correlation time based on the integral time scale of the turbulence appears as a first way to define a practical metric to evaluate the statistic convergence on-fly during simulations. Next step should be to explore procedures to accelerate the statistics accumulation step.

* '''Sub-project 4: Aerodynamics of floating wind turbines (R. Amaral, F. Houtin-Mongrolle, E. Muller)'''

* '''Sub-project 5: Implementation of the Risoe dynamic stall model for YALES2 (V. Maronnier, E. Muller, B. Duboc)'''

* '''Sub-project 6: YALES2-OpenFAST coupling (A. Parinam, A. Viré, D. Von Terzi, B. Duboc, F. Houtin-Mongrolle, P. Bénard)'''

* '''Sub-project 7: Wall law for Immersed Boundaries & Rough surfaces (M. Cailler, A. Cuffaro, P. Benez, S. Meynet)'''

Conservative Lagrangian Immersed Boundaries (CLIB) are now a useful way to take into account complex geometries in YALES2. During the workshop, a brand-new data-structure for modular and generic immersed-body has been developed. This data-structure paves the way for various new capabilities for IB methods: penalization mask shape optimization for improved velocity imposition, better control of near wall discretization based on a reliable evaluation of wall units, wall-modeling, etc... For this purpose the periodic hill test case has been considered. Simulations of this configuration has been performed by using body-fitted meshes, and CLIB for both smooth and rough surfaces. This will allow to assess the accuracy of the IB methods, and will constitute a database for IB models improvement, and the development of wall-modeling strategies.

* '''Sub-project 8: Atmospheric flow (U. Vigny, L. Voivenel, P. Benard, S. Zeoli)'''

Atmospheric flow such as Atmospheric Boundary Layer (ABL) and thermal stratification have an impact on wind turbines aerodynamic and wakes. Mostly at a wind farm scale, the change of wind turbine wake size and recovery can modify the global power production. During the workshop, the Coriolis force implementation has been validated through neutral case (where no thermal stratification i.e. no temperature gradient). It also allowed to validate the pressure forcing term, needed to drive the flow in a periodic box. YALES2 results showed a good agreement with other numerical and experimental results. Afterwards, the stable case (i.e. temperature gradient downwards) has been studied. A surface temperature as boundary condition has been developed. Yet, results are not as expected and further investigation is needed.

=== Two Phase Flow - C. Merlin, Ariane Group & M. Cailler, Safran Tech ===

* '''Sub-project 1: Convergent computation of interface curvature (G. Ghigliotti, M. Benard, G. Balarac, J. Carmona, R. Mercier, G. Lartigue)'''

Though Level-set distance evaluation through GPMM (Janodet et al., 2022) converges at order 2, the interface curvature convergence is as best 0 using the non-compact Goldman formulation.
Following progresses obtained during ECFD5, a strategy based on parabolic fit of the interface has been explored during the workshhop. This method aims at fitting a parabola through least squares using the interface markers stored in the interface vicinity. First the method was applied on a 2-D perfectly spherical droplet with exact projection of the marker on the circle. This results in a first order convergent curvature. Without projection of the markers, the fiting strategy allows a slight decrease of the error but no improve on the curvature convergence order in comparison with the standard non-compact formulation. As a persective, these results will be validated on dynamic and 3-D cases (MMG3D meshes). Also, the sensitivity on the number of markers and their redundancy will be investigated.

=== Combustion - K. Bioche, CORIA & R. Mercier, Safran Tech ===

* '''Sub-project 1: Multi-regime F-TACLES (S. Dillon, R. Mercier, B. Fiorina)'''

Filtered tabulated chemistry for large eddy simulations is currently a common tool to model premixed flames or diffusion flames. Tabulation using 1D counterflow flames, as a function of the mixture fraction and progress variable, was previously tested on laminar and turbulent cases. It resulted in difficulties to describe the outer mixing zone and yield a very stiff evolution of SGS source terms in the phase space. The model was modified to include the mixture fraction scalar dissipation rate as a table dimension. This solves previous limitations, but using 1D counterflow flames yields empty table zones, making the method numerically infeasible. Tabulation using both 1D counterflow flames and 1D partially premixed flames gives well-built tables, and was tested on 1D flames for various strain rates.

* '''Sub-project 2: Limiter model for turbulence combustion interaction in MILD combustion (E. Stendardo, L. Bricteux, K. Bioche)'''

MILD combustion yields intense turbulence and widespread reaction zones, requiring expensive mesh refinement over large areas. Practical mesh won’t be fine enough, leading to sub-grid heterogeneousness and effects of sub-grid turbulent fluctuations. A generic limiter type combustion model was implemented to solve for turbulence combustion interaction. This family of models includes Partially Stirred Reactor, Quasi Laminar Finite Rate and Laminar Finite Rate models. In these models, the source term is multiplied by a limiter factor and the residence time in inner cell reactive structure can be modelled. This implementation will permit testing the various limiter formulations in the future.

* '''Sub-project 3: Evaluate spatial discretization schemes on scalar transport (K. Bioche, Y. Bechane, R. Mercier, G. Lartigue, V. Moureau, J. Carmona, M. Bernard, L. Voivenel)'''

Common practice in combustion solvers is to use centred spatial schemes. Such low-dissipation schemes can prove unstable when applied to under-resolved scalar transport in presence of strong gradients. This is typically the case for H2/air combustion. Initial low-resolution simulations require thus adapted numerical schemes. Various spatial schemes were evaluated on the scalar transport problem, including: 4th order, 3rd order, 2nd order, WENO3, high order schemes, MUSCL schemes with various limiters (overbee, superbee, sweby, van leer, minmod). Their application to various configurations was discussed to emphasize on their robustness and accuracy. Tests cases include: 1D scalar convection Jiang Shu test case, 2D scalar bump convection for convergence analysis, a 2D reactive Bunsen burner and finally the 3D Preccinsta burner.

* '''Sub-project 4: Phenomenological plasma model for reacting systems (S. Wang, Y. Bechane, B. Fiorina)'''

Plasma assisted combustion consists in stabilizing flames in near extinction conditions thanks to electric discharges. Stabilization of lean premixed flames with Nanosecond Repetitively Pulsed electric Discharges is a strategy to reduce NOx emissions. Full 3D simulations of plasma assisted combustion are extremely expensive, so that the use of a semi-empirical strategy to model NRPD is preferred in CFD solvers. During the workshop, Castela’s model was implemented in a variable density solver. This model was extended to an explicit compressible solver. The model of Blanchard was also implemented in both frameworks. A 2D pin-to-pin configuration was successfully simulated with both models and frameworks.

* '''Sub-project 5: Development and assessment of combustion in an explicit compressible solver (Y. Bechane, L. Voivenel, R. Mercier, K. Bioche)'''

The implementation of reactive physics in the Explicit Compressible Solver (ECS) of the YALES2 platform was undertaken. To this aim, reactive gases thermochemical functions were implemented. Specific schemes were developed to increase the temperature and species diffusion schemes from 2nd to 4th order. Finally, a 2D methane-air Bunsen flame was simulated with low order numerical schemes (RK1 and SLAU2).

* '''Sub-project 6: Clustering for finite rate chemistry using PCA (R. Mercier, A. Stock)'''
To reduce the cost of species source terms computation, a clustering method was adopted. It consists in detecting nodes with similar properties and compute chemical source terms only once for these. Still, considering each species in this process creates a high dimensional cluster, while replacing species by a user-set progress variable may not well describe species. The strategy adopted here relies on the application of a PCA on species. It can be viewed as an automated “progress variable” creation. The use of such strategy was shown to reduce the simulation cost of source term computation by a factor 6 on a simple 2D flame ball case.

=== User Experience - J. Leparoux, Safran Tech & A. Pushkarev, GE Renewable Energy===

* '''Sub-project 1: External coupling with CWIPI (R. Letournel, V. Moureau, C. Merlin, M. Cailler, P. Bégou, S. Meynet)'''

* '''Sub-project 2: Automated Grid Convergence refactoring (J. Leparoux, M. Cailler, R. Letournel)'''

* '''Sub-project 3: Advanced Liquid spray post-processing (J. Carmona, J. Leparoux, N. Gasnier, C. Brunet, I. El Yamani)'''

* '''Sub-project 4: YALES2 as industrial solver for GE design optimization tools (A. Pushkarev, H. Lam, G. Balarac)'''

* '''Sub-project 5: YALES2 History and Geography (T. Marzlin, A. Dauptain, P. Bénard)'''

* '''Sub-project 6: Improve the HT solver: refactoring of linear solver operators & Robin BC (C. Merlin, V. Moureau, R. Letournel)'''

Ecfd:ecfd 6th edition

2023-02-02T17:58:55Z

Benard: /* Projects */

Ecfd:ecfd 6th edition

2023-02-02T17:56:28Z

Benard:

Ecfd:ecfd 5th edition

2022-02-01T08:05:21Z

Benard: /* News */

{{DISPLAYTITLE: ECFD workshop, 5th edition, 2022}}

== Description ==
{| align="right" style="text-align:center;" cellpadding="2"
| [[File:Logo_ECFD5.png | center | thumb | 350px | ECFD5 workshop logo.]]
|}
* Event from '''23th to 28th of January 2022'''
* Location: [https://www.bonsejour-laplage.com/vacances-tout-compris Centre Bonséjour], Merville-Franceville, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points.
** mini-workshops. Potential workshops are listed below.
* Free of charge
* More than 50 participants from academics (CERFACS, CORIA, IMAG, LEGI, EM2C, UMONS, UVM, VUB, UCL, TUDelft), HPC center/experts (GENCI, AMD, CINES, CRIANN) and industry (Safran, Ariane Group, Siemens-Gamesa).

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes

== News ==

* 03/11/2021: First announcement of the '''5th Extreme CFD Workshop & Hackathon''' !

[[File:Banniere_ECFD5_sponso.png|text-bottom|600px]]

* 13/01/2022: After discussions with the participants, the '''5th Extreme CFD Workshop & Hackathon''' is maintained as an in-person event! It will be also possible to attend to the plenary sessions and participate remotely to the workshop.

* 14/01/2022: The [[#Agenda|ECFD5 program]] is online! The plenary sessions will be announced soon!

* 20/01/2022: The plenary sessions are now defined:
** P1 - 24/01/2022: GPU porting challenges and quantum computing, présentation machine Adastra by G. Staffelbach (CERFACS) + Presentation of the new cluster from CINES called Adastra by C. Andrieu (CINES)
** P2 - 25/01/2022: News, perspectives and future of GPU computing applied to CFD by A. Toure (AMD)
** P3 - 26/01/2022: Theory, applications and perspectives of the Lattice-Boltzmann Method by P. Boivin (M2P2)
** P4 - 27/01/2022: Concepts and notions of mesh adaptation by C. Dapogny (LJK)

* 23/04/2022: '''The ECFD5 event has now started!!''' [https://www.linkedin.com/feed/update/urn:li:activity:6891053385072594944| LinkedIn post]

* 28/04/2022: '''The ECFD5 event is now finished!''' It was again a successful scientific week. See you next year for the '''ECFD6'''! [https://www.linkedin.com/posts/l%C3%A9a-voivenel-642ab7186_avbp-yales2-yales2-activity-6892778892801716224-3zxn| LinkedIn post]

[[File:ECFD5_PhotoGroupe.jpeg|text-bottom|600px]]

== Agenda ==

[[File:ECFD5_program.png|text-bottom|600px]]

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

=== Combustion - K. Bioche, VUB ===

* '''Sub-project 1: H2/air jet-in-cross-flow numerical simulations (R. Le Dortz, E. Riber, Q. Douasbin)'''

The use of hydrogen as an aviation fuel requires new combustion chamber design. Among strategies to prevent flame flashback and low flame residence time, the micromix injection system is further studied by ENABLEH2. This systems corresponds to a multitude of H2/air jet-in-cross-flow configurations. A 3D numerical simulation with realistic thermodynamics and kinetics is now tractable thanks to massively parralel computing. This week saw the completion of the first steps towards the establishment of a complete simulation. (I) The non-reactive air injection in the combustion chamber. (II) The cross-injection of H2 without ignition. (III) The ignition of this mixture modeled with the skeletal kinetic mechanism of Boivin (H2, H, O2, OH, O, H2O, HO2, H2O2, N2). Further work will be realised concerning mesh refinement, modelling of NOx and porting of the computation on GPU.

* '''Sub-project 2: LES calculation of the MICADO test rig with multicomponent jetA1 (S. Puggelli, T. Lesaffre, E. Riber, B. Cuenot)'''

The EU-funded project ALTERNATE has the goal of exploring the possibility for a wider utilisation of aviation sustainable fuels. A part of the project deals with the assessment of the effect of SAFs on soot production: using the experimental information obtained at ONERA in high-pressure conditions on the MICADO test rig, the effect of Alcohol to Jet (ATJ-SPK) fuel on soot levels are assessed and compared with standard jet A1 emissions. During the project, STech and CERFACS are working jointly on the numerical modelling of soot emissions for jet-A1 and ATJ-SPK combustion in AVBP. Starting from the numerical setup under-development for jet-A1, the worshop permitted to: (I) Switch from a 2-step kinetic mechanism to a complex 29 species, 233 reacs and 15 QSS mechanism. This transition was efficiently conducted with the tool Multi Table Generator. (II) At this stage, an assessment of the effects of the flame sensor on the calculation results was carried out, indicating the consistent behaviour of a recently developped sensor w.r.t classical tools. (III) Switch towards a multicomponent formulation of jet-A1 and assessment of the effect of such advanced approach with respect to the single-component formulation previously employed. Further work will be realised to manage the stiffness of employed kinetics and to compare jet-A1 and ATJ-SPK fuels from a chemical point of view.

* '''Sub-project 3: Euler-Lagrange Multigrid Simulation (T. Lesaffre, O. Vermorel, E. Riber, B. Cuenot)'''

In Lagrange simulations, the point-source approach is based on a ponctual approximation of the particule and requires this last to be smaller than the mesh. The very fine meshes required to represent the Eulerian phase of Euler-Lagrange two-phase flow simulations can lead to a non-validity of the point-source hypothesis. This project aimed at implementing, in the AVBP solver, the simultaneous management and coupling of several simulations. During this week, the Eulerian and Lagrangian phase were successfuly computed on two different meshes and coupled via the CWIPI library. The good behaviour of this framework was assessed on a 1D Evaporation of kerosene droplets in an air stream test case. Encouraging preliminary performance results were obtained on a 3D injection case and require further work.

* '''Sub-project 4: Devolatilization modelling for biomass combustion (K. Bioche, L. Bricteux)'''

Biomass combustion simulations require the modelling of numerous physical phenomena: particle drying, devolatilization, gas-phase combustion, chars oxidation. Besides, the valorisation chains for biomass include fluidized bed reactors, fixed bed reactors and pulverized fuel burners. The Granular Flow Solver of YALES2 offers a good framework for the simulation of fluidized bed reactors and is functionnaly coupled with the reactive gas-phase solver of the same code. This week permitted to partically implement the modelling of devolatilization in this solver. A single-step kinetic scheme is considered for the particle mass evolution equation while the particle diameter evolves during the process. Further work is necessary to account for the thermal and mass couplings with the fluid phase.

* '''Sub-project 5: Thickened-Flame LES model in a Lattice-Boltzmann Method framework (P. Boivin, S. Zhao, M. Le Boursicaud)'''

The TFLES framework of the hybrid Lattice-Boltzmann sover ProLB was extended to account for recent sensor methods. During this week, a smooth flame sensor based on the curvature of the norm of the advancement variable gradient was developped. Also for filtering operations, the lattice requires to access data over three neighboring layers. A precise and continuous thickening factor was obtained with such method.

* '''Sub-project 6: NOx modeling applied to KIAI combustion chamber (J. Obando, P. Bénard, V. Moureau)'''

This project treated of the implementation of NOx modeling into simulations of the KIAI combustion chamber, experimentaly studied at CORIA lab. During this week, various NOx modeling strategies were listed. Associated kinetic mechanisms, among which analytical chemisty, were employed for 1D flame simulations in YALES2 solver. Further work include the use of such methods on the 3D computational case.

=== Static and dynamic mesh adaptation - G. Balarac, LEGI ===

=== Multi-phase flows - M. Cailler, SAFRAN TECH ===

* '''Sub-project 1: Hybrid E-E/E-L two-phase flow method (M. Cailler, F. Pecquery, I. El Yamani, V. Moureau)'''

The High-Fidelity approach based on ACLS & DMA allows a reliable description of interface dynamics. For design exploration, low-CPU methods with controlled level of fidelity are required. An interesting approach to reduce CPU cost relies on an hybrid approach based on an Eulerian representation of the gas & and a Lagrangian description for the liquid phase. Objective of the ECFD5 was to explore the capability to reconstruct the interface normal of a liquid droplet made of particles on an Eulerian grid. First, a level-set based strategy relying on Geometric Multiple Markers Projection (Janodet et al., 2022) has been first tested showing good capabilities providing that the iso-surface distance equal 0 is well captured on the mesh. An alternative strategy based on the liquid volume fraction has been tested. This information was then used to build a velocity correction that is used to transport particles and ensures regularized particle positions. This preliminary banchmark work will be pursued on a liquid jet propagation problem.

* '''Sub-project 2: Jet atomization with a diffuse interface mathod (N. Odier, B. Péden, J. Carmona, P. Boivin)'''

A so-called diffuse interface method, using the multi-fluid formalism, coupled with a Riemann solver HLLC and a thermodynamic closure of the NASG type, was implemented in AVBP during the thesis of Carmona. Objective of the project was to couple the NASG thermodynamic implementation in an improved HLLC solver based on nodal information only.

