Extreme CFD workshop - User contributions [en]

Ecfd:ecfd 9th edition

2026-02-16T10:02:14Z

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

{{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.

==== N2 - Traction Open Boundary Conditions - JB. Lagaert (IMO) & G. Balarac (LEGI) ====
In CFD, artificial boundaries are used as free-exit conditions and should disrupt the upstream flow as little as possible. During previous ECFDs, a traction boundary condition was implemented and coupled with a model estimating the outlet traction. This approach allowed steady flows to be correctly captured even when the outlet was located close to the region of interest, but it proved to be less accurate for unsteady flows.

This year was devoted to the implementation of a new formulation better suited to capturing temporal flow fluctuations. As in Bozonnet et al. (2021), traction is imposed at the prediction stage, and the velocity correction is performed so as to preserve the prescribed outlet traction, in addition to enforcing incompressibility. In order to generalize their approach to arbitrary meshes, the pressure equation is reformulated as a “heat-like” equation on the outlet. The numerical tools required to solve such a surface equation were developed during the ECFD. Future work will focus on coupling these building blocks with the existing traction model to validate the new approach on a range of test cases.

==== 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.

==== N4 - High fidelity simulation of a cone calorimeter - A.E. Simon (ONERA/CMAP), L. François (ONERA), R. Letournel (Safran), N. Dellinger (ONERA), B. Andrieu (ONERA) ====

Multiphysics simulations often require the use of multiple specialized codes, which need to be coupled by exchanging some data, the coupling variables (heat fluxes, source term, temperature...), at regular time interval. The traditional partitioned coupling approach holds these values constant in-between two consecutive exchanges, thus producing a first-order error in time. The multistep coupling strategy developed at ONERA aims at improving on this situation by using polynomial-in-time representations of the exchanged quantities, enabling arbitrary order in time and error estimation for automatic adaptation of the coupling time step.

During this year's event, the implementation of this multistep technique as an additional layer in the CWIPI coupling library has been greatly advanced. The new functionality has been incorporated in Yales2's low-Mach (VDS) and heat conduction (HTS) solvers, as well as ONERA's MoDeTheC thermal decomposition software. A few conjugate heat transfer simulations have given promising results for the new coupling strategy and pave the way for the cone calorimeter simulation coupling Yales2 and MoDeTheC. Future work will be aimed at developing specialized coupling procedures for quasi-steady-state couplings, increasing the efficiency of the implicit variant of coupling scheme, and improving the handling of highly fluctuating coupling variables, as encountered in LES simulations.

==== 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. Illuminati (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.

==== C10 – Anisotropic mesh adaptation for reacting flows- N. Moslimani (CORIA), R. Barbera (LEGI), K. Bioche (CORIA), G. Lartigue (CORIA) ====

Anisotropic mesh adaptation (AMA) with high aspect ratios has been shown in multiphase-flow simulations to reduce accuracy in regions of high interface curvature, motivating the use of locally isotropic meshes. This work investigated whether a similar limitation arises in combustion simulations with curved flame fronts when using a level-set–based AMA strategy. A two-dimensional planar premixed flame subjected to inlet mixture inhomogeneities was considered, leading to flame wrinkling and localized regions of high curvature. A sensitivity study with respect to the imposed aspect ratio was performed, comparing isotropic adaptation with anisotropic adaptation at increasing aspect ratios. All configurations yielded nearly identical flame shapes and accurately resolved curvature, showing no reduction of solution quality with increasing anisotropy. This indicates that varying the aspect ratio as a function of curvature is not necessary for combustion applications. However, the distance-based remeshing criterion used in level-set–based AMA was found to delay adaptation during rapid flame tilting, leading to temporary misalignment between anisotropic cells and the flame front. To address this issue, a new remeshing criterion based on local metric comparison was introduced, triggering adaptation whenever the desired flame-aligned metric is not included in the expanded actual mesh metric within a narrow band around the flame, ensuring robust alignment and resolution during flame reorientation.

==== 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) ===

==== M1 – OpenM3S: toward an open-source multiscale, multiphysics, multiphase flows solver with AMR - A. Chadil (MSME), A. Hakkoum (MSME) & S. Elkanih (MSME) ====

This project develops OpenM3S, a new open-source Fortran framework for multiscale, multiphysics, multiphase flows simulations and especially particle-resolved direct numerical simulations using a Viscous Penalty Method, targeting applications involving catalytic reactions and triboelectrification of polarized particles — phenomena that exhibit strong spatial localization near particle surfaces and require Adaptive Mesh Refinement. During ECFD9, the legacy codebase was entirely restructured into a modular, object-oriented architecture comprising 46 modules (9,000+ lines), 198 unit tests using pFUnit (4,400+ test lines, 29.5% coverage), and a five-stage GitLab CI/CD pipeline covering linting, security analysis, build/test with coverage enforcement, and automated Sphinx documentation deployment. A polymorphic mesh architecture was designed to support uniform grids as well as patch-based and cell-based AMR approaches. For the patch-based strategy, hierarchical time subcycling, automatic coarse–fine synchronization, and inter-level operators (prolongation, restriction) were implemented. Convergence tests confirmed that the interpolation procedures preserve the formal order of accuracy of the underlying schemes; first order for Upwind and second order for Lax-Wendroff, on both uniform grids and multi-level AMR configurations with up to four refinement levels. Ongoing work focuses on leveraging the AMReX and p4est libraries for efficient cell connectivity management, dynamic load balancing, and scalability on HPC platforms, as well as improving numerical accuracy in the cell-based AMR framework and ensuring strict conservation of physical quantities. On the other hand some work was also done in the legacy code, the MUMPS solver (sequential and MPI) was integrated alongside a refactoring of the MPI modules, enabling the simulation of an endothermic fixed-bed reactor composed of 60 catalytic particles with a detailed surface reaction mechanism (42 reactions, 6 gas species, 12 surface species) for PR-DNS of Dry Reforming of Methane, building upon validated 2D and 3D catalytic particle flow simulations.

==== 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.

==== M6 – Static mesh adaptation workflow based on on-the-fly computed metrics - Q. Douasbin (CERFACS), A. Pestre(CERFACS), P. Picard(CERFACS), L. Carbajal-Carrasco (Safran) & T. Duranton (Safran) ====

Reactive Large Eddy Simulations (LES) for industrial applications are computationally expensive, and mesh adaptation represents an effective strategy to reduce this cost while preserving solution accuracy. The current Adaptative Static Mesh Refinement (ASMR) workflow suffers from 1) a history effect that keeps cells in regions previously refined and 2) a global cell size constraint that prevents fine control of the local cell size.
The objective of this project is to address these limitations by developing a new workflow for AVBP driven by physics-based criteria and capable of performing each adaptation from the initial coarse mesh. In this context, the objectives of the project are twofold: 1) develop a new ASMR workflow and 2) define a target mesh for each Quantity of Interest.
A prototype of a flexible ASMR workflow was developed to run multiple simulation scenarios. It is based on the Lemmings and Tekigo libraries, developed at CERFACS, to handle the orchestration of successive simulation jobs and the generation of the mesh adaptation process, respectively. Several phases of simulations, each composed of several stages, allow an efficient mesh convergence.
For each physics of interest a target mesh is determined and the adapted local cell size is the smallest cell size of all metrics. The turbulent combustion mesh is based on the TFLES model and consists of a target flame thickening factor that takes into account the probability of flame presence. The pressure drop and turbulent flow mesh is based on the work of H. Lam and G. Balarac which defines the metric based on two criteria i) a cell Reynolds number for DNS. ii) When the Kolmogorov to integral length scale ratio is high enough in a cell, the flow is deemed suitable for LES and the cell size is fixed via a constant number of cells per local integral length scale. The implementation and validation of this second metric are currently ongoing.

==== 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) ====

Ceramic core displacement and deformation during the casting process is a major source of cooled blades manufacturing scrap. Predictive numerical simulations of the casting process would be an essential asset to increase the efficiency of the conception and industrial processes. At ECFD7, the numerical setup to simulate the filling process with YALES2 was drawn and results were compared to simplified casting experiment on a test blade. And at ECFD8, the deformation of the test blade was addressed via a numerical chaining between YALES2 and ABAQUS (FEM software). During this ECFD9 workshop, two objectives were targeted: 1) study the impact of different physics (contact angles, partial vacuum) on the fluidic forces exerted on the core 2) linked to the TP3 project, challenge a new SPS-ALE solver in YALES2 concerning the shifting of the core. About the first objective, several simulations have been conducted on a test case and on an industrial configuration. The first results showed that it was important to take surface tension and contact angles into account with regard to force amplitudes. Partial vacuum also has an influence. However, the force curves show a similar trend to those obtained without these different models, and the cost of the simulation is lower. It is therefore necessary to find a compromise between the best possible accuracy and the cost for future simulations. Concerning the second objective, test simulations have highlighted the need of debugging / cleaning sessions for the SPS-ALE solver. The later have been done by Mélody Cailler and Vincent Moureau and have led to a functional solver. This work paves the way for accurate two-phase flows and moving bodies simulations with YALES2.

==== 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 ====

The main objective of this project was to model the rotation of a gear wheel immersed in an oil bath using YALES2. Such a phenomenon occurs in aeronautical power transmission systems, either intentionally for the lubrication of solid contacts, or unintentionally, leading to gulping effects that may critically impact aircraft performance. The ECAM configuration serves as a well-studied, simplified representation of scenarios encountered within engines; it provides diagnostics on both oil projection quantities and the torque exerted by the oil on the gear.
During ECFD9, the necessary components to enable the modelling of this phenomenon were established, thus allowing computation for this configuration. The coupling between the ALE and SPS solvers was adapted to the numerical schemes recommended for two-phase flow simulations, allowing initial iterations on the final setup. Nevertheless, modelling the contact point at the liquid–gas–solid interface remains a challenge to ensure numerical stability, and will be addressed in future work.

==== 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) ====

This project focuses on the modeling of kerosene cavitation using the MultiFluid Solver (MFS) of YALES2. This solver is based on a diffuse-interface approach for phase mixing and relies on a NASG thermodynamic closure. Prior to this ECFD, it had been tested and validated on several theoretical benchmark cases from the literature. During this ECFD9, the method was applied to an academic case of Jet-A1 cavitation in an external gear pump. The project was structured around two main lines of investigation. The first one addresses the improvement of computational performance, for which several computational acceleration strategies were evaluated. In particular, it was shown that the Pressure Gradient Scaling (PGS) strategy is counterproductive for this type of configuration. The most promising approach identified is the use of implicit time-integration schemes for advancing the conservation equations, whose implementation was initiated during this ECFD and will be pursued in future work. The second line of investigation aims at making the phase-change process more stable and more physically realistic by focusing on the relaxation time of the liquid–vapor mixture pressure toward the saturated vapor pressure. Increasing this relaxation time was found to be beneficial both for numerical stability and for the physical representation of cavitation, as it allows a better capture of shear effects within cavitation pockets.