* '''Sub-project 3: Convergence of the interface curvature computation (G. Ghigliotti, J. Carmona, G. Balarac, G. Lartigue)'''

The computation of interface curvature in a level-set framework is based on the classic formula as divergence of the gradient of the levelset function. This function being computed at 2nd order, one obtains a O(0) curvature, meaning that the error does not decrease with mesh refinement.
We have implemented in YALES2 a strategy proposed by Emmanuel Maître and collaborators in a finite element method based on the regularization (filtering) of the level-set gradient and curvature.
This strategy has been tested for the simple test case of a static circular interface.
We used two types of filters (simple gather-scatter or bilaplacian as developed by Lola Guedot (PhD thesis 2015)) on different mesh types (split quadrilaterals, isotropic triangular mesh, unstructured triangular mesh).
The results are encouraging since a O(1) convergence is obtained in all cases.
Further work is needed to tune the filter properties (amplitude and size) for different spatial resolutions and levelset "narrow band" width.

* '''Sub-project 4: Conservative two-fluid momentum transport (F. Pecquery, C. Merlin, M. Cailler, J. Carmona, V. Moureau)'''

The aim of this project was to investigate the applicability of the conservative two-fluid transport framework developed for passive scalars to the momentum conservation equation. First a complete two-fluid framework for the incompressible two-phase Navier-Stokes equations was proposed. Then the discontinuous algorithms for discontinuous scalars (data extension, reinitialization and transport) have been extended to discontinuous vectors. Moreover, some improvement of the data-structure were implemented to further generalize the framework and improve user-experience. To eventually conclude on the applicability of the proposed two-fluid momentum approach, the proposed interfacial momentum flux model and correction step will be implemented in YALES2

=== Numerics - G. Lartigue, CORIA ===

* '''Sub-project 4: Avoiding the acoustic timestep restriction in two-phase flow simulations (V. Boniou, J. Paris, A. Vié, T. Schmitt, C. Tenaud, Y. Béchane)'''
In the context of simulating compressible two-phase flows with phase change, the CFL constraint related to the acoustic part of an explicit solver, especially in the liquid phase, can be prohibitive for evaporation studies implying large timescale separation between evaporation and acoustic wave propagation.
The acoustic CFL constraint can be either relaxed by considering an implicit treatment of the acoustic part or avoided by deriving the low Mach limit of the multifluid model.

During this week, the first steps of this vast work have been initiated on single-phase flows by implementing the variable-density low Mach solver and the IMEX method applied on the Euler equations in TITAN.

The IMEX method was shown to provide degraded solutions in acoustic-driven cases, compared to conventional Riemann solutions. However, in transport-dominated cases, the increase of the transport CFL leads to a better numerical precision. A 2D cylindrical shock has been simulated with an acoustic CFL of 40 to assess the ability to run multidimensional compressible computations with large time step. The method was also extented to 4eq and 2nd order spatial accuracy.

Finally, the extension of the variable density solver to two-phase flows was performed using a VOF interface capturing method, considering a dilatable gas phase along with an incompressible liquid phase. Particular attention has been drawn to preserving liquid volume by treating the VOF transport accordingly with an adapted velocity field.

=== Turbulent flows - P. Bénard, CORIA ===
* '''Sub-project 1: Optimization of the actuator set for several wind turbines in YALES2 (F. Houtin Mongrolle, S. Gremmo, E. Muller, B. Duboc)'''
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified:
(i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges.
(ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications.
(iii) Adapt the YALES2-BHawC aero-servo-elastic coupling to make it consistent with the new ALM implementation.
During the workshop, the (i)-algorithm has been implemented with additional optimizations. Tests on a 30-turbine wind farm showed a 60% improvement compared to previous performances, which is very promising. The (ii)-feature still needs to be developed but should lead to the performances objective. The tests with the coupled code YALES2-BHawC were not conclusive yet and still require some work.

* '''Sub-project 2: Thermal effect in an atmospheric solver (U. Vigny, L. Voivenel, S. Zeoli, P. Benard)'''
Given the current environmental and energy challenges, maximising the wind farm electricity production is essential. Therefore, it becomes necessary to develop the most reliable and accurate prediction and simulation tools. Following this tenet, an atmospheric solver, which will take into account meteorological phenomena, should be developed. The preliminary work, going from bibliography study to road map was performed during the extreme cod workshop. Thus five parts have been identified:
(I) The YALES2 Variable Density Solver (VDS) will be used because of the need to take into account buoyancy effect including for big density differences.
(II) A wall law correction term, relative to atmospheric boundary layer will be added.
(III) The actuator line method used to simulate wind turbine will be extended to VDS, modifying the velocity source term to a momentum source term.
(IV) The Coriolis effects, depending on the latitude will be implemented.
(V) The wall heat flux, allowing to simulate diurnal and nocturnal cycles on various terrains, is more realistic than a target wall temperature.
From this work, future development are now clear and just waiting to be developed.

* '''Sub-project 3: Dynamic stall correction models for Horizontal Axis Wind Turbine (S. Gremmo, E. Muller, B. Duboc, F. Houtin-Mongrolle)'''
The Actuator Line Method implementation into YALES2 library suffers from the lack of correction models for some 3D unsteady effects. An important one, the dynamic stall, implies an unsteady modification of the aerodynamic loads, i.e. the polars, with the angle of attack. With the standard ALM model, static 2D polars are used. Adding a dynamic stall correction model allow to have an hysteresis loop on the of the forces coefficient with the angle of attack. Several models exists in the literature: Oye, BeddoesLeishman, Sheng, Risoe, ONERA... During the workshop, the Oye model was selected, as it is the simplest model, and was implemented. First, new ingredients necessary to the model were added in the polars look-up table generator used to describe the blades. Then, the Oye correction was integrated into the YALES2 library. Finally, the model was tested on simple configurations. It is now essential to validate more the implementation.

* '''Sub-project4: Immersed Boundary solvers uniformisation into YALES2 (I. Tsetoglou, M. Cailler, P. Benez, S. Mendez, P. Benard)'''
A novel Conservative Lagrangian Immersed Boundary (CLIB) method [Tsetoglou et al., 2021] has been developed by CORIA & Safran Tech for wind turbines & gearbox lubrication simulations. Nevertheless, two different implementations of the same method were created that tends to diverge. This sub-project aims to develop a shared numerical kernel (CLIB) on which 2 solvers are based: CIB for aerodynamic applications (ICS + CLIB) and TPS for two-phase flows (SPS+CLIB). The newly developed kernel and solvers were tested and validated on test cases: 2D flows around static/rotating/oscillating disk, 3D flow around a rotating cylinder and 2D flows around counter-rotating ellipses. The newt steps involve the continuation of the validation of both solvers and its documentation, as well as the development of wall model for immersed bodies.

* '''Sub-project 5: TBLE wall model for LES with pressure gradient on a simple turbomachinery geometry (M. Cizeron, N. Odier, R. Vicquelin)'''
Wall modeling is often used in LES to alleviate the computational cost that would be required to resolve all the length scales up to the solid boundaries of the domain. The classical way of doing it is by using an algebraic model to provide the wall friction and heat flux, with a coupling to the LES solver at the first off-wall nodes. The wall model was designed from analyzing RANS equation with strong assumptions such as planar flow, equilibrium and no pressure gradient. These assumptions are often far from true in real applications, such as turbomachinery applications, where the use of a wall model is mandatory due to the size of the calculation. During this workshop, a wall model relying on the resolution of the Thin Boundary Layer Equations (TBLE) was studied, which had been implemented by EM2C. The addition of a pressure gradient to these equations has been conducted and tested, at first only for the 1D wall model solver, then on a 3D turbulent channel. It remains to be tested on a diffuser configuration with a real pressure gradient to quantify the effect of the new wall model. The influence of the point considered to do the coupling between the LES and the wall model (ie. its distance to the wall) has also been tested both for the TBLE and the original algebraic model, showing that coupling farther from the wall yields better results and reduces the so-called log-layer mismatch.

* '''Sub-project 6: Tools for rough wall modelling (A. Barge, S. Meynet)'''
Within the STREAM project framework, a roughness-resolved Large-Eddy Simulation (RRLES) database is being built. The aim of this latter is to be representative of rough channel flows, especially for additive-manufacturing heat exchangers. First RRLES have already been performed. From turbulence and rough wall stress statistics analysis of the results, a first stochastic model, which reproduces the statistical behavior of the wall stress vector, have been proposed. The modeled wall stress allows a better prediction of the pressure drop in a flat wall channel compared to the use of the mean value of the wall stress measured in RRLES alone. However, the near wall region is still mispredicted and the model is correlated in time but not in space. The aim of this ECFD5 was to develop tools to improve modelling and explore new ways. A roughness mapping tool for smooth surfaces have been implemented into YALES2 to get local surface height. This tool is based on an existing in-house surface roughness generator developed for the STREAM project. The idea is to use the map to generated space correlated fluctuations for the wall shear stress. Some bugs still remain to fully use this tool. In parallel, the modelling approach was extended to passive scalar, especially for temperature. To this end, new random tools as white noise, unit sphere random walk and Gaussian / Log-normal stochastic processes have been coded. Finally, the idea of using walls as velocity source terms emerged during this ECFD5. The principle is to mask a grid layer above the wall and to transport the rough map on this grid to estimate the roughness effects above the wall. Parametrizing and testing these tools remained to be done at the end of ECFD5.

=== User experience - J. Leparoux, SAFRAN TECH ===

* '''Sub-project 1: Multi combustion model chemtable generator (S. Dillon, R. Mercier)'''

Due to the high computational cost of LES of turbulent combustion with detailed chemistry, tabulated chemistry approaches remain a popular choice within the combustion community. This popularity of tabulated chemistry leads to research into the development of novel combustion models for which a platform to test these models is required. The objective of the workshop was to therefore begin developing an easy-to-use chemistry table generator within Python. Given that there exists numerous turbulent combustion models (F-TACLES Diffusion, F-TACLES Premixed, 2PFT, etc.), a generalised multi-model code was necessary. Along with multi-model capabilities, the code was required to function with multiple solvers for the creation of the flame database (REGATH and CANTERA). With these objectives in mind, the code “TabulatEd Chemistry GenERator for Aeronautical CombusTion” (TECERACT) was created. Advancements were made in all key areas using code already developed by Safran Tech. TECERACT includes a parallel flame database generator and current developments are focused on multi-model functionality and simplified mesh management within Python. Once finished, the TECERACT code will provide a user-friendly chemistry table generator capable of producing chemistry tables for LES simulations and its multi-model structure will serve as a platform for future turbulent combustion model development/testing.

* '''Sub-project 2: Task-driven automatic run sequence (R. Mercier, J. Leparoux, M. Cailler, R. Letournel)'''

The principal objective of this project was to develop a new simulation workflow based on Task-driven approach. This approach could offer a higher flexibility to manage CFD simulations by decomposing the whole simulation on small sequences (run initialization, fuel injection, etc.)

This week showed the achievement of the first steps towards a complete task-driven simulation. (i) The whole simulation workflow was set-up (ii) Missing actions were implemented allowing to update simulation status on-the-fly (from non reactive to reactive simulation) (iii) State are now embedded from the restart solution allowing to restart from the last known state. Further work is still needed to obtain the full implementation of the AMC framework based on task-driven approach. Especially to automate the adaptation loop

* '''Sub-project 3: YALES2 Tools & Gitlab CI (J. Leparoux, A. Tstetoglou)'''

* '''Sub-project 4: Wind energy tools (E. Muller, S. Gremmo, F. Houtin-Mongrolle, B. Duboc)'''

Set-up phase of simulations involving several wind turbines is very demanding and error-prone, especially for simulations carried out with the coupled solver YALES2-BHawC*. This type of simulation can involve several dozen input files and then, an assistant tool is required.

The objectives of this projet were (i) develop a python tools to set all necessary ACTUATOR_SET objects (ii) provide basic visualization plots allowing to monitor simulations on-the-fly (by post-processing YALES2 temporals and BHawC results files).

''*BHawC : aero-servo-elastic solver used and developped at Siemens Gamesa Renewable Energy for wind turbine design and certification
''

=== Hackathon - G. Staffelbach, CERFACS ===
* '''Participants: I. d'Ast, J. Legaux, G. Staffelbach, P. Begou, G. Lartigue, V. Moureau, A. Toure, C. Laurie, S. Delamare, C. Andrieu, C. Jourdain'''
AMD GPU hardware is still relatively unknown in our CFD community. This hackathon was the opportunity to deep dive into the AMD dev environment to prepare the arrival of AdAstra at CINES.
Both YALES and AVBP have been ported to the AOMP framework using ROCm 4.5 on the GRID5000 Neowise system.
CPU execution posed no issues and we were able to focus on GPU Offloading using OpenMP.
On the YALES2 side, a mini-app encompassing the typical YALES2 structure hierarchy and loop execution was extracted from the code to evaluate different porting strategies and on the AVBP side the current OpenACC GPU offloading was translated to OpenMP focusing on the viscosity computation kernel.
We learnt that the current supported standard of OpenMP in ROCm 4.5 does not allow for direct offloading of reference values inside an derived type structure but is was possible to use aliases such as pointers or flat array copies to do the job. This should be solved with the support of OpenMP 5.0
Another troublesome issues, was the lack of support for offloading of array vector operations (ex : array(:) = 1.0 ) rendering the explicitation of the loops for these manadatory.

Some bugs remain and it is encouraged to use the latest compiler version when working on the porting ( the release of flang 14.0.1 saved us a lot of time as we had started with 14.0.0 ).
Offloading of the miniapp of YALES2 yielded a times 60 acceleration of the kernel whereas the offloading of the viscosity model in a full avbp simulation yielded an 7 times factor in performance when comparing on core to one GPU. These results are to be taken with a grain of salt but are really encouraging.

For the next steps, a porting strategy for both codes will be implemented (depending on the OpenMP 5 support ) and discussions are underway with CINES and other partners so as to offer the best experience to both code's communities on AdAstra at its release.

File:ECFD5 PhotoGroupe.jpeg

2022-02-01T08:05:03Z

Benard:

Ecfd:ecfd 5th edition

2022-02-01T07:46:31Z

Benard: /* Multi-phase flows - M. Cailler, SAFRAN TECH */

{{DISPLAYTITLE: ECFD workshop, 5th edition, 2022}}

== Description ==
{| align="right" style="text-align:center;" cellpadding="2"
| [[File:Logo_ECFD5.png | center | thumb | 350px | ECFD5 workshop logo.]]
|}
* Event from '''23th to 28th of January 2022'''
* Location: [https://www.bonsejour-laplage.com/vacances-tout-compris Centre Bonséjour], Merville-Franceville, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points.
** mini-workshops. Potential workshops are listed below.
* Free of charge
* More than 50 participants from academics (CERFACS, CORIA, IMAG, LEGI, EM2C, UMONS, UVM, VUB, UCL, TUDelft), HPC center/experts (GENCI, AMD, CINES, CRIANN) and industry (Safran, Ariane Group, Siemens-Gamesa).

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes

== News ==

* 03/11/2021: First announcement of the '''5th Extreme CFD Workshop & Hackathon''' !

[[File:Banniere_ECFD5_sponso.png|text-bottom|600px]]

* 13/01/2022: After discussions with the participants, the '''5th Extreme CFD Workshop & Hackathon''' is maintained as an in-person event! It will be also possible to attend to the plenary sessions and participate remotely to the workshop.

* 14/01/2022: The [[#Agenda|ECFD5 program]] is online! The plenary sessions will be announced soon!

* 20/01/2022: The plenary sessions are now defined:
** P1 - 24/01/2022: GPU porting challenges and quantum computing, présentation machine Adastra by G. Staffelbach (CERFACS) + Presentation of the new cluster from CINES called Adastra by C. Andrieu (CINES)
** P2 - 25/01/2022: News, perspectives and future of GPU computing applied to CFD by A. Toure (AMD)
** P3 - 26/01/2022: Theory, applications and perspectives of the Lattice-Boltzmann Method by P. Boivin (M2P2)
** P4 - 27/01/2022: Concepts and notions of mesh adaptation by C. Dapogny (LJK)

* 23/04/2022: '''The ECFD5 event has now started !!''' [https://www.linkedin.com/feed/update/urn:li:activity:6891053385072594944| LinkedIn post]

== Agenda ==

[[File:ECFD5_program.png|text-bottom|600px]]

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

=== Combustion - K. Bioche, VUB ===

* '''Sub-project 1: H2/air jet-in-cross-flow numerical simulations (R. Le Dortz, E. Riber, Q. Douasbin)'''

The use of hydrogen as an aviation fuel requires new combustion chamber design. Among strategies to prevent flame flashback and low flame residence time, the micromix injection system is further studied by ENABLEH2. This systems corresponds to a multitude of H2/air jet-in-cross-flow configurations. A 3D numerical simulation with realistic thermodynamics and kinetics is now tractable thanks to massively parralel computing. This week saw the completion of the first steps towards the establishment of a complete simulation. (I) The non-reactive air injection in the combustion chamber. (II) The cross-injection of H2 without ignition. (III) The ignition of this mixture modeled with the skeletal kinetic mechanism of Boivin (H2, H, O2, OH, O, H2O, HO2, H2O2, N2). Further work will be realised concerning mesh refinement, modelling of NOx and porting of the computation on GPU.

* '''Sub-project 2: LES calculation of the MICADO test rig with multicomponent jetA1 (S. Puggelli, T. Lesaffre, E. Riber, B. Cuenot)'''

The EU-funded project ALTERNATE has the goal of exploring the possibility for a wider utilisation of aviation sustainable fuels. A part of the project deals with the assessment of the effect of SAFs on soot production: using the experimental information obtained at ONERA in high-pressure conditions on the MICADO test rig, the effect of Alcohol to Jet (ATJ-SPK) fuel on soot levels are assessed and compared with standard jet A1 emissions. During the project, STech and CERFACS are working jointly on the numerical modelling of soot emissions for jet-A1 and ATJ-SPK combustion in AVBP. Starting from the numerical setup under-development for jet-A1, the worshop permitted to: (I) Switch from a 2-step kinetic mechanism to a complex 29 species, 233 reacs and 15 QSS mechanism. This transition was efficiently conducted with the tool Multi Table Generator. (II) At this stage, an assessment of the effects of the flame sensor on the calculation results was carried out, indicating the consistent behaviour of a recently developped sensor w.r.t classical tools. (III) Switch towards a multicomponent formulation of jet-A1 and assessment of the effect of such advanced approach with respect to the single-component formulation previously employed. Further work will be realised to manage the stiffness of employed kinetics and to compare jet-A1 and ATJ-SPK fuels from a chemical point of view.

* '''Sub-project 3: Euler-Lagrange Multigrid Simulation (T. Lesaffre, O. Vermorel, E. Riber, B. Cuenot)'''

In Lagrange simulations, the point-source approach is based on a ponctual approximation of the particule and requires this last to be smaller than the mesh. The very fine meshes required to represent the Eulerian phase of Euler-Lagrange two-phase flow simulations can lead to a non-validity of the point-source hypothesis. This project aimed at implementing, in the AVBP solver, the simultaneous management and coupling of several simulations. During this week, the Eulerian and Lagrangian phase were successfuly computed on two different meshes and coupled via the CWIPI library. The good behaviour of this framework was assessed on a 1D Evaporation of kerosene droplets in an air stream test case. Encouraging preliminary performance results were obtained on a 3D injection case and require further work.

* '''Sub-project 4: Devolatilization modelling for biomass combustion (K. Bioche, L. Bricteux)'''

Biomass combustion simulations require the modelling of numerous physical phenomena: particle drying, devolatilization, gas-phase combustion, chars oxidation. Besides, the valorisation chains for biomass include fluidized bed reactors, fixed bed reactors and pulverized fuel burners. The Granular Flow Solver of YALES2 offers a good framework for the simulation of fluidized bed reactors and is functionnaly coupled with the reactive gas-phase solver of the same code. This week permitted to partically implement the modelling of devolatilization in this solver. A single-step kinetic scheme is considered for the particle mass evolution equation while the particle diameter evolves during the process. Further work is necessary to account for the thermal and mass couplings with the fluid phase.

* '''Sub-project 5: Thickened-Flame LES model in a Lattice-Boltzmann Method framework (P. Boivin, S. Zhao, M. Le Boursicaud)'''

The TFLES framework of the hybrid Lattice-Boltzmann sover ProLB was extended to account for recent sensor methods. During this week, a smooth flame sensor based on the curvature of the norm of the advancement variable gradient was developped. Also for filtering operations, the lattice requires to access data over three neighboring layers. A precise and continuous thickening factor was obtained with such method.

* '''Sub-project 6: NOx modeling applied to KIAI combustion chamber (J. Obando, P. Bénard, V. Moureau)'''

This project treated of the implementation of NOx modeling into simulations of the KIAI combustion chamber, experimentaly studied at CORIA lab. During this week, various NOx modeling strategies were listed. Associated kinetic mechanisms, among which analytical chemisty, were employed for 1D flame simulations in YALES2 solver. Further work include the use of such methods on the 3D computational case.

=== Static and dynamic mesh adaptation - G. Balarac, LEGI ===

=== Multi-phase flows - M. Cailler, SAFRAN TECH ===

* '''Sub-project 1: Hybrid E-E/E-L two-phase flow method (M. Cailler, F. Pecquery, I. El Yamani, V. Moureau)'''

The High-Fidelity approach based on ACLS & DMA allows a reliable description of interface dynamics. For design exploration, low-CPU methods with controlled level of fidelity are required. An interesting approach to reduce CPU cost relies on an hybrid approach based on an Eulerian representation of the gas & and a Lagrangian description for the liquid phase. Objective of the ECFD5 was to explore the capability to reconstruct the interface normal of a liquid droplet made of particles on an Eulerian grid. First, a level-set based strategy relying on Geometric Multiple Markers Projection (Janodet et al., 2022) has been first tested showing good capabilities providing that the iso-surface distance equal 0 is well captured on the mesh. An alternative strategy based on the liquid volume fraction has been tested. This information was then used to build a velocity correction that is used to transport particles and ensures regularized particle positions. This preliminary banchmark work will be pursued on a liquid jet propagation problem.

* '''Sub-project 2: Jet atomization with a diffuse interface mathod (N. Odier, B. Péden, J. Carmona, P. Boivin)'''

A so-called diffuse interface method, using the multi-fluid formalism, coupled with a Riemann solver HLLC and a thermodynamic closure of the NASG type, was implemented in AVBP during the thesis of Carmona. Objective of the project was to couple the NASG thermodynamic implementation in an improved HLLC solver based on nodal information only.

* '''Sub-project 3: Convergence of the interface curvature computation (G. Ghigliotti, J. Carmona, G. Balarac, G. Lartigue)'''

The computation of interface curvature in a level-set framework is based on the classic formula as divergence of the gradient of the levelset function. This function being computed at 2nd order, one obtains a O(0) curvature, meaning that the error does not decrease with mesh refinement.
We have implemented in YALES2 a strategy proposed by Emmanuel Maître and collaborators in a finite element method based on the regularization (filtering) of the level-set gradient and curvature.
This strategy has been tested for the simple test case of a static circular interface.
We used two types of filters (simple gather-scatter or bilaplacian as developed by Lola Guedot (PhD thesis 2015)) on different mesh types (split quadrilaterals, isotropic triangular mesh, unstructured triangular mesh).
The results are encouraging since a O(1) convergence is obtained in all cases.
Further work is needed to tune the filter properties (amplitude and size) for different spatial resolutions and levelset "narrow band" width.

* '''Sub-project 4: Conservative two-fluid momentum transport (F. Pecquery, C. Merlin, M. Cailler, J. Carmona, V. Moureau)'''

The aim of this project was to investigate the applicability of the conservative two-fluid transport framework developed for passive scalars to the momentum conservation equation. First a complete two-fluid framework for the incompressible two-phase Navier-Stokes equations was proposed. Then the discontinuous algorithms for discontinuous scalars (data extension, reinitialization and transport) have been extended to discontinuous vectors. Moreover, some improvement of the data-structure were implemented to further generalize the framework and improve user-experience. To eventually conclude on the applicability of the proposed two-fluid momentum approach, the proposed interfacial momentum flux model and correction step will be implemented in YALES2

=== Numerics - G. Lartigue, CORIA ===

* '''Sub-project 4: Avoiding the acoustic timestep restriction in two-phase flow simulations (V. Boniou, J. Paris, A. Vié, T. Schmitt, C. Tenaud, Y. Béchane)‘’'
In the context of simulating compressible two-phase flows with phase change, the CFL constraint related to the acoustic part of an explicit solver, especially in the liquid phase, can be prohibitive for evaporation studies implying large timescale separation between evaporation and acoustic wave propagation.
The acoustic CFL constraint can be either relaxed by considering an implicit treatment of the acoustic part or avoided by deriving the low Mach limit of the multifluid model.

During this week, the first steps of this vast work have been initiated on single-phase flows by implementing the variable-density low Mach solver and the IMEX method applied on the Euler equations in TITAN.

The IMEX method was shown to provide degraded solutions in acoustic-driven cases, compared to conventional Riemann solutions. However, in transport-dominated cases, the increase of the transport CFL leads to a better numerical precision. A 2D cylindrical shock has been simulated with an acoustic CFL of 40 to assess the ability to run multidimensional compressible computations with large time step. The method was also extented to 4eq and 2nd order spatial accuracy.

Finally, the extension of the variable density solver to two-phase flows was performed using a VOF interface capturing method, considering a dilatable gas phase along with an incompressible liquid phase. Particular attention has been drawn to preserving liquid volume by treating the VOF transport accordingly with an adapted velocity field.

=== Turbulent flows - P. Bénard, CORIA ===
* '''Sub-project 1: Optimization of the actuator set for several wind turbines in YALES2 (F. Houtin Mongrolle, S. Gremmo, E. Muller, B. Duboc)'''
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified:
(i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges.
(ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications.
(iii) Adapt the YALES2-BHawC aero-servo-elastic coupling to make it consistent with the new ALM implementation.
During the workshop, the (i)-algorithm has been implemented with additional optimizations. Tests on a 30-turbine wind farm showed a 60% improvement compared to previous performances, which is very promising. The (ii)-feature still needs to be developed but should lead to the performances objective. The tests with the coupled code YALES2-BHawC were not conclusive yet and still require some work.

* '''Sub-project 2: Thermal effect in an atmospheric solver (U. Vigny, L. Voivenel, S. Zeoli, P. Benard)'''
Given the current environmental and energy challenges, maximising the wind farm electricity production is essential. Therefore, it becomes necessary to develop the most reliable and accurate prediction and simulation tools. Following this tenet, an atmospheric solver, which will take into account meteorological phenomena, should be developed. The preliminary work, going from bibliography study to road map was performed during the extreme cod workshop. Thus five parts have been identified:
(I) The YALES2 Variable Density Solver (VDS) will be used because of the need to take into account buoyancy effect including for big density differences.
(II) A wall law correction term, relative to atmospheric boundary layer will be added.
(III) The actuator line method used to simulate wind turbine will be extended to VDS, modifying the velocity source term to a momentum source term.
(IV) The Coriolis effects, depending on the latitude will be implemented.
(V) The wall heat flux, allowing to simulate diurnal and nocturnal cycles on various terrains, is more realistic than a target wall temperature.
From this work, future development are now clear and just waiting to be developed.

* '''Sub-project 3: Dynamic stall correction models for Horizontal Axis Wind Turbine (S. Gremmo, E. Muller, B. Duboc, F. Houtin-Mongrolle)'''
The Actuator Line Method implementation into YALES2 library suffers from the lack of correction models for some 3D unsteady effects. An important one, the dynamic stall, implies an unsteady modification of the aerodynamic loads, i.e. the polars, with the angle of attack. With the standard ALM model, static 2D polars are used. Adding a dynamic stall correction model allow to have an hysteresis loop on the of the forces coefficient with the angle of attack. Several models exists in the literature: Oye, BeddoesLeishman, Sheng, Risoe, ONERA... During the workshop, the Oye model was selected, as it is the simplest model, and was implemented. First, new ingredients necessary to the model were added in the polars look-up table generator used to describe the blades. Then, the Oye correction was integrated into the YALES2 library. Finally, the model was tested on simple configurations. It is now essential to validate more the implementation.

* '''Sub-project4: Immersed Boundary solvers uniformisation into YALES2 (I. Tsetoglou, M. Cailler, P. Benez, S. Mendez, P. Benard)'''
A novel Conservative Lagrangian Immersed Boundary (CLIB) method [Tsetoglou et al., 2021] has been developed by CORIA & Safran Tech for wind turbines & gearbox lubrication simulations. Nevertheless, two different implementations of the same method were created that tends to diverge. This sub-project aims to develop a shared numerical kernel (CLIB) on which 2 solvers are based: CIB for aerodynamic applications (ICS + CLIB) and TPS for two-phase flows (SPS+CLIB). The newly developed kernel and solvers were tested and validated on test cases: 2D flows around static/rotating/oscillating disk, 3D flow around a rotating cylinder and 2D flows around counter-rotating ellipses. The newt steps involve the continuation of the validation of both solvers and its documentation, as well as the development of wall model for immersed bodies.

* '''Sub-project 5: TBLE wall model for LES with pressure gradient on a simple turbomachinery geometry (M. Cizeron, N. Odier, R. Vicquelin)'''
Wall modeling is often used in LES to alleviate the computational cost that would be required to resolve all the length scales up to the solid boundaries of the domain. The classical way of doing it is by using an algebraic model to provide the wall friction and heat flux, with a coupling to the LES solver at the first off-wall nodes. The wall model was designed from analyzing RANS equation with strong assumptions such as planar flow, equilibrium and no pressure gradient. These assumptions are often far from true in real applications, such as turbomachinery applications, where the use of a wall model is mandatory due to the size of the calculation. During this workshop, a wall model relying on the resolution of the Thin Boundary Layer Equations (TBLE) was studied, which had been implemented by EM2C. The addition of a pressure gradient to these equations has been conducted and tested, at first only for the 1D wall model solver, then on a 3D turbulent channel. It remains to be tested on a diffuser configuration with a real pressure gradient to quantify the effect of the new wall model. The influence of the point considered to do the coupling between the LES and the wall model (ie. its distance to the wall) has also been tested both for the TBLE and the original algebraic model, showing that coupling farther from the wall yields better results and reduces the so-called log-layer mismatch.

* '''Sub-project 6: Tools for rough wall modelling (A. Barge, S. Meynet)'''
Within the STREAM project framework, a roughness-resolved Large-Eddy Simulation (RRLES) database is being built. The aim of this latter is to be representative of rough channel flows, especially for additive-manufacturing heat exchangers. First RRLES have already been performed. From turbulence and rough wall stress statistics analysis of the results, a first stochastic model, which reproduces the statistical behavior of the wall stress vector, have been proposed. The modeled wall stress allows a better prediction of the pressure drop in a flat wall channel compared to the use of the mean value of the wall stress measured in RRLES alone. However, the near wall region is still mispredicted and the model is correlated in time but not in space. The aim of this ECFD5 was to develop tools to improve modelling and explore new ways. A roughness mapping tool for smooth surfaces have been implemented into YALES2 to get local surface height. This tool is based on an existing in-house surface roughness generator developed for the STREAM project. The idea is to use the map to generated space correlated fluctuations for the wall shear stress. Some bugs still remain to fully use this tool. In parallel, the modelling approach was extended to passive scalar, especially for temperature. To this end, new random tools as white noise, unit sphere random walk and Gaussian / Log-normal stochastic processes have been coded. Finally, the idea of using walls as velocity source terms emerged during this ECFD5. The principle is to mask a grid layer above the wall and to transport the rough map on this grid to estimate the roughness effects above the wall. Parametrizing and testing these tools remained to be done at the end of ECFD5.

=== User experience - J. Leparoux, SAFRAN TECH ===

* '''Sub-project 1: Multi combustion model chemtable generator (S. Dillon, R. Mercier)'''

Due to the high computational cost of LES of turbulent combustion with detailed chemistry, tabulated chemistry approaches remain a popular choice within the combustion community. This popularity of tabulated chemistry leads to research into the development of novel combustion models for which a platform to test these models is required. The objective of the workshop was to therefore begin developing an easy-to-use chemistry table generator within Python. Given that there exists numerous turbulent combustion models (F-TACLES Diffusion, F-TACLES Premixed, 2PFT, etc.), a generalised multi-model code was necessary. Along with multi-model capabilities, the code was required to function with multiple solvers for the creation of the flame database (REGATH and CANTERA). With these objectives in mind, the code “TabulatEd Chemistry GenERator for Aeronautical CombusTion” (TECERACT) was created. Advancements were made in all key areas using code already developed by Safran Tech. TECERACT includes a parallel flame database generator and current developments are focused on multi-model functionality and simplified mesh management within Python. Once finished, the TECERACT code will provide a user-friendly chemistry table generator capable of producing chemistry tables for LES simulations and its multi-model structure will serve as a platform for future turbulent combustion model development/testing.

* '''Sub-project 2: Task-driven automatic run sequence (R. Mercier, J. Leparoux, M. Cailler, R. Letournel)'''

The principal objective of this project was to develop a new simulation workflow based on Task-driven approach. This approach could offer a higher flexibility to manage CFD simulations by decomposing the whole simulation on small sequences (run initialization, fuel injection, etc.)

This week showed the achievement of the first steps towards a complete task-driven simulation. (i) The whole simulation workflow was set-up (ii) Missing actions were implemented allowing to update simulation status on-the-fly (from non reactive to reactive simulation) (iii) State are now embedded from the restart solution allowing to restart from the last known state. Further work is still needed to obtain the full implementation of the AMC framework based on task-driven approach. Especially to automate the adaptation loop

* '''Sub-project 3: YALES2 Tools & Gitlab CI (J. Leparoux, A. Tstetoglou)'''

* '''Sub-project 4: Wind energy tools (E. Muller, S. Gremmo, F. Houtin-Mongrolle, B. Duboc)'''

Set-up phase of simulations involving several wind turbines is very demanding and error-prone, especially for simulations carried out with the coupled solver YALES2-BHawC*. This type of simulation can involve several dozen input files and then, an assistant tool is required.

The objectives of this projet were (i) develop a python tools to set all necessary ACTUATOR_SET objects (ii) provide basic visualization plots allowing to monitor simulations on-the-fly (by post-processing YALES2 temporals and BHawC results files).