==== 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, C. Merlin (AGS), V. Moureau (CORIA), T. Laurent (CORIA/AGS), D. Fouquet (CORIA), J. Carmona (CORIA) ====
Modelling of two phase flow with heat and mass transfer is key for modeling of cryogenic tank. This project aimed at implementing mass transfer due to evaporation/condensation in the phase change solver (PCS). This solver relies on a two fluid approach for energy and species and a one fluid approach for velocity in a low Mach framework. The Accurate Conservative Level Set (ACLS) is used to track the interface as well as reinitialization to keep a hyperbolic tangent profile. In the two fluid approach, the thermodynamic quantities are transported with the phase indicator and reinitialized accordingly. The project was divided into two parts. The first one was focussed on the implementation of phase change models for various equations of state. Different analytical cases have been investigated for monospecies gas. Due to the use of one velocity, different strategies to take into account the velocity jump were tested. The second part was devoted to the implementation of a conservative reinitialization for the conservative thermodynamic quantities. It resulted in a huge transformation of the PCS solver with adaptation of the data structure, boundary conditions and pressure homogenization loop. The initially planned model for multi species phase change was then postponed.

==== TP8 - Jet-in-crossflow simulation with the Hybrid SPH-FVM solver - M. Helal (CORIA/Safran), M. Cailler (Safran), V. Moureau (CORIA) ====
Over the past two years, a new solver has been developed in YALES2, based on a two-way coupling between incompressible SPH (Cummins & Rudman, 1999) and a diffuse-interface finite-volume method (FVM). The aim is to capture the global dynamics of multiscale liquid–gas flows while maintaining a controlled trade-off between physical fidelity and computational cost. The Lagrangian representation of the liquid phase enables accurate resolution of its dynamics and interface deformations without requiring additional interface-tracking procedures. Meanwhile, the Eulerian description of the gas phase allows efficient simulation of the strong dynamics of the large turbulent scales on a coarser grid.
The objective of this project is to validate this new framework on a jet-in-crossflow (JICF) configuration. This requires implementing a new inlet boundary condition, which raises challenges in achieving a formulation that is consistent for both the Lagrangian and Eulerian descriptions. A method based on mirror ghost particles (Hirschler et al., 2015) has been implemented. The results obtained for the JICF case, compared with the spray solver, are satisfactory.

==== 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) ====
We are developing a sub-grid scale contact line model for nucleate boiling simulations in YALES2, which includes closures for heat and mass fluxes near the contact line. For substrates with high thermal conductivity, the assumption of no temperature variation across the solid-fluid interface is reasonable, allowing to model the solid simply through an isothermal wall boundary condition for the fluid. However, this approximation does not hold for substrates with low thermal conductivity, where significant temperature variations and localized cold spots can occur near the contact line. This project aims to couple the boiling and heat transfer solvers to enable conjugate heat transfer across the solid-fluid interface.
An initial test of the coupling was conducted for a single nitrogen bubble undergoing nucleate boiling on a low-conductivity substrate, without the aforementioned closures at the contact line. The results were analysed in terms of bubble growth rate and revealed consistent behaviour—specifically, the emergence of localized cold spots in the solid substrate at the position of the contact line. Subsequently, the sub-grid contact line model was coupled with the combined boiling and heat transfer solver. Initial results showed the expected increase in bubble growth rate.
We also implemented the possibility for the boiling solver to use axisymmetric coordinates. Initial tests give the expected results, but also show that the computation of curvature close to the symmetry axis may need some improvement, as for the general-purpose two-phase solver in YALES2.

Ecfd:ecfd 9th edition

2026-02-04T10:30:02Z

Lartigue: /* U1 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 */

Ecfd:ecfd 9th edition

2026-02-04T10:29:24Z

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

Ecfd:ecfd 5th edition

2022-02-03T01:24:41Z

Lartigue: /* Numerics - G. Lartigue, 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]

* 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.

== Projects ==

=== 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'''
While hybrid CPU/GPU computing has seen growing interest in the CFD community, AMD GPU hardware is a newcomer for this application. This hackathon was the opportunity to deep dive into the AMD development environment and to prepare the arrival of AdAstra at CINES, a new super-computer which relies on AMD CPU and GPU hardware. Two flagship codes of the applied CFD community, namely YALES2 and AVBP, have been ported to the AOMP framework using ROCm 4.5 on the GRID5000 Neowise system. This machine [https://www.grid5000.fr/w/GENCI-Neowise_Usage] was donated by AMD to GENCI and INRIA to help in fighting the COVID pandemic and it is operated by the Grid'5000 infrastructure.

Part of the project was spent in porting the codes to the new development environment. This work posed no real issues and CPU execution was achieved fast so that 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. 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 a derived type structure but is was possible to use aliases such as pointers or flat array copies to offload. 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.

=== 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 ===

* '''Sub-project 1: Anisotropic mesh adaptation in YALES2 (A. Grenouilloux, A. Stock, G. Lartigue, G. Balarac, A. Froehly, C. Dapogny, J. Leparoux)'''

Mesh adaptation based on isotropic meshes is now a mature tool in YALES2 based on the Mmg library. This parallel mesh adaptation method is very effective for increasing the resolution in sensitive regions of the flow even when using billion-cell meshes. In many configurations, a significant gain in terms of computational cost can be expected through the use of anisotropic mesh. However, this requires to address several aspects: (i) the ability to generate anisotropic meshes via the coupling between YALES2 and Mmg, (ii) to define an anisotropic metric adapted to the simulated flow, and (iii) to assess the properties of the numerical methods (prediction/stability) of YALES2 on such meshes. This project has addressed the two first points (see sub-project 5, in Numerics for the last one). During the workshop, a full anisotropic mesh adaptation strategy has been implemented in YALES2. Anisotropic mesh adaptation needs to prescribe a metric tensor to define the ellipsoid characterizing the anisotropic tetrahedral element. First developments were then devoted to the manipulation and the control of this metric tensor (metric intersection, graduation control, aspect ratio control, etc...). Second part of the project was devoted to the determination of an anisotropic mesh based on computed fields. The Hessian-based metric definition has been implemented for this purpose. Successfull demonstrations have been conducted for 2D and 3D meshes during the workshop paving the way to upcoming applications.

=== 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 benchmark 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 type Noble-Abel Stiffened Gas (NASG), was implemented in AVBP during the thesis of Julien Carmona. During the ECFD5, the NASG thermodynamic was coupled with an improved HLLC solver implementation based on nodal information only, therefore leading to better performances in parallel. Moreover, the NSCBC terms expressed in the framework of the NASG thermodynamics were accurately derived. Future works include validation of the new solver capability on 1-D shock tube and isolated bubble test cases.

* '''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 divergence of the gradient of the distance to the interface. 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. This approach, implemented in a finite element code FreeFEM++, is 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 algorithms for discontinuous scalars (data extension in the other phase, 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 two-fluid momentum approach, the proposed interfacial momentum flux model and correction step will be implemented in YALES2.

=== Numerics - G. Lartigue, CORIA ===

* '''Sub-project 1 : High order schemes for distorted meshes (M. Bernard, G. Balarac, G. Lartigue)'''
The high order framework (HOF) based on deconvolution recently developed and implemented in Yales2 permits to increase the accuracy of spatial numerical schemes on distorted meshes.
Nevertheless, while dealing with highly anisotropic meshes (aspect-ratio above 10), the conditioning of the convolution matrix gets worse, complicating its inversion.

The aim of this project was too improve the robustness of the HO3 numerical scheme while dealing with highly anisotropic meshes.
The strategy was to ensure third order accuracy as far as possible, then if conditioning gets too bad, to locally decrease the solution at 2nd order, using the HO2 convolution matrix which is unconditionally invertible.
Then, solution must be reconstructed with care at control volume interface to account for this mixing between HO2 and HO3.
Implementation has been successfully tested on transport of a scalar field in a non-uniform flow field with aspect-ratio up to 100.

* '''Sub-project 2: A coupling between YALES2 solver and a VPM method for the simulation of external flows (P. Billuart, G. Lartigue, P. Bénard)'''
Numerical simulations of flows past immersed objects are ubiquitous in both research and industry, spanning e.g. vehicle aero- or hydrodynamics, biological propulsion, wind energy, civil engineering, etc. Such problems are typically solved using either a (1) body-fitted grid solver, like the YALES2 finite volumes solver, or via (2) a vortex method, like the VPM (Vortex Particle Mesh) solver. Vortex methods are indeed quite appealing in that problem because they yield very low dispersion and diffusion errors which puts them among the most efficient methods to accurately advect vortical structures over long distances. In addition, their vorticity-based formulation accommodates quite naturally the enforcement of the unbounded conditions of external flows. However, vortex methods are much less suited to capture boundary layers than classical body-fitted grid solvers, e.g. finite elements, finite volumes, etc, because of their isotropic computational elements. This leads to the conclusion that one could exploit the advantages of vortex methods and body fitted-grid solvers in a coupled approach; using YALES2 for solving the near-wall part of the flow, while solving the wake using the VPM method.
This project aimed to implement such a coupling using the CWIPI library. It requires two information exchanges between both solvers; (a) an interpolation of vorticity and velocity VPM fields onto the outer YALES2 boundary and, (b) the interpolation of the YALES2 velocity field onto the VPM grid. While tests regarding the MPI communicator splitting, CWIPI initializations and CWIPI interpolation initializations demonstrate a correct implementation of those, they also point out that a bug still remains in the interpolation process from VPM to YALES2.

The next steps will be: (1) keeping on identifying this bug, (2) implementing the information exchange from YALES2 to VPM and (3) synchronizing both solvers on the global communicator via MPI calls.

* '''Sub-Project 3: Assessment of the YALES2 compressible flow solvers (CPS,ECS,SIC) for turbomachinery applications (P. Tene, L. Bricteux, S. Lavagnoli, Y. Béchane, P. Bénez)'''
High propulsive efficiency calls for Ultra-High By-Pass ratio geared turbofans. A huge fan ensuring the high dilution is rotating slowly. This fan is driven by a low-pressure turbine (LPT) rotating fast (diminishing stage count and weight). Both components are linked using a gearbox. As the fan produces 80% of the thrust, the LPT that drives it is a key component to achieve efficiency. LPT operates at transonic exit Mach numbers and low-Reynolds numbers (60-80k). Within this range of operating conditions, there is a critical shortage of aerodynamic and performance measurements.

This week we assessed the ability of the YALES2 Explicit compressible flow solver (ECS) to ensure consistent results for such applications. The first results were very promising, and proposals were made (e.g imposing total pressure as a boundary condition) to improve the development of this solver. In addition, an assessment of the different numerical schemes of this solver in terms of diffusion has also been carried out. Advances have also been made in the development of the YALES2 semi-implicit compressible solver (SIC). This solver will eventually (in the context of this application), improve the computational cost because unlike ECS, the time step here is not limited by the acoustic CFL.

* '''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.

* '''Sub-project 5 : Numerical schemes for anisotropic meshes (R. Letournel, G. Lartigue, G. Balarac, M. Bernard)'''
Many efforts are devoted to the development of mesh adaptation based on anisotropic meshes in Yales2. For many configurations (e.g. shocks, mixing layers, near wall flows), a significant gain in terms of computational cost can be expected through the use of such meshes. The aim of this project was to characterize Yales2 numerical schemes on anisotropic meshes in comparison with isotropic meshes.

The strategy was at first to test different Laplacian operators on analytic functions (linear, quatratic, bump in one or two directions) in direct sense (apply the Laplacian operator) and indirect (solve the Poisson equation), to verify the order of convergence of the operators. Different meshes were considered: Isomesh, Quad2tri, MMG meshes, with different stretching (corresponding to anisotropic ratios). This exhaustive study showed that only the SIMPLEX operator remains of order 2 for the Poisson solver, while the IGNORE_SKEWNESS and PAIR_BASED operators do not converge.