''*BHawC : aero-servo-elastic solver used and developped at Siemens Gamesa Renewable Energy for wind turbine design and certification
''

=== Hackathon - G. Staffelbach, CERFACS ===
* '''Participants: I. d'Ast, J. Legaux, G. Staffelbach, P. Begou, G. Lartigue, V. Moureau, A. Toure, C. Laurie, S. Delamare, C. Andrieu, C. Jourdain'''
AMD GPU hardware is still relatively unknown in our CFD community. This hackathon was the opportunity to deep dive into the AMD dev environment to prepare the arrival of AdAstra at CINES.
Both YALES and AVBP have been ported to the AOMP framework using ROCm 4.5 on the GRID5000 Neowise system.
CPU execution posed no issues and we were able to focus on GPU Offloading using OpenMP.
On the YALES2 side, a mini-app encompassing the typical YALES2 structure hierarchy and loop execution was extracted from the code to evaluate different porting strategies and on the AVBP side the current OpenACC GPU offloading was translated to OpenMP focusing on the viscosity computation kernel.
We learnt that the current supported standard of OpenMP in ROCm 4.5 does not allow for direct offloading of reference values inside an derived type structure but is was possible to use aliases such as pointers or flat array copies to do the job. This should be solved with the support of OpenMP 5.0
Another troublesome issues, was the lack of support for offloading of array vector operations (ex : array(:) = 1.0 ) rendering the explicitation of the loops for these manadatory.

Some bugs remain and it is encouraged to use the latest compiler version when working on the porting ( the release of flang 14.0.1 saved us a lot of time as we had started with 14.0.0 ).
Offloading of the miniapp of YALES2 yielded a times 60 acceleration of the kernel whereas the offloading of the viscosity model in a full avbp simulation yielded an 7 times factor in performance when comparing on core to one GPU. These results are to be taken with a grain of salt but are really encouraging.

For the next steps, a porting strategy for both codes will be implemented (depending on the OpenMP 5 support ) and discussions are underway with CINES and other partners so as to offer the best experience to both code's communities on AdAstra at its release.

Ecfd:ecfd 5th edition

2022-02-01T07:42:55Z

Benard: /* Turbulent flows - P. Bénard, CORIA */

{{DISPLAYTITLE: ECFD workshop, 5th edition, 2022}}

== Description ==
{| align="right" style="text-align:center;" cellpadding="2"
| [[File:Logo_ECFD5.png | center | thumb | 350px | ECFD5 workshop logo.]]
|}
* Event from '''23th to 28th of January 2022'''
* Location: [https://www.bonsejour-laplage.com/vacances-tout-compris Centre Bonséjour], Merville-Franceville, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points.
** mini-workshops. Potential workshops are listed below.
* Free of charge
* More than 50 participants from academics (CERFACS, CORIA, IMAG, LEGI, EM2C, UMONS, UVM, VUB, UCL, TUDelft), HPC center/experts (GENCI, AMD, CINES, CRIANN) and industry (Safran, Ariane Group, Siemens-Gamesa).

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes

== News ==

* 03/11/2021: First announcement of the '''5th Extreme CFD Workshop & Hackathon''' !

[[File:Banniere_ECFD5_sponso.png|text-bottom|600px]]

* 13/01/2022: After discussions with the participants, the '''5th Extreme CFD Workshop & Hackathon''' is maintained as an in-person event! It will be also possible to attend to the plenary sessions and participate remotely to the workshop.

* 14/01/2022: The [[#Agenda|ECFD5 program]] is online! The plenary sessions will be announced soon!

* 20/01/2022: The plenary sessions are now defined:
** P1 - 24/01/2022: GPU porting challenges and quantum computing, présentation machine Adastra by G. Staffelbach (CERFACS) + Presentation of the new cluster from CINES called Adastra by C. Andrieu (CINES)
** P2 - 25/01/2022: News, perspectives and future of GPU computing applied to CFD by A. Toure (AMD)
** P3 - 26/01/2022: Theory, applications and perspectives of the Lattice-Boltzmann Method by P. Boivin (M2P2)
** P4 - 27/01/2022: Concepts and notions of mesh adaptation by C. Dapogny (LJK)

* 23/04/2022: '''The ECFD5 event has now started !!''' [https://www.linkedin.com/feed/update/urn:li:activity:6891053385072594944| LinkedIn post]

== Agenda ==

[[File:ECFD5_program.png|text-bottom|600px]]

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

=== Combustion - K. Bioche, VUB ===

* '''Sub-project 1: H2/air jet-in-cross-flow numerical simulations (R. Le Dortz, E. Riber, Q. Douasbin)'''

The use of hydrogen as an aviation fuel requires new combustion chamber design. Among strategies to prevent flame flashback and low flame residence time, the micromix injection system is further studied by ENABLEH2. This systems corresponds to a multitude of H2/air jet-in-cross-flow configurations. A 3D numerical simulation with realistic thermodynamics and kinetics is now tractable thanks to massively parralel computing. This week saw the completion of the first steps towards the establishment of a complete simulation. (I) The non-reactive air injection in the combustion chamber. (II) The cross-injection of H2 without ignition. (III) The ignition of this mixture modeled with the skeletal kinetic mechanism of Boivin (H2, H, O2, OH, O, H2O, HO2, H2O2, N2). Further work will be realised concerning mesh refinement, modelling of NOx and porting of the computation on GPU.

* '''Sub-project 2: LES calculation of the MICADO test rig with multicomponent jetA1 (S. Puggelli, T. Lesaffre, E. Riber, B. Cuenot)'''

The EU-funded project ALTERNATE has the goal of exploring the possibility for a wider utilisation of aviation sustainable fuels. A part of the project deals with the assessment of the effect of SAFs on soot production: using the experimental information obtained at ONERA in high-pressure conditions on the MICADO test rig, the effect of Alcohol to Jet (ATJ-SPK) fuel on soot levels are assessed and compared with standard jet A1 emissions. During the project, STech and CERFACS are working jointly on the numerical modelling of soot emissions for jet-A1 and ATJ-SPK combustion in AVBP. Starting from the numerical setup under-development for jet-A1, the worshop permitted to: (I) Switch from a 2-step kinetic mechanism to a complex 29 species, 233 reacs and 15 QSS mechanism. This transition was efficiently conducted with the tool Multi Table Generator. (II) At this stage, an assessment of the effects of the flame sensor on the calculation results was carried out, indicating the consistent behaviour of a recently developped sensor w.r.t classical tools. (III) Switch towards a multicomponent formulation of jet-A1 and assessment of the effect of such advanced approach with respect to the single-component formulation previously employed. Further work will be realised to manage the stiffness of employed kinetics and to compare jet-A1 and ATJ-SPK fuels from a chemical point of view.

* '''Sub-project 3: Euler-Lagrange Multigrid Simulation (T. Lesaffre, O. Vermorel, E. Riber, B. Cuenot)'''

In Lagrange simulations, the point-source approach is based on a ponctual approximation of the particule and requires this last to be smaller than the mesh. The very fine meshes required to represent the Eulerian phase of Euler-Lagrange two-phase flow simulations can lead to a non-validity of the point-source hypothesis. This project aimed at implementing, in the AVBP solver, the simultaneous management and coupling of several simulations. During this week, the Eulerian and Lagrangian phase were successfuly computed on two different meshes and coupled via the CWIPI library. The good behaviour of this framework was assessed on a 1D Evaporation of kerosene droplets in an air stream test case. Encouraging preliminary performance results were obtained on a 3D injection case and require further work.

* '''Sub-project 4: Devolatilization modelling for biomass combustion (K. Bioche, L. Bricteux)'''

Biomass combustion simulations require the modelling of numerous physical phenomena: particle drying, devolatilization, gas-phase combustion, chars oxidation. Besides, the valorisation chains for biomass include fluidized bed reactors, fixed bed reactors and pulverized fuel burners. The Granular Flow Solver of YALES2 offers a good framework for the simulation of fluidized bed reactors and is functionnaly coupled with the reactive gas-phase solver of the same code. This week permitted to partically implement the modelling of devolatilization in this solver. A single-step kinetic scheme is considered for the particle mass evolution equation while the particle diameter evolves during the process. Further work is necessary to account for the thermal and mass couplings with the fluid phase.

* '''Sub-project 5: Thickened-Flame LES model in a Lattice-Boltzmann Method framework (P. Boivin, S. Zhao, M. Le Boursicaud)'''

The TFLES framework of the hybrid Lattice-Boltzmann sover ProLB was extended to account for recent sensor methods. During this week, a smooth flame sensor based on the curvature of the norm of the advancement variable gradient was developped. Also for filtering operations, the lattice requires to access data over three neighboring layers. A precise and continuous thickening factor was obtained with such method.

* '''Sub-project 6: NOx modeling applied to KIAI combustion chamber (J. Obando, P. Bénard, V. Moureau)'''

This project treated of the implementation of NOx modeling into simulations of the KIAI combustion chamber, experimentaly studied at CORIA lab. During this week, various NOx modeling strategies were listed. Associated kinetic mechanisms, among which analytical chemisty, were employed for 1D flame simulations in YALES2 solver. Further work include the use of such methods on the 3D computational case.

=== Static and dynamic mesh adaptation - G. Balarac, LEGI ===

=== Multi-phase flows - M. Cailler, SAFRAN TECH ===

* '''Sub-project 1: Hybrid E-E/E-L two-phase flow method (M. Cailler, F. Pecquery, I. El Yamani, V. Moureau)'''

The High-Fidelity approach based on ACLS & DMA allows a reliable description of interface dynamics. For design exploration, low-CPU methods with controlled level of fidelity are required. An interesting approach to reduce CPU cost relies on an hybrid approach based on an Eulerian representation of the gas & and a Lagrangian description for the liquid phase. Objective of the ECFD5 was to explore the capability to reconstruct the interface normal of a liquid droplet made of particles on an Eulerian grid. First, a level-set based strategy relying on Geometric Multiple Markers Projection (Janodet et al., 2022) has been first tested showing good capabilities providing that the iso-surface distance equal 0 is well captured on the mesh. An alternative strategy based on the liquid volume fraction has been tested. This information was then used to build a velocity correction that is used to transport particles and ensures regularized particle positions. This preliminary banchmark work will be pursued on a liquid jet propagation problem.

* '''Sub-project 2: Jet atomization with a diffuse interface mathod (N. Odier, B. Péden, J. Carmona, P. Boivin)'''

A so-called diffuse interface method, using the multi-fluid formalism, coupled with a Riemann solver HLLC and a thermodynamic closure of the NASG type, was implemented in AVBP during the thesis of Carmona. Objective of the project was to couple the NASG thermodynamic implementation in an improved HLLC solver based on nodal information only.

* '''Sub-project 3: Convergence of the interface curvature computation (G. Ghigliotti, J. Carmona, G. Balarac, G. Lartigue)'''

The computation of interface curvature in a level-set framework is based on the classic formula as divergence of the gradient of the levelset function. This function being computed at 2nd order, one obtains a O(0) curvature, meaning that the error does not decrease with mesh refinement.
We have implemented in YALES2 a strategy proposed by Emmanuel Maître and collaborators in a finite element method based on the regularization (filtering) of the level-set gradient and curvature.
This strategy has been tested for the simple test case of a static circular interface.
We used two types of filters (simple gather-scatter or bilaplacian as developed by Lola Guedot (PhD thesis 2015)) on different mesh types (split quadrilaterals, isotropic triangular mesh, unstructured triangular mesh).
The results are encouraging since a O(1) convergence is obtained in all cases.
Further work is needed to tune the filter properties (amplitude and size) for different spatial resolutions and levelset "narrow band" width.

* '''Sub-project 4: Conservative two-fluid momentum transport (F. Pecquery, C. Merlin, M. Cailler, J. Carmona, V. Moureau)'''

The aim of this project was to investigate the applicability of the conservative two-fluid transport framework developed for passive scalars to the momentum conservation equation. First a complete two-fluid framework for the incompressible two-phase Navier-Stokes equations was proposed. Then the discontinuous algorithms for discontinuous scalars (data extension, reinitialization and transport) have been extended to discontinuous vectors. Moreover, some improvement of the data-structure were implemented to further generalize the framework and improve user-experience. To eventually conclude on the applicability of the proposed two-fluid momentum approach, the proposed interfacial momentum flux model and correction step will be implemented in YALES2

=== Numerics - G. Lartigue, CORIA ===

* '''Sub-project 4: Avoiding the acoustic timestep restriction in two-phase flow simulations (V. Boniou, J. Paris, A. Vié, T. Schmitt, C. Tenaud, Y. Béchane)‘’'
In the context of simulating compressible two-phase flows with phase change, the CFL constraint related to the acoustic part of an explicit solver, especially in the liquid phase, can be prohibitive for evaporation studies implying large timescale separation between evaporation and acoustic wave propagation.
The acoustic CFL constraint can be either relaxed by considering an implicit treatment of the acoustic part or avoided by deriving the low Mach limit of the multifluid model.

During this week, the first steps of this vast work have been initiated on single-phase flows by implementing the variable-density low Mach solver and the IMEX method applied on the Euler equations in TITAN.

The IMEX method was shown to provide degraded solutions in acoustic-driven cases, compared to conventional Riemann solutions. However, in transport-dominated cases, the increase of the transport CFL leads to a better numerical precision. A 2D cylindrical shock has been simulated with an acoustic CFL of 40 to assess the ability to run multidimensional compressible computations with large time step. The method was also extented to 4eq and 2nd order spatial accuracy.

Finally, the extension of the variable density solver to two-phase flows was performed using a VOF interface capturing method, considering a dilatable gas phase along with an incompressible liquid phase. Particular attention has been drawn to preserving liquid volume by treating the VOF transport accordingly with an adapted velocity field.

=== Turbulent flows - P. Bénard, CORIA ===
* '''Sub-project 1: Optimization of the actuator set for several wind turbines in YALES2 (F. Houtin Mongrolle, S. Gremmo, E. Muller, B. Duboc)'''
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified:
(i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges.
(ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications.
(iii) Adapt the YALES2-BHawC aero-servo-elastic coupling to make it consistent with the new ALM implementation.
During the workshop, the (i)-algorithm has been implemented with additional optimizations. Tests on a 30-turbine wind farm showed a 60% improvement compared to previous performances, which is very promising. The (ii)-feature still needs to be developed but should lead to the performances objective. The tests with the coupled code YALES2-BHawC were not conclusive yet and still require some work.

* '''Sub-project 2: Thermal effect in an atmospheric solver (U. Vigny, L. Voivenel, S. Zeoli, P. Benard)'''
Given the current environmental and energy challenges, maximising the wind farm electricity production is essential. Therefore, it becomes necessary to develop the most reliable and accurate prediction and simulation tools. Following this tenet, an atmospheric solver, which will take into account meteorological phenomena, should be developed. The preliminary work, going from bibliography study to road map was performed during the extreme cod workshop. Thus five parts have been identified:
(I) The YALES2 Variable Density Solver (VDS) will be used because of the need to take into account buoyancy effect including for big density differences.
(II) A wall law correction term, relative to atmospheric boundary layer will be added.
(III) The actuator line method used to simulate wind turbine will be extended to VDS, modifying the velocity source term to a momentum source term.
(IV) The Coriolis effects, depending on the latitude will be implemented.
(V) The wall heat flux, allowing to simulate diurnal and nocturnal cycles on various terrains, is more realistic than a target wall temperature.
From this work, future development are now clear and just waiting to be developed.

* '''Sub-project 3: Dynamic stall correction models for Horizontal Axis Wind Turbine (S. Gremmo, E. Muller, B. Duboc, F. Houtin-Mongrolle)'''
The Actuator Line Method implementation into YALES2 library suffers from the lack of correction models for some 3D unsteady effects. An important one, the dynamic stall, implies an unsteady modification of the aerodynamic loads, i.e. the polars, with the angle of attack. With the standard ALM model, static 2D polars are used. Adding a dynamic stall correction model allow to have an hysteresis loop on the of the forces coefficient with the angle of attack. Several models exists in the literature: Oye, BeddoesLeishman, Sheng, Risoe, ONERA... During the workshop, the Oye model was selected, as it is the simplest model, and was implemented. First, new ingredients necessary to the model were added in the polars look-up table generator used to describe the blades. Then, the Oye correction was integrated into the YALES2 library. Finally, the model was tested on simple configurations. It is now essential to validate more the implementation.

* '''Sub-project4: Immersed Boundary solvers uniformisation into YALES2 (I. Tsetoglou, M. Cailler, P. Benez, S. Mendez, P. Benard)'''
A novel Conservative Lagrangian Immersed Boundary (CLIB) method [Tsetoglou et al., 2021] has been developed by CORIA & Safran Tech for wind turbines & gearbox lubrication simulations. Nevertheless, two different implementations of the same method were created that tends to diverge. This sub-project aims to develop a shared numerical kernel (CLIB) on which 2 solvers are based: CIB for aerodynamic applications (ICS + CLIB) and TPS for two-phase flows (SPS+CLIB). The newly developed kernel and solvers were tested and validated on test cases: 2D flows around static/rotating/oscillating disk, 3D flow around a rotating cylinder and 2D flows around counter-rotating ellipses. The newt steps involve the continuation of the validation of both solvers and its documentation, as well as the development of wall model for immersed bodies.