In a second step, the performance of the incompressible solver on a 2D Poiseuille flow have been evaluated for increasingly stretched meshes. The coarsening in the preferential direction of the flow allows to decrease the convective time step and thus to reduce the RTTS (Return Time To Solution), while keeping the same error. These conclusions are valid for anisotropic ratios of the order of 10, beyond which the time step no longer decreases and further studies are needed to understand the limitations in these cases.

* '''Sub-project 6 : Poisson solver comparison for a two-phase flow CFD code (J. C. Brändle de Motta, A. Poux, T. Ménard)'''
Poisson solvers known to be highly sensitive to discontinuities and represent the main computational costs of incompressible codes.
Archer is a mature two-phase flow CFD code. Its historical Poisson solver is currently based on a Conjugate gradient with multigrid preconditioner (MGCG).
The aim of this project was to couple Archer and the PETSc library, assess its performances and challenge the historical solver of Archer.

The coupling with PETSc has been implemented during the workshop and the comparison with MGCG has been done up to 2 thousands cores in single and two phase turbulent flow.
Multiple PETSc configuration has been tested, and show that MGCG is very competitive (one order of magnitude faster than PETSc)

=== 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 allows to have an hysteresis loop on the evolution of the forces coefficients with the angle of attack. Several models exist in the literature: Oye, BeddoesLeishman, Sheng, Risoe, ONERA... During the workshop, the Oye model was selected, because of its simplicity, and 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 further validate 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, R. Mercier)'''

Y2 smart archive is a useful tool which gives the user more freedom when it comes storing their data. Y2 smart archive allows for automatic smart archiving and has the capability of taking user-specified maximum number of tar files and/or the maximum tar file size. The python tool conserves the original tree structure of the folder and has the ability to automatically detect the tree depth where the tar files are to be saved. During the project, y2 smart archive was tested on a windows computer. This implementation verified the ability to archive save a whole Windows computer with automatic handling of multi-platform symlinks. In the case where files may be skipped, log files are stored for reference. Current developments of the tool include automatic background launching, which will result in a reduction of required user-input hours.

* '''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
''

Ecfd:ecfd 5th edition

2022-02-02T03:09:59Z

Lartigue: /* Numerics - G. Lartigue, 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]

* 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 benchmark 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 type Noble-Abel Stiffened Gas (NASG), was implemented in AVBP during the thesis of Julien Carmona. During the ECFD5, the NASG thermodynamic was coupled with an improved HLLC solver implementation based on nodal information only, therefore leading to better performances in parallel. Moreover, the NSCBC terms expressed in the framework of the NASG thermodynamics were accurately derived. Future works include validation of the new solver capability on 1-D shock tube and isolated bubble test cases.

* '''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 divergence of the gradient of the distance to the interface. 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. This approach, implemented in a finite element code FreeFEM++, is 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 algorithms for discontinuous scalars (data extension in the other phase, 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 two-fluid momentum approach, the proposed interfacial momentum flux model and correction step will be implemented in YALES2.

=== Numerics - G. Lartigue, CORIA ===

* '''Sub-project 1 : High order schemes for distorted meshes (M. Bernard, G. Balarac, G. Lartigue)'''
The high order framework (HOF) based on deconvolution recently developed and implemented in Yales2 permits to increase the accuracy of spatial numerical schemes on distorted meshes.
Nevertheless, while dealing with highly anisotropic meshes (aspect-ratio above 10), the conditioning of the convolution matrix gets worse, complicating its inversion.

The aim of this project was too improve the robustness of the HO3 numerical scheme while dealing with highly anisotropic meshes.
The strategy was to ensure third order accuracy as far as possible, then if conditioning gets too bad, to locally decrease the solution at 2nd order, using the HO2 convolution matrix which is unconditionally invertible.
Then, solution must be reconstructed with care at control volume interface to account for this mixing between HO2 and HO3.
Implementation has been successfully tested on transport of a scalar field in a non-uniform flow field with aspect-ratio up to 100.

* '''Sub-project 2: A coupling between YALES2 solver and a VPM method for the simulation of external flows (P. Billuart, G. Lartigue, P. Bénard)'''
Numerical simulations of flows past immersed objects are ubiquitous in both research and industry, spanning e.g. vehicle aero- or hydrodynamics, biological propulsion, wind energy, civil engineering, etc. Such problems are typically solved using either a (1) body-fitted grid solver, like the YALES2 finite volumes solver, or via (2) a vortex method, like the VPM (Vortex Particle Mesh) solver. Vortex methods are indeed quite appealing in that problem because they yield very low dispersion and diffusion errors which puts them among the most efficient methods to accurately advect vortical structures over long distances. In addition, their vorticity-based formulation accommodates quite naturally the enforcement of the unbounded conditions of external flows. However, vortex methods are much less suited to capture boundary layers than classical body-fitted grid solvers, e.g. finite elements, finite volumes, etc, because of their isotropic computational elements. This leads to the conclusion that one could exploit the advantages of vortex methods and body fitted-grid solvers in a coupled approach; using YALES2 for solving the near-wall part of the flow, while solving the wake using the VPM method.
This project aimed to implement such a coupling using the CWIPI library. It requires two information exchanges between both solvers; (a) an interpolation of vorticity and velocity VPM fields onto the outer YALES2 boundary and, (b) the interpolation of the YALES2 velocity field onto the VPM grid. While tests regarding the MPI communicator splitting, CWIPI initializations and CWIPI interpolation initializations demonstrate a correct implementation of those, they also point out that a bug still remains in the interpolation process from VPM to YALES2.

The next steps will be: (1) keeping on identifying this bug, (2) implementing the information exchange from YALES2 to VPM and (3) synchronizing both solvers on the global communicator via MPI calls.

* '''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.

* '''Sub-project 5 : Numerical schemes for anisotropic meshes (R. Letournel, G. Lartigue, G. Balarac, M. Bernard)'''
Many efforts are devoted to the development of mesh adaptation based on anisotropic meshes in Yales2. For many configurations (e.g. shocks, mixing layers, near wall flows), a significant gain in terms of computational cost can be expected through the use of such meshes. The aim of this project was to characterize Yales2 numerical schemes on anisotropic meshes in comparison with isotropic meshes.

The strategy was at first to test different Laplacian operators on analytic functions (linear, quatratic, bump in one or two directions) in direct sense (apply the Laplacian operator) and indirect (solve the Poisson equation), to verify the order of convergence of the operators. Different meshes were considered: Isomesh, Quad2tri, MMG meshes, with different stretching (corresponding to anisotropic ratios). This exhaustive study showed that only the SIMPLEX operator remains of order 2 for the Poisson solver, while the IGNORE_SKEWNESS and PAIR_BASED operators do not converge.

In a second step, the performance of the incompressible solver on a 2D Poiseuille flow have been evaluated for increasingly stretched meshes. The coarsening in the preferential direction of the flow allows to decrease the convective time step and thus to reduce the RTTS (Return Time To Solution), while keeping the same error. These conclusions are valid for anisotropic ratios of the order of 10, beyond which the time step no longer decreases and further studies are needed to understand the limitations in these cases.

* '''Sub-project 6 : Poisson solver comparison for a two-phase flow CFD code (J. C. Brändle de Motta, A. Poux, T. Ménard)'''
Poisson solvers known to be highly sensitive to discontinuities and represent the main computational costs of incompressible codes.
Archer is a mature two-phase flow CFD code. Its historical Poisson solver is currently based on a Conjugate gradient with multigrid preconditioner (MGCG).
The aim of this project was to couple Archer and the PETSc library, assess its performances and challenge the historical solver of Archer.

The coupling with PETSc has been implemented during the workshop and the comparison with MGCG has been done up to 2 thousands cores in single and two phase turbulent flow.
Multiple PETSc configuration has been tested, and show that MGCG is very competitive (one order of magnitude faster than PETSc)

=== 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 allows to have an hysteresis loop on the evolution of the forces coefficients with the angle of attack. Several models exist in the literature: Oye, BeddoesLeishman, Sheng, Risoe, ONERA... During the workshop, the Oye model was selected, because of its simplicity, and 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 further validate 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, R. Mercier)'''

Y2 smart archive is a useful tool which gives the user more freedom when it comes storing their data. Y2 smart archive allows for automatic smart archiving and has the capability of taking user-specified maximum number of tar files and/or the maximum tar file size. The python tool conserves the original tree structure of the folder and has the ability to automatically detect the tree depth where the tar files are to be saved. During the project, y2 smart archive was tested on a windows computer. This implementation verified the ability to archive save a whole Windows computer with automatic handling of multi-platform symlinks. In the case where files may be skipped, log files are stored for reference. Current developments of the tool include automatic background launching, which will result in a reduction of required user-input hours.

* '''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

2022-02-02T03:09:42Z

Lartigue: /* Numerics - G. Lartigue, 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]

* 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 benchmark 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 type Noble-Abel Stiffened Gas (NASG), was implemented in AVBP during the thesis of Julien Carmona. During the ECFD5, the NASG thermodynamic was coupled with an improved HLLC solver implementation based on nodal information only, therefore leading to better performances in parallel. Moreover, the NSCBC terms expressed in the framework of the NASG thermodynamics were accurately derived. Future works include validation of the new solver capability on 1-D shock tube and isolated bubble test cases.

* '''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 divergence of the gradient of the distance to the interface. 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. This approach, implemented in a finite element code FreeFEM++, is 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 algorithms for discontinuous scalars (data extension in the other phase, 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 two-fluid momentum approach, the proposed interfacial momentum flux model and correction step will be implemented in YALES2.

=== Numerics - G. Lartigue, CORIA ===

* '''Sub-project 1 : High order schemes for distorted meshes (M. Bernard, G. Balarac, G. Lartigue)'''
The high order framework (HOF) based on deconvolution recently developed and implemented in Yales2 permits to increase the accuracy of spatial numerical schemes on distorted meshes.
Nevertheless, while dealing with highly anisotropic meshes (aspect-ratio above 10), the conditioning of the convolution matrix gets worse, complicating its inversion.

The aim of this project was too improve the robustness of the HO3 numerical scheme while dealing with highly anisotropic meshes.
The strategy was to ensure third order accuracy as far as possible, then if conditioning gets too bad, to locally decrease the solution at 2nd order, using the HO2 convolution matrix which is unconditionally invertible.
Then, solution must be reconstructed with care at control volume interface to account for this mixing between HO2 and HO3.
Implementation has been successfully tested on transport of a scalar field in a non-uniform flow field with aspect-ratio up to 100.

* '''A coupling between YALES2 solver and a VPM method for the simulation of external flows (P. Billuart, G. Lartigue, P. Bénard)'''
Numerical simulations of flows past immersed objects are ubiquitous in both research and industry, spanning e.g. vehicle aero- or hydrodynamics, biological propulsion, wind energy, civil engineering, etc. Such problems are typically solved using either a (1) body-fitted grid solver, like the YALES2 finite volumes solver, or via (2) a vortex method, like the VPM (Vortex Particle Mesh) solver. Vortex methods are indeed quite appealing in that problem because they yield very low dispersion and diffusion errors which puts them among the most efficient methods to accurately advect vortical structures over long distances. In addition, their vorticity-based formulation accommodates quite naturally the enforcement of the unbounded conditions of external flows. However, vortex methods are much less suited to capture boundary layers than classical body-fitted grid solvers, e.g. finite elements, finite volumes, etc, because of their isotropic computational elements. This leads to the conclusion that one could exploit the advantages of vortex methods and body fitted-grid solvers in a coupled approach; using YALES2 for solving the near-wall part of the flow, while solving the wake using the VPM method.
This project aimed to implement such a coupling using the CWIPI library. It requires two information exchanges between both solvers; (a) an interpolation of vorticity and velocity VPM fields onto the outer YALES2 boundary and, (b) the interpolation of the YALES2 velocity field onto the VPM grid. While tests regarding the MPI communicator splitting, CWIPI initializations and CWIPI interpolation initializations demonstrate a correct implementation of those, they also point out that a bug still remains in the interpolation process from VPM to YALES2.