* '''Sub-project 5: TBLE wall model for LES with pressure gradient on a simple turbomachinery geometry (M. Cizeron, N. Odier, R. Vicquelin)'''
Wall modeling is often used in LES to alleviate the computational cost that would be required to resolve all the length scales up to the solid boundaries of the domain. The classical way of doing it is by using an algebraic model to provide the wall friction and heat flux, with a coupling to the LES solver at the first off-wall nodes. The wall model was designed from analyzing RANS equation with strong assumptions such as planar flow, equilibrium and no pressure gradient. These assumptions are often far from true in real applications, such as turbomachinery applications, where the use of a wall model is mandatory due to the size of the calculation. During this workshop, a wall model relying on the resolution of the Thin Boundary Layer Equations (TBLE) was studied, which had been implemented by EM2C. The addition of a pressure gradient to these equations has been conducted and tested, at first only for the 1D wall model solver, then on a 3D turbulent channel. It remains to be tested on a diffuser configuration with a real pressure gradient to quantify the effect of the new wall model. The influence of the point considered to do the coupling between the LES and the wall model (ie. its distance to the wall) has also been tested both for the TBLE and the original algebraic model, showing that coupling farther from the wall yields better results and reduces the so-called log-layer mismatch.

* '''Sub-project 6: Tools for rough wall modelling (A. Barge, S. Meynet)'''
Within the STREAM project framework, a roughness-resolved Large-Eddy Simulation (RRLES) database is being built. The aim of this latter is to be representative of rough channel flows, especially for additive-manufacturing heat exchangers. First RRLES have already been performed. From turbulence and rough wall stress statistics analysis of the results, a first stochastic model, which reproduces the statistical behavior of the wall stress vector, have been proposed. The modeled wall stress allows a better prediction of the pressure drop in a flat wall channel compared to the use of the mean value of the wall stress measured in RRLES alone. However, the near wall region is still mispredicted and the model is correlated in time but not in space. The aim of this ECFD5 was to develop tools to improve modelling and explore new ways. A roughness mapping tool for smooth surfaces have been implemented into YALES2 to get local surface height. This tool is based on an existing in-house surface roughness generator developed for the STREAM project. The idea is to use the map to generated space correlated fluctuations for the wall shear stress. Some bugs still remain to fully use this tool. In parallel, the modelling approach was extended to passive scalar, especially for temperature. To this end, new random tools as white noise, unit sphere random walk and Gaussian / Log-normal stochastic processes have been coded. Finally, the idea of using walls as velocity source terms emerged during this ECFD5. The principle is to mask a grid layer above the wall and to transport the rough map on this grid to estimate the roughness effects above the wall. Parametrizing and testing these tools remained to be done at the end of ECFD5.

=== User experience - J. Leparoux, SAFRAN TECH ===

* '''Sub-project 1: Multi combustion model chemtable generator (S. Dillon, R. Mercier)'''

Due to the high computational cost of LES of turbulent combustion with detailed chemistry, tabulated chemistry approaches remain a popular choice within the combustion community. This popularity of tabulated chemistry leads to research into the development of novel combustion models for which a platform to test these models is required. The objective of the workshop was to therefore begin developing an easy-to-use chemistry table generator within Python. Given that there exists numerous turbulent combustion models (F-TACLES Diffusion, F-TACLES Premixed, 2PFT, etc.), a generalised multi-model code was necessary. Along with multi-model capabilities, the code was required to function with multiple solvers for the creation of the flame database (REGATH and CANTERA). With these objectives in mind, the code “TabulatEd Chemistry GenERator for Aeronautical CombusTion” (TECERACT) was created. Advancements were made in all key areas using code already developed by Safran Tech. TECERACT includes a parallel flame database generator and current developments are focused on multi-model functionality and simplified mesh management within Python. Once finished, the TECERACT code will provide a user-friendly chemistry table generator capable of producing chemistry tables for LES simulations and its multi-model structure will serve as a platform for future turbulent combustion model development/testing.

* '''Sub-project 2: Task-driven automatic run sequence (R. Mercier, J. Leparoux, M. Cailler, R. Letournel)'''

The principal objective of this project was to develop a new simulation workflow based on Task-driven approach. This approach could offer a higher flexibility to manage CFD simulations by decomposing the whole simulation on small sequences (run initialization, fuel injection, etc.)

This week showed the achievement of the first steps towards a complete task-driven simulation. (i) The whole simulation workflow was set-up (ii) Missing actions were implemented allowing to update simulation status on-the-fly (from non reactive to reactive simulation) (iii) State are now embedded from the restart solution allowing to restart from the last known state. Further work is still needed to obtain the full implementation of the AMC framework based on task-driven approach. Especially to automate the adaptation loop

* '''Sub-project 3: YALES2 Tools & Gitlab CI (J. Leparoux, A. Tstetoglou)'''

* '''Sub-project 4: Wind energy tools (E. Muller, S. Gremmo, F. Houtin-Mongrolle, B. Duboc)'''

Set-up phase of simulations involving several wind turbines is very demanding and error-prone, especially for simulations carried out with the coupled solver YALES2-BHawC*. This type of simulation can involve several dozen input files and then, an assistant tool is required.

The objectives of this projet were (i) develop a python tools to set all necessary ACTUATOR_SET objects (ii) provide basic visualization plots allowing to monitor simulations on-the-fly (by post-processing YALES2 temporals and BHawC results files).

''*BHawC : aero-servo-elastic solver used and developped at Siemens Gamesa Renewable Energy for wind turbine design and certification
''

=== Hackathon - G. Staffelbach, CERFACS ===
* '''Participants: I. d'Ast, J. Legaux, G. Staffelbach, P. Begou, G. Lartigue, V. Moureau, A. Toure, C. Laurie, S. Delamare, C. Andrieu, C. Jourdain'''
AMD GPU hardware is still relatively unknown in our CFD community. This hackathon was the opportunity to deep dive into the AMD dev environment to prepare the arrival of AdAstra at CINES.
Both YALES and AVBP have been ported to the AOMP framework using ROCm 4.5 on the GRID5000 Neowise system.
CPU execution posed no issues and we were able to focus on GPU Offloading using OpenMP.
On the YALES2 side, a mini-app encompassing the typical YALES2 structure hierarchy and loop execution was extracted from the code to evaluate different porting strategies and on the AVBP side the current OpenACC GPU offloading was translated to OpenMP focusing on the viscosity computation kernel.
We learnt that the current supported standard of OpenMP in ROCm 4.5 does not allow for direct offloading of reference values inside an derived type structure but is was possible to use aliases such as pointers or flat array copies to do the job. This should be solved with the support of OpenMP 5.0
Another troublesome issues, was the lack of support for offloading of array vector operations (ex : array(:) = 1.0 ) rendering the explicitation of the loops for these manadatory.

Some bugs remain and it is encouraged to use the latest compiler version when working on the porting ( the release of flang 14.0.1 saved us a lot of time as we had started with 14.0.0 ).
Offloading of the miniapp of YALES2 yielded a times 60 acceleration of the kernel whereas the offloading of the viscosity model in a full avbp simulation yielded an 7 times factor in performance when comparing on core to one GPU. These results are to be taken with a grain of salt but are really encouraging.

For the next steps, a porting strategy for both codes will be implemented (depending on the OpenMP 5 support ) and discussions are underway with CINES and other partners so as to offer the best experience to both code's communities on AdAstra at its release.

Ecfd:ecfd 5th edition

2022-02-01T07:30:34Z

Benard: /* Turbulent flows - P. Bénard, CORIA */

{{DISPLAYTITLE: ECFD workshop, 5th edition, 2022}}

== Description ==
{| align="right" style="text-align:center;" cellpadding="2"
| [[File:Logo_ECFD5.png | center | thumb | 350px | ECFD5 workshop logo.]]
|}
* Event from '''23th to 28th of January 2022'''
* Location: [https://www.bonsejour-laplage.com/vacances-tout-compris Centre Bonséjour], Merville-Franceville, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points.
** mini-workshops. Potential workshops are listed below.
* Free of charge
* More than 50 participants from academics (CERFACS, CORIA, IMAG, LEGI, EM2C, UMONS, UVM, VUB, UCL, TUDelft), HPC center/experts (GENCI, AMD, CINES, CRIANN) and industry (Safran, Ariane Group, Siemens-Gamesa).

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes

== News ==

* 03/11/2021: First announcement of the '''5th Extreme CFD Workshop & Hackathon''' !

[[File:Banniere_ECFD5_sponso.png|text-bottom|600px]]

* 13/01/2022: After discussions with the participants, the '''5th Extreme CFD Workshop & Hackathon''' is maintained as an in-person event! It will be also possible to attend to the plenary sessions and participate remotely to the workshop.

* 14/01/2022: The [[#Agenda|ECFD5 program]] is online! The plenary sessions will be announced soon!

* 20/01/2022: The plenary sessions are now defined:
** P1 - 24/01/2022: GPU porting challenges and quantum computing, présentation machine Adastra by G. Staffelbach (CERFACS) + Presentation of the new cluster from CINES called Adastra by C. Andrieu (CINES)
** P2 - 25/01/2022: News, perspectives and future of GPU computing applied to CFD by A. Toure (AMD)
** P3 - 26/01/2022: Theory, applications and perspectives of the Lattice-Boltzmann Method by P. Boivin (M2P2)
** P4 - 27/01/2022: Concepts and notions of mesh adaptation by C. Dapogny (LJK)

* 23/04/2022: '''The ECFD5 event has now started !!''' [https://www.linkedin.com/feed/update/urn:li:activity:6891053385072594944| LinkedIn post]

== Agenda ==

[[File:ECFD5_program.png|text-bottom|600px]]

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

=== Combustion - K. Bioche, VUB ===

* '''Sub-project 1: H2/air jet-in-cross-flow numerical simulations (R. Le Dortz, E. Riber, Q. Douasbin)'''

The use of hydrogen as an aviation fuel requires new combustion chamber design. Among strategies to prevent flame flashback and low flame residence time, the micromix injection system is further studied by ENABLEH2. This systems corresponds to a multitude of H2/air jet-in-cross-flow configurations. A 3D numerical simulation with realistic thermodynamics and kinetics is now tractable thanks to massively parralel computing. This week saw the completion of the first steps towards the establishment of a complete simulation. (I) The non-reactive air injection in the combustion chamber. (II) The cross-injection of H2 without ignition. (III) The ignition of this mixture modeled with the skeletal kinetic mechanism of Boivin (H2, H, O2, OH, O, H2O, HO2, H2O2, N2). Further work will be realised concerning mesh refinement, modelling of NOx and porting of the computation on GPU.

* '''Sub-project 2: LES calculation of the MICADO test rig with multicomponent jetA1 (S. Puggelli, T. Lesaffre, E. Riber, B. Cuenot)'''

The EU-funded project ALTERNATE has the goal of exploring the possibility for a wider utilisation of aviation sustainable fuels. A part of the project deals with the assessment of the effect of SAFs on soot production: using the experimental information obtained at ONERA in high-pressure conditions on the MICADO test rig, the effect of Alcohol to Jet (ATJ-SPK) fuel on soot levels are assessed and compared with standard jet A1 emissions. During the project, STech and CERFACS are working jointly on the numerical modelling of soot emissions for jet-A1 and ATJ-SPK combustion in AVBP. Starting from the numerical setup under-development for jet-A1, the worshop permitted to: (I) Switch from a 2-step kinetic mechanism to a complex 29 species, 233 reacs and 15 QSS mechanism. This transition was efficiently conducted with the tool Multi Table Generator. (II) At this stage, an assessment of the effects of the flame sensor on the calculation results was carried out, indicating the consistent behaviour of a recently developped sensor w.r.t classical tools. (III) Switch towards a multicomponent formulation of jet-A1 and assessment of the effect of such advanced approach with respect to the single-component formulation previously employed. Further work will be realised to manage the stiffness of employed kinetics and to compare jet-A1 and ATJ-SPK fuels from a chemical point of view.

* '''Sub-project 3: Euler-Lagrange Multigrid Simulation (T. Lesaffre, O. Vermorel, E. Riber, B. Cuenot)'''

In Lagrange simulations, the point-source approach is based on a ponctual approximation of the particule and requires this last to be smaller than the mesh. The very fine meshes required to represent the Eulerian phase of Euler-Lagrange two-phase flow simulations can lead to a non-validity of the point-source hypothesis. This project aimed at implementing, in the AVBP solver, the simultaneous management and coupling of several simulations. During this week, the Eulerian and Lagrangian phase were successfuly computed on two different meshes and coupled via the CWIPI library. The good behaviour of this framework was assessed on a 1D Evaporation of kerosene droplets in an air stream test case. Encouraging preliminary performance results were obtained on a 3D injection case and require further work.

* '''Sub-project 4: Devolatilization modelling for biomass combustion (K. Bioche, L. Bricteux)'''

Biomass combustion simulations require the modelling of numerous physical phenomena: particle drying, devolatilization, gas-phase combustion, chars oxidation. Besides, the valorisation chains for biomass include fluidized bed reactors, fixed bed reactors and pulverized fuel burners. The Granular Flow Solver of YALES2 offers a good framework for the simulation of fluidized bed reactors and is functionnaly coupled with the reactive gas-phase solver of the same code. This week permitted to partically implement the modelling of devolatilization in this solver. A single-step kinetic scheme is considered for the particle mass evolution equation while the particle diameter evolves during the process. Further work is necessary to account for the thermal and mass couplings with the fluid phase.

* '''Sub-project 5: Thickened-Flame LES model in a Lattice-Boltzmann Method framework (P. Boivin, S. Zhao, M. Le Boursicaud)'''

The TFLES framework of the hybrid Lattice-Boltzmann sover ProLB was extended to account for recent sensor methods. During this week, a smooth flame sensor based on the curvature of the norm of the advancement variable gradient was developped. Also for filtering operations, the lattice requires to access data over three neighboring layers. A precise and continuous thickening factor was obtained with such method.

* '''Sub-project 6: NOx modeling applied to KIAI combustion chamber (J. Obando, P. Bénard, V. Moureau)'''

This project treated of the implementation of NOx modeling into simulations of the KIAI combustion chamber, experimentaly studied at CORIA lab. During this week, various NOx modeling strategies were listed. Associated kinetic mechanisms, among which analytical chemisty, were employed for 1D flame simulations in YALES2 solver. Further work include the use of such methods on the 3D computational case.

=== Static and dynamic mesh adaptation - G. Balarac, LEGI ===

=== Multi-phase flows - M. Cailler, SAFRAN TECH ===

* '''Sub-project 1: Hybrid E-E/E-L two-phase flow method (M. Cailler, F. Pecquery, I. El Yamani, V. Moureau)'''

The High-Fidelity approach based on ACLS & DMA allows a reliable description of interface dynamics. For design exploration, low-CPU methods with controlled level of fidelity are required. An interesting approach to reduce CPU cost relies on an hybrid approach based on an Eulerian representation of the gas & and a Lagrangian description for the liquid phase. Objective of the ECFD5 was to explore the capability to reconstruct the interface normal of a liquid droplet made of particles on an Eulerian grid. First, a level-set based strategy relying on Geometric Multiple Markers Projection (Janodet et al., 2022) has been first tested showing good capabilities providing that the iso-surface distance equal 0 is well captured on the mesh. An alternative strategy based on the liquid volume fraction has been tested. This information was then used to build a velocity correction that is used to transport particles and ensures regularized particle positions. This preliminary banchmark work will be pursued on a liquid jet propagation problem.

* '''Sub-project 2: Jet atomization with a diffuse interface mathod (N. Odier, B. Péden, J. Carmona, P. Boivin)'''

A so-called diffuse interface method, using the multi-fluid formalism, coupled with a Riemann solver HLLC and a thermodynamic closure of the NASG type, was implemented in AVBP during the thesis of Carmona. Objective of the project was to couple the NASG thermodynamic implementation in an improved HLLC solver based on nodal information only.

* '''Sub-project 3: Convergence of the interface curvature computation (G. Ghigliotti, J. Carmona, G. Balarac, G. Lartigue)'''

The computation of interface curvature in a level-set framework is based on the classic formula as divergence of the gradient of the levelset function. This function being computed at 2nd order, one obtains a O(0) curvature, meaning that the error does not decrease with mesh refinement.
We have implemented in YALES2 a strategy proposed by Emmanuel Maître and collaborators in a finite element method based on the regularization (filtering) of the level-set gradient and curvature.
This strategy has been tested for the simple test case of a static circular interface.
We used two types of filters (simple gather-scatter or bilaplacian as developed by Lola Guedot (PhD thesis 2015)) on different mesh types (split quadrilaterals, isotropic triangular mesh, unstructured triangular mesh).
The results are encouraging since a O(1) convergence is obtained in all cases.
Further work is needed to tune the filter properties (amplitude and size) for different spatial resolutions and levelset "narrow band" width.

* '''Sub-project 4: Conservative two-fluid momentum transport (F. Pecquery, C. Merlin, M. Cailler, J. Carmona, V. Moureau)'''

The aim of this project was to investigate the applicability of the conservative two-fluid transport framework developed for passive scalars to the momentum conservation equation. First a complete two-fluid framework for the incompressible two-phase Navier-Stokes equations was proposed. Then the discontinuous algorithms for discontinuous scalars (data extension, reinitialization and transport) have been extended to discontinuous vectors. Moreover, some improvement of the data-structure were implemented to further generalize the framework and improve user-experience. To eventually conclude on the applicability of the proposed two-fluid momentum approach, the proposed interfacial momentum flux model and correction step will be implemented in YALES2

=== Numerics - G. Lartigue, CORIA ===

* '''Sub-project 4: Avoiding the acoustic timestep restriction in two-phase flow simulations (V. Boniou, J. Paris, A. Vié, T. Schmitt, C. Tenaud, Y. Béchane)‘’'
In the context of simulating compressible two-phase flows with phase change, the CFL constraint related to the acoustic part of an explicit solver, especially in the liquid phase, can be prohibitive for evaporation studies implying large timescale separation between evaporation and acoustic wave propagation.
The acoustic CFL constraint can be either relaxed by considering an implicit treatment of the acoustic part or avoided by deriving the low Mach limit of the multifluid model.

During this week, the first steps of this vast work have been initiated on single-phase flows by implementing the variable-density low Mach solver and the IMEX method applied on the Euler equations in TITAN.

The IMEX method was shown to provide degraded solutions in acoustic-driven cases, compared to conventional Riemann solutions. However, in transport-dominated cases, the increase of the transport CFL leads to a better numerical precision. A 2D cylindrical shock has been simulated with an acoustic CFL of 40 to assess the ability to run multidimensional compressible computations with large time step. The method was also extented to 4eq and 2nd order spatial accuracy.

Finally, the extension of the variable density solver to two-phase flows was performed using a VOF interface capturing method, considering a dilatable gas phase along with an incompressible liquid phase. Particular attention has been drawn to preserving liquid volume by treating the VOF transport accordingly with an adapted velocity field.

=== Turbulent flows - P. Bénard, CORIA ===
* '''Sub-project 1: Optimization of the actuator set for several wind turbines in YALES2 (F. Houtin Mongrolle, S. Gremmo, E. Muller, B. Duboc)'''
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified:
(i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges.
(ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications.
(iii) Adapt the YALES2-BHawC aero-servo-elastic coupling to make it consistent with the new ALM implementation.
During the workshop, the (i)-algorithm has been implemented with additional optimizations. Tests on a 30-turbine wind farm showed a 60% improvement compared to previous performances, which is very promising. The (ii)-feature still needs to be developed but should lead to the performances objective. The tests with the coupled code YALES2-BHawC were not conclusive yet and still require some work.

* '''Sub-project 2: Thermal effect in an atmospheric solver (U. Vigny, L. Voivenel, S. Zeoli, P. Benard)'''
Given the current environmental and energy challenges, maximising the wind farm electricity production is essential. Therefore, it becomes necessary to develop the most reliable and accurate prediction and simulation tools. Following this tenet, an atmospheric solver, which will take into account meteorological phenomena, should be developed. The preliminary work, going from bibliography study to road map was performed during the extreme cod workshop. Thus five parts have been identified:
(I) The YALES2 Variable Density Solver (VDS) will be used because of the need to take into account buoyancy effect including for big density differences.
(II) A wall law correction term, relative to atmospheric boundary layer will be added.
(III) The actuator line method used to simulate wind turbine will be extended to VDS, modifying the velocity source term to a momentum source term.
(IV) The Coriolis effects, depending on the latitude will be implemented.
(V) The wall heat flux, allowing to simulate diurnal and nocturnal cycles on various terrains, is more realistic than a target wall temperature.
From this work, future development are now clear and just waiting to be developed.

* '''Sub-project 3: Dynamic stall correction models for Horizontal Axis Wind Turbine (U. Vigny, L. Voivenel, S. Zeoli, P. Benard)'''
The Actuator Line Method implementation into YALES2 library suffers from the lack of correction models for some 3D unsteady effects. An important one, the dynamic stall, implies an unsteady modification of the aerodynamic loads, i.e. the polars, with the angle of attack. With the standard ALM model, static 2D polars are used. Adding a dynamic stall correction model allow to have an hysteresis loop on the of the forces coefficient with the angle of attack. Several models exists in the literature: Oye, BeddoesLeishman, Sheng, Risoe, ONERA... During the workshop, the Oye model was selected, as it is the simplest model, and was implemented. First, new ingredients necessary to the model were added in the polars look-up table generator used to describe the blades. Then, the Oye correction was integrated into the YALES2 library. Finally, the model was tested on simple configurations. It is now essential to validate more the implementation.

* '''Sub-project 5: TBLE wall model for LES with pressure gradient on a simple turbomachinery geometry (M. Cizeron, N. Odier, R. Vicquelin)'''
Wall modeling is often used in LES to alleviate the computational cost that would be required to resolve all the length scales up to the solid boundaries of the domain. The classical way of doing it is by using an algebraic model to provide the wall friction and heat flux, with a coupling to the LES solver at the first off-wall nodes. The wall model was designed from analyzing RANS equation with strong assumptions such as planar flow, equilibrium and no pressure gradient. These assumptions are often far from true in real applications, such as turbomachinery applications, where the use of a wall model is mandatory due to the size of the calculation. During this workshop, a wall model relying on the resolution of the Thin Boundary Layer Equations (TBLE) was studied, which had been implemented by EM2C. The addition of a pressure gradient to these equations has been conducted and tested, at first only for the 1D wall model solver, then on a 3D turbulent channel. It remains to be tested on a diffuser configuration with a real pressure gradient to quantify the effect of the new wall model. The influence of the point considered to do the coupling between the LES and the wall model (ie. its distance to the wall) has also been tested both for the TBLE and the original algebraic model, showing that coupling farther from the wall yields better results and reduces the so-called log-layer mismatch.

* '''Sub-project 6: Tools for rough wall modelling (A. Barge, S. Meynet)'''
Within the STREAM project framework, a roughness-resolved Large-Eddy Simulation (RRLES) database is being built. The aim of this latter is to be representative of rough channel flows, especially for additive-manufacturing heat exchangers. First RRLES have already been performed. From turbulence and rough wall stress statistics analysis of the results, a first stochastic model, which reproduces the statistical behavior of the wall stress vector, have been proposed. The modeled wall stress allows a better prediction of the pressure drop in a flat wall channel compared to the use of the mean value of the wall stress measured in RRLES alone. However, the near wall region is still mispredicted and the model is correlated in time but not in space. The aim of this ECFD5 was to develop tools to improve modelling and explore new ways. A roughness mapping tool for smooth surfaces have been implemented into YALES2 to get local surface height. This tool is based on an existing in-house surface roughness generator developed for the STREAM project. The idea is to use the map to generated space correlated fluctuations for the wall shear stress. Some bugs still remain to fully use this tool. In parallel, the modelling approach was extended to passive scalar, especially for temperature. To this end, new random tools as white noise, unit sphere random walk and Gaussian / Log-normal stochastic processes have been coded. Finally, the idea of using walls as velocity source terms emerged during this ECFD5. The principle is to mask a grid layer above the wall and to transport the rough map on this grid to estimate the roughness effects above the wall. Parametrizing and testing these tools remained to be done at the end of ECFD5.

=== User experience - J. Leparoux, SAFRAN TECH ===

* '''Sub-project 1: Multi combustion model chemtable generator (S. Dillon, R. Mercier)'''

Due to the high computational cost of LES of turbulent combustion with detailed chemistry, tabulated chemistry approaches remain a popular choice within the combustion community. This popularity of tabulated chemistry leads to research into the development of novel combustion models for which a platform to test these models is required. The objective of the workshop was to therefore begin developing an easy-to-use chemistry table generator within Python. Given that there exists numerous turbulent combustion models (F-TACLES Diffusion, F-TACLES Premixed, 2PFT, etc.), a generalised multi-model code was necessary. Along with multi-model capabilities, the code was required to function with multiple solvers for the creation of the flame database (REGATH and CANTERA). With these objectives in mind, the code “TabulatEd Chemistry GenERator for Aeronautical CombusTion” (TECERACT) was created. Advancements were made in all key areas using code already developed by Safran Tech. TECERACT includes a parallel flame database generator and current developments are focused on multi-model functionality and simplified mesh management within Python. Once finished, the TECERACT code will provide a user-friendly chemistry table generator capable of producing chemistry tables for LES simulations and its multi-model structure will serve as a platform for future turbulent combustion model development/testing.

* '''Sub-project 2: Task-driven automatic run sequence (R. Mercier, J. Leparoux, M. Cailler, R. Letournel)'''

The principal objective of this project was to develop a new simulation workflow based on Task-driven approach. This approach could offer a higher flexibility to manage CFD simulations by decomposing the whole simulation on small sequences (run initialization, fuel injection, etc.)

This week showed the achievement of the first steps towards a complete task-driven simulation. (i) The whole simulation workflow was set-up (ii) Missing actions were implemented allowing to update simulation status on-the-fly (from non reactive to reactive simulation) (iii) State are now embedded from the restart solution allowing to restart from the last known state. Further work is still needed to obtain the full implementation of the AMC framework based on task-driven approach. Especially to automate the adaptation loop

* '''Sub-project 3: YALES2 Tools & Gitlab CI (J. Leparoux, A. Tstetoglou)'''

* '''Sub-project 4: Wind energy tools (E. Muller, S. Gremmo, F. Houtin-Mongrolle, B. Duboc)'''

Set-up phase of simulations involving several wind turbines is very demanding and error-prone, especially for simulations carried out with the coupled solver YALES2-BHawC*. This type of simulation can involve several dozen input files and then, an assistant tool is required.

The objectives of this projet were (i) develop a python tools to set all necessary ACTUATOR_SET objects (ii) provide basic visualization plots allowing to monitor simulations on-the-fly (by post-processing YALES2 temporals and BHawC results files).

''*BHawC : aero-servo-elastic solver used and developped at Siemens Gamesa Renewable Energy for wind turbine design and certification
''

=== Hackathon - G. Staffelbach, CERFACS ===
* '''Participants: I. d'Ast, J. Legaux, G. Staffelbach, P. Begou, G. Lartigue, V. Moureau, A. Toure, C. Laurie, S. Delamare, C. Andrieu, C. Jourdain'''
AMD GPU hardware is still relatively unknown in our CFD community. This hackathon was the opportunity to deep dive into the AMD dev environment to prepare the arrival of AdAstra at CINES.
Both YALES and AVBP have been ported to the AOMP framework using ROCm 4.5 on the GRID5000 Neowise system.
CPU execution posed no issues and we were able to focus on GPU Offloading using OpenMP.
On the YALES2 side, a mini-app encompassing the typical YALES2 structure hierarchy and loop execution was extracted from the code to evaluate different porting strategies and on the AVBP side the current OpenACC GPU offloading was translated to OpenMP focusing on the viscosity computation kernel.
We learnt that the current supported standard of OpenMP in ROCm 4.5 does not allow for direct offloading of reference values inside an derived type structure but is was possible to use aliases such as pointers or flat array copies to do the job. This should be solved with the support of OpenMP 5.0
Another troublesome issues, was the lack of support for offloading of array vector operations (ex : array(:) = 1.0 ) rendering the explicitation of the loops for these manadatory.

Some bugs remain and it is encouraged to use the latest compiler version when working on the porting ( the release of flang 14.0.1 saved us a lot of time as we had started with 14.0.0 ).
Offloading of the miniapp of YALES2 yielded a times 60 acceleration of the kernel whereas the offloading of the viscosity model in a full avbp simulation yielded an 7 times factor in performance when comparing on core to one GPU. These results are to be taken with a grain of salt but are really encouraging.

For the next steps, a porting strategy for both codes will be implemented (depending on the OpenMP 5 support ) and discussions are underway with CINES and other partners so as to offer the best experience to both code's communities on AdAstra at its release.

Ecfd:ecfd 5th edition

2022-02-01T07:20:22Z

Benard: /* Turbulent flows - P. Bénard, CORIA */

{{DISPLAYTITLE: ECFD workshop, 5th edition, 2022}}

== Description ==
{| align="right" style="text-align:center;" cellpadding="2"
| [[File:Logo_ECFD5.png | center | thumb | 350px | ECFD5 workshop logo.]]
|}
* Event from '''23th to 28th of January 2022'''
* Location: [https://www.bonsejour-laplage.com/vacances-tout-compris Centre Bonséjour], Merville-Franceville, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points.
** mini-workshops. Potential workshops are listed below.
* Free of charge
* More than 50 participants from academics (CERFACS, CORIA, IMAG, LEGI, EM2C, UMONS, UVM, VUB, UCL, TUDelft), HPC center/experts (GENCI, AMD, CINES, CRIANN) and industry (Safran, Ariane Group, Siemens-Gamesa).

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes

== News ==

* 03/11/2021: First announcement of the '''5th Extreme CFD Workshop & Hackathon''' !

[[File:Banniere_ECFD5_sponso.png|text-bottom|600px]]

* 13/01/2022: After discussions with the participants, the '''5th Extreme CFD Workshop & Hackathon''' is maintained as an in-person event! It will be also possible to attend to the plenary sessions and participate remotely to the workshop.

* 14/01/2022: The [[#Agenda|ECFD5 program]] is online! The plenary sessions will be announced soon!

* 20/01/2022: The plenary sessions are now defined:
** P1 - 24/01/2022: GPU porting challenges and quantum computing, présentation machine Adastra by G. Staffelbach (CERFACS) + Presentation of the new cluster from CINES called Adastra by C. Andrieu (CINES)
** P2 - 25/01/2022: News, perspectives and future of GPU computing applied to CFD by A. Toure (AMD)
** P3 - 26/01/2022: Theory, applications and perspectives of the Lattice-Boltzmann Method by P. Boivin (M2P2)
** P4 - 27/01/2022: Concepts and notions of mesh adaptation by C. Dapogny (LJK)

* 23/04/2022: '''The ECFD5 event has now started !!''' [https://www.linkedin.com/feed/update/urn:li:activity:6891053385072594944| LinkedIn post]

== Agenda ==

[[File:ECFD5_program.png|text-bottom|600px]]

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

=== Combustion - K. Bioche, VUB ===

* '''Sub-project 1: H2/air jet-in-cross-flow numerical simulations (R. Le Dortz, E. Riber, Q. Douasbin)'''

The use of hydrogen as an aviation fuel requires new combustion chamber design. Among strategies to prevent flame flashback and low flame residence time, the micromix injection system is further studied by ENABLEH2. This systems corresponds to a multitude of H2/air jet-in-cross-flow configurations. A 3D numerical simulation with realistic thermodynamics and kinetics is now tractable thanks to massively parralel computing. This week saw the completion of the first steps towards the establishment of a complete simulation. (I) The non-reactive air injection in the combustion chamber. (II) The cross-injection of H2 without ignition. (III) The ignition of this mixture modeled with the skeletal kinetic mechanism of Boivin (H2, H, O2, OH, O, H2O, HO2, H2O2, N2). Further work will be realised concerning mesh refinement, modelling of NOx and porting of the computation on GPU.

* '''Sub-project 2: LES calculation of the MICADO test rig with multicomponent jetA1 (S. Puggelli, T. Lesaffre, E. Riber, B. Cuenot)'''

The EU-funded project ALTERNATE has the goal of exploring the possibility for a wider utilisation of aviation sustainable fuels. A part of the project deals with the assessment of the effect of SAFs on soot production: using the experimental information obtained at ONERA in high-pressure conditions on the MICADO test rig, the effect of Alcohol to Jet (ATJ-SPK) fuel on soot levels are assessed and compared with standard jet A1 emissions. During the project, STech and CERFACS are working jointly on the numerical modelling of soot emissions for jet-A1 and ATJ-SPK combustion in AVBP. Starting from the numerical setup under-development for jet-A1, the worshop permitted to: (I) Switch from a 2-step kinetic mechanism to a complex 29 species, 233 reacs and 15 QSS mechanism. This transition was efficiently conducted with the tool Multi Table Generator. (II) At this stage, an assessment of the effects of the flame sensor on the calculation results was carried out, indicating the consistent behaviour of a recently developped sensor w.r.t classical tools. (III) Switch towards a multicomponent formulation of jet-A1 and assessment of the effect of such advanced approach with respect to the single-component formulation previously employed. Further work will be realised to manage the stiffness of employed kinetics and to compare jet-A1 and ATJ-SPK fuels from a chemical point of view.

* '''Sub-project 3: Euler-Lagrange Multigrid Simulation (T. Lesaffre, O. Vermorel, E. Riber, B. Cuenot)'''

In Lagrange simulations, the point-source approach is based on a ponctual approximation of the particule and requires this last to be smaller than the mesh. The very fine meshes required to represent the Eulerian phase of Euler-Lagrange two-phase flow simulations can lead to a non-validity of the point-source hypothesis. This project aimed at implementing, in the AVBP solver, the simultaneous management and coupling of several simulations. During this week, the Eulerian and Lagrangian phase were successfuly computed on two different meshes and coupled via the CWIPI library. The good behaviour of this framework was assessed on a 1D Evaporation of kerosene droplets in an air stream test case. Encouraging preliminary performance results were obtained on a 3D injection case and require further work.

* '''Sub-project 4: Devolatilization modelling for biomass combustion (K. Bioche, L. Bricteux)'''

Biomass combustion simulations require the modelling of numerous physical phenomena: particle drying, devolatilization, gas-phase combustion, chars oxidation. Besides, the valorisation chains for biomass include fluidized bed reactors, fixed bed reactors and pulverized fuel burners. The Granular Flow Solver of YALES2 offers a good framework for the simulation of fluidized bed reactors and is functionnaly coupled with the reactive gas-phase solver of the same code. This week permitted to partically implement the modelling of devolatilization in this solver. A single-step kinetic scheme is considered for the particle mass evolution equation while the particle diameter evolves during the process. Further work is necessary to account for the thermal and mass couplings with the fluid phase.

* '''Sub-project 5: Thickened-Flame LES model in a Lattice-Boltzmann Method framework (P. Boivin, S. Zhao, M. Le Boursicaud)'''

The TFLES framework of the hybrid Lattice-Boltzmann sover ProLB was extended to account for recent sensor methods. During this week, a smooth flame sensor based on the curvature of the norm of the advancement variable gradient was developped. Also for filtering operations, the lattice requires to access data over three neighboring layers. A precise and continuous thickening factor was obtained with such method.