The next steps will be: (1) keeping on identifying this bug, (2) implementing the information exchange from YALES2 to VPM and (3) synchronizing both solvers on the global communicator via MPI calls.

* '''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.

* '''Sub-project 5 : Numerical schemes for anisotropic meshes (R. Letournel, G. Lartigue, G. Balarac, M. Bernard)'''
Many efforts are devoted to the development of mesh adaptation based on anisotropic meshes in Yales2. For many configurations (e.g. shocks, mixing layers, near wall flows), a significant gain in terms of computational cost can be expected through the use of such meshes. The aim of this project was to characterize Yales2 numerical schemes on anisotropic meshes in comparison with isotropic meshes.

The strategy was at first to test different Laplacian operators on analytic functions (linear, quatratic, bump in one or two directions) in direct sense (apply the Laplacian operator) and indirect (solve the Poisson equation), to verify the order of convergence of the operators. Different meshes were considered: Isomesh, Quad2tri, MMG meshes, with different stretching (corresponding to anisotropic ratios). This exhaustive study showed that only the SIMPLEX operator remains of order 2 for the Poisson solver, while the IGNORE_SKEWNESS and PAIR_BASED operators do not converge.

In a second step, the performance of the incompressible solver on a 2D Poiseuille flow have been evaluated for increasingly stretched meshes. The coarsening in the preferential direction of the flow allows to decrease the convective time step and thus to reduce the RTTS (Return Time To Solution), while keeping the same error. These conclusions are valid for anisotropic ratios of the order of 10, beyond which the time step no longer decreases and further studies are needed to understand the limitations in these cases.

* '''Sub-project 6 : Poisson solver comparison for a two-phase flow CFD code (J. C. Brändle de Motta, A. Poux, T. Ménard)'''
Poisson solvers known to be highly sensitive to discontinuities and represent the main computational costs of incompressible codes.
Archer is a mature two-phase flow CFD code. Its historical Poisson solver is currently based on a Conjugate gradient with multigrid preconditioner (MGCG).
The aim of this project was to couple Archer and the PETSc library, assess its performances and challenge the historical solver of Archer.

The coupling with PETSc has been implemented during the workshop and the comparison with MGCG has been done up to 2 thousands cores in single and two phase turbulent flow.
Multiple PETSc configuration has been tested, and show that MGCG is very competitive (one order of magnitude faster than PETSc)

=== 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 allows to have an hysteresis loop on the evolution of the forces coefficients with the angle of attack. Several models exist in the literature: Oye, BeddoesLeishman, Sheng, Risoe, ONERA... During the workshop, the Oye model was selected, because of its simplicity, and 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 further validate 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, R. Mercier)'''

Y2 smart archive is a useful tool which gives the user more freedom when it comes storing their data. Y2 smart archive allows for automatic smart archiving and has the capability of taking user-specified maximum number of tar files and/or the maximum tar file size. The python tool conserves the original tree structure of the folder and has the ability to automatically detect the tree depth where the tar files are to be saved. During the project, y2 smart archive was tested on a windows computer. This implementation verified the ability to archive save a whole Windows computer with automatic handling of multi-platform symlinks. In the case where files may be skipped, log files are stored for reference. Current developments of the tool include automatic background launching, which will result in a reduction of required user-input hours.

* '''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

2022-02-01T07:53:47Z

Lartigue: /* Numerics - G. Lartigue, 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 4th edition

2021-03-30T08:06:21Z

Lartigue: /* Numerics - G. Lartigue, CORIA */

{{DISPLAYTITLE: ECFD workshop, 4th edition, 2021}}

== Description ==
{| align="right" style="text-align:center;" cellpadding="2"
| [[File:logo_ecfd4.png | center | thumb | 300px | ECFD4 workshop logo.]]
|}
* Virtual event from '''22nd to 26th of March 2021'''
* 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, UMONS, UVM, VUB), HPC center/experts (GENCI, IDRIS, NVIDIA, HPE) and industry (Safran, Ariane Group).
* Web TV: [https://webtv.insa-rouen.fr/channels/#ecfd4 https://webtv.insa-rouen.fr/channels/#ecfd4]

== News ==

Annoncements on Linkedin
* [https://www.linkedin.com/posts/christelle-piechurski-429b3925_the-4th-extreme-computational-fluid-dynamics-activity-6777492300546236416-njcV '''First annoncement''']
* [https://www.linkedin.com/posts/christelle-piechurski-429b3925_2nd-day-of-extreme-computational-fluid-dynamics-activity-6780117155796017152-Epr5 '''Second day annoncement''']
* [https://www.linkedin.com/posts/christelle-piechurski-429b3925_4th-day-of-ecfd4-starting-with-a-plenary-activity-6781048446448140288-wvlz '''Fourth day annoncement''']


== 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 AVBP and YALES2 codes

== Agenda ==

[[File:Agenda ECFDW4.png | 800px | CFDW4 agenda]]

==== Plénière 1 ====
Lundi 22/03/2021 9h00-9h20

'''Introduction (organisation, agenda semaine, etc.)'''

''V. Moureau (CORIA), G. Balarac (LEGI), C. Piechurski (GENCI)''

==== Plénière 2 ====

Lundi 22/03/2021 9h20-11h20

'''Présentation des projets du workshop et Présentation des thématiques du hackathon'''

''Responsables de projets''

==== Plénière 3 ====

Lundi 22/03/2021 11h20-12h00

'''Contrat de Progrès Jean Zay: Véhicule d'accompagnement des utilisateurs au portage des applications sur les nouvelles technologies'''

''P.-F. Lavallée (IDRIS)''

==== Plénière 4 ====

Mardi 23/03/2021 9h00-10h00

'''Evolution de la programmation GPU – CUDA, OpenACC, Standard Langages (C++, Fortran)'''

''F. Courteille (NVIDIA)''

==== Plénière 5 ====

Mercredi 24/03/2021 13h00-14h00

'''Le portage applicatif sur GPU de AVBP et Yales 2: Concrêtement comment cela se matérialise?'''

''G. Staffelbach (CERFACS) & V. Moureau (CORIA)''

==== Plénière 6 ====

Jeudi 25/03/2021 9h00-10h00

'''Approche et démarche pour accompagner le portage d'un code sur GPU NVIDIA'''

''P.-E. Bernard (HPE)''

==== Plénière 7 ====

Vendredi 26/03/2021 9h00-10h00

'''Roadmaps YALES2 & AVBP'''

''V. Moureau (CORIA) & N. Odier (CERFACS)''

==== Plénière 8 ====

Vendredi 26/03/2021 15h00-17h00

'''Wrap-up : présentation des résultats et conclusion générale'''

''Responsables de projets + V. Moureau (CORIA)''

== Thematics / Mini-workshops ==

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

=== Combustion - B. Cuenot, CERFACS ===
* H2 and alternative fuels combustion
* turbulent combustion modeling

=== Static and dynamic mesh adaptation - G. Balarac, LEGI ===

''Participants: G. Balarac and M. Bernard (LEGI), Y. Dubief (Vermont U.), U. Vigny and L. Bricteux (Mons U.), A. Grenouilloux, S. Meynet and P. Bernard (CORIA), R. Mercier and J. Leparoux (Safran Tech), P. Mohanamuraly, G. Staffelbach and N. Odier (CERFACS))''

Mesh adaptation is now an essential procedure to be able toi perform numerical simulations in complex geometries. The aim of mesh adaptation is to be able to define an "objective" mesh allowing the best compromise between accuracy and computational cost, with a reproducibility property, i.e. independent of the user. This project gathered thus six sub-projects related to static and dynamic mesh adaptation, with the main objectives to improve mesh adaptation capabilities of codes (sub-projects 1 and 2), to allow automatic mesh convergence (sub-projects 3 and 4), and to perform dynamic mesh adaptation for specific cases (sub-projects 5 and 6).

* '''Sub-project 1: Coupling TreeAdapt / AVBP (P. Mohanamuraly, G. Staffelbach)'''
The main objective of this sub-project was to couple the TreeAdapt library with the AVBP code. TreeAdapt is a library based on the partitioning library TreePart. This allows a hierarchical topology-aware massively parallel, online interface for unstructured mesh adaption. During the workshop the one-way coupling with AVBP has been performed with success and the two-way coupling has been started.

* '''Sub-project 2: New features in YALES2 (A. Grenouilloux, S. Meynet, M. Bernard, R. Mercier):'''
The main objectives of this sub-project was to develop in YALES2 (i) anisotropic mesh adaptation and (ii) a new partitioning algorithm for a more performant mesh adaptation procedure. To allow anisotropic mesh adaptation a new metric definition based on a tensor at cells has been proposed. The new partitioning has been developed to create halos around bad quality cells and to ensure contiguity.

* '''Sub-project 3: Criteria based on statistical quantities for static mesh adaptation in LES (G. Balarac, N. Odier, A. Grenouilloux):'''
The main objective of this sub-project was to develop a strategy for automatic mesh convergence based on statistical quantities. The proposed strategy is independent of the flwo case and of the user. It is defined to guarantee that the energy balance of the overall system is independent of the mesh. This strategy combine criteria already proposed by Benard et al. (2015) and Daviller et al. (2017).

* '''Sub-project 4: Automated Mesh Convergence plugin re-integration (R. Mercier, J. Leparoux, A. Grenouilloux):'''
The main objective of this sub-project was to integrate the Automated Mesh Convergence (AMC) plugin developed by Safran Tech in YALES2 distribution. This was done with success during the workshop. Moreover, additional criteria were integrated. In particular, the y_plus criterion from Duprat law (A. Grenouilloux PhD) was considered to be able to control cells size in boundary layers.

* '''Sub-project 5: Dynamic mesh adaptation for DNS/LES of isolated vortices (L. Bricteux, G. Balarac):'''
The main objective of this sub-project was to develop dynamic mesh adaptation strategy for simulation of isolated vortices, and to compare with DNS on static mesh, or with vortex methods. A well docuimented test case of a 2D vortice has been considered. Criteria based on the Palinstrophy have been proposed with success, allowing to perform simulation with dynamic mesh adaptation having the same accuracy as reference methods.

* '''Sub-project 6: Dynamic mesh adaptation for non-statistically stationary turbulence (U. Vigny, L. Bricteux, Y. Dubief, P. Benard):'''
The main objective of this sub-project was to test dynamic mesh adaptation strategies for flow configurations where statistical quantities are unavailable (conversely to SP3), and where various vortices on a broad range of scales exist (conversely to SP5). Various quantities based on velocity gradient, Q criterion, or passive scalar have been tested. But no unified strategy has been proposed yet. A procedure has been initiated based on a multiobjective genetic algorithm (GA) to identify the optimum dynamic mesh adaptation parameters to minimize computational cost and maximize solution quality.

=== Multi-phase flows - M. Cailler, SAFRAN TECH and V. Moureau, CORIA ===
''Participants: G. Ghigliotti, G. Sahut, S. Pertant (LEGI), Y. Dubief (Vermont U.), S. Mendez (IMAG), R. Mercier, M. Cailler, J. Leparoux (Safran Tech), F. Pecquery, C. Merlin (ARIANE GROUP), V. Moureau, R. Janodet, I. Tsetoglou, P. Benez, Y. Atmani (CORIA)''

The modeling of two-phase flows has always been a tedious task because of the differences in thermo-physical properties between the fluids. While two-phase flow numerics based on interface capturing methods have reached maturity for simple thermodynamics, the focus in this field is now on how to deal with multi-physics. Most of the sub-projects of this event have addressed this need.

* '''Sub-project 1: Thermodynamics for two-phase flows (Y. Atmani, F. Pecquery, M. Cailler, C. Merlin, G. Sahut, S. Pertant, V. Moureau)'''
The main objective of this sub-project was to continue the development in YALES2 of the conservative transport of scalars in two-phase flows using a two-fluid approach. To this aim, new data structures for the "discontinuous scalars" have been derived in order to include various equations of state. The transport of the discontinuous scalars has also been augmented with dilatation. The calculation of surface tension has also been coupled to the scalars in order to start the modeling of Marangoni effects.

* '''Sub-project 2: Contact angle/triple line (S. Pertant, G. Sahut, G. Ghigliotti, C. Merlin, V. Moureau)'''
In this sub-project, the boiling solver of YALES2 has been coupled to the contact angle model of Wang & Desjardins 2018 based on the accurate conservative levelset framework. The discontinuous scalar transport has also been added to the boiling solver. With these new features, the solver has been used to perform the first simulation of nucleate boiling with dynamic mesh adaptation. The merging of the contact angle model into master has also progressed during the event.

* '''Sub-project 3: Heat-flux modeling for two-fluid conservative method (Y. Atmani, R. Janodet, M. Cailler, V. Moureau)'''
This sub-project aimed at improving the heat flux model used at the interface in the two-fluid scalar transport framework in YALES2. The work consisted in evaluating the heat flux at the interface instead in the volume. The interface is here materialized by the intersection of the level set iso-surface with the edges of the mesh. The flux is thus evaluated at this intersection and then extended in the volume where it is used to compute the various terms in the transport and reinitialization equations. These developments have been tested successfully for the transport of a 2D water droplet in hot air.

* '''Sub-project 4: Two-phase flows with polymers (Y. Dubief, S. Mendez, V. Moureau)'''
In this sub-project, the FENE-P model, which as been merged into the master branch of YALES2, has been revisited. While the stiff integration of the non-linear spring, which represents the polymer dynamics, is very efficient and accurate at maximum stretch, the Gibbs phenomenon occurs at zero-stretch and leads to negative values of the trace of the conformation tensor. A new form of the non-linear spring has been derived and tested which prevents the conformation tensor trace to become negative. This new model has been implemented and tested successfully for the flow behind a 2D cylinder.

=== Numerics - G. Lartigue, CORIA ===
''Participants: Ghislain LARTIGUE and Vincent MOUREAU (CORIA), Manuel BERNARD and Guillaume BALARAC (LEGI), Nicolas ODIER and Benjamin MARTIN (CERFACS)''

This project gathered four sub-projects related to Numerical Methods. Most of these activities are related to the use of high-order schemes presented in [1] in the context of Finite-Volumes Method.

* '''Sub-project 1 (N. Odier, B. Martin, G. Lartigue):''' The main objective of this sub-project was to implement a '''High-Order Finite-Volume method in the Cell-Vertex compressible code AVBP'''.

During the workshop, we focused on the improvement of the convective flux computation. Using the high-order reconstruction of [1], we were able to express the conservative variables as high-order polynomials within each nodal volume. In the case of the Euler equations, a specific scheme is required to compute the correct numerical flux crossing each edge. At the edge, the polynomials obtained for the two neighbouring nodes are not continuous and it hence corresponds to a Riemann problem. Two implementations of the numerical fluxes have been tested: a Roe solver and a Lax-Wendroff scheme. Both schemes achieve third order accuracy on regular and distorted triangular grids. The Roe scheme has also been tested on hybrid triangular/quadrilateral grids, with a resulting second order accuracy in accordance with the high-order reconstruction theory of [1].

* '''Sub-project 2 (M. Bernard, G. Balarac, G. Lartigue):''' The main objective of this sub-project was to work on the use of '''High-Order Finite-Volume method to solve the Poisson Equation in the incompressible code YALES2'''.

In the context of projection method, a special attention needs to be paid to the accuracy of the coupling between pressure and velocity fields.
To achieve this goal, the keystone is to be able to solve efficiently the Poisson problem for the pressure.
During the workshop, we focused on resolution of a generic Poisson problem by use of conjugated gradient algorithm (CJ).
Idea was to use, at each iteration of the CG, the high-order Laplacian operator recently developed on the basis of high-order schemes [1].
This high-order Laplacian operator shows a better accuracy than the classical one used in YALES2 (SIMPLEX [3])
However, its usage during conjugated gradient algorithm does not improve the accuracy of the solution of the Poisson problem.
Further investigations are ongoing to evaluate the potential improvement on the correction of the velocity field with the pressure arising from the inversion of the high-order Laplacian operator.

* '''Sub-project 3 (G. Sahut, G. Balarac, G. Lartigue):''' The main objective of this sub-project was to implement a '''URANS method with a semi-implicit solver in YALES2'''.

G. Sahut: merci de compléter ta partie stp

* '''Sub-project 4 (G. Lartigue, V. Moureau):''' The main objective of this sub-project was to '''improve the precision and robustness of the Laplacian Operator in YALES2'''.

There is two class of operators in YALES2: '''ROBUST''' (a.k.a. PAIR_BASED and IGNORE_SKEWNESS) and '''PRECISE''' (a.k.a. SIMPLEX). It has been shown that for operators with constant coefficients (as in ICS and VDS solvers), the '''PRECISE''' approach is unconditionally stable and must be used in all situations. However, in the SPS solver, the density variations across a pair of vertex can lead to a non-PSD operator. A major achievement of the workshop was to propose an hybrid operator that mixes both operators to achieve both precision and robustness. This operator will be implemented in a near future.

* '''Discussion (All):''' A two-hours discussion on Tuesday afternoon have been dedicated to the analysis of the paper [2]. This paper deals with an optimal way of mixing a robust low-order numerical scheme with high-order scheme. The major interest of this mixing technique is that it preserves the boundedness of the solution with a so-called convex-limiting. This is similar to WENO techniques but it relies on the resolution of the interface Riemann

[1] Manuel Bernard, Ghislain Lartigue, Guillaume Balarac, Vincent Moureau, Guillaume Puigt. '''A framework to perform high-order deconvolution for finite-volume method on simplicial meshes'''. ''International Journal for Numerical Methods in Fluids'', Wiley, 2020, 92 (11), pp.1551-1583. [https://onlinelibrary.wiley.com/doi/10.1002/fld.4839] [https://hal.archives-ouvertes.fr/hal-02558814v2]

[2] Jean-Luc Guermond, Bojan Popov, Ignacio Tomas. '''Invariant domain preserving discretization-independent schemes and convex limiting for hyperbolic systems'''. ''Comput. Methods Appl. Mech. Engrg''. 347 (2019) 143–175. [https://www.math.tamu.edu/~guermond/PUBLICATIONS/guermond_popov_tomas_CMAME_2019.pdf]

[3] Ruben Specogna, Francesco Trevisan. '''A discrete geometric approach to solving time independent Schrödinger equation'''. '''Journal of Computational Physics''' 2011, 1370-1381. [https://www.sciencedirect.com/science/article/pii/S0021999110006091]

=== Turbulent flows - P. Bénard, CORIA ===
* turbulence injection
* wall modeling
* rotor modeling for wind or hydro turbines applications
* advanced post processing for unsteady turbulence

=== User experience - R. Mercier, SAFRAN TECH ===
* automation & workflows for HPC
* on-line and off-line analysis of massive datasets

=== Fluid structure interaction - S. Mendez, IMAG ===
''Participants: Thomas Fabbri and Guillaume Balarac, LEGI, Barthélémy Thibaud and Simon Mendez, IMAG, Likhitha Ramesh Reddy and Axelle Viré, TU Delft and Pierre Bénard, CORIA''

This project gathered three sub-projects related to fluid-structure interactions (FSI). Their common feature was the FSI solver from YALES2, which is based on a partitioned approach. The FSI solver couples an Arbitrary Lagrangian-Eulerian solver for predicting the fluid motion in a moving domain (FSI_ALE) and a solver for structural dynamics (FSI_SMS), which are both YALES2 solvers. The FSI solver has been initiated by Thomas Fabbri (LEGI, Grenoble) and the objectives of ECFD4 were to optimize it and generalize its use among several teams, by improving its performances, demonstrating its versatility and adding multiphysics effects. All the projects made interesting progree and will continue over the newt weeks/months.
* '''Sub-project 1 (Thomas Fabbri and Guillaume Balarac, LEGI):''' The aim of this sub-project was '''to decrease the time spent in computing the fluid grid deformation''', which is currently the most expensive part of the calculation. The strategy is to solve a deformation field on a coarse mesh and apply it to a fine mesh after interpolation. Many pieces exist in YALES2 related to such a task (using several grids, performing interpolations...), but they are currently not appropriate for this application. The work performed during the workshop consisted in identifying the different subroutines of interest and start coding the method. Many parts of the method are functional and the next step is to properly compbine them and test its efficiency.
* '''Sub-project 2 (Barthélémy Thibaud and Simon Mendez, IMAG):''' The aim of this sub-project was '''to validate the FSI solver in the case of a flexible valve''' bent by a pulsatile flow. A proper workflow (sequence of runs) has been defined during the week to be able to run this simulation and the first results are extremely promising, with already fair comparisons with the reference results from the literature. This workshop has also contributed in enhacing the experience of the solver at IMAG.
* '''Sub-project 3 (Likhitha Ramesh Reddy and Axelle Viré, TU Delft and Pierre Bénard, CORIA):''' The long-term aim of this sub-project is to perform simulations of the flow around floating wind turbines, which constitutes a huge challenge, as it gathers the difficulties of wind tubines flows, two-phase flows, and fluid-structure interactions between a fluid and a solid. During the workshop, the aim was '''to progress on two aspects: the use of the two-phase flow solver of YALES2, SPS, in a moving domain (coupling SPS and ALE) and the coupling with FSI'''. Both tasks were tackled: preliminary validation simulations were performed for the SPS-ALE solver, and the strategy to couple the SPS-ALE solver with the FSI has been clearly identified within the group.
* '''Common work:''' TU Delft (Sub-project 3) needs to perform FSI without deformation of the structure, so that the coupling with the SMS solver may not be indispensable. Tests were performed to study the ability of the SMS to work in a regime of very stiff material to mimic rigid bodies, and first tests were very convincing. In the future however, it is planned to implement a rigid-body motion solver in YALES2 as an alternative to SMS. This task gathers the four teams of the project and is a clear shared objective of the next months.
* '''Bugs and cleaning:''' minor bugs were identified in the FSI solver, mostly related to options rarely used. There were corrected and pushed in the YALES2 gitlab.
* '''Documentation:''' the information shared between participants for the use and understanding of the SMS and FSI solvers has been directly gathered in the YALES2 wiki.

=== GENCI Hackathon - G. Staffelbach, CERFACS ===

Participants : V. Moureau (CORIA), P. Bégou (LEGI), J. Legaux, G. Staffelbach (CERFACS), L. Stuber, F. Courteille (NVIDIA), T. Braconnier, P.E Bernard (HPE).

GPU acceleration is the keystone towards exascale computing as evidenced by the top500 where two thirds of the top50 systems are now accelerated. Within this workshop the objective was to reevaluate the performance of both AVBP and YALES2 following their initial port under a contrat de progrés between GENCI and HPE with the support of IDRIS conducted in 2019. Then update as much as possible the codes to todays versions, assess new porting and optimisation possibilities and carry them out when possible.

'''YALES2'''
The YALES2 solver has evolved immensely since the 2019 port and most of the time was spent merging and updated the code to todays standards. Two updated branches with the current source code have been released idris/openacc_node2pair et idris/openacc_pair2node and profiling and optimisation tools have been tested on CORIA and LEGI platforms.
In parallel, using the CVODE GPU-enabled library to accelerate the chemistry solver in YALES2 was investigated. This proved more complex than anticipated as the library did not build as is with the latest release of the NVIDIA SDK. This issue was promptly solved with the help of NVIDIA. Coupling YALES2 with the accelerated library seems to require more extensive knowledge in OpenACC and CUDA, the team is highly motivated to pursue this train of though and will probably participate to the IDRIS hackathon initiative in May 2020 to continue this effort.

'''AVBP'''
Efforts to port AVBP to GPU have continued through an second grand challenge on the JEANZAY system targeting the port of a complex industrial type combustion chamber (DGENCC). In preparation for this workshop, the new models required for the DGENCC simulation were ported to GPU and performance analysis was undertaken. A new branch WIP/GC_JZ2 is currently available allowing for the accelerated simulation of this type of workflow.
Under the guidance of NVIDIA and HPE, optimisation venues have been identified:
* removal of extended temporary arrays.
* remplacement of implicit vector assignements.
* Collapsable compute driven loops.

Integrating this efforts in some of the kernels has yieled a 4.2 acceleration between a full cpu compute node with 40 cascade lake cores and a the accelerated counter part using 4 NVIDIA V100 GPUs. Further more the case has been strong scaling tested up to 1024 gpus with excellent performance.

Ecfd:ecfd 4th edition

2021-03-30T08:06:03Z

Lartigue: /* Numerics - G. Lartigue, CORIA */

Ecfd:ecfd 4th edition

2021-03-29T06:04:56Z

Lartigue: /* Numerics - G. Lartigue, CORIA */

Ecfd:ecfd 4th edition

2021-03-29T06:04:31Z

Lartigue: /* Numerics - G. Lartigue, CORIA */

Ecfd:ecfd 4th edition

2021-03-29T06:01:25Z

Lartigue: /* Numerics - G. Lartigue, CORIA */

Ecfd:ecfd 4th edition

2021-03-29T06:01:00Z

Lartigue: /* Numerics - G. Lartigue, CORIA */

Ecfd:ecfd 4th edition

2021-03-29T06:00:30Z

Lartigue: /* Numerics - G. Lartigue, CORIA */

Ecfd:ecfd 3rd edition

2020-01-31T10:20:28Z

Lartigue: /* Project #4: Conservative Heat Transfers in the the Accurate Conservative Level-Set framework */

ECFD workshop, 3rd edition, 2020

== Sponsors ==

[[File:ecfd3_sponsors.png|center|frameless|800px]]

== Participants ==

[[File:ecfd3_participants.png|center|frameless|1000px]]

== Flyer ==

* [[media:ecfd3_flyer.pdf | Flyer]]

== Presentations ==

* [[media:ecfd3_intro.pdf | Introduction workshop]]
* [[media:ecfd3_intro_genci.pdf | Introduction GENCI]]
* [[media:ecfd3_avbp_roadmap_HPC.pdf | Roadmap AVBP (HPC)]]
* [[media:ecfd3_yales2_roadmap.pdf | Roadmap YALES2]]

== Booklet ==

* [[media:ecfd3_booklet_template.zip | Template]]

== Project achievements ==

=== Project #1: Hackathon GENCI/ATOS/AMD/CERFACS on AVBP ===

''C. Piechurski (GENCI), S. Jauré (ATOS), B. Pajot (ATOS), P.-A. Harraud (AMD), P. Mohanamuraly (CERFACS), G. Staffelbach (CERFACS), J. Legaux (CERFACS)''

We ported the AVBP solver to the AMD Rome system available at GENCI -TGCC ( IRENE Joliot Curie).
Characterisation of the application on the architecture showed a 1/3 performance dependency to bandwidth and 2/3 to compute.
Strong scaling performance up to 130k cores was measured with openmpi and provided an acceleration of 75% without optimisations.
Weak scaling up to 32k MPI ranks suggests that decimation of the processes by a factor 2 improves computational efficiency by up to 30%.
This suggests a trade off between mpi imbalance and decimation is possible if imbalance is higher than 30% to improve time to solution.

Currently Openmpi offers the best perfofrmance, intelmpi is still a bit unstable.

During the Hackathon we also introduced colour based cache blocking using ColPack in the code in order to use OpenMP without critical sections.
On a 2x18 core Skylake processor the new implementation offered similar speedup using full threading versus full MPI with the best trade off being 4 MPI and 9 threads per MPI.
On AMD Rome, Full threading did not offer much acceleration and needs to be inversigated but 8 MPI and 16 threads per MPI seem quite promising.

[[media:ecfd3_final_project1.pdf | Final presentation of project #1]]

=== Project #2: Hackathon GENCI/ATOS/AMD/CORIA on YALES2 ===
''C. Piechurski (GENCI), S. Jauré (ATOS), P.-A. Harraud (AMD), P. Mohanamuraly (CERFACS), G.Lartigue (CORIA), F. Gava (CORIA), P. Begou (LEGI)''

[[media:ecfd3_final_project2.pdf | Final presentation of project #2]]

=== Project #3: Implementation of a secondary atomization model in YALES2 ===

''C. G. Guillamon (Safran Tech), L .Voivenel (Safran Tech), R. Mercier (Safran Tech)''

In Lagrangian simulations, droplets are transported following a ballistic motion in an eulerian mesh. For non-reactive environments, droplets might undergo secondary atomization due to the aerodynamic interaction. In this work, we implement in YALES2 a breakup model known as Taylor-Analogy Breakup (TAB). This model is based on the analogy between a droplet and a second-order mechanical system, hence making possible to determine the breakup behaviour by means of Newton's second law.

Another model, the stochastic breakup model by Gorokhovski, is also suggested for future work and will be implemented in YALES2.

[[media:ecfd3_final_project3.pdf | Final presentation of project #3]]

=== Project #4: Conservative Heat Transfers in the the Accurate Conservative Level-Set framework ===

[[media:ecfd3_final_project4.pdf | Final presentation of project #4]]

Authors: François Pecquery, Mélody Cailler, Romain Janodet and Vincent Moureau

Objectives of the project was to introduce conservative heat transfers in the Accurate Conservative Level-Set framework to be able to describe heat transfers and liquid dynamics in an accurate, robust and conservative manner. A Multi-Phase Transport solver is introduced relying on the conserving and level-set coherent transport of the temperature. The solution is to use the fluxes of a phase indicator that may be sharp, contrarily to the level-set. The new solver was used on a simplified test case where a liquid droplet is transported in a temperature stratified environment. Results show promising capabilities of the new framework. Next work include improvement of the transport equation stability, and of the jump condition at the interface.

=== Project #5: Jet-in-crossflow par une méthode d’interface diffuse ===

In the context of fuel injection in an aircraft engine, liquid fuel is injected through a swirler, and sheared by a high-speed oxyder which destabilizes the liquid interface. This interaction induces liquid ligaments, which break up into large droplets (primary atomization), and then themselves break into small droplets (secondary atomization)
This project deals with the implementation of a diffuse-interface method in the massively parallel solver AVBP to represent the liquid interface destabilization during primary atomization for compressible applications. This methodology is found to be very efficient, however a control of the interface diffusion is mandatory as soon as convective effects are added. During this workshop, the methodology proposed by Chiodi and Desjardins ( A reformulation of the conservative level set reinitialization equation for accurate and robust simulation of complex multiphase flows, JCP 2017) to control the interface thickness has been implemented in AVBP, and is currently under validation on a periodic liquid jet with surface tension effects.

[[media:ecfd3_final_project5.pdf | Final presentation of project #5]]

=== Project #6: Accurate numerical predicti􏴇on of vorti􏴇cal flows using AMR ===

[[media:ecfd3_final_project6.pdf | Final presentation of project #6]]
We try to demonstrate that Eulerian method YALES2 using AMR can do a very good job to capture complex vortical flows at moderate Re=10k
Here we use an AMR strategy based on vorticity. We investigate the problem of vortex ring collision. We have a gain of 1000 on the numbers of elements compared
to a non adaptative approach. We are able to capture the transition from a very simple laminar flow to a complex turbulent flow.

=== Project #7: Modélisation de parois pour la simulation des grandes échelles ===

[[media:ecfd3_final_project7.pdf | Final presentation of project #7]]

=== Project #8: Accurate numerical simulation of contact lines with dynamic mesh adaptation ===
''S. Pertant (LEGI), G. Ghigliotti (LEGI), G. Balarac (LEGI)''

The main objective of this project was to develop a methodology to simulate contact lines on unstructured meshes. We especially wanted to get rid of mesh influence on contact line movement when the flow is driven by surface tension and the contact line close to its equilibrium position. A slight modification in the Ghost Fluid Method to apply the pressure jump has been tested and seems promising. The pressure gradient at contact line is indeed less sensitive to mesh elements for high density ratios. Furthermore, dynamic mesh adaptation has been used to simulate a 2D vapour bubble lying on a wall. Due to gravity, the two contact lines are receding until their merging and the bubble departure. The mesh remains fine to capture the contact line dynamics. As a future work, we plan to perform mesh adaptation on 3D contact line cases and to include additional physics such as contact angle imposition (already implemented but not used yet with mesh adaptation).

[[media:ecfd3_final_project8.pdf | Final presentation of project #8]]

=== Project #9: Remeshed particle method at high Schmidt and Reynolds number ===

''S. Santoso (LJK), J.-B. Lagaert (Math Orsay), G.Balarac (LEGI)''

We study the advection of a scalar function in turbulent flows with a multimesh method. The finite volume method is used to solve Navier-Stokes equations on an unstructured mesh (YALES2). The advection equation is solved with remeshed particle method on a cartesian mesh. In the context of parallel computing, we face a very unbalanced problem since a large number of particles are created in a very fine meshed zone. Our strategy to load-balance the problem is to give a weight to every element group which is equal to the density of particle.

[[media:ecfd3_final_project9.pdf | Final presentation of project #9]]

=== Project #10: Adaptive mesh refinement for turbulent premixed combustion ===
''W. Agostinelli, O. Dounia, , T. Jaravel, O. Vermorel

The objective of the project was to evaluate the potential of adaptive mesh refinement (AMR) for premixed combustion in unsteady systems. Three target cases were identified: a semi-vented deflagration with laminar to turbulent transition, a planar detonation wave, and a bluff-body stabilized burner subjected to thermoacoustic oscillations. The simulations were performed with AVBP and coupled to the AMR implementation of YALES2. Several metrics and remeshing criterions were developed to identify and correctly resolve both the combustion wave front and the turbulent flow. The comparison of numerical results with reference simulations showed that the main features of the physics could be recovered with a significant speed-up in term of computational cost.

[[media:ecfd3_final_project10.pdf | Final presentation of project #10]]

=== Project #11: Multiphysics coupling for wind turbine wake modeling ===

''F.Houtin-Mongrolle (CORIA), B. Duboc (SGRE), P. Benard (CORIA)''

The goal of this project was to evaluate the coupling of YALES2 (flow solver) and BHawC(Aero-Servo-Elastic solver).

[[media:ecfd3_final_project11.pdf | Final presentation of project #11]]

=== Project #12: Stability of a semi-implicit compressible cavitation solver ===
The compressible cavitation solver is used to simulate cavitation inception in an initially liquid flow behind an obstacle.
This solver is based on the implicit compressible solver, that has been modified to include a « barotropic » pressure-density relationship playing the role of an equation of state independent from the temperature.
While this strategy has proven to be effective for DNS simulations of the implosion of vapour bubbles, the simulation of cavitation inception in an initially liquid flow was leading to strong instabilities in the simulation shortly after the appearence of vapour.
The test case chosen is a flow behind a 2D cylinder.
The analysis of the results has shown that instabilities were correlated with very low (and even unphysically negative) values of the pressure, that were triggering negative density values leading to code instability.
Using limiters to ensure a positive pressure and a density within the range of the equation of state improved the stability and allowed to perform a preliminary simulation of a cavitating flow behind an obstacle.
Ultimately instabilities appear anyways, so that the spatial discretisation is now under study, notably through upwinding.

[[media:ecfd3_final_project12.pdf | Final presentation of project #12]]

=== Project #13: Validations and comparisons of Diffuse / Sharp interface methods in a structured DNS solver (Titan) ===
''V. Boniou (EM2C), J.M. Dupays (EM2C), M. Pelletier (EM2C), T. Schmitt (EM2C), A. Vié (EM2C)

The project aimed at using academic test cases to compare the sharp (incompressible) and diffuse (compressible) models. In particular, the test case of an inviscid initially elliptical oscillating droplet has been carried out.
The solvers features are the following:

- incompressible VOF solver (sharp): Numerical Method: Projection Method, Interface reconstruction: VOF, Surface tension: CSF

- compressible multifluid solver (diffuse): Advection scheme: MUSCL + RK2 + minmod limiter, Surface tension: CSF.

The source term is integrated with operator-splitting, and the curvature computation relies on a 2nd-order differentiation of the liquid volume fraction, which is previously smooth by filtering.
This test case showed good agreement on the oscillation period, while exhibiting a slight numerical diffusion in the incompressible case and a strong numerical diffusion in the compressible case.
In the compressible case, the use of higher-order splitting (Strang [SIAM Num. An. 1968]) has been tested, yielding no noticeable improvement. Reduction of the number of filtering iterations on the liquid volume fraction provides a slight improvement, which may indicate that a better curvature computation could participate to reduce the numerical diffusion.

[[media:ecfd3_final_project13.pdf | Final presentation of project #13]]

=== Project #14: Méthode d'ordre élevé ===
''M. Bernard (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI), V. Moureau (CORIA)''

[[media:ecfd3_final_project14.pdf | Final presentation of project #14]]

=== Project #15: Utilisation d’éléments finis du second ordre dans le SMS ===
''T. Fabbri (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI), V. Moureau (CORIA)''

[[media:ecfd3_final_project15.pdf | Final presentation of project #15]]

=== Project #16: Development of a RANS solver in YALES2 ===
''G. Sahut (LEGI), G. Balarac (LEGI), V. Moureau (CORIA), G. Lartigue (CORIA), P. Bénard (CORIA), A. Grenouilloux (CORIA)''

While the accuracy of LES usually approaches the one of DNS, LES are still too time-consuming for daily use in industrial applications. In this context, we started the development of a RANS solver in YALES2. We are first only interested in the steady state of the solution. In order to remove the CFL constraint, we developed, implemented and validated an implicit projection method for the resolution of the Navier-Stokes equations without turbulence models. The method is based on the implicitation of the velocity predictor ; the Poisson equation and the correction step of the velocity are then solved and applied as in the explicit incompressible solver. We validated the method on a stationary 2D Poiseuille flow with periodic boundary conditions: the simulation runs fine for CFL and Fourier numbers which are inaccessible with the explicit incompressible solver. The advection-diffusion equation for scalars has also been implicited and will be used to add turbulence models to the new implicit incompressible solver developped during this Workshop. More complex boundary conditions will also be addressed in a near future.

[[media:ecfd3_final_project16.pdf | Final presentation of project #16]]

=== Project #17: COUPLING OF A FLUID PLASMA SOLVER WITH A LAGRANGIAN SOLVER FOR THE MODELING OF DUSTY ===

[[media:ecfd3_final_project17.pdf | Final presentation of project #17]]

=== Project #18: L’Evaporo O Maıtre ===

[[media:ecfd3_final_project18.pdf | Final presentation of project #18]]

=== Project #19: The Clone Wars ===
''H. Maldonado Colman (EM2C), C. Nguyen Van (EM2C - Safran-Tech), R. Mercier (Safran-Tech), B. Fiorina (EM2C)''

The aim of this project was to increase the computation performance using virtual chemistry approach in the YALES2 solver.

In order to reach this goal three test cases where identified:

- 1D laminar premixed flame (methane / air combustion with carbon monoxide prediction)

- 2D laminar premixed bunsen flame (methane / air combustion with carbon monoxide prediction)

- 3D two phase and turbulent flame (nheptane / air combustion with nitrogen monoxide prediction)

Several ways were explored:

- Profiling of reactive simulations when using Virtual Chemistry

- Effect of redundant species transport

- Effect of the size and the numbers of jacobian matrix to compute and solve

- Effect of the correction functions smoothing

Conclusions of the study are:

- String trimming and concatenation heavily affect computing performances

- Redundant species transport and source terms computations has a minor impact on performances

[[media:ecfd3_final_project19.pdf | Final presentation of project #19]]

=== Project #20: Stiff complex fluid simulation with YALES2 ===
''Sam Whitmore, Yves Dubief, M2CE, University of Vermont''

The objective was to simulate (1) ionized gases and (2) polymer solutions in flows using YALES2. Both problems are challenging owing to their stiff thermodynamics (1) or polymer dynamics (2). Significant gains were achieved in the implementation of the respective models thanks to the stiff integrator library CVODE. The plasma flow demonstrated an increase in time step of two orders of magnitude compared to previous implementation of the plasma chemistry in the variable density solver. Polymer models are notoriously prone to numerical instability. Again the use of CVODE showed equivalent if not superior stability of the solution at a fraction of the cost of commonly employed algorithms designed to address the stiffness of the problem.

[[media:ecfd3_final_project20.pdf | Final presentation of project #20]]

=== Project #21: AVBP Dense Gases ===

[[media:ecfd3_final_project21.pdf | Final presentation of project #21]]

=== Project #22: Numerical prediction of wind turbine wakes using AMR ===

[[media:ecfd3_final_project22.pdf | Final presentation of project #22]]

Ecfd:ecfd 3rd edition

2020-01-31T10:19:05Z

Lartigue: /* Project #4: Conservative Heat Transfers in the the Accurate Conservative Level-Set framework */

ECFD workshop, 3rd edition, 2020

== Sponsors ==

[[File:ecfd3_sponsors.png|center|frameless|800px]]

== Participants ==

[[File:ecfd3_participants.png|center|frameless|1000px]]

== Flyer ==

* [[media:ecfd3_flyer.pdf | Flyer]]

== Presentations ==

* [[media:ecfd3_intro.pdf | Introduction workshop]]
* [[media:ecfd3_intro_genci.pdf | Introduction GENCI]]
* [[media:ecfd3_avbp_roadmap_HPC.pdf | Roadmap AVBP (HPC)]]
* [[media:ecfd3_yales2_roadmap.pdf | Roadmap YALES2]]

== Booklet ==

* [[media:ecfd3_booklet_template.zip | Template]]

== Project achievements ==

=== Project #1: Hackathon GENCI/ATOS/AMD/CERFACS on AVBP ===

''C. Piechurski (GENCI), S. Jauré (ATOS), B. Pajot (ATOS), P.-A. Harraud (AMD), P. Mohanamuraly (CERFACS), G. Staffelbach (CERFACS), J. Legaux (CERFACS)''

We ported the AVBP solver to the AMD Rome system available at GENCI -TGCC ( IRENE Joliot Curie).
Characterisation of the application on the architecture showed a 1/3 performance dependency to bandwidth and 2/3 to compute.
Strong scaling performance up to 130k cores was measured with openmpi and provided an acceleration of 75% without optimisations.
Weak scaling up to 32k MPI ranks suggests that decimation of the processes by a factor 2 improves computational efficiency by up to 30%.
This suggests a trade off between mpi imbalance and decimation is possible if imbalance is higher than 30% to improve time to solution.

Currently Openmpi offers the best perfofrmance, intelmpi is still a bit unstable.

During the Hackathon we also introduced colour based cache blocking using ColPack in the code in order to use OpenMP without critical sections.
On a 2x18 core Skylake processor the new implementation offered similar speedup using full threading versus full MPI with the best trade off being 4 MPI and 9 threads per MPI.
On AMD Rome, Full threading did not offer much acceleration and needs to be inversigated but 8 MPI and 16 threads per MPI seem quite promising.

[[media:ecfd3_final_project1.pdf | Final presentation of project #1]]

=== Project #2: Hackathon GENCI/ATOS/AMD/CORIA on YALES2 ===
''C. Piechurski (GENCI), S. Jauré (ATOS), P.-A. Harraud (AMD), P. Mohanamuraly (CERFACS), G.Lartigue (CORIA), F. Gava (CORIA), P. Begou (LEGI)''

[[media:ecfd3_final_project2.pdf | Final presentation of project #2]]

=== Project #3: Implementation of a secondary atomization model in YALES2 ===

''C. G. Guillamon (Safran Tech), L .Voivenel (Safran Tech), R. Mercier (Safran Tech)''

In Lagrangian simulations, droplets are transported following a ballistic motion in an eulerian mesh. For non-reactive environments, droplets might undergo secondary atomization due to the aerodynamic interaction. In this work, we implement in YALES2 a breakup model known as Taylor-Analogy Breakup (TAB). This model is based on the analogy between a droplet and a second-order mechanical system, hence making possible to determine the breakup behaviour by means of Newton's second law.

Another model, the stochastic breakup model by Gorokhovski, is also suggested for future work and will be implemented in YALES2.

[[media:ecfd3_final_project3.pdf | Final presentation of project #3]]

=== Project #4: Conservative Heat Transfers in the the Accurate Conservative Level-Set framework ===

[[media:ecfd3_final_project4.pdf | Final presentation of project #4]]

Authors: François Pecquery, Mélody Cailler, Romain Janodet and Vincent Moureau

Objectives of the project was to introduce conservative heat transfers in the Accurate ConservativeLevel-Set framework to be able to describe heat transfers and liquid dynamics in an accurate, robust and conservative manner. A Multi-Phase Transport solver is introduced relying on the conserving and level-set coherent transport of the temperature. The solution is to use the fluxes of a phase indicator that may be sharp, contrarily to the level-set. The new solver was used on a simplified test case where a liquid droplet is transported in a temperature stratified environment. Results show promising capabilities of the new framework. Next work include improvement of the transport equation stability, and of the jump condition at the interface.

=== Project #5: Jet-in-crossflow par une méthode d’interface diffuse ===

In the context of fuel injection in an aircraft engine, liquid fuel is injected through a swirler, and sheared by a high-speed oxyder which destabilizes the liquid interface. This interaction induces liquid ligaments, which break up into large droplets (primary atomization), and then themselves break into small droplets (secondary atomization)
This project deals with the implementation of a diffuse-interface method in the massively parallel solver AVBP to represent the liquid interface destabilization during primary atomization for compressible applications. This methodology is found to be very efficient, however a control of the interface diffusion is mandatory as soon as convective effects are added. During this workshop, the methodology proposed by Chiodi and Desjardins ( A reformulation of the conservative level set reinitialization equation for accurate and robust simulation of complex multiphase flows, JCP 2017) to control the interface thickness has been implemented in AVBP, and is currently under validation on a periodic liquid jet with surface tension effects.

[[media:ecfd3_final_project5.pdf | Final presentation of project #5]]

=== Project #6: Accurate numerical predicti􏴇on of vorti􏴇cal flows using AMR ===

[[media:ecfd3_final_project6.pdf | Final presentation of project #6]]
We try to demonstrate that Eulerian method YALES2 using AMR can do a very good job to capture complex vortical flows at moderate Re=10k
Here we use an AMR strategy based on vorticity. We investigate the problem of vortex ring collision. We have a gain of 1000 on the numbers of elements compared
to a non adaptative approach. We are able to capture the transition from a very simple laminar flow to a complex turbulent flow.

=== Project #7: Modélisation de parois pour la simulation des grandes échelles ===

[[media:ecfd3_final_project7.pdf | Final presentation of project #7]]

=== Project #8: Accurate numerical simulation of contact lines with dynamic mesh adaptation ===
''S. Pertant (LEGI), G. Ghigliotti (LEGI), G. Balarac (LEGI)''

The main objective of this project was to develop a methodology to simulate contact lines on unstructured meshes. We especially wanted to get rid of mesh influence on contact line movement when the flow is driven by surface tension and the contact line close to its equilibrium position. A slight modification in the Ghost Fluid Method to apply the pressure jump has been tested and seems promising. The pressure gradient at contact line is indeed less sensitive to mesh elements for high density ratios. Furthermore, dynamic mesh adaptation has been used to simulate a 2D vapour bubble lying on a wall. Due to gravity, the two contact lines are receding until their merging and the bubble departure. The mesh remains fine to capture the contact line dynamics. As a future work, we plan to perform mesh adaptation on 3D contact line cases and to include additional physics such as contact angle imposition (already implemented but not used yet with mesh adaptation).

[[media:ecfd3_final_project8.pdf | Final presentation of project #8]]

=== Project #9: Remeshed particle method at high Schmidt and Reynolds number ===

''S. Santoso (LJK), J.-B. Lagaert (Math Orsay), G.Balarac (LEGI)''

We study the advection of a scalar function in turbulent flows with a multimesh method. The finite volume method is used to solve Navier-Stokes equations on an unstructured mesh (YALES2). The advection equation is solved with remeshed particle method on a cartesian mesh. In the context of parallel computing, we face a very unbalanced problem since a large number of particles are created in a very fine meshed zone. Our strategy to load-balance the problem is to give a weight to every element group which is equal to the density of particle.

[[media:ecfd3_final_project9.pdf | Final presentation of project #9]]

=== Project #10: Adaptive mesh refinement for turbulent premixed combustion ===
''W. Agostinelli, O. Dounia, , T. Jaravel, O. Vermorel

The objective of the project was to evaluate the potential of adaptive mesh refinement (AMR) for premixed combustion in unsteady systems. Three target cases were identified: a semi-vented deflagration with laminar to turbulent transition, a planar detonation wave, and a bluff-body stabilized burner subjected to thermoacoustic oscillations. The simulations were performed with AVBP and coupled to the AMR implementation of YALES2. Several metrics and remeshing criterions were developed to identify and correctly resolve both the combustion wave front and the turbulent flow. The comparison of numerical results with reference simulations showed that the main features of the physics could be recovered with a significant speed-up in term of computational cost.

[[media:ecfd3_final_project10.pdf | Final presentation of project #10]]

=== Project #11: Multiphysics coupling for wind turbine wake modeling ===

''F.Houtin-Mongrolle (CORIA), B. Duboc (SGRE), P. Benard (CORIA)''

The goal of this project was to evaluate the coupling of YALES2 (flow solver) and BHawC(Aero-Servo-Elastic solver).

[[media:ecfd3_final_project11.pdf | Final presentation of project #11]]

=== Project #12: Stability of a semi-implicit compressible cavitation solver ===
The compressible cavitation solver is used to simulate cavitation inception in an initially liquid flow behind an obstacle.
This solver is based on the implicit compressible solver, that has been modified to include a « barotropic » pressure-density relationship playing the role of an equation of state independent from the temperature.
While this strategy has proven to be effective for DNS simulations of the implosion of vapour bubbles, the simulation of cavitation inception in an initially liquid flow was leading to strong instabilities in the simulation shortly after the appearence of vapour.
The test case chosen is a flow behind a 2D cylinder.
The analysis of the results has shown that instabilities were correlated with very low (and even unphysically negative) values of the pressure, that were triggering negative density values leading to code instability.
Using limiters to ensure a positive pressure and a density within the range of the equation of state improved the stability and allowed to perform a preliminary simulation of a cavitating flow behind an obstacle.
Ultimately instabilities appear anyways, so that the spatial discretisation is now under study, notably through upwinding.

[[media:ecfd3_final_project12.pdf | Final presentation of project #12]]

=== Project #13: Validations and comparisons of Diffuse / Sharp interface methods in a structured DNS solver (Titan) ===
''V. Boniou (EM2C), J.M. Dupays (EM2C), M. Pelletier (EM2C), T. Schmitt (EM2C), A. Vié (EM2C)

The project aimed at using academic test cases to compare the sharp (incompressible) and diffuse (compressible) models. In particular, the test case of an inviscid initially elliptical oscillating droplet has been carried out.
The solvers features are the following:

- incompressible VOF solver (sharp): Numerical Method: Projection Method, Interface reconstruction: VOF, Surface tension: CSF

- compressible multifluid solver (diffuse): Advection scheme: MUSCL + RK2 + minmod limiter, Surface tension: CSF.

The source term is integrated with operator-splitting, and the curvature computation relies on a 2nd-order differentiation of the liquid volume fraction, which is previously smooth by filtering.
This test case showed good agreement on the oscillation period, while exhibiting a slight numerical diffusion in the incompressible case and a strong numerical diffusion in the compressible case.
In the compressible case, the use of higher-order splitting (Strang [SIAM Num. An. 1968]) has been tested, yielding no noticeable improvement. Reduction of the number of filtering iterations on the liquid volume fraction provides a slight improvement, which may indicate that a better curvature computation could participate to reduce the numerical diffusion.

[[media:ecfd3_final_project13.pdf | Final presentation of project #13]]

=== Project #14: Méthode d'ordre élevé ===
''M. Bernard (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI), V. Moureau (CORIA)''

[[media:ecfd3_final_project14.pdf | Final presentation of project #14]]

=== Project #15: Utilisation d’éléments finis du second ordre dans le SMS ===
''T. Fabbri (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI), V. Moureau (CORIA)''

[[media:ecfd3_final_project15.pdf | Final presentation of project #15]]

=== Project #16: Development of a RANS solver in YALES2 ===
''G. Sahut (LEGI), G. Balarac (LEGI), V. Moureau (CORIA), G. Lartigue (CORIA), P. Bénard (CORIA), A. Grenouilloux (CORIA)''

While the accuracy of LES usually approaches the one of DNS, LES are still too time-consuming for daily use in industrial applications. In this context, we started the development of a RANS solver in YALES2. We are first only interested in the steady state of the solution. In order to remove the CFL constraint, we developed, implemented and validated an implicit projection method for the resolution of the Navier-Stokes equations without turbulence models. The method is based on the implicitation of the velocity predictor ; the Poisson equation and the correction step of the velocity are then solved and applied as in the explicit incompressible solver. We validated the method on a stationary 2D Poiseuille flow with periodic boundary conditions: the simulation runs fine for CFL and Fourier numbers which are inaccessible with the explicit incompressible solver. The advection-diffusion equation for scalars has also been implicited and will be used to add turbulence models to the new implicit incompressible solver developped during this Workshop. More complex boundary conditions will also be addressed in a near future.

[[media:ecfd3_final_project16.pdf | Final presentation of project #16]]

=== Project #17: COUPLING OF A FLUID PLASMA SOLVER WITH A LAGRANGIAN SOLVER FOR THE MODELING OF DUSTY ===

[[media:ecfd3_final_project17.pdf | Final presentation of project #17]]

=== Project #18: L’Evaporo O Maıtre ===

[[media:ecfd3_final_project18.pdf | Final presentation of project #18]]

=== Project #19: The Clone Wars ===
''H. Maldonado Colman (EM2C), C. Nguyen Van (EM2C - Safran-Tech), R. Mercier (Safran-Tech), B. Fiorina (EM2C)''

The aim of this project was to increase the computation performance using virtual chemistry approach in the YALES2 solver.

In order to reach this goal three test cases where identified:

- 1D laminar premixed flame (methane / air combustion with carbon monoxide prediction)

- 2D laminar premixed bunsen flame (methane / air combustion with carbon monoxide prediction)

- 3D two phase and turbulent flame (nheptane / air combustion with nitrogen monoxide prediction)

Several ways were explored:

- Profiling of reactive simulations when using Virtual Chemistry

- Effect of redundant species transport

- Effect of the size and the numbers of jacobian matrix to compute and solve

- Effect of the correction functions smoothing

Conclusions of the study are:

- String trimming and concatenation heavily affect computing performances

- Redundant species transport and source terms computations has a minor impact on performances

[[media:ecfd3_final_project19.pdf | Final presentation of project #19]]

=== Project #20: Stiff complex fluid simulation with YALES2 ===
''Sam Whitmore, Yves Dubief, M2CE, University of Vermont''

The objective was to simulate (1) ionized gases and (2) polymer solutions in flows using YALES2. Both problems are challenging owing to their stiff thermodynamics (1) or polymer dynamics (2). Significant gains were achieved in the implementation of the respective models thanks to the stiff integrator library CVODE. The plasma flow demonstrated an increase in time step of two orders of magnitude compared to previous implementation of the plasma chemistry in the variable density solver. Polymer models are notoriously prone to numerical instability. Again the use of CVODE showed equivalent if not superior stability of the solution at a fraction of the cost of commonly employed algorithms designed to address the stiffness of the problem.

[[media:ecfd3_final_project20.pdf | Final presentation of project #20]]

=== Project #21: AVBP Dense Gases ===

[[media:ecfd3_final_project21.pdf | Final presentation of project #21]]

=== Project #22: Numerical prediction of wind turbine wakes using AMR ===

[[media:ecfd3_final_project22.pdf | Final presentation of project #22]]

Ecfd:ecfd 3rd edition

2020-01-31T08:03:17Z

Lartigue: /* Project #15: Utilisation d’éléments finis du second ordre dans le SMS */

Ecfd:ecfd 3rd edition

2020-01-31T08:02:41Z

Lartigue: /* Project #14: Méthode d'ordre élevé */

Ecfd:ecfd 3rd edition

2020-01-31T08:01:03Z

Lartigue: /* Project #2: Hackathon GENCI/ATOS/AMD/CORIA on YALES2 */