* '''Sub-project 6: NOx modeling applied to KIAI combustion chamber (J. Obando, P. Bénard, V. Moureau)'''

This project treated of the implementation of NOx modeling into simulations of the KIAI combustion chamber, experimentaly studied at CORIA lab. During this week, various NOx modeling strategies were listed. Associated kinetic mechanisms, among which analytical chemisty, were employed for 1D flame simulations in YALES2 solver. Further work include the use of such methods on the 3D computational case.

=== Static and dynamic mesh adaptation - G. Balarac, LEGI ===

=== Multi-phase flows - M. Cailler, SAFRAN TECH ===

* '''Sub-project 1: Hybrid E-E/E-L two-phase flow method (M. Cailler, F. Pecquery, I. El Yamani, V. Moureau)'''

The High-Fidelity approach based on ACLS & DMA allows a reliable description of interface dynamics. For design exploration, low-CPU methods with controlled level of fidelity are required. An interesting approach to reduce CPU cost relies on an hybrid approach based on an Eulerian representation of the gas & and a Lagrangian description for the liquid phase. Objective of the ECFD5 was to explore the capability to reconstruct the interface normal of a liquid droplet made of particles on an Eulerian grid. First, a level-set based strategy relying on Geometric Multiple Markers Projection (Janodet et al., 2022) has been first tested showing good capabilities providing that the iso-surface distance equal 0 is well captured on the mesh. An alternative strategy based on the liquid volume fraction has been tested. This information was then used to build a velocity correction that is used to transport particles and ensures regularized particle positions. This preliminary banchmark work will be pursued on a liquid jet propagation problem.

* '''Sub-project 2: Jet atomization with a diffuse interface mathod (N. Odier, B. Péden, J. Carmona, P. Boivin)'''

A so-called diffuse interface method, using the multi-fluid formalism, coupled with a Riemann solver HLLC and a thermodynamic closure of the NASG type, was implemented in AVBP during the thesis of Carmona. Objective of the project was to couple the NASG thermodynamic implementation in an improved HLLC solver based on nodal information only.

* '''Sub-project 3: Convergence of the interface curvature computation (G. Ghigliotti, J. Carmona, G. Balarac, G. Lartigue)'''

The computation of interface curvature in a level-set framework is based on the classic formula as divergence of the gradient of the levelset function. This function being computed at 2nd order, one obtains a O(0) curvature, meaning that the error does not decrease with mesh refinement.
We have implemented in YALES2 a strategy proposed by Emmanuel Maître and collaborators in a finite element method based on the regularization (filtering) of the level-set gradient and curvature.
This strategy has been tested for the simple test case of a static circular interface.
We used two types of filters (simple gather-scatter or bilaplacian as developed by Lola Guedot (PhD thesis 2015)) on different mesh types (split quadrilaterals, isotropic triangular mesh, unstructured triangular mesh).
The results are encouraging since a O(1) convergence is obtained in all cases.
Further work is needed to tune the filter properties (amplitude and size) for different spatial resolutions and levelset "narrow band" width.

* '''Sub-project 4: Conservative two-fluid momentum transport (F. Pecquery, C. Merlin, M. Cailler, J. Carmona, V. Moureau)'''

The aim of this project was to investigate the applicability of the conservative two-fluid transport framework developed for passive scalars to the momentum conservation equation. First a complete two-fluid framework for the incompressible two-phase Navier-Stokes equations was proposed. Then the discontinuous algorithms for discontinuous scalars (data extension, reinitialization and transport) have been extended to discontinuous vectors. Moreover, some improvement of the data-structure were implemented to further generalize the framework and improve user-experience. To eventually conclude on the applicability of the proposed two-fluid momentum approach, the proposed interfacial momentum flux model and correction step will be implemented in YALES2

=== Numerics - G. Lartigue, CORIA ===

* '''Sub-project 4: Avoiding the acoustic timestep restriction in two-phase flow simulations (V. Boniou, J. Paris, A. Vié, T. Schmitt, C. Tenaud, Y. Béchane)‘’'
In the context of simulating compressible two-phase flows with phase change, the CFL constraint related to the acoustic part of an explicit solver, especially in the liquid phase, can be prohibitive for evaporation studies implying large timescale separation between evaporation and acoustic wave propagation.
The acoustic CFL constraint can be either relaxed by considering an implicit treatment of the acoustic part or avoided by deriving the low Mach limit of the multifluid model.

During this week, the first steps of this vast work have been initiated on single-phase flows by implementing the variable-density low Mach solver and the IMEX method applied on the Euler equations in TITAN.

The IMEX method was shown to provide degraded solutions in acoustic-driven cases, compared to conventional Riemann solutions. However, in transport-dominated cases, the increase of the transport CFL leads to a better numerical precision. A 2D cylindrical shock has been simulated with an acoustic CFL of 40 to assess the ability to run multidimensional compressible computations with large time step. The method was also extented to 4eq and 2nd order spatial accuracy.

Finally, the extension of the variable density solver to two-phase flows was performed using a VOF interface capturing method, considering a dilatable gas phase along with an incompressible liquid phase. Particular attention has been drawn to preserving liquid volume by treating the VOF transport accordingly with an adapted velocity field.

=== Turbulent flows - P. Bénard, CORIA ===
* '''Sub-project 1: Optimization of the actuator set for several wind turbines in YALES2 (F. Houtin Mongrolle, S. Gremmo, E. Muller & B. Duboc)'''
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified:
(i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges.
(ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications.
(iii) Adapt the YALES2-BHawC aero-servo-elastic coupling to make it consistent with the new ALM implementation.
During the workshop, the (i)-algorithm has been implemented with additional optimizations. Tests on a 30-turbine wind farm showed a 60% improvement compared to previous performances, which is very promising. The (ii)-feature still needs to be developed but should lead to the performances objective. The tests with the coupled code YALES2-BHawC were not conclusive yet and still require some work.

* '''Sub-project 2: Thermal effect in an atmospheric solver (U. Vigny, L. Voivenel, S. Zeoli, P. Benard)'''
Given the current environmental and energy challenges, maximising the wind farm electricity production is essential. Therefore, it becomes necessary to develop the most reliable and accurate prediction and simulation tools. Following this tenet, an atmospheric solver, which will take into account meteorological phenomena, should be developed. The preliminary work, going from bibliography study to road map was performed during the extreme cod workshop. Thus five parts have been identified:
(I) The YALES2 Variable Density Solver (VDS) will be used because of the need to take into account buoyancy effect including for big density differences.
(II) A wall law correction term, relative to atmospheric boundary layer will be added.
(III) The actuator line method used to simulate wind turbine will be extended to VDS, modifying the velocity source term to a momentum source term.
(IV) The Coriolis effects, depending on the latitude will be implemented.
(V) The wall heat flux, allowing to simulate diurnal and nocturnal cycles on various terrains, is more realistic than a target wall temperature.
From this work, future development are now clear and just waiting to be developed.

* '''Sub-project 5: TBLE wall model for LES with pressure gradient on a simple turbomachinery geometry (M. Cizeron, N. Odier, R. Vicquelin)'''
Wall modeling is often used in LES to alleviate the computational cost that would be required to resolve all the length scales up to the solid boundaries of the domain. The classical way of doing it is by using an algebraic model to provide the wall friction and heat flux, with a coupling to the LES solver at the first off-wall nodes. The wall model was designed from analyzing RANS equation with strong assumptions such as planar flow, equilibrium and no pressure gradient. These assumptions are often far from true in real applications, such as turbomachinery applications, where the use of a wall model is mandatory due to the size of the calculation. During this workshop, a wall model relying on the resolution of the Thin Boundary Layer Equations (TBLE) was studied, which had been implemented by EM2C. The addition of a pressure gradient to these equations has been conducted and tested, at first only for the 1D wall model solver, then on a 3D turbulent channel. It remains to be tested on a diffuser configuration with a real pressure gradient to quantify the effect of the new wall model. The influence of the point considered to do the coupling between the LES and the wall model (ie. its distance to the wall) has also been tested both for the TBLE and the original algebraic model, showing that coupling farther from the wall yields better results and reduces the so-called log-layer mismatch.

* '''Sub-project 6: Tools for rough wall modelling (A. Barge, S. Meynet)'''
Within the STREAM project framework, a roughness-resolved Large-Eddy Simulation (RRLES) database is being built. The aim of this latter is to be representative of rough channel flows, especially for additive-manufacturing heat exchangers. First RRLES have already been performed. From turbulence and rough wall stress statistics analysis of the results, a first stochastic model, which reproduces the statistical behavior of the wall stress vector, have been proposed. The modeled wall stress allows a better prediction of the pressure drop in a flat wall channel compared to the use of the mean value of the wall stress measured in RRLES alone. However, the near wall region is still mispredicted and the model is correlated in time but not in space. The aim of this ECFD5 was to develop tools to improve modelling and explore new ways. A roughness mapping tool for smooth surfaces have been implemented into YALES2 to get local surface height. This tool is based on an existing in-house surface roughness generator developed for the STREAM project. The idea is to use the map to generated space correlated fluctuations for the wall shear stress. Some bugs still remain to fully use this tool. In parallel, the modelling approach was extended to passive scalar, especially for temperature. To this end, new random tools as white noise, unit sphere random walk and Gaussian / Log-normal stochastic processes have been coded. Finally, the idea of using walls as velocity source terms emerged during this ECFD5. The principle is to mask a grid layer above the wall and to transport the rough map on this grid to estimate the roughness effects above the wall. Parametrizing and testing these tools remained to be done at the end of ECFD5.

=== User experience - J. Leparoux, SAFRAN TECH ===

* '''Sub-project 1: Multi combustion model chemtable generator (S. Dillon, R. Mercier)'''

Due to the high computational cost of LES of turbulent combustion with detailed chemistry, tabulated chemistry approaches remain a popular choice within the combustion community. This popularity of tabulated chemistry leads to research into the development of novel combustion models for which a platform to test these models is required. The objective of the workshop was to therefore begin developing an easy-to-use chemistry table generator within Python. Given that there exists numerous turbulent combustion models (F-TACLES Diffusion, F-TACLES Premixed, 2PFT, etc.), a generalised multi-model code was necessary. Along with multi-model capabilities, the code was required to function with multiple solvers for the creation of the flame database (REGATH and CANTERA). With these objectives in mind, the code “TabulatEd Chemistry GenERator for Aeronautical CombusTion” (TECERACT) was created. Advancements were made in all key areas using code already developed by Safran Tech. TECERACT includes a parallel flame database generator and current developments are focused on multi-model functionality and simplified mesh management within Python. Once finished, the TECERACT code will provide a user-friendly chemistry table generator capable of producing chemistry tables for LES simulations and its multi-model structure will serve as a platform for future turbulent combustion model development/testing.

* '''Sub-project 2: Task-driven automatic run sequence (R. Mercier, J. Leparoux, M. Cailler, R. Letournel)'''

The principal objective of this project was to develop a new simulation workflow based on Task-driven approach. This approach could offer a higher flexibility to manage CFD simulations by decomposing the whole simulation on small sequences (run initialization, fuel injection, etc.)

This week showed the achievement of the first steps towards a complete task-driven simulation. (i) The whole simulation workflow was set-up (ii) Missing actions were implemented allowing to update simulation status on-the-fly (from non reactive to reactive simulation) (iii) State are now embedded from the restart solution allowing to restart from the last known state. Further work is still needed to obtain the full implementation of the AMC framework based on task-driven approach. Especially to automate the adaptation loop

* '''Sub-project 3: YALES2 Tools & Gitlab CI (J. Leparoux, A. Tstetoglou)'''

* '''Sub-project 4: Wind energy tools (E. Muller, S. Gremmo, F. Houtin-Mongrolle, B. Duboc)'''

Set-up phase of simulations involving several wind turbines is very demanding and error-prone, especially for simulations carried out with the coupled solver YALES2-BHawC*. This type of simulation can involve several dozen input files and then, an assistant tool is required.

The objectives of this projet were (i) develop a python tools to set all necessary ACTUATOR_SET objects (ii) provide basic visualization plots allowing to monitor simulations on-the-fly (by post-processing YALES2 temporals and BHawC results files).

''*BHawC : aero-servo-elastic solver used and developped at Siemens Gamesa Renewable Energy for wind turbine design and certification
''

=== Hackathon - G. Staffelbach, CERFACS ===
* '''Participants: I. d'Ast, J. Legaux, G. Staffelbach, P. Begou, G. Lartigue, V. Moureau, A. Toure, C. Laurie, S. Delamare, C. Andrieu, C. Jourdain'''
AMD GPU hardware is still relatively unknown in our CFD community. This hackathon was the opportunity to deep dive into the AMD dev environment to prepare the arrival of AdAstra at CINES.
Both YALES and AVBP have been ported to the AOMP framework using ROCm 4.5 on the GRID5000 Neowise system.
CPU execution posed no issues and we were able to focus on GPU Offloading using OpenMP.
On the YALES2 side, a mini-app encompassing the typical YALES2 structure hierarchy and loop execution was extracted from the code to evaluate different porting strategies and on the AVBP side the current OpenACC GPU offloading was translated to OpenMP focusing on the viscosity computation kernel.
We learnt that the current supported standard of OpenMP in ROCm 4.5 does not allow for direct offloading of reference values inside an derived type structure but is was possible to use aliases such as pointers or flat array copies to do the job. This should be solved with the support of OpenMP 5.0
Another troublesome issues, was the lack of support for offloading of array vector operations (ex : array(:) = 1.0 ) rendering the explicitation of the loops for these manadatory.

Some bugs remain and it is encouraged to use the latest compiler version when working on the porting ( the release of flang 14.0.1 saved us a lot of time as we had started with 14.0.0 ).
Offloading of the miniapp of YALES2 yielded a times 60 acceleration of the kernel whereas the offloading of the viscosity model in a full avbp simulation yielded an 7 times factor in performance when comparing on core to one GPU. These results are to be taken with a grain of salt but are really encouraging.

For the next steps, a porting strategy for both codes will be implemented (depending on the OpenMP 5 support ) and discussions are underway with CINES and other partners so as to offer the best experience to both code's communities on AdAstra at its release.

Ecfd:ecfd 5th edition

2022-02-01T06:58:14Z

Benard: /* Thematics / Mini-workshops */

{{DISPLAYTITLE: ECFD workshop, 5th edition, 2022}}

== Description ==
{| align="right" style="text-align:center;" cellpadding="2"
| [[File:Logo_ECFD5.png | center | thumb | 350px | ECFD5 workshop logo.]]
|}
* Event from '''23th to 28th of January 2022'''
* Location: [https://www.bonsejour-laplage.com/vacances-tout-compris Centre Bonséjour], Merville-Franceville, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points.
** mini-workshops. Potential workshops are listed below.
* Free of charge
* More than 50 participants from academics (CERFACS, CORIA, IMAG, LEGI, EM2C, UMONS, UVM, VUB, UCL, TUDelft), HPC center/experts (GENCI, AMD, CINES, CRIANN) and industry (Safran, Ariane Group, Siemens-Gamesa).

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes

== News ==

* 03/11/2021: First announcement of the '''5th Extreme CFD Workshop & Hackathon''' !

[[File:Banniere_ECFD5_sponso.png|text-bottom|600px]]

* 13/01/2022: After discussions with the participants, the '''5th Extreme CFD Workshop & Hackathon''' is maintained as an in-person event! It will be also possible to attend to the plenary sessions and participate remotely to the workshop.

* 14/01/2022: The [[#Agenda|ECFD5 program]] is online! The plenary sessions will be announced soon!

* 20/01/2022: The plenary sessions are now defined:
** P1 - 24/01/2022: GPU porting challenges and quantum computing, présentation machine Adastra by G. Staffelbach (CERFACS) + Presentation of the new cluster from CINES called Adastra by C. Andrieu (CINES)
** P2 - 25/01/2022: News, perspectives and future of GPU computing applied to CFD by A. Toure (AMD)
** P3 - 26/01/2022: Theory, applications and perspectives of the Lattice-Boltzmann Method by P. Boivin (M2P2)
** P4 - 27/01/2022: Concepts and notions of mesh adaptation by C. Dapogny (LJK)

* 23/04/2022: '''The ECFD5 event has now started !!''' [https://www.linkedin.com/feed/update/urn:li:activity:6891053385072594944| LinkedIn post]

== Agenda ==

[[File:ECFD5_program.png|text-bottom|600px]]

== Thematics / Mini-workshops ==

These mini-workshops may change and cover more or less topics. This page will be adapted according to your feedback.

=== Combustion - K. Bioche, VUB ===

* '''Sub-project 1: H2/air jet-in-cross-flow numerical simulations (R. Le Dortz, E. Riber, Q. Douasbin)'''

The use of hydrogen as an aviation fuel requires new combustion chamber design. Among strategies to prevent flame flashback and low flame residence time, the micromix injection system is further studied by ENABLEH2. This systems corresponds to a multitude of H2/air jet-in-cross-flow configurations. A 3D numerical simulation with realistic thermodynamics and kinetics is now tractable thanks to massively parralel computing. This week saw the completion of the first steps towards the establishment of a complete simulation. (I) The non-reactive air injection in the combustion chamber. (II) The cross-injection of H2 without ignition. (III) The ignition of this mixture modeled with the skeletal kinetic mechanism of Boivin (H2, H, O2, OH, O, H2O, HO2, H2O2, N2). Further work will be realised concerning mesh refinement, modelling of NOx and porting of the computation on GPU.

* '''Sub-project 2: LES calculation of the MICADO test rig with multicomponent jetA1 (S. Puggelli, T. Lesaffre, E. Riber, B. Cuenot)'''

The EU-funded project ALTERNATE has the goal of exploring the possibility for a wider utilisation of aviation sustainable fuels. A part of the project deals with the assessment of the effect of SAFs on soot production: using the experimental information obtained at ONERA in high-pressure conditions on the MICADO test rig, the effect of Alcohol to Jet (ATJ-SPK) fuel on soot levels are assessed and compared with standard jet A1 emissions. During the project, STech and CERFACS are working jointly on the numerical modelling of soot emissions for jet-A1 and ATJ-SPK combustion in AVBP. Starting from the numerical setup under-development for jet-A1, the worshop permitted to: (I) Switch from a 2-step kinetic mechanism to a complex 29 species, 233 reacs and 15 QSS mechanism. This transition was efficiently conducted with the tool Multi Table Generator. (II) At this stage, an assessment of the effects of the flame sensor on the calculation results was carried out, indicating the consistent behaviour of a recently developped sensor w.r.t classical tools. (III) Switch towards a multicomponent formulation of jet-A1 and assessment of the effect of such advanced approach with respect to the single-component formulation previously employed. Further work will be realised to manage the stiffness of employed kinetics and to compare jet-A1 and ATJ-SPK fuels from a chemical point of view.

* '''Sub-project 3: Euler-Lagrange Multigrid Simulation (T. Lesaffre, O. Vermorel, E. Riber, B. Cuenot)'''

In Lagrange simulations, the point-source approach is based on a ponctual approximation of the particule and requires this last to be smaller than the mesh. The very fine meshes required to represent the Eulerian phase of Euler-Lagrange two-phase flow simulations can lead to a non-validity of the point-source hypothesis. This project aimed at implementing, in the AVBP solver, the simultaneous management and coupling of several simulations. During this week, the Eulerian and Lagrangian phase were successfuly computed on two different meshes and coupled via the CWIPI library. The good behaviour of this framework was assessed on a 1D Evaporation of kerosene droplets in an air stream test case. Encouraging preliminary performance results were obtained on a 3D injection case and require further work.

* '''Sub-project 4: Devolatilization modelling for biomass combustion (K. Bioche, L. Bricteux)'''

Biomass combustion simulations require the modelling of numerous physical phenomena: particle drying, devolatilization, gas-phase combustion, chars oxidation. Besides, the valorisation chains for biomass include fluidized bed reactors, fixed bed reactors and pulverized fuel burners. The Granular Flow Solver of YALES2 offers a good framework for the simulation of fluidized bed reactors and is functionnaly coupled with the reactive gas-phase solver of the same code. This week permitted to partically implement the modelling of devolatilization in this solver. A single-step kinetic scheme is considered for the particle mass evolution equation while the particle diameter evolves during the process. Further work is necessary to account for the thermal and mass couplings with the fluid phase.

* '''Sub-project 5: Thickened-Flame LES model in a Lattice-Boltzmann Method framework (P. Boivin, S. Zhao, M. Le Boursicaud)'''

The TFLES framework of the hybrid Lattice-Boltzmann sover ProLB was extended to account for recent sensor methods. During this week, a smooth flame sensor based on the curvature of the norm of the advancement variable gradient was developped. Also for filtering operations, the lattice requires to access data over three neighboring layers. A precise and continuous thickening factor was obtained with such method.

* '''Sub-project 6: NOx modeling applied to KIAI combustion chamber (J. Obando, P. Bénard, V. Moureau)'''

This project treated of the implementation of NOx modeling into simulations of the KIAI combustion chamber, experimentaly studied at CORIA lab. During this week, various NOx modeling strategies were listed. Associated kinetic mechanisms, among which analytical chemisty, were employed for 1D flame simulations in YALES2 solver. Further work include the use of such methods on the 3D computational case.

=== Static and dynamic mesh adaptation - G. Balarac, LEGI ===

=== Multi-phase flows - M. Cailler, SAFRAN TECH ===

* '''Sub-project 1: Hybrid E-E/E-L two-phase flow method (M. Cailler, F. Pecquery, I. El Yamani, V. Moureau)'''

The High-Fidelity approach based on ACLS & DMA allows a reliable description of interface dynamics. For design exploration, low-CPU methods with controlled level of fidelity are required. An interesting approach to reduce CPU cost relies on an hybrid approach based on an Eulerian representation of the gas & and a Lagrangian description for the liquid phase. Objective of the ECFD5 was to explore the capability to reconstruct the interface normal of a liquid droplet made of particles on an Eulerian grid. First, a level-set based strategy relying on Geometric Multiple Markers Projection (Janodet et al., 2022) has been first tested showing good capabilities providing that the iso-surface distance equal 0 is well captured on the mesh. An alternative strategy based on the liquid volume fraction has been tested. This information was then used to build a velocity correction that is used to transport particles and ensures regularized particle positions. This preliminary banchmark work will be pursued on a liquid jet propagation problem.

* '''Sub-project 2: Jet atomization with a diffuse interface mathod (N. Odier, B. Péden, J. Carmona, P. Boivin)'''

A so-called diffuse interface method, using the multi-fluid formalism, coupled with a Riemann solver HLLC and a thermodynamic closure of the NASG type, was implemented in AVBP during the thesis of Carmona. Objective of the project was to couple the NASG thermodynamic implementation in an improved HLLC solver based on nodal information only.

* '''Sub-project 3: Convergence of the interface curvature computation (G. Ghigliotti, J. Carmona, G. Balarac, G. Lartigue)'''

The computation of interface curvature in a level-set framework is based on the classic formula as divergence of the gradient of the levelset function. This function being computed at 2nd order, one obtains a O(0) curvature, meaning that the error does not decrease with mesh refinement.
We have implemented in YALES2 a strategy proposed by Emmanuel Maître and collaborators in a finite element method based on the regularization (filtering) of the level-set gradient and curvature.
This strategy has been tested for the simple test case of a static circular interface.
We used two types of filters (simple gather-scatter or bilaplacian as developed by Lola Guedot (PhD thesis 2015)) on different mesh types (split quadrilaterals, isotropic triangular mesh, unstructured triangular mesh).
The results are encouraging since a O(1) convergence is obtained in all cases.
Further work is needed to tune the filter properties (amplitude and size) for different spatial resolutions and levelset "narrow band" width.

* '''Sub-project 4: Conservative two-fluid momentum transport (F. Pecquery, C. Merlin, M. Cailler, J. Carmona, V. Moureau)'''

The aim of this project was to investigate the applicability of the conservative two-fluid transport framework developed for passive scalars to the momentum conservation equation. First a complete two-fluid framework for the incompressible two-phase Navier-Stokes equations was proposed. Then the discontinuous algorithms for discontinuous scalars (data extension, reinitialization and transport) have been extended to discontinuous vectors. Moreover, some improvement of the data-structure were implemented to further generalize the framework and improve user-experience. To eventually conclude on the applicability of the proposed two-fluid momentum approach, the proposed interfacial momentum flux model and correction step will be implemented in YALES2

=== Numerics - G. Lartigue, CORIA ===

* '''Sub-project 4: Avoiding the acoustic timestep restriction in two-phase flow simulations (V. Boniou, J. Paris, A. Vié, T. Schmitt, C. Tenaud, Y. Béchane)‘’'
In the context of simulating compressible two-phase flows with phase change, the CFL constraint related to the acoustic part of an explicit solver, especially in the liquid phase, can be prohibitive for evaporation studies implying large timescale separation between evaporation and acoustic wave propagation.
The acoustic CFL constraint can be either relaxed by considering an implicit treatment of the acoustic part or avoided by deriving the low Mach limit of the multifluid model.

During this week, the first steps of this vast work have been initiated on single-phase flows by implementing the variable-density low Mach solver and the IMEX method applied on the Euler equations in TITAN.

The IMEX method was shown to provide degraded solutions in acoustic-driven cases, compared to conventional Riemann solutions. However, in transport-dominated cases, the increase of the transport CFL leads to a better numerical precision. A 2D cylindrical shock has been simulated with an acoustic CFL of 40 to assess the ability to run multidimensional compressible computations with large time step. The method was also extented to 4eq and 2nd order spatial accuracy.

Finally, the extension of the variable density solver to two-phase flows was performed using a VOF interface capturing method, considering a dilatable gas phase along with an incompressible liquid phase. Particular attention has been drawn to preserving liquid volume by treating the VOF transport accordingly with an adapted velocity field.

=== Turbulent flows - P. Bénard, CORIA ===
* '''Sub-project 1: Optimization of the actuator set for several wind turbines in YALES2 (F. Houtin Mongrolle, S. Gremmo, E. Muller & B. Duboc)'''

* '''Sub-project 2: Thermal effect in an atmospheric solver (U. Vigny, L. Voivenel, S. Zeoli, P. Benard)'''
Given the current environmental and energy challenges, maximising the wind farm electricity production is essential. Therefore, it becomes necessary to develop the most reliable and accurate prediction and simulation tools. Following this tenet, an atmospheric solver, which will take into account meteorological phenomena, should be developed. The preliminary work, going from bibliography study to road map was performed during the extreme cod workshop. Thus five parts have been identified:
(I) The YALES2 Variable Density Solver (VDS) will be used because of the need to take into account buoyancy effect including for big density differences.
(II) A wall law correction term, relative to atmospheric boundary layer will be added.
(III) The actuator line method used to simulate wind turbine will be extended to VDS, modifying the velocity source term to a momentum source term.
(IV) The Coriolis effects, depending on the latitude will be implemented.
(V) The wall heat flux, allowing to simulate diurnal and nocturnal cycles on various terrains, is more realistic than a target wall temperature.
From this work, future development are now clear and just waiting to be developed.

* '''Sub-project 5: TBLE wall model for LES with pressure gradient on a simple turbomachinery geometry (M. Cizeron, N. Odier, R. Vicquelin)'''
Wall modeling is often used in LES to alleviate the computational cost that would be required to resolve all the length scales up to the solid boundaries of the domain. The classical way of doing it is by using an algebraic model to provide the wall friction and heat flux, with a coupling to the LES solver at the first off-wall nodes. The wall model was designed from analyzing RANS equation with strong assumptions such as planar flow, equilibrium and no pressure gradient. These assumptions are often far from true in real applications, such as turbomachinery applications, where the use of a wall model is mandatory due to the size of the calculation. During this workshop, a wall model relying on the resolution of the Thin Boundary Layer Equations (TBLE) was studied, which had been implemented by EM2C. The addition of a pressure gradient to these equations has been conducted and tested, at first only for the 1D wall model solver, then on a 3D turbulent channel. It remains to be tested on a diffuser configuration with a real pressure gradient to quantify the effect of the new wall model. The influence of the point considered to do the coupling between the LES and the wall model (ie. its distance to the wall) has also been tested both for the TBLE and the original algebraic model, showing that coupling farther from the wall yields better results and reduces the so-called log-layer mismatch.

* '''Sub-project 6: Tools for rough wall modelling (A. Barge, S. Meynet)'''
Within the STREAM project framework, a roughness-resolved Large-Eddy Simulation (RRLES) database is being built. The aim of this latter is to be representative of rough channel flows, especially for additive-manufacturing heat exchangers. First RRLES have already been performed. From turbulence and rough wall stress statistics analysis of the results, a first stochastic model, which reproduces the statistical behavior of the wall stress vector, have been proposed. The modeled wall stress allows a better prediction of the pressure drop in a flat wall channel compared to the use of the mean value of the wall stress measured in RRLES alone. However, the near wall region is still mispredicted and the model is correlated in time but not in space. The aim of this ECFD5 was to develop tools to improve modelling and explore new ways. A roughness mapping tool for smooth surfaces have been implemented into YALES2 to get local surface height. This tool is based on an existing in-house surface roughness generator developed for the STREAM project. The idea is to use the map to generated space correlated fluctuations for the wall shear stress. Some bugs still remain to fully use this tool. In parallel, the modelling approach was extended to passive scalar, especially for temperature. To this end, new random tools as white noise, unit sphere random walk and Gaussian / Log-normal stochastic processes have been coded. Finally, the idea of using walls as velocity source terms emerged during this ECFD5. The principle is to mask a grid layer above the wall and to transport the rough map on this grid to estimate the roughness effects above the wall. Parametrizing and testing these tools remained to be done at the end of ECFD5.

=== User experience - J. Leparoux, SAFRAN TECH ===

* '''Sub-project 1: Multi combustion model chemtable generator (S. Dillon, R. Mercier)'''

Due to the high computational cost of LES of turbulent combustion with detailed chemistry, tabulated chemistry approaches remain a popular choice within the combustion community. This popularity of tabulated chemistry leads to research into the development of novel combustion models for which a platform to test these models is required. The objective of the workshop was to therefore begin developing an easy-to-use chemistry table generator within Python. Given that there exists numerous turbulent combustion models (F-TACLES Diffusion, F-TACLES Premixed, 2PFT, etc.), a generalised multi-model code was necessary. Along with multi-model capabilities, the code was required to function with multiple solvers for the creation of the flame database (REGATH and CANTERA). With these objectives in mind, the code “TabulatEd Chemistry GenERator for Aeronautical CombusTion” (TECERACT) was created. Advancements were made in all key areas using code already developed by Safran Tech. TECERACT includes a parallel flame database generator and current developments are focused on multi-model functionality and simplified mesh management within Python. Once finished, the TECERACT code will provide a user-friendly chemistry table generator capable of producing chemistry tables for LES simulations and its multi-model structure will serve as a platform for future turbulent combustion model development/testing.

* '''Sub-project 2: Task-driven automatic run sequence (R. Mercier, J. Leparoux, M. Cailler, R. Letournel)'''

The principal objective of this project was to develop a new simulation workflow based on Task-driven approach. This approach could offer a higher flexibility to manage CFD simulations by decomposing the whole simulation on small sequences (run initialization, fuel injection, etc.)

This week showed the achievement of the first steps towards a complete task-driven simulation. (i) The whole simulation workflow was set-up (ii) Missing actions were implemented allowing to update simulation status on-the-fly (from non reactive to reactive simulation) (iii) State are now embedded from the restart solution allowing to restart from the last known state. Further work is still needed to obtain the full implementation of the AMC framework based on task-driven approach. Especially to automate the adaptation loop

* '''Sub-project 3: YALES2 Tools & Gitlab CI (J. Leparoux, A. Tstetoglou)'''

* '''Sub-project 4: Wind energy tools (E. Muller, S. Gremmo, F. Houtin-Mongrolle, B. Duboc)'''

Set-up phase of simulations involving several wind turbines is very demanding and error-prone, especially for simulations carried out with the coupled solver YALES2-BHawC*. This type of simulation can involve several dozen input files and then, an assistant tool is required.

The objectives of this projet were (i) develop a python tools to set all necessary ACTUATOR_SET objects (ii) provide basic visualization plots allowing to monitor simulations on-the-fly (by post-processing YALES2 temporals and BHawC results files).

''*BHawC : aero-servo-elastic solver used and developped at Siemens Gamesa Renewable Energy for wind turbine design and certification
''

=== Hackathon - G. Staffelbach, CERFACS ===
* '''Participants: I. d'Ast, J. Legaux, G. Staffelbach, P. Begou, G. Lartigue, V. Moureau, A. Toure, C. Laurie, S. Delamare, C. Andrieu, C. Jourdain'''
AMD GPU hardware is still relatively unknown in our CFD community. This hackathon was the opportunity to deep dive into the AMD dev environment to prepare the arrival of AdAstra at CINES.
Both YALES and AVBP have been ported to the AOMP framework using ROCm 4.5 on the GRID5000 Neowise system.
CPU execution posed no issues and we were able to focus on GPU Offloading using OpenMP.
On the YALES2 side, a mini-app encompassing the typical YALES2 structure hierarchy and loop execution was extracted from the code to evaluate different porting strategies and on the AVBP side the current OpenACC GPU offloading was translated to OpenMP focusing on the viscosity computation kernel.
We learnt that the current supported standard of OpenMP in ROCm 4.5 does not allow for direct offloading of reference values inside an derived type structure but is was possible to use aliases such as pointers or flat array copies to do the job. This should be solved with the support of OpenMP 5.0
Another troublesome issues, was the lack of support for offloading of array vector operations (ex : array(:) = 1.0 ) rendering the explicitation of the loops for these manadatory.

Some bugs remain and it is encouraged to use the latest compiler version when working on the porting ( the release of flang 14.0.1 saved us a lot of time as we had started with 14.0.0 ).
Offloading of the miniapp of YALES2 yielded a times 60 acceleration of the kernel whereas the offloading of the viscosity model in a full avbp simulation yielded an 7 times factor in performance when comparing on core to one GPU. These results are to be taken with a grain of salt but are really encouraging.

For the next steps, a porting strategy for both codes will be implemented (depending on the OpenMP 5 support ) and discussions are underway with CINES and other partners so as to offer the best experience to both code's communities on AdAstra at its release.

Ecfd:ecfd 5th edition

2022-01-24T09:59:28Z

Benard: /* Thematics / Mini-workshops */

Ecfd:ecfd 5th edition

2022-01-24T09:54:08Z

Benard:

Ecfd:ecfd 5th edition

2022-01-24T09:53:30Z

Benard:

Ecfd:ecfd 5th edition

2022-01-24T09:51:22Z

Benard:

File:ECFD5 program.png

2022-01-22T15:41:59Z

Benard: Benard uploaded a new version of File:ECFD5 program.png

Ecfd:ecfd 5th edition

2022-01-14T06:31:58Z

Benard: