Extreme CFD workshop - User contributions [en]

Ecfd:ecfd 8th edition

2025-02-24T15:01:32Z

Dillon: /* C11 - Exploring efficient tabulation strategies for detailed chemistry */

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

== Description ==

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

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

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

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

== News ==

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

== Thematics / Mini-workshops ==

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

To come...

== Projects ==

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

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

In this edition of the ECFD, our team envisioned extending the CFD code SoNICS, developed by ONERA, to be able to run on multiple GPU accelerators. SoNICS' unique architecture is based on large tasks called "operators" that can be linked via a dependency graph according to their required inputs and outputs. To optimize performance on GPUs, this graph is translated into a CUDAGraph, a generic feature of NVIDIA GPUs, also available on AMD accelerators via HIP (then called HIPgraph). The use of CUDAGraph allows for optimal efficiency by reducing latency and optimizing redundant operations.

Initially, SoNICS could only work using one GPU. To add parallelism on the CPU, the code relies on MPI. Unfortunately, the use of MPI message passing inside the graph was not recommended. Therefore, the execution graphs were split to include intermediate MPI calls wherever needed. This implementation was successfully tested during the ECFD on simple test cases such as the NACA12 profile and a compressor SRV2 blade using A30 NVIDIA nodes on the JUNO cluster at ONERA.

The multi-GPU implementation was also ported and tested on the TOPAZE cluster at CCRT on A100 GPUs. Additionally, the code was ported to the Grace Hopper architecture, which uses Arm-based processors and H100 GPUs (on the Calypso cluster at CERFACS).

Profiling, optimization, and performance tests are ongoing to evaluate and improve the multi-GPU implementation.

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

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

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

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

==== M1 - Simulation of core shifting during investment casting, Y. Mayi (Safran Tech), S. Meynet (GDTech), M. Cailler (Safran Tech), R. Mercier (Safran Tech) ====
Ceramic core displacement and deformation during the casting process is a major source of cooled blades manufacturing scrap. Simplified casting experiment on a test blade has already been led with the help of our academic partners. During this project, two topics have been addressed: compute the shifting and the deformation of the test blade with YALES2.
Concerning the shifting, dynamic mesh adaptation is required. This is why a coupling has been done between spray (for the filling) and mesh movement (for the shifting) solvers within YALES2. Tests cases have shown promising results but forces on the blade by fluids will have to be integrated later.
About the deformation, the chosen strategy is to run filling simulation with YALES2 and ABAQUS afterwards (FEM software). This implies a numerical chaining but mesh interpolation is needed as meshes are different. As ABAQUS requires input files, the work consisted in writing this kind of ABAQUS files during a YALES2 simulation. For this purpose, four steps are considered during a time step: 1) Parse ABAQUS mesh 2) Create particles at face centers of ABAQUS mesh 3) Interpolate pressure between particles and the YALES2 mesh at the considered blade 4) Write a ABAQUS input file. Finally, the chaining was a success and this paves the way for ABAQUS simulations from YALES2 runs in the future.

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

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

==== M4 - Anisotropic mesh adaptation of interfaces in YALES2 (R. Barbera (LEGI/Safran Tech), G. Balarac (LEGI), G. Ghigliotti (LPSC), M. Cailler (Safran Tech)) ====
Mesh adaptation is now a key feature for simulations of complex industrial flows. For transient flows such as multiphase and/or reactive flows, where regions of interest are strongly moving in space, dynamic mesh adaptation appears as the most suitable strategy. This strategy is now widely used in YALES2 based on isotropic mesh definition. The purpose of this project is to adapt this strategy to an anisotropic framework to reduce the overall simulation costs (in terms of memory consumption, CPU cost and time to solution). In order to be able to handle multiphase flows, the main objective of the project is to study the conditions for accurately describing the dynamics of the level-set function with an anisotropic mesh. Based on the work conducted during ECFD7, in which we assessed several anisotropic remeshing strategy for simple flow (ie droplet advection and dam break test case), we worked on the more complex case of the jet-in-crossflow. We highlighted that our strategy is able to accurately describe the main features of the flow (hydrodynamic instabilities at the basis of the jet and ligaments atomization in the upper region). But that a particular focus needs to be made on the accurate resolution of turbulence in the gaseous phase in order to accurately catch the complex coupling between the liquid phase and turbulence in the gas phase. This strategy showed a significant reduction in terms of number of elements in the domain with respect to isotropic mesh adaptation. The issue of mass conservation still needs to be dealt with but encouraging results has been conducted on this matter in the N3 project.

==== M6 - Dynamic mesh adaptation of turbulent flows (G. Balarac, L. Bricteux, P. Bénard, S. Mendez) ====
This work focuses on developing dynamic mesh adaptation strategies for turbulent flows, particularly in cases where static (statistical-based) adaptation is inadequate. The objective is to design a dynamic mesh metric capable of computing the required physical metric directly, without the need for an explicitly prescribed mesh size.
The method, based on QC6 and developed at LEGI, has been assessed with encouraging results. Validation was performed on hemisphere flow at Re=55k —a regime for which LDV experimental measurements are available—yielding promising results in terms of Drag coefficient. Additionally, the approach was tested on vortex ring collisions, a canonical case where static adaptation is ineffective. The results demonstrated that the adaptation strategy performs well in capturing vortical flow regions. Furthermore, the strategy was applied to an elliptic jet flow (aspect ratio 3) generating vortex rings at Re=30k.
The dynamically generated meshes successfully tracked the regions of interest, confirming the method’s robustness and effectiveness in complex flow configurations.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

* WP1: Improve inflow generation

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Participants: M. Préteseille (EM2C), E. Espada (EM2C), N. Galand (EM2C), N. Darabiha (EM2C), B. Fiorina (EM2C)

Virtual chemistry presents a promising approach by creating optimized reduced mechanisms of chemical species and reactions to mimic specific flame characteristics. This method has successfully modeled the combustion of various fuels, including complex pollutants like NOx. However, its reliance on tabulated parameters has limited its adoption due to the need for modifications in traditional CFD solvers. This work aims to revise the formalism to eliminate parameter tabulation, creating highly reduced virtual mechanisms that emulate detailed schemes used in software like CHEMKIN and Cantera. The methodology is divided into three steps. A first optimization is achieved, focusing on mixture's thermodynamic properties to recover gas thermochemical equilibrium states across various equivalence ratios. The optimization of Arrhenius reaction rates on reference 0D reactors is then carried out to match temperature and heat release rate profiles. A final optimization is undertaken to find the optimal set of species' transport properties to capture complex diffusion phenomena on 1D laminar premixed flames. This methodology is applied to optimize a virtual scheme dedicated to Sustainable Aviation Fuel (SAF), illustrating the potential and versatility of the method to create highly reduced kinetic mechanisms for any desired fuel.

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

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

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

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

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

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

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

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

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

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

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

==== C12 - Dynamic sub-grid-scale modelling of multi-regime flame wrinkling ====
Large-eddy-simulation (LES) of reactive flows is widely used in both academic and industrial applications. Combustion phenomena occur at a scale often smaller than the LES mesh size, therefore, turbulent combustion models are required to account for unresolved turbulent flame interactions. The modeling of sub-grid-scale (SGS) flame turbulence interactions can be described with a flame surface wrinkling factor which measures the ratio of the total flame surface area to the resolved flame surface area. Flame surface wrinkling models are often expressed by assuming equilibrium between turbulent motions and flame surface wrinkling, however, in realistic burners non-equilibrium is present and dynamic models are needed to adapt model parameters. Current dynamic models identify flame surface areas from scalars such as the progress variable <math>\widetilde{c}</math> or the mixture fraction <math>\widetilde{z}</math>, however, in multi-regime flames where filtered gradients <math>\overline{|\nabla c|}</math> and <math>\overline{|\nabla z|}</math> coexist, such approaches tend to fail. In this project we introduced a multi-regime dynamic formalism for determining flame wrinkling factors based on a mixed approach between premixed and diffusion flame surfaces. Model validation was performed using a DNS database of the HYLON configuration, a dual-swirl coaxial H2/air injector. The operating condition of interest was the lifted flame, HYLON L, which was studied in the framework of the turbulent flame workshop TNF. This flame has characteristic behavior of both non-premixed and partially-premixed flames, which allowed the model to be tested under conditions where iso-surfaces of <math>\widetilde{c}</math> and <math>\widetilde{z}</math> coexist.

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

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

==== U1 - Low-fidelity (RANS) rotor/stator simulations, application to Kaplan Turbine - ====

Participants: Yayha Lakrifi, Guillaume Balarac (LEGI), Renaud Mercier (SAFRAN), Vincent Moureau (CORIA)

RANS rotor/stator coupling simulation has recently been developed within YALES2. This approach involves coupling the rotor, in the rotational frame, with the stator using a patch located at the domain interface. This patch allows interaction between the two regions and enables azimuthal averaging to account for azimuthal periodicity.

This year, the main objective was to improve the automatic mesh convergence (AMC) procedure for coupled RANS simulations by managing the AMC of coupled runs, integrating coupling runs into the workflow, which was not previously supported and, finally, implementing parallel remeshing of periodic boundaries.

==== U2 - Coupling PyTorch/YALES2, combustion cartesian look-up tables ====

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

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

==== U3 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 ====

Participants: Laurent Korzeczek, Serge Meynet (GDTECH), Julien Leparoux, Melody Cailler (SAFRAN)

Ecfd:ecfd 8th edition

2025-02-24T15:01:01Z

Dillon: /* C12 - Dynamic sub-grid-scale modelling of multi-regime flame wrinkling */

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

== Description ==

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

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

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

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

== News ==

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

== Thematics / Mini-workshops ==

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

To come...

== Projects ==

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

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

In this edition of the ECFD, our team envisioned extending the CFD code SoNICS, developed by ONERA, to be able to run on multiple GPU accelerators. SoNICS' unique architecture is based on large tasks called "operators" that can be linked via a dependency graph according to their required inputs and outputs. To optimize performance on GPUs, this graph is translated into a CUDAGraph, a generic feature of NVIDIA GPUs, also available on AMD accelerators via HIP (then called HIPgraph). The use of CUDAGraph allows for optimal efficiency by reducing latency and optimizing redundant operations.

Initially, SoNICS could only work using one GPU. To add parallelism on the CPU, the code relies on MPI. Unfortunately, the use of MPI message passing inside the graph was not recommended. Therefore, the execution graphs were split to include intermediate MPI calls wherever needed. This implementation was successfully tested during the ECFD on simple test cases such as the NACA12 profile and a compressor SRV2 blade using A30 NVIDIA nodes on the JUNO cluster at ONERA.

The multi-GPU implementation was also ported and tested on the TOPAZE cluster at CCRT on A100 GPUs. Additionally, the code was ported to the Grace Hopper architecture, which uses Arm-based processors and H100 GPUs (on the Calypso cluster at CERFACS).

Profiling, optimization, and performance tests are ongoing to evaluate and improve the multi-GPU implementation.

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

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

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

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

==== M1 - Simulation of core shifting during investment casting, Y. Mayi (Safran Tech), S. Meynet (GDTech), M. Cailler (Safran Tech), R. Mercier (Safran Tech) ====
Ceramic core displacement and deformation during the casting process is a major source of cooled blades manufacturing scrap. Simplified casting experiment on a test blade has already been led with the help of our academic partners. During this project, two topics have been addressed: compute the shifting and the deformation of the test blade with YALES2.
Concerning the shifting, dynamic mesh adaptation is required. This is why a coupling has been done between spray (for the filling) and mesh movement (for the shifting) solvers within YALES2. Tests cases have shown promising results but forces on the blade by fluids will have to be integrated later.
About the deformation, the chosen strategy is to run filling simulation with YALES2 and ABAQUS afterwards (FEM software). This implies a numerical chaining but mesh interpolation is needed as meshes are different. As ABAQUS requires input files, the work consisted in writing this kind of ABAQUS files during a YALES2 simulation. For this purpose, four steps are considered during a time step: 1) Parse ABAQUS mesh 2) Create particles at face centers of ABAQUS mesh 3) Interpolate pressure between particles and the YALES2 mesh at the considered blade 4) Write a ABAQUS input file. Finally, the chaining was a success and this paves the way for ABAQUS simulations from YALES2 runs in the future.

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

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

==== M4 - Anisotropic mesh adaptation of interfaces in YALES2 (R. Barbera (LEGI/Safran Tech), G. Balarac (LEGI), G. Ghigliotti (LPSC), M. Cailler (Safran Tech)) ====
Mesh adaptation is now a key feature for simulations of complex industrial flows. For transient flows such as multiphase and/or reactive flows, where regions of interest are strongly moving in space, dynamic mesh adaptation appears as the most suitable strategy. This strategy is now widely used in YALES2 based on isotropic mesh definition. The purpose of this project is to adapt this strategy to an anisotropic framework to reduce the overall simulation costs (in terms of memory consumption, CPU cost and time to solution). In order to be able to handle multiphase flows, the main objective of the project is to study the conditions for accurately describing the dynamics of the level-set function with an anisotropic mesh. Based on the work conducted during ECFD7, in which we assessed several anisotropic remeshing strategy for simple flow (ie droplet advection and dam break test case), we worked on the more complex case of the jet-in-crossflow. We highlighted that our strategy is able to accurately describe the main features of the flow (hydrodynamic instabilities at the basis of the jet and ligaments atomization in the upper region). But that a particular focus needs to be made on the accurate resolution of turbulence in the gaseous phase in order to accurately catch the complex coupling between the liquid phase and turbulence in the gas phase. This strategy showed a significant reduction in terms of number of elements in the domain with respect to isotropic mesh adaptation. The issue of mass conservation still needs to be dealt with but encouraging results has been conducted on this matter in the N3 project.

==== M6 - Dynamic mesh adaptation of turbulent flows (G. Balarac, L. Bricteux, P. Bénard, S. Mendez) ====
This work focuses on developing dynamic mesh adaptation strategies for turbulent flows, particularly in cases where static (statistical-based) adaptation is inadequate. The objective is to design a dynamic mesh metric capable of computing the required physical metric directly, without the need for an explicitly prescribed mesh size.
The method, based on QC6 and developed at LEGI, has been assessed with encouraging results. Validation was performed on hemisphere flow at Re=55k —a regime for which LDV experimental measurements are available—yielding promising results in terms of Drag coefficient. Additionally, the approach was tested on vortex ring collisions, a canonical case where static adaptation is ineffective. The results demonstrated that the adaptation strategy performs well in capturing vortical flow regions. Furthermore, the strategy was applied to an elliptic jet flow (aspect ratio 3) generating vortex rings at Re=30k.
The dynamically generated meshes successfully tracked the regions of interest, confirming the method’s robustness and effectiveness in complex flow configurations.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

* WP1: Improve inflow generation

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Participants: M. Préteseille (EM2C), E. Espada (EM2C), N. Galand (EM2C), N. Darabiha (EM2C), B. Fiorina (EM2C)

Virtual chemistry presents a promising approach by creating optimized reduced mechanisms of chemical species and reactions to mimic specific flame characteristics. This method has successfully modeled the combustion of various fuels, including complex pollutants like NOx. However, its reliance on tabulated parameters has limited its adoption due to the need for modifications in traditional CFD solvers. This work aims to revise the formalism to eliminate parameter tabulation, creating highly reduced virtual mechanisms that emulate detailed schemes used in software like CHEMKIN and Cantera. The methodology is divided into three steps. A first optimization is achieved, focusing on mixture's thermodynamic properties to recover gas thermochemical equilibrium states across various equivalence ratios. The optimization of Arrhenius reaction rates on reference 0D reactors is then carried out to match temperature and heat release rate profiles. A final optimization is undertaken to find the optimal set of species' transport properties to capture complex diffusion phenomena on 1D laminar premixed flames. This methodology is applied to optimize a virtual scheme dedicated to Sustainable Aviation Fuel (SAF), illustrating the potential and versatility of the method to create highly reduced kinetic mechanisms for any desired fuel.

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

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

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

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

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

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

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

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

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

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

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

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

==== C12 - Dynamic sub-grid-scale modelling of multi-regime flame wrinkling ====
Large-eddy-simulation (LES) of reactive flows is widely used in both academic and industrial applications. Combustion phenomena occur at a scale often smaller than the LES mesh size, therefore, turbulent combustion models are required to account for unresolved turbulent flame interactions. The modeling of sub-grid-scale (SGS) flame turbulence interactions can be described with a flame surface wrinkling factor which measures the ratio of the total flame surface area to the resolved flame surface area. Flame surface wrinkling models are often expressed by assuming equilibrium between turbulent motions and flame surface wrinkling, however, in realistic burners non-equilibrium is present and dynamic models are needed to adapt model parameters. Current dynamic models identify flame surface areas from scalars such as the progress variable <math>\widetilde{c}</math> or the mixture fraction <math>\widetilde{z}</math>, however, in multi-regime flames where filtered gradients <math>\overline{|\nabla c|}</math> and <math>\overline{|\nabla z|}</math> coexist, such approaches tend to fail. In this project we introduced a multi-regime dynamic formalism for determining flame wrinkling factors based on a mixed approach between premixed and diffusion flame surfaces. Model validation was performed using a DNS database of the HYLON configuration, a dual-swirl coaxial H2/air injector. The operating condition of interest was the lifted flame, HYLON L, which was studied in the framework of the turbulent flame workshop TNF. This flame has characteristic behavior of both non-premixed and partially-premixed flames, which allowed the model to be tested under conditions where iso-surfaces of <math>\widetilde{c}</math> and <math>\widetilde{z}</math> coexist.

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

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

==== U1 - Low-fidelity (RANS) rotor/stator simulations, application to Kaplan Turbine - ====

Participants: Yayha Lakrifi, Guillaume Balarac (LEGI), Renaud Mercier (SAFRAN), Vincent Moureau (CORIA)

RANS rotor/stator coupling simulation has recently been developed within YALES2. This approach involves coupling the rotor, in the rotational frame, with the stator using a patch located at the domain interface. This patch allows interaction between the two regions and enables azimuthal averaging to account for azimuthal periodicity.

This year, the main objective was to improve the automatic mesh convergence (AMC) procedure for coupled RANS simulations by managing the AMC of coupled runs, integrating coupling runs into the workflow, which was not previously supported and, finally, implementing parallel remeshing of periodic boundaries.

==== U2 - Coupling PyTorch/YALES2, combustion cartesian look-up tables ====

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

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

==== U3 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 ====

Participants: Laurent Korzeczek, Serge Meynet (GDTECH), Julien Leparoux, Melody Cailler (SAFRAN)

Ecfd:ecfd 7th edition

2024-02-16T09:17:58Z

Dillon: /* C3: Dynamic sub-grid-scale wrinkling for diffusion flames - S. Dillon (EM2C/Safran), R. Mercier (Safran), E. Espada, B. Fiorina, D. Veynante (EM2C) */

{{DISPLAYTITLE: ECFD workshop, 7th edition, 2024}}

== Description ==

* Event from '''22th of January to 2nd of February 2024'''
* Location: [https://www.hotelclubdelaplage.com Hôtel Club de la Plage], Merville-Franceville, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* More than 70 participants from academics, HPC center/experts and industry.

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes
* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Viovenel (CORIA).
[[File:ecfd7.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_6th_edition]]

[[File:sponsor_ecfd7.png|text-bottom|600px]]



== Agenda ==

[[File:agenda_ecfd7.png|text-bottom|600px]]

== Thematics / Mini-workshops ==

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

To come...

== Projects ==

=== Hackathon GENCI - P. Begou, LEGI ===
The '''GENCI Hackathon''' will be devoted to porting two CFD codes to the Mi250 GPUs of the Adastra supercomputer deployed by GENCI at CINES.

For the '''YALES2''' code the goal is to obtain a first reference version giving the expected results then, if possible, to start its optimization to gain performance. The approach is OpenACC based with the objective of an implementation as least intrusive as possible in the existing code and which remains portable with the work done on the Nvidia GPUs of the Jean-Zay supercomputer at IDRIS.

The porting of the '''AVBP''' code is more advanced with a prototype already functional on Adastra but "hard-coded". The objective is to rationalize this first implementation, to integrate the latest developments in the code, to centralize memory management (host and device), to work on porting the Lagrangian part of the code and, of course, to improve the global performance.

This Hackathon is supported by GENCI, HPE, AMD and CINES with the presence on site of several development experts on AMD GPUS.

=== Mesh adaptation - R. Letournel, Safran ===

==== M1: ASMR for reheat chamber applications - Paul Pouech (CERFACS), Thibault Duranton, Luis Carbajal Carrasco (Safran) ====

Combustion in reheat chambers feature a wide range of lenght scales. Mesh refinement is thus mandatory to capture the flow characteristics within a reasonnable CPU cost for LES computations using the AVBP code. The purpose of this project is to consolidate mesh refinement criteria and strategy in an academic reference case. The retained workflow is supported by the [https://lemmings.readthedocs.io/en/latest/readme_copy.html Lemmings] code that calls the Tékigô wrapper for the mesh adaptations. During the ECFD7, the convergence time needed to have significant distribution of quantities of interest was analysed. An optimum runtime, based on a characteristic flow time-scale, was thus identified and led to a reduced running time for each adaptation step. As a second step, discussions with the ECFD7 participants led to the identification of interesting refinement criteria, namely the flame sensor or the mach rms for instance. Parametric analysis showed the robustness of the workflow based on a ponderation of different criteria. Finally, in order to facilitate the use of the workflow, efforts were made to improve the user experience by making it more human readable.

==== M2: Parallel remeshing - B. Andrieu, C. Benazet, K. Hoogveld, B. Maugars, E. Quémerais (ONERA) ====

Mesh adaptation is a crucial tool in order to automate industrial RANS numerical simulations. To meet this need, we need to carry out mesh adaptation as quickly as possible by setting up an efficient, parallel solution. To this end, we have explored two avenues: a parallel edge-splitting algorithm that has recently been initiated in the ParaDiGM library, and a solution based on [https://github.com/nasa/refine the refine library] for adapting meshes with MPI implementation. On the one hand, we fixed several bugs in our split operator, and validated it on test cases of increasing complexity with a node-centered solver. In addition, we've added interfaces to refine so as to avoid using files, and call directly in library mode. We also investigated geometric projection issues during the mesh adaptation procedure, notably by looking at solutions such as EGADS, which offers a simplified API for CAD interrogation. We finally implemented metric gradation (in serial), metric intersection and complexity computations. All the ingredients we've tested give us a clearer picture of the entire mesh adaptation process.

==== M3: Anisotropic mesh refinement - R. Barbera (LEGI/Safran), G. Ghigliotti, G. Balarac (LEGI), R. Letournel (Safran) ====

Mesh adaptation is now a key feature for simulations of complex industrial flows. For transient flows such as multiphase and/or reactive flows, where regions of interest are strongly moving in space, dynamic mesh adaptation appears as the most suitable strategy. This strategy is now widely used in YALES2 based on isotropic mesh definition. The purpose of this project is to adapt this strategy to an anisotropic framework to reduce the overall simulation costs (in term of memory consumption, cpu cost and time to solution). In order to be able to handle multiphase flows, the main objective of the project is to study the conditions for accurately describing the dynamics of the level-set function with an anisotropic mesh. Accuracy is mainly assessed in terms of interface position and mass conservation. The inaccuracy of mass conservation is mainly due to interpolation errors after the adaptation step. Furthermore, inaccuracy in interface position may be due to misalignment between the anisotropic mesh elements and the interface normal. The first methodological corrections have been proposed, as an adaptation of the level-set reinitialization algorithm to the anisotropic mesh.

=== Numerics - S. Mendez, IMAG & G. Balarac, LEGI ===

==== N1: Treatment of boundary conditions for high-order schemes - M. Bernard & G. Balarac (LEGI), G. Lartigue (Total Energies) ====

In the context of Finite Volumes Method, spacial accuracy of a numerical scheme depends on ability to evaluate accurately fluxes through interface of each control volume (CV).
Such accurate evaluation is not straightforward, especially when dealing with distorted grids.
This project follows the work of [1] where fluxes use pointwise quantities, which are reconstructed from integrated quantities advanced in time.
During the workshop, task force was dedicated to the treatment of **inlet** boundary conditions (BC) and **non-planar walls**.
For inlet BC, the key resides in the spatial integration of convective flux over discrete faces of the CV touching the boundary.
Such treatment lead to exact integration for linear inlet profile and large error reduction on other profiles.
Concerning non-planar walls, the strategy adopted consists in the enforcement of the BC on each discrete face, by modifying the normal component of the wall gradient in order to evaluate accurately the diffusive flux.
Again, a large reduction of this error has been observed.

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

==== N2: Implementation of linearised implicit time integration in ALE solver - T. Berthelon, G. Balarac (LEGI) ====

An linearised implicit time integration has recently been developed in the incompressible solver of YALES2. This new integration scheme allows to use larger time-step that the ones constraints by classic stability criteria inherent to explicit time integration method. This allows to reduce the restitution time of Large Eddy Simulations [1].
The objective of this project was to implement this new time integration in the ale solver in order to be able to reduce restitution time of moving mesh configuration.

The developments were validated on a scalar advection case and on a rotor-stator interaction case. Although the results seem to be in line with the explicit integration methods, the validation of the temporal convergence to 2nd order remains to be shown.

[1] ''Toward the use of LES for industrial complex geometries. Part II: Reduce the time-to-solution by using a linearised implicit time advancement, Berthelon et al., JoT, 2023''

==== N4: Non-uniform outlet pressure and coupling with CWIPI - J. B. Lagaert (LMO), Y. Lakrifi, T. Berthelon, G.Balarac (LEGI) , R. Letournel (Safran) ====

In simulations, artificial boundaries need to be introduced due to the limited size of computational domains. At these boundaries, flow variables need to be calculated in a way that will not induce any perturbation of the interior solution. During ECFD#7, a generic outlet boundary condition defined from non-uniform pressure has been implemented in Yales2. This non-uniform pressure can de determined from a traction model (null or advected from the interior domain, for example). This non-uniform pressure can also be deducted through a coupling between two simulations. In this case a coupling via CWIPI is performed where the velocity and the pressure are exchanged at the common boundary to define the inlet and outlet conditions, respectively.

==== N5: Optimization of the RBC solver - F. Rojas, S. Mendez (IMAG) ====

In the study of blood diseases, the mechanical behaviour of Red Blood Cells (RBCs) is one of the most relevant effects to take into account in the numerical models but also in experimental setups. Our system of interest is the thin gap of a rheometer where RBC suspensions are placed to explore their properties. To interpret the experimental data, the simulations of large suspensions of RBC are required to determine the blood’s microstructure (spatial arrangement of cells) and its rheological properties.

Currently, YALES2BIO’s RBC solver is capable to manage thousands of cells, but in order to approach closer to the experimental scales, we propose the characterisation and optimisation of its performance to reduce the computational requirements and increase the RBC’s number and domain sizes in our simulations. During the workshop a parametric study was carried out to obtain the strong and weak scaling. Studying the increase in the volume fraction allowed us to quantify how the cost of the simulation increases rapidly with the RBC’s number and identify which routines have the biggest impact on the performance. One conclusion is that the cost is spread of several routines, which makes code optimization more cumbersome. However, the amount of RBCs and RBC nodes duplicated over processors is identified as a key factor for performance. Indeed, as RBCs may interact with several partitions, it is duplicated as much as needed based on criteria of boundaing box intersections. However, the current criteria have been shown to be too loose.

In order to limit the amount of work during the RBC processing, stricter criteria were introduced to avoid unnecessary calculations at the level of the nodes with a small gain in performance. On the other hand, much better results were obtained using cartesian partitioning to optimise the bounding box of each processor, reducing the involved RBC operations: this demonstrates that the performances of the RBC solver may be optimized by a stricter selection of RBC duplicates over processors.

We thank Ghislain Lartigue and Renaud Mercier for helpful discussions.

==== N6: Electrodeformation of red blood cells, extension to 3D and improved accuracy at membrane - A. Spadotto, S. Mendez (IMAG), M. Bernard (LEGI) ====

The Leaky Dielectric Model is a popular framework to describe electric stresses over micro-scale membranes. We have adopted it to simulate the effect of a DC electric field on a red blood cell using the YALES2BIO solver. The goal of the project is to reproduce the electric charging process of the membrane, as well as the resulting stresses, which may yield to electrodeformation of the cell. From the point of view of the implementation, the grid is represented by a 2D surface mesh embedded in a 3D eulerian grid. The need to make variables stored on the surface interact with quantities stored on the Eulerian grid calls for a proper bidirectional 2D-membrane/3D-grid dynamic connectivity. The advancement of theis task during this ECFD has led to the first 3D simulation of a charging fixed spherical shell. Moreover, the estimation of grid variables on elements cut by the membrane has been improved thanks to a High-Order extrapolation. The latter has been successfully tested on 2D configurations. The project opens the way for a series of validation tests. In particular, future work will demand treatment of instabilities emerging in symmetrical configurations.

==== N7: Optimisation Dorothy - M. Roperch, G. Pinon (LOMC), B. Gaston (CRIANN), P. Benard (CORIA) ====

Dorothy is a Lagrangian code using the particle vortex method. This method must have a homogeneous distribution of particles in space. To achieve this, at regular intervals during the simulation a Cartesian grid with new particles is created. The weights of the old particles are interpolated for each of the new particles. Before ECFD7, all the processors knew the general grid and the new particles. The aim of ECFD was to parallelize this module to avoid memory problem. To do this, each processor creates a grid corresponding to the particles it knows. They then exchange data on the supperposition zones. This solves the issue because the quantity of new particles known is smaller. During ECFD7, a trial on a ring vortex case was successfully carried out to test domain communications and supperposition. The next step will be to implement this new method in the Dorothy code.

=== Turbulence - P. Benard, CORIA & L. Bricteux, UMONS ===

==== T1: Wall Law for immersed boundaries – P. Bénez (CORIA), M. Cailler (Safran), S. Meynet (GDTech), J. Carmona (CORIA), Y. Bechane (CORIA) ====
Conservative Lagrangian Immersed Boundaries (CLIB) are now a useful way to take into account complex geometries in YALES2. In order to study highly turbulent configurations, it appears necessary to implement wall law models adapted to this method. If we consider a non-moving immersed body, developing wall-law models in a conservative immersed boundary formalism presents numerous challenges related to the diffuse interface property of the solid and the continuous formulation of the penalty force. During the ECFD, a new formulation of the penalty force has been established to ensure the imposition of the wall shear stress across the immersed solid interface. A strategy based on the use of two near-wall level sets was implemented to estimate the wall shear stress from the LES fluid velocity field at a distance D from the solid interface. At the end of the ECFD, turbulent flat plate cases were set up to start the validation of the strategy implemented for a logarithmic wall law. Future works will focus on validating this strategy for fixed solids.

==== T2: Turbulence injection Compressible flows – P. Tene Hedje (UMONS), J. Carmona (CORIA), Y. Bechane (CORIA), L. Bricteux (UMONS) ====

Turbulence injection for compressible flows remains a real challenge. Indeed, In these types of flow, the acoustic waves must also be controlled on boundaries. In addition, the non-reflective formulation of the Navier-Stokes characteristic Boundary Conditions (NSCBC) generally used in compressible solvers produce spurious pressure oscillations when applied to turbulent flows, making turbulence injection difficult for such applications. During the ECFD, two turbulence injection approaches were investigated and applied within the framework of the Explicit compressible solver (ECS) of YALES2. The first involved modifying the NSCBC formulation to inject turbulence from the inlet of the domain. To this end, the vortical-flow characteristic boundary condition [1] was implemented in ECS and the first validations were performed. The second was to use AL to generate a turbulence grid in the flow [2]. Future works will focus on further validating these approaches.

[1] ''Guézennec et al., Acoustically nonreflecting and reflecting boundary conditions for vortcity injection in compressible solvers, AIAA journal, 47(7), 1709-1722, 2009.''

[2] ''Houtin-Mongrolle et al., Actuator line method applied to grid turbulence generation for large-Eddy simulations, Journal of Turbulence, 21(8), 407-433, (2020).''

==== T3: Aero-servo-elastic simulations of wind turbines including atmospheric effects – E. Muller (SGRE), U. Vigny (UMONS), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE) ====
Aero-servo-elastic engineering solvers used in the industry (i.e., BHawC) for structural response and power assessments are unsuited for wake simulations, as aerodynamic loads are usually derived from a BEM-like method. To tackle this, the YALES2 library was coupled (P11-ECFD3) to BHawC, the Siemens Gamesa Renewable Energy (SGRE) in-house certification code for wind turbines. This allowed the investigation of neutral atmospheric conditions. This project aims to include stable and unstable atmospheric conditions into this coupling based on the development done in T4-ECFD7. Therefore, this project is divided into three work packages:
Work package 1: Adjustment and refactoring of the existing coupling library between YALES2 and BHawC.
Work package 2: Rethink how turbulence is injected into the simulation (recycling in SGRE setup) to consider thermal and Coriolis effects.
Work package 3: Adapt how the blade forces are computed in the coupling to consider the resulting density fluctuations.

==== T4: Atmospheric solver – U. Vigny (UMONS), L. Voivenel (CORIA), S. Zeoli (UMONS), P. Benard (CORIA) ====
Wind turbines, bigger and bigger, are now influenced by atmospheric flows. An atmospheric solver has already been developed in YALES2 to represents some of its effects (Coriolis, veer, thermal stratification). In this continuum, the project has been divided into two work-packages.
- Work-package 1: The use of the Variable density solver (VDS).
Before ECFD7, thermal stratification was taken into account using the Boussinesq buoyancy approximation within the incompressible solver framework. Now, VDS can be used, taking into account all thermal effect. Results are promissing.
- Work-package 2: Wall law velocity filtering.
Wall law are using velocity at the first grid node to compute wall shear stress. Before ECFD7, atmospheric wall law were using the local velocity, leading sometimes to convergence errors. Now a gather-scatter filter can be used to average velocity (and temperature) at first grid node.

==== T5: Implementation of the RVMs-WALE model in YALES2 – L. Bricteux (UMONS), P. Benard (CORIA), Y. Bechane (CORIA) ====
This study focused on the implementation of an advanced multiscale variational subgrid-scale model, incorporating scaling based on the WALE (Wall-Adapting Local Eddy-viscosity) model within YALES2. This model has demonstrated efficiency across various flow configurations, and it is anticipated that its multiscale nature can enhance the spectral selectivity. The aim is to ensure that its dissipative effects specifically target the smallest scales near the cut-off point.

Additionally, collaborative work with G. Balarac aimed to enhance the mesh adaptation strategy for wall-bounded flows with massive boundary layer detachment and vortical wake.
This new strategy based on vortex detection was developed during the ECFD6 and ECFD4 workshops. We have now shown that this strategy is effective.
Flow simulations around a hemisphere at Reynolds number Re=55K have been conducted, and we anticipate publishing the results soon."

==== T6: Development of coupling between YALES2-OpenFAST – A. Parinam (TUDelft/CORIA), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE) ====

==== T7: Confidence intervals for estimators – C. Papagiannis, G.Balarac (LEGI), R. Letournel (Safran) ====

=== Two Phase Flow - M. Cailler, Safran & V. Moureau, CORIA ===

==== P1: Level set reinitialization at the contact line for boiling flows - H. Lam, M. Benard, G. Ghigliotti (LEGI) ====

In Savinien Pertant's PhD thesis (2022), DNS of nucleate boiling at the bubble scale were performed, but suffered some lack of accuracy in the imposition of the contact angle.
Indeed, the contact angle was not well respected, with a difference of around 10 degrees between the desired angle and the angle measured on the solution.
This lack of accuracy, that contrast with the accurate imposition obtained in the spray solver (SPS), is due to fluctuations of the contact line. This behavior that was traced back to the modifications of the level set reinitialisation needed to take correctly into account the triple line, and for which the solution applied in 2022 was to revert to the standard Janodet reinitialisation.
S. Pertant tested, at the very end of its PhD, a correction which nullifies the temperature transport term at the first node from the wall of the contact line. This correction was introduced to overcome an instability of the code when the contact line velocity on the substrate changes direction, from receding to advancing.
It turned out at the ECFD7 that this correction proves to be very efficient to stabilise the contact line for contact angles between 50 to 90 degrees even in the case of the use of the level set reinitialization. We were able to simulate nucleate boiling with a smooth contact line at the triple line and a precision in the angle of the contact angle of around +-0.5 degrees. More work remains to be able to run DNS of nucleate boiling for extreme contact angles (<50° and >100°). Moreover, longer runs will be needed to further confirm these results.

==== P2: Compatibility of Boiling solver with PCS and MPH structure - H. Lam, M. Benard, G. Ghigliotti (LEGI) ====

The boiling solver does not work since the introduction of MPH data structure and PCS solver in March 2022. An investigation work was carried out to understand the changes made between the previous and the new version of the different solvers. A simple test case was created to show potential differences between the working version of the code and the new one. Several problems were spotted: the order of level set declaration became important as it is the first one declared which is advected. Sign convention was chosen differently for the mass transfer rate. The temporal discretization of the level set was different.
A test case with no flow and at an imposed mass transfer rate (i.e., no coupling of the level set with the temperature field) was run successfully and the results of the commit prior to the March 2022 modifications were retrieved. More work is needed to find the origin of the differences between the two solvers when the temperature field is solved and coupled with the level set and the velocity field. New common test cases for the two solvers will have to be implemented in order to cross-validate the results and avoid such cases happening again (i.e., cross-fertilization).

==== P3: Blood platelets adhesion model - C. Raveleau, S. Mendez, F. Nicoud (IMAG) ====

Medical devices in contact with blood (e.g. artificial valves) are used to treat various cardiovascular diseases, but their thrombogenicity remains the main unresolved issue in their development. A numerical model of blood platelets is being constructed to help to understand the effect of microstructuration on the thrombogenicity of artificial surface. The Force Coupling Method (FCM) was previously implemented and allows the modelisation of ellipsoidal particle and their interaction with the surrounding fluid. During the workshop, the particle model was extended to include adhesive and repulsive interactions with walls or with other particles. The adhesive bonds are modeled with springs forming when the distance between a node of a particle surface and a node of the wall or another particle is smaller than a given threshold. The stiffness of the bond is increased after a given formation time to mimic the 2-step adhesion process of platelets to von Willebrand Factor. A Lennard-Jones potential was used to model the collision of particles. Future work will aim at generalizing these implementations for an arbitrary number of particles (currently only working for 2 particles) and ensuring the interactions are unaltered by the crossing of a periodic boundary.

==== P4: vWF Unfolding - C. Raveleau, S. Mendez, F. Nicoud (IMAG) ====

==== P5: Towards even more efficient particle algorithms - M. Helal (CORIA/Safran), M. Cailler (Safran) ====

Lagrangian particles are widely used in the YALES2 plateform to model: liquid spray, granular flow, two-phase flows with SPH approach or solids in IB method.
Though important developments to handle efficiently high number of particles in massively parallel simulations, the growing use of particles in Yales2 make necessary to re-evaluate and optimize the performances of Lagrangian particles algorithms handling.
Objective of this project was twofold: analyze and improve the performance and robustness of the newly developed SPH solver of YALES2 and improve the performance of the Lagrangian particle relocation (identification of connectivity between Lagrangian and Eulerian grid) during the Dynamic Mesh Adaptation.
Regarding the first subject, profiling tools have been used to identify the hot-spots and bottle-necks in the SPH solver. Optimizations including code factorization, removal of string comparison allows to reduce the computational cost by a factor 3. Moreover, robustification of the solver was achieved.
In the second sub-project, a new implicit 4th-level decomposition has been introduced. This implicit decomposition consists in contiguous coloring of sub-el_grp in element group. The availability of smallest group of elements has been used to improve the local particle relocation algorithm that mainly relies on bounding-box comparison. This new relocation algorithm has been tested for various number of sub-el_grp on a representative case of gear lubrication showing a decrease by a factor 3 to 5 of the relocation algorithm. Perspective is to extend the use of sub-el_grp to the interpolation algorithm.

==== P6: Two fluid and phase change in PCS - C. Merlin (Ariane Group), J. Carmona (CORIA), V. Moureau (CORIA) ====

==== P8: Wall liquid film numerical model - N. Gasnier (EM2C/Safran), J. Leparoux (Safran), J. Carmona (CORIA) ====
Wall liquid films are likely to be formed when fuel sprays impact the walls of aeronautical fuel injection systems. Such phenomenon may have a significant influence on the whole combustion process, however the small scales involved prevent from performing high fidelity simulations of film flows in the context of industrial geometries. Therefore, a low order model is required to model the dynamics of thin liquid flows under the action of spray droplets and of a turbulent gas shear. During ECFD7, a liquid film numerical model accounting for the influence of surface tension as well as gas shear, and based on the 2-dimensional Shallow Water Equations was implemented in Yales2. This model was then coupled to an algorithm ensuring a proper transition between fully resolved liquid structures (levelset) and film model during liquid droplet impacts on a solid wall.

==== P9: Casting simulation for the study of ceramic core displacement - S. Sirot, R. Mercier, M. Cailler (Safran), S. Meynet (GDTech) ====

Ceramic core displacement and deformation during the casting process is a major source of cooled blades manufacturing scrap. A possible source of core deformation may be the fluidic forces due to the filling of the mold with the liquid alloy. Predictive numerical simulations of the casting process would be an essential asset to increase the efficiency of the conception and industrial processes. During the workshop, a numerical methodology to simulate the filling process was drawn, with several modelling levels (with or without surface tension and slipping-wall conditions), in order to estimate the relevance of each of these models. Numerical results were then compared to available experimental results. Numerical deformation of the core was approximated as a beam flexion. Despite this post-processing approximation, the correlation between experimental measurements and numerical simulations is satisfying. The evolution of the core displacement with the inlet velocity of the fluid also has the same behaviour in the experiments and in the simulation. Future work will aim at including the dynamic contact angles in the simulations, in order to evaluate the relevance of this finer modelling, as well as correlating simulations with experiments on cases more representative of the industrial process.

==== P10: Velocity regularization for Euler-Lagrange conversion - I. El Yamani (CORIA/Safran), M. Cailler (Safran), L. Voivenel, J. Carmona (CORIA) ====

The Euler Lagrange multi scale approach aims to reduce the computational costs when simulating two phase flow. To reduce the cost even more, more droplets have to be converted in the Lagrangian formalism where droplets are seen as point forces. It implies that droplets can not always check the hypothesis of the LPP (Lagrangian Particle Point) formalism which is that the diameter of the particle has to be much smaller than the cell size. This hypothesis allows to have a good approximation of the undisturbed velocity for the Lagrangian particle. If the hypothesis is not checked when a Eulerian droplet is converted into a Lagrangian particle a residual velocity field can exists and therefore the velocity given to the particle is impacted by itself. This project aims to filter the gaseous velocity field through a gaussian filtering to remove the contribution of the Eulerian droplet to better approximate the undisturbed velocity.

=== Combustion - K. Bioche, CORIA & R. Mercier, Safran ===

==== C1: Plasma discharge models for reacting system - S. Wang, B. Kruljevic, B. Fiorina (EM2C), Y. Bechane (CORIA) ====

To reduce the expensive computational cost of Plasma-Assisted Combustion (PAC) full 3D simulations, the EM2C laboratory has developed phenomenological approaches to model Nanosecond Repetitively Pulsed (NRP) plasma discharges in reacting flows (Castela 2016 & Blanchard 2023). As part of previous works and ECFDs, both models were implemented and validated in the Low-Mach number framework (YALES2-VDS). While they were also implemented in the Compressible framework (YALES2-ECS), the validation against existing measurements or computations remained. During the workshop, numerical simulations of pin-to-pin configurations were performed with different numerical schemes and reactive mixtures to validate both models in ECS. The energy deposition was relatively well-validated through 2D simulations in the conditions of Castela et al. CNF 2016 and Rusterholtz et al. JPhysD 2013. A glimpse of baroclinic instabilities was observed through 3D simulations in the conditions of Castela et al. PROCI 2017.

==== C3: Dynamic sub-grid-scale wrinkling model for diffusion flames - S. Dillon (EM2C/Safran), R. Mercier (Safran), E. Espada, B. Fiorina, D. Veynante (EM2C) ====

Large-eddy-simulation (LES) of reactive flows is widely used in both academic and industrial applications. Combustion phenomena occur at a scale often smaller than the LES mesh size, therefore, turbulent combustion models are required to account for unresolved turbulent flame interactions. The modeling of sub-grid-scale (SGS) flame turbulence interactions can be described with a flame surface wrinkling factor which measures the ratio of the total flame surface area to the resolved flame surface area. Flame surface wrinkling models are often expressed by assuming equilibrium between turbulent motions and flame surface wrinkling, however, in realistic burners non-equilibrium is present and dynamic models are needed to adapt model parameters. Fractal-like models require information about the outer and inner cut-off length scales along with a fractal exponent, which is determined dynamically from resolved scales in the LES. The dynamic formalism can be coupled with the Filtered Tabulated Chemistry for LES (F-TACLES) model, where the required cut-off length scales are tabulated in the F-TACLES table along with other filtered thermochemical variables. The coupling of the F-TACLES model with the dynamic formalism has been previously applied to premixed flames in the past, however, the formal extension to non-premixed flames has never been investigated. The objective of this project is to investigate the performance of the dynamic SGS flame surface wrinkling model coupled with the F-TACLES model for non-premixed flames. A priori tests are conducted on a 2D H2/Air reactive mixing layer and HYLON, a 3D turbulent dual-swirl coaxial H2/Air injector. In both 2D and 3D cases, the modelled flame surface density shows good agreement with the filtered flame surface density extracted from the DNS. Moreover, the variation of the fractal model exponent in the HYLON test case is significant, highlighting the importance of the dynamic procedure. A posteriori tests were also conducted, and modelled chemical reaction rates show promising results.

==== C4: Developement of an automated virtual scheme generator for CFD - T. Luu, M. Hustache, N. Darabiha, B. Fiorina (EM2C) ====

In reactive CFD simulations, a non-negligible part of the time cost is spent in the resolution of the chemical system. Simplified chemistry models aim to reduce the number of transported species while still ensuring a correct representation of the phenomena of interest. Among them, the virtual chemistry method consists of using “virtual” species and reactions to reproduce detailed chemistry results through a mechanism of drastically smaller size. These “virtual” species and reactions are optimized to target quantities of interest such as temperature, laminar flame speed or pollutants. In practice, the optimization is done using a learning database composed of representative canonical reactive configurations computed with detailed chemistry. The objective of this project was to develop a tool to easily generate virtual schemes. The tool, named VISION (Virtual Scheme optimizatION), is currently able to both generate a user-defined database of wide reactive configurations and optimize a given scheme structure using either CANTERA or REGATH.

==== C5: Partially-Stirred reactor model for MILD combustion - E. Stendardo, L. Bricteux (UMONS), M. Laignel, K. Bioche (CORIA), J. Blondeau (VUB) ====

MILD combustion produces intense turbulence and extensive reaction zones, necessitating costly mesh refinement over large areas. Practical mesh lacks precision, leading to sub-grid heterogeneity and turbulent fluctuations. A Partially Stirred Reactor model was implemented to address turbulence-combustion interaction. This model multiplies the source term by a limiter factor, allowing modelling of residence time in the inner cell reactive structure. Testing various limiter formulations based on mixing and chemical timescales revealed increased computational costs. Future work aims to reduce costs by utilizing the model only where necessary. This ongoing research seeks to optimize performance while minimizing computational overhead for efficient application in engineering scenarios.

==== C6: Static Mesh Adaptation for Hydrogen High pressure combustion using GPUs - G. Hexilar, C. Brunet, R. Mari, S. Richard (Safran), P. Pouech, Q. Douasbin, G. Staffelbach (Cerfacs) ====

This research project focuses on advancing the understanding of hydrogen combustion under high-pressure conditions (up to 10bars), employing an automated workflow coupled with static mesh adaptation to tailor computational simulations to specific requirements. The study aims to enhance the accuracy and efficiency of combustion models by utilizing both Central Processing Units (CPUs) and Graphics Processing Units (GPUs). The automated workflow streamlines the simulation process, optimizing resource utilization and minimizing manual intervention. Static mesh adaptation further refines the computational mesh based on evolving combustion dynamics, ensuring accurate representation of high-pressure hydrogen combustion phenomena. By leveraging the parallel processing capabilities of GPUs alongside traditional CPUs, the research team aims to achieve significant computational speedup. This innovative approach not only contributes to fundamental insights into high-pressure hydrogen combustion but also establishes a robust framework for scalable and efficient simulations in complex reactive flow scenarios.

==== C7: High fidelity simulation of a cone calorimeter - A. Grenouilloux, K. Bioche (CORIA), N. Dellinger (ONERA), R. Letournel (Safran) ====

A methodology to simulate the decomposition of a composite sample in a calorimeter cone has been proposed. The configuration consists in the imposition of an incident radiative flux that heats the test coupon until it degrades. During test campaigns, the composite degradation can lead to the auto-ignition of the outgassed species, a phenomenon that needs to be predicted by the simulation. The variety of physical phenomena encountered, as well as the different characteristic time-scales, require the implementation of a coupled simulation. Hence, the proposed methodology relies on the coupling between two solvers of the massively parallel library YALES2 (fluid, radiation) and the MoDeThec solver developed at ONERA (solid degradation). First, a set of elementary validation tests to characterize the composite’s properties has been performed, showing good agreement with experimental data. A reduced three-equation kinetic scheme for the ignition delay has been derived, which aligns with experimental observations. Additionally, the framework for the coupled simulation involving the three solvers has been implemented.

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

==== U1: Refactoring the YALES2 tools - J. Leparoux, M. Cailler (Safran), L. Voivenel, J. Carmona, I. El Yamani (CORIA), S. Meynet, L. Korzeczek (GDTech) ====

The YALES2 distribution "tools" are becoming difficult to read and are a mixture of several types of tools. This leaves developers and end users unaware of what exists and how to use it (duplicated functions or tools) and makes it impossible to propose generic data analysis tools (FFT, confidence intervals, ...) that can be easily applied to YALES2 data structures. The main efforts have been concentrated on promoting a new architecture for YALES2 distribution tools with an object-oriented structure, including a refactored version of the main readers of YALES2 data. Several tutorials using Jupyter notebooks have been published for demonstration and explanation. A new CLI is now available under the name 'y2tools'. More work is needed before this structure can be pushed to the master trunk.

==== U2: Improved USEX for Multi-Scale Eulerian-Lagrangian simulation - L. Voivenel, J. Carmona, I. El Yamani (CORIA) J. Leparoux, M. Cailler (Safran) ====

The multi-scale Eulerian-Lagrangian approach has now reached a certain maturity and is being used to simulate fuel spray atomization. Post-treatments of these multi-scale simulations require the development of specific tools that track liquid structures either described in an Eulerian or Lagrangian way. In this project, we implemented a strategy to register in a post-treatment particle-set all Eulerian droplets crossing an arbitrarily shaped surface (described with an interior-boundary). The strategy is based on artificial Eulerian droplet advancement (using a Lagrangian representation) and verification of the new Eulerian droplet position compared to the surface of interest. We used this strategy to build a new post-treatment that allows to track both Eulerian and Lagrangian structures and build particle size or velocity distributions.

==== U3: Evaluate technological debt - P. Pouech, T. Marzlin, A. Dauptain (CERFACS) ====

The tool dedicated to cleaning and extracting meta data from the codebase (Tucan) is now able to interpret the most common #Ifdef statements for C and Fortran. Is has be tested successfully on the codebases of Yales2 , Neko and some elements of the Chemkin II package, for Fortran language. It has also included more tests, especially on C++ samples and large files. The call-graph aggregator “Marauder’s map” also evolved a lot. It was used as a refactoring monitoring tool for the UX project U1 “Refactoring Yales2 tools”, on a codebase mixing Python and Fortran files, providing feedback to both U3 and U1 teams. With this experience, we plan to add at least two additional complexity metrics in the months to come : the single component #Ifdef footprint , and the Custom Structures footprint : declarations vs usage in the code.

==== U4: CWIPI 1.0 porting - N. Dellinger, B. Andrieu, K. Hoogveld, E. Quémerais (ONERA), A. Grenouilloux (CORIA), R. Letournel (Safran) ====

Coupling is a cornerstone of numerical simulation, especially for addressing multi-physics problems using highly-specialized solvers for each phenomenon. The CWIPI library, developed at ONERA for coupling codes in a massively parallel environment, has been used in YALES2 for many years for internal and external coupling.
Significant developments have been carried out in recent years to improve the performance and usability of CWIPI, resulting in the release of version 1 in july 2023. This version features a completely revised API to overcome the limitations of version 0.12 and offer more possibilities to users.
The goal of this project was to support users in their transition to version 1. A training course based on Jupyter Notebooks was first organized. Assistance was then provided to successfully port MoDeTheC's and YALES2's internal couplings to the new version. Some fixes were made in CWIPI along the way, and will be reported in a new patched version.

==== U5: Integration of YALES2 in PRESTO supervisor - A. Pushkarev (GE Vernova), G. Balarac (LEGI) ====

A Graphical User Interface (GUI) exists at GE Vernova (Hydro) provides a user-friendly solution to perform numerical simulations for typical geometries of hydroelectric turbine in varied operation regimes. Previously, we implemented an interface for YALES2 code as alternative of CFX solver for this GUI client. Actual project is dedicated to implementation of the automatic mesh generation process for the runner section of the turbine using only section profile files of geometry such as blade profiles, meridional channel section, guide vane profile, etc... The algorithm should be able to generate a new *.msh mesh file once geometry profiles are updated as well as to setup standard named sections of the numerical domain.

==== U6: Optimization of YALES2 compilation time - R. Mercier (Safran), G. Lartigue (Total Energy) ====

YALES2 Compilation time are very different depending on the compilation options but also depending on the machine where the code is built. In particular, compiling on cluster always takes more time than on local station because of the slower filesystems. On these machines, the compilation time scales with the number of .f90 files to compile. 1) During this project we proposed a detailed compilation timing system activated with Y2_COMPILE_TIMER = TRUE and plotted using the tool y2_compilation_gantt.py. 2) This new profiling tool allowed us to spot some very large modules including a lot a dependencies. To enhance the tasks parallelization, the expl_comp_numerics module has been spliced in several modules. An important gain in compilation time has been obtained. 3) Another optimization of the compilation time have been developed using an on-the-fly modification of the .f90 tree. The use of _h.f90 is dynamically removed and replaced by including equivalent .defs files allowing the divide by 2 the number of .f90 files to compile. Some demonstrations have been performed but this compilation mode still have to be cross-check with different compilation option to ensure its robustness.
At the end of the project, the sum of all optimization allowed us to divide by 2 the compilation time.

Ecfd:ecfd 7th edition

2024-02-16T09:17:12Z

Dillon: /* Combustion - K. Bioche, CORIA & R. Mercier, Safran */

{{DISPLAYTITLE: ECFD workshop, 7th edition, 2024}}

== Description ==

* Event from '''22th of January to 2nd of February 2024'''
* Location: [https://www.hotelclubdelaplage.com Hôtel Club de la Plage], Merville-Franceville, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* More than 70 participants from academics, HPC center/experts and industry.

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes
* Organizers
** Guillaume Balarac (LEGI), Simon Mendez (IMAG), Pierre Bénard, Vincent Moureau, Léa Viovenel (CORIA).
[[File:ecfd7.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_6th_edition]]

[[File:sponsor_ecfd7.png|text-bottom|600px]]



== Agenda ==

[[File:agenda_ecfd7.png|text-bottom|600px]]

== Thematics / Mini-workshops ==

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

To come...

== Projects ==

=== Hackathon GENCI - P. Begou, LEGI ===
The '''GENCI Hackathon''' will be devoted to porting two CFD codes to the Mi250 GPUs of the Adastra supercomputer deployed by GENCI at CINES.

For the '''YALES2''' code the goal is to obtain a first reference version giving the expected results then, if possible, to start its optimization to gain performance. The approach is OpenACC based with the objective of an implementation as least intrusive as possible in the existing code and which remains portable with the work done on the Nvidia GPUs of the Jean-Zay supercomputer at IDRIS.

The porting of the '''AVBP''' code is more advanced with a prototype already functional on Adastra but "hard-coded". The objective is to rationalize this first implementation, to integrate the latest developments in the code, to centralize memory management (host and device), to work on porting the Lagrangian part of the code and, of course, to improve the global performance.

This Hackathon is supported by GENCI, HPE, AMD and CINES with the presence on site of several development experts on AMD GPUS.

=== Mesh adaptation - R. Letournel, Safran ===

==== M1: ASMR for reheat chamber applications - Paul Pouech (CERFACS), Thibault Duranton, Luis Carbajal Carrasco (Safran) ====

Combustion in reheat chambers feature a wide range of lenght scales. Mesh refinement is thus mandatory to capture the flow characteristics within a reasonnable CPU cost for LES computations using the AVBP code. The purpose of this project is to consolidate mesh refinement criteria and strategy in an academic reference case. The retained workflow is supported by the [https://lemmings.readthedocs.io/en/latest/readme_copy.html Lemmings] code that calls the Tékigô wrapper for the mesh adaptations. During the ECFD7, the convergence time needed to have significant distribution of quantities of interest was analysed. An optimum runtime, based on a characteristic flow time-scale, was thus identified and led to a reduced running time for each adaptation step. As a second step, discussions with the ECFD7 participants led to the identification of interesting refinement criteria, namely the flame sensor or the mach rms for instance. Parametric analysis showed the robustness of the workflow based on a ponderation of different criteria. Finally, in order to facilitate the use of the workflow, efforts were made to improve the user experience by making it more human readable.

==== M2: Parallel remeshing - B. Andrieu, C. Benazet, K. Hoogveld, B. Maugars, E. Quémerais (ONERA) ====

Mesh adaptation is a crucial tool in order to automate industrial RANS numerical simulations. To meet this need, we need to carry out mesh adaptation as quickly as possible by setting up an efficient, parallel solution. To this end, we have explored two avenues: a parallel edge-splitting algorithm that has recently been initiated in the ParaDiGM library, and a solution based on [https://github.com/nasa/refine the refine library] for adapting meshes with MPI implementation. On the one hand, we fixed several bugs in our split operator, and validated it on test cases of increasing complexity with a node-centered solver. In addition, we've added interfaces to refine so as to avoid using files, and call directly in library mode. We also investigated geometric projection issues during the mesh adaptation procedure, notably by looking at solutions such as EGADS, which offers a simplified API for CAD interrogation. We finally implemented metric gradation (in serial), metric intersection and complexity computations. All the ingredients we've tested give us a clearer picture of the entire mesh adaptation process.

==== M3: Anisotropic mesh refinement - R. Barbera (LEGI/Safran), G. Ghigliotti, G. Balarac (LEGI), R. Letournel (Safran) ====

Mesh adaptation is now a key feature for simulations of complex industrial flows. For transient flows such as multiphase and/or reactive flows, where regions of interest are strongly moving in space, dynamic mesh adaptation appears as the most suitable strategy. This strategy is now widely used in YALES2 based on isotropic mesh definition. The purpose of this project is to adapt this strategy to an anisotropic framework to reduce the overall simulation costs (in term of memory consumption, cpu cost and time to solution). In order to be able to handle multiphase flows, the main objective of the project is to study the conditions for accurately describing the dynamics of the level-set function with an anisotropic mesh. Accuracy is mainly assessed in terms of interface position and mass conservation. The inaccuracy of mass conservation is mainly due to interpolation errors after the adaptation step. Furthermore, inaccuracy in interface position may be due to misalignment between the anisotropic mesh elements and the interface normal. The first methodological corrections have been proposed, as an adaptation of the level-set reinitialization algorithm to the anisotropic mesh.

=== Numerics - S. Mendez, IMAG & G. Balarac, LEGI ===

==== N1: Treatment of boundary conditions for high-order schemes - M. Bernard & G. Balarac (LEGI), G. Lartigue (Total Energies) ====

In the context of Finite Volumes Method, spacial accuracy of a numerical scheme depends on ability to evaluate accurately fluxes through interface of each control volume (CV).
Such accurate evaluation is not straightforward, especially when dealing with distorted grids.
This project follows the work of [1] where fluxes use pointwise quantities, which are reconstructed from integrated quantities advanced in time.
During the workshop, task force was dedicated to the treatment of **inlet** boundary conditions (BC) and **non-planar walls**.
For inlet BC, the key resides in the spatial integration of convective flux over discrete faces of the CV touching the boundary.
Such treatment lead to exact integration for linear inlet profile and large error reduction on other profiles.
Concerning non-planar walls, the strategy adopted consists in the enforcement of the BC on each discrete face, by modifying the normal component of the wall gradient in order to evaluate accurately the diffusive flux.
Again, a large reduction of this error has been observed.

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

==== N2: Implementation of linearised implicit time integration in ALE solver - T. Berthelon, G. Balarac (LEGI) ====

An linearised implicit time integration has recently been developed in the incompressible solver of YALES2. This new integration scheme allows to use larger time-step that the ones constraints by classic stability criteria inherent to explicit time integration method. This allows to reduce the restitution time of Large Eddy Simulations [1].
The objective of this project was to implement this new time integration in the ale solver in order to be able to reduce restitution time of moving mesh configuration.

The developments were validated on a scalar advection case and on a rotor-stator interaction case. Although the results seem to be in line with the explicit integration methods, the validation of the temporal convergence to 2nd order remains to be shown.

[1] ''Toward the use of LES for industrial complex geometries. Part II: Reduce the time-to-solution by using a linearised implicit time advancement, Berthelon et al., JoT, 2023''

==== N4: Non-uniform outlet pressure and coupling with CWIPI - J. B. Lagaert (LMO), Y. Lakrifi, T. Berthelon, G.Balarac (LEGI) , R. Letournel (Safran) ====

In simulations, artificial boundaries need to be introduced due to the limited size of computational domains. At these boundaries, flow variables need to be calculated in a way that will not induce any perturbation of the interior solution. During ECFD#7, a generic outlet boundary condition defined from non-uniform pressure has been implemented in Yales2. This non-uniform pressure can de determined from a traction model (null or advected from the interior domain, for example). This non-uniform pressure can also be deducted through a coupling between two simulations. In this case a coupling via CWIPI is performed where the velocity and the pressure are exchanged at the common boundary to define the inlet and outlet conditions, respectively.

==== N5: Optimization of the RBC solver - F. Rojas, S. Mendez (IMAG) ====

In the study of blood diseases, the mechanical behaviour of Red Blood Cells (RBCs) is one of the most relevant effects to take into account in the numerical models but also in experimental setups. Our system of interest is the thin gap of a rheometer where RBC suspensions are placed to explore their properties. To interpret the experimental data, the simulations of large suspensions of RBC are required to determine the blood’s microstructure (spatial arrangement of cells) and its rheological properties.

Currently, YALES2BIO’s RBC solver is capable to manage thousands of cells, but in order to approach closer to the experimental scales, we propose the characterisation and optimisation of its performance to reduce the computational requirements and increase the RBC’s number and domain sizes in our simulations. During the workshop a parametric study was carried out to obtain the strong and weak scaling. Studying the increase in the volume fraction allowed us to quantify how the cost of the simulation increases rapidly with the RBC’s number and identify which routines have the biggest impact on the performance. One conclusion is that the cost is spread of several routines, which makes code optimization more cumbersome. However, the amount of RBCs and RBC nodes duplicated over processors is identified as a key factor for performance. Indeed, as RBCs may interact with several partitions, it is duplicated as much as needed based on criteria of boundaing box intersections. However, the current criteria have been shown to be too loose.

In order to limit the amount of work during the RBC processing, stricter criteria were introduced to avoid unnecessary calculations at the level of the nodes with a small gain in performance. On the other hand, much better results were obtained using cartesian partitioning to optimise the bounding box of each processor, reducing the involved RBC operations: this demonstrates that the performances of the RBC solver may be optimized by a stricter selection of RBC duplicates over processors.

We thank Ghislain Lartigue and Renaud Mercier for helpful discussions.

==== N6: Electrodeformation of red blood cells, extension to 3D and improved accuracy at membrane - A. Spadotto, S. Mendez (IMAG), M. Bernard (LEGI) ====

The Leaky Dielectric Model is a popular framework to describe electric stresses over micro-scale membranes. We have adopted it to simulate the effect of a DC electric field on a red blood cell using the YALES2BIO solver. The goal of the project is to reproduce the electric charging process of the membrane, as well as the resulting stresses, which may yield to electrodeformation of the cell. From the point of view of the implementation, the grid is represented by a 2D surface mesh embedded in a 3D eulerian grid. The need to make variables stored on the surface interact with quantities stored on the Eulerian grid calls for a proper bidirectional 2D-membrane/3D-grid dynamic connectivity. The advancement of theis task during this ECFD has led to the first 3D simulation of a charging fixed spherical shell. Moreover, the estimation of grid variables on elements cut by the membrane has been improved thanks to a High-Order extrapolation. The latter has been successfully tested on 2D configurations. The project opens the way for a series of validation tests. In particular, future work will demand treatment of instabilities emerging in symmetrical configurations.

==== N7: Optimisation Dorothy - M. Roperch, G. Pinon (LOMC), B. Gaston (CRIANN), P. Benard (CORIA) ====

Dorothy is a Lagrangian code using the particle vortex method. This method must have a homogeneous distribution of particles in space. To achieve this, at regular intervals during the simulation a Cartesian grid with new particles is created. The weights of the old particles are interpolated for each of the new particles. Before ECFD7, all the processors knew the general grid and the new particles. The aim of ECFD was to parallelize this module to avoid memory problem. To do this, each processor creates a grid corresponding to the particles it knows. They then exchange data on the supperposition zones. This solves the issue because the quantity of new particles known is smaller. During ECFD7, a trial on a ring vortex case was successfully carried out to test domain communications and supperposition. The next step will be to implement this new method in the Dorothy code.

=== Turbulence - P. Benard, CORIA & L. Bricteux, UMONS ===

==== T1: Wall Law for immersed boundaries – P. Bénez (CORIA), M. Cailler (Safran), S. Meynet (GDTech), J. Carmona (CORIA), Y. Bechane (CORIA) ====
Conservative Lagrangian Immersed Boundaries (CLIB) are now a useful way to take into account complex geometries in YALES2. In order to study highly turbulent configurations, it appears necessary to implement wall law models adapted to this method. If we consider a non-moving immersed body, developing wall-law models in a conservative immersed boundary formalism presents numerous challenges related to the diffuse interface property of the solid and the continuous formulation of the penalty force. During the ECFD, a new formulation of the penalty force has been established to ensure the imposition of the wall shear stress across the immersed solid interface. A strategy based on the use of two near-wall level sets was implemented to estimate the wall shear stress from the LES fluid velocity field at a distance D from the solid interface. At the end of the ECFD, turbulent flat plate cases were set up to start the validation of the strategy implemented for a logarithmic wall law. Future works will focus on validating this strategy for fixed solids.

==== T2: Turbulence injection Compressible flows – P. Tene Hedje (UMONS), J. Carmona (CORIA), Y. Bechane (CORIA), L. Bricteux (UMONS) ====

Turbulence injection for compressible flows remains a real challenge. Indeed, In these types of flow, the acoustic waves must also be controlled on boundaries. In addition, the non-reflective formulation of the Navier-Stokes characteristic Boundary Conditions (NSCBC) generally used in compressible solvers produce spurious pressure oscillations when applied to turbulent flows, making turbulence injection difficult for such applications. During the ECFD, two turbulence injection approaches were investigated and applied within the framework of the Explicit compressible solver (ECS) of YALES2. The first involved modifying the NSCBC formulation to inject turbulence from the inlet of the domain. To this end, the vortical-flow characteristic boundary condition [1] was implemented in ECS and the first validations were performed. The second was to use AL to generate a turbulence grid in the flow [2]. Future works will focus on further validating these approaches.

[1] ''Guézennec et al., Acoustically nonreflecting and reflecting boundary conditions for vortcity injection in compressible solvers, AIAA journal, 47(7), 1709-1722, 2009.''

[2] ''Houtin-Mongrolle et al., Actuator line method applied to grid turbulence generation for large-Eddy simulations, Journal of Turbulence, 21(8), 407-433, (2020).''

==== T3: Aero-servo-elastic simulations of wind turbines including atmospheric effects – E. Muller (SGRE), U. Vigny (UMONS), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE) ====
Aero-servo-elastic engineering solvers used in the industry (i.e., BHawC) for structural response and power assessments are unsuited for wake simulations, as aerodynamic loads are usually derived from a BEM-like method. To tackle this, the YALES2 library was coupled (P11-ECFD3) to BHawC, the Siemens Gamesa Renewable Energy (SGRE) in-house certification code for wind turbines. This allowed the investigation of neutral atmospheric conditions. This project aims to include stable and unstable atmospheric conditions into this coupling based on the development done in T4-ECFD7. Therefore, this project is divided into three work packages:
Work package 1: Adjustment and refactoring of the existing coupling library between YALES2 and BHawC.
Work package 2: Rethink how turbulence is injected into the simulation (recycling in SGRE setup) to consider thermal and Coriolis effects.
Work package 3: Adapt how the blade forces are computed in the coupling to consider the resulting density fluctuations.

==== T4: Atmospheric solver – U. Vigny (UMONS), L. Voivenel (CORIA), S. Zeoli (UMONS), P. Benard (CORIA) ====
Wind turbines, bigger and bigger, are now influenced by atmospheric flows. An atmospheric solver has already been developed in YALES2 to represents some of its effects (Coriolis, veer, thermal stratification). In this continuum, the project has been divided into two work-packages.
- Work-package 1: The use of the Variable density solver (VDS).
Before ECFD7, thermal stratification was taken into account using the Boussinesq buoyancy approximation within the incompressible solver framework. Now, VDS can be used, taking into account all thermal effect. Results are promissing.
- Work-package 2: Wall law velocity filtering.
Wall law are using velocity at the first grid node to compute wall shear stress. Before ECFD7, atmospheric wall law were using the local velocity, leading sometimes to convergence errors. Now a gather-scatter filter can be used to average velocity (and temperature) at first grid node.

==== T5: Implementation of the RVMs-WALE model in YALES2 – L. Bricteux (UMONS), P. Benard (CORIA), Y. Bechane (CORIA) ====
This study focused on the implementation of an advanced multiscale variational subgrid-scale model, incorporating scaling based on the WALE (Wall-Adapting Local Eddy-viscosity) model within YALES2. This model has demonstrated efficiency across various flow configurations, and it is anticipated that its multiscale nature can enhance the spectral selectivity. The aim is to ensure that its dissipative effects specifically target the smallest scales near the cut-off point.

Additionally, collaborative work with G. Balarac aimed to enhance the mesh adaptation strategy for wall-bounded flows with massive boundary layer detachment and vortical wake.
This new strategy based on vortex detection was developed during the ECFD6 and ECFD4 workshops. We have now shown that this strategy is effective.
Flow simulations around a hemisphere at Reynolds number Re=55K have been conducted, and we anticipate publishing the results soon."

==== T6: Development of coupling between YALES2-OpenFAST – A. Parinam (TUDelft/CORIA), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE) ====

==== T7: Confidence intervals for estimators – C. Papagiannis, G.Balarac (LEGI), R. Letournel (Safran) ====

=== Two Phase Flow - M. Cailler, Safran & V. Moureau, CORIA ===

==== P1: Level set reinitialization at the contact line for boiling flows - H. Lam, M. Benard, G. Ghigliotti (LEGI) ====

In Savinien Pertant's PhD thesis (2022), DNS of nucleate boiling at the bubble scale were performed, but suffered some lack of accuracy in the imposition of the contact angle.
Indeed, the contact angle was not well respected, with a difference of around 10 degrees between the desired angle and the angle measured on the solution.
This lack of accuracy, that contrast with the accurate imposition obtained in the spray solver (SPS), is due to fluctuations of the contact line. This behavior that was traced back to the modifications of the level set reinitialisation needed to take correctly into account the triple line, and for which the solution applied in 2022 was to revert to the standard Janodet reinitialisation.
S. Pertant tested, at the very end of its PhD, a correction which nullifies the temperature transport term at the first node from the wall of the contact line. This correction was introduced to overcome an instability of the code when the contact line velocity on the substrate changes direction, from receding to advancing.
It turned out at the ECFD7 that this correction proves to be very efficient to stabilise the contact line for contact angles between 50 to 90 degrees even in the case of the use of the level set reinitialization. We were able to simulate nucleate boiling with a smooth contact line at the triple line and a precision in the angle of the contact angle of around +-0.5 degrees. More work remains to be able to run DNS of nucleate boiling for extreme contact angles (<50° and >100°). Moreover, longer runs will be needed to further confirm these results.

==== P2: Compatibility of Boiling solver with PCS and MPH structure - H. Lam, M. Benard, G. Ghigliotti (LEGI) ====

The boiling solver does not work since the introduction of MPH data structure and PCS solver in March 2022. An investigation work was carried out to understand the changes made between the previous and the new version of the different solvers. A simple test case was created to show potential differences between the working version of the code and the new one. Several problems were spotted: the order of level set declaration became important as it is the first one declared which is advected. Sign convention was chosen differently for the mass transfer rate. The temporal discretization of the level set was different.
A test case with no flow and at an imposed mass transfer rate (i.e., no coupling of the level set with the temperature field) was run successfully and the results of the commit prior to the March 2022 modifications were retrieved. More work is needed to find the origin of the differences between the two solvers when the temperature field is solved and coupled with the level set and the velocity field. New common test cases for the two solvers will have to be implemented in order to cross-validate the results and avoid such cases happening again (i.e., cross-fertilization).

==== P3: Blood platelets adhesion model - C. Raveleau, S. Mendez, F. Nicoud (IMAG) ====

Medical devices in contact with blood (e.g. artificial valves) are used to treat various cardiovascular diseases, but their thrombogenicity remains the main unresolved issue in their development. A numerical model of blood platelets is being constructed to help to understand the effect of microstructuration on the thrombogenicity of artificial surface. The Force Coupling Method (FCM) was previously implemented and allows the modelisation of ellipsoidal particle and their interaction with the surrounding fluid. During the workshop, the particle model was extended to include adhesive and repulsive interactions with walls or with other particles. The adhesive bonds are modeled with springs forming when the distance between a node of a particle surface and a node of the wall or another particle is smaller than a given threshold. The stiffness of the bond is increased after a given formation time to mimic the 2-step adhesion process of platelets to von Willebrand Factor. A Lennard-Jones potential was used to model the collision of particles. Future work will aim at generalizing these implementations for an arbitrary number of particles (currently only working for 2 particles) and ensuring the interactions are unaltered by the crossing of a periodic boundary.

==== P4: vWF Unfolding - C. Raveleau, S. Mendez, F. Nicoud (IMAG) ====

==== P5: Towards even more efficient particle algorithms - M. Helal (CORIA/Safran), M. Cailler (Safran) ====

Lagrangian particles are widely used in the YALES2 plateform to model: liquid spray, granular flow, two-phase flows with SPH approach or solids in IB method.
Though important developments to handle efficiently high number of particles in massively parallel simulations, the growing use of particles in Yales2 make necessary to re-evaluate and optimize the performances of Lagrangian particles algorithms handling.
Objective of this project was twofold: analyze and improve the performance and robustness of the newly developed SPH solver of YALES2 and improve the performance of the Lagrangian particle relocation (identification of connectivity between Lagrangian and Eulerian grid) during the Dynamic Mesh Adaptation.
Regarding the first subject, profiling tools have been used to identify the hot-spots and bottle-necks in the SPH solver. Optimizations including code factorization, removal of string comparison allows to reduce the computational cost by a factor 3. Moreover, robustification of the solver was achieved.
In the second sub-project, a new implicit 4th-level decomposition has been introduced. This implicit decomposition consists in contiguous coloring of sub-el_grp in element group. The availability of smallest group of elements has been used to improve the local particle relocation algorithm that mainly relies on bounding-box comparison. This new relocation algorithm has been tested for various number of sub-el_grp on a representative case of gear lubrication showing a decrease by a factor 3 to 5 of the relocation algorithm. Perspective is to extend the use of sub-el_grp to the interpolation algorithm.

==== P6: Two fluid and phase change in PCS - C. Merlin (Ariane Group), J. Carmona (CORIA), V. Moureau (CORIA) ====

==== P8: Wall liquid film numerical model - N. Gasnier (EM2C/Safran), J. Leparoux (Safran), J. Carmona (CORIA) ====
Wall liquid films are likely to be formed when fuel sprays impact the walls of aeronautical fuel injection systems. Such phenomenon may have a significant influence on the whole combustion process, however the small scales involved prevent from performing high fidelity simulations of film flows in the context of industrial geometries. Therefore, a low order model is required to model the dynamics of thin liquid flows under the action of spray droplets and of a turbulent gas shear. During ECFD7, a liquid film numerical model accounting for the influence of surface tension as well as gas shear, and based on the 2-dimensional Shallow Water Equations was implemented in Yales2. This model was then coupled to an algorithm ensuring a proper transition between fully resolved liquid structures (levelset) and film model during liquid droplet impacts on a solid wall.

==== P9: Casting simulation for the study of ceramic core displacement - S. Sirot, R. Mercier, M. Cailler (Safran), S. Meynet (GDTech) ====

Ceramic core displacement and deformation during the casting process is a major source of cooled blades manufacturing scrap. A possible source of core deformation may be the fluidic forces due to the filling of the mold with the liquid alloy. Predictive numerical simulations of the casting process would be an essential asset to increase the efficiency of the conception and industrial processes. During the workshop, a numerical methodology to simulate the filling process was drawn, with several modelling levels (with or without surface tension and slipping-wall conditions), in order to estimate the relevance of each of these models. Numerical results were then compared to available experimental results. Numerical deformation of the core was approximated as a beam flexion. Despite this post-processing approximation, the correlation between experimental measurements and numerical simulations is satisfying. The evolution of the core displacement with the inlet velocity of the fluid also has the same behaviour in the experiments and in the simulation. Future work will aim at including the dynamic contact angles in the simulations, in order to evaluate the relevance of this finer modelling, as well as correlating simulations with experiments on cases more representative of the industrial process.

==== P10: Velocity regularization for Euler-Lagrange conversion - I. El Yamani (CORIA/Safran), M. Cailler (Safran), L. Voivenel, J. Carmona (CORIA) ====

The Euler Lagrange multi scale approach aims to reduce the computational costs when simulating two phase flow. To reduce the cost even more, more droplets have to be converted in the Lagrangian formalism where droplets are seen as point forces. It implies that droplets can not always check the hypothesis of the LPP (Lagrangian Particle Point) formalism which is that the diameter of the particle has to be much smaller than the cell size. This hypothesis allows to have a good approximation of the undisturbed velocity for the Lagrangian particle. If the hypothesis is not checked when a Eulerian droplet is converted into a Lagrangian particle a residual velocity field can exists and therefore the velocity given to the particle is impacted by itself. This project aims to filter the gaseous velocity field through a gaussian filtering to remove the contribution of the Eulerian droplet to better approximate the undisturbed velocity.

=== Combustion - K. Bioche, CORIA & R. Mercier, Safran ===

==== C1: Plasma discharge models for reacting system - S. Wang, B. Kruljevic, B. Fiorina (EM2C), Y. Bechane (CORIA) ====

To reduce the expensive computational cost of Plasma-Assisted Combustion (PAC) full 3D simulations, the EM2C laboratory has developed phenomenological approaches to model Nanosecond Repetitively Pulsed (NRP) plasma discharges in reacting flows (Castela 2016 & Blanchard 2023). As part of previous works and ECFDs, both models were implemented and validated in the Low-Mach number framework (YALES2-VDS). While they were also implemented in the Compressible framework (YALES2-ECS), the validation against existing measurements or computations remained. During the workshop, numerical simulations of pin-to-pin configurations were performed with different numerical schemes and reactive mixtures to validate both models in ECS. The energy deposition was relatively well-validated through 2D simulations in the conditions of Castela et al. CNF 2016 and Rusterholtz et al. JPhysD 2013. A glimpse of baroclinic instabilities was observed through 3D simulations in the conditions of Castela et al. PROCI 2017.

==== C3: Dynamic sub-grid-scale wrinkling for diffusion flames - S. Dillon (EM2C/Safran), R. Mercier (Safran), E. Espada, B. Fiorina, D. Veynante (EM2C) ====

Large-eddy-simulation (LES) of reactive flows is widely used in both academic and industrial applications. Combustion phenomena occur at a scale often smaller than the LES mesh size, therefore, turbulent combustion models are required to account for unresolved turbulent flame interactions. The modeling of sub-grid-scale (SGS) flame turbulence interactions can be described with a flame surface wrinkling factor which measures the ratio of the total flame surface area to the resolved flame surface area. Flame surface wrinkling models are often expressed by assuming equilibrium between turbulent motions and flame surface wrinkling, however, in realistic burners non-equilibrium is present and dynamic models are needed to adapt model parameters. Fractal-like models require information about the outer and inner cut-off length scales along with a fractal exponent, which is determined dynamically from resolved scales in the LES. The dynamic formalism can be coupled with the Filtered Tabulated Chemistry for LES (F-TACLES) model, where the required cut-off length scales are tabulated in the F-TACLES table along with other filtered thermochemical variables. The coupling of the F-TACLES model with the dynamic formalism has been previously applied to premixed flames in the past, however, the formal extension to non-premixed flames has never been investigated. The objective of this project is to investigate the performance of the dynamic SGS flame surface wrinkling model coupled with the F-TACLES model for non-premixed flames. A priori tests are conducted on a 2D H2/Air reactive mixing layer and HYLON, a 3D turbulent dual-swirl coaxial H2/Air injector. In both 2D and 3D cases, the modelled flame surface density shows good agreement with the filtered flame surface density extracted from the DNS. Moreover, the variation of the fractal model exponent in the HYLON test case is significant, highlighting the importance of the dynamic procedure. A posteriori tests were also conducted, and modelled chemical reaction rates show promising results.

==== C4: Developement of an automated virtual scheme generator for CFD - T. Luu, M. Hustache, N. Darabiha, B. Fiorina (EM2C) ====

In reactive CFD simulations, a non-negligible part of the time cost is spent in the resolution of the chemical system. Simplified chemistry models aim to reduce the number of transported species while still ensuring a correct representation of the phenomena of interest. Among them, the virtual chemistry method consists of using “virtual” species and reactions to reproduce detailed chemistry results through a mechanism of drastically smaller size. These “virtual” species and reactions are optimized to target quantities of interest such as temperature, laminar flame speed or pollutants. In practice, the optimization is done using a learning database composed of representative canonical reactive configurations computed with detailed chemistry. The objective of this project was to develop a tool to easily generate virtual schemes. The tool, named VISION (Virtual Scheme optimizatION), is currently able to both generate a user-defined database of wide reactive configurations and optimize a given scheme structure using either CANTERA or REGATH.

==== C5: Partially-Stirred reactor model for MILD combustion - E. Stendardo, L. Bricteux (UMONS), M. Laignel, K. Bioche (CORIA), J. Blondeau (VUB) ====

MILD combustion produces intense turbulence and extensive reaction zones, necessitating costly mesh refinement over large areas. Practical mesh lacks precision, leading to sub-grid heterogeneity and turbulent fluctuations. A Partially Stirred Reactor model was implemented to address turbulence-combustion interaction. This model multiplies the source term by a limiter factor, allowing modelling of residence time in the inner cell reactive structure. Testing various limiter formulations based on mixing and chemical timescales revealed increased computational costs. Future work aims to reduce costs by utilizing the model only where necessary. This ongoing research seeks to optimize performance while minimizing computational overhead for efficient application in engineering scenarios.

==== C6: Static Mesh Adaptation for Hydrogen High pressure combustion using GPUs - G. Hexilar, C. Brunet, R. Mari, S. Richard (Safran), P. Pouech, Q. Douasbin, G. Staffelbach (Cerfacs) ====

This research project focuses on advancing the understanding of hydrogen combustion under high-pressure conditions (up to 10bars), employing an automated workflow coupled with static mesh adaptation to tailor computational simulations to specific requirements. The study aims to enhance the accuracy and efficiency of combustion models by utilizing both Central Processing Units (CPUs) and Graphics Processing Units (GPUs). The automated workflow streamlines the simulation process, optimizing resource utilization and minimizing manual intervention. Static mesh adaptation further refines the computational mesh based on evolving combustion dynamics, ensuring accurate representation of high-pressure hydrogen combustion phenomena. By leveraging the parallel processing capabilities of GPUs alongside traditional CPUs, the research team aims to achieve significant computational speedup. This innovative approach not only contributes to fundamental insights into high-pressure hydrogen combustion but also establishes a robust framework for scalable and efficient simulations in complex reactive flow scenarios.

==== C7: High fidelity simulation of a cone calorimeter - A. Grenouilloux, K. Bioche (CORIA), N. Dellinger (ONERA), R. Letournel (Safran) ====

A methodology to simulate the decomposition of a composite sample in a calorimeter cone has been proposed. The configuration consists in the imposition of an incident radiative flux that heats the test coupon until it degrades. During test campaigns, the composite degradation can lead to the auto-ignition of the outgassed species, a phenomenon that needs to be predicted by the simulation. The variety of physical phenomena encountered, as well as the different characteristic time-scales, require the implementation of a coupled simulation. Hence, the proposed methodology relies on the coupling between two solvers of the massively parallel library YALES2 (fluid, radiation) and the MoDeThec solver developed at ONERA (solid degradation). First, a set of elementary validation tests to characterize the composite’s properties has been performed, showing good agreement with experimental data. A reduced three-equation kinetic scheme for the ignition delay has been derived, which aligns with experimental observations. Additionally, the framework for the coupled simulation involving the three solvers has been implemented.

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

==== U1: Refactoring the YALES2 tools - J. Leparoux, M. Cailler (Safran), L. Voivenel, J. Carmona, I. El Yamani (CORIA), S. Meynet, L. Korzeczek (GDTech) ====

The YALES2 distribution "tools" are becoming difficult to read and are a mixture of several types of tools. This leaves developers and end users unaware of what exists and how to use it (duplicated functions or tools) and makes it impossible to propose generic data analysis tools (FFT, confidence intervals, ...) that can be easily applied to YALES2 data structures. The main efforts have been concentrated on promoting a new architecture for YALES2 distribution tools with an object-oriented structure, including a refactored version of the main readers of YALES2 data. Several tutorials using Jupyter notebooks have been published for demonstration and explanation. A new CLI is now available under the name 'y2tools'. More work is needed before this structure can be pushed to the master trunk.

==== U2: Improved USEX for Multi-Scale Eulerian-Lagrangian simulation - L. Voivenel, J. Carmona, I. El Yamani (CORIA) J. Leparoux, M. Cailler (Safran) ====

The multi-scale Eulerian-Lagrangian approach has now reached a certain maturity and is being used to simulate fuel spray atomization. Post-treatments of these multi-scale simulations require the development of specific tools that track liquid structures either described in an Eulerian or Lagrangian way. In this project, we implemented a strategy to register in a post-treatment particle-set all Eulerian droplets crossing an arbitrarily shaped surface (described with an interior-boundary). The strategy is based on artificial Eulerian droplet advancement (using a Lagrangian representation) and verification of the new Eulerian droplet position compared to the surface of interest. We used this strategy to build a new post-treatment that allows to track both Eulerian and Lagrangian structures and build particle size or velocity distributions.

==== U3: Evaluate technological debt - P. Pouech, T. Marzlin, A. Dauptain (CERFACS) ====

The tool dedicated to cleaning and extracting meta data from the codebase (Tucan) is now able to interpret the most common #Ifdef statements for C and Fortran. Is has be tested successfully on the codebases of Yales2 , Neko and some elements of the Chemkin II package, for Fortran language. It has also included more tests, especially on C++ samples and large files. The call-graph aggregator “Marauder’s map” also evolved a lot. It was used as a refactoring monitoring tool for the UX project U1 “Refactoring Yales2 tools”, on a codebase mixing Python and Fortran files, providing feedback to both U3 and U1 teams. With this experience, we plan to add at least two additional complexity metrics in the months to come : the single component #Ifdef footprint , and the Custom Structures footprint : declarations vs usage in the code.

==== U4: CWIPI 1.0 porting - N. Dellinger, B. Andrieu, K. Hoogveld, E. Quémerais (ONERA), A. Grenouilloux (CORIA), R. Letournel (Safran) ====

Coupling is a cornerstone of numerical simulation, especially for addressing multi-physics problems using highly-specialized solvers for each phenomenon. The CWIPI library, developed at ONERA for coupling codes in a massively parallel environment, has been used in YALES2 for many years for internal and external coupling.
Significant developments have been carried out in recent years to improve the performance and usability of CWIPI, resulting in the release of version 1 in july 2023. This version features a completely revised API to overcome the limitations of version 0.12 and offer more possibilities to users.
The goal of this project was to support users in their transition to version 1. A training course based on Jupyter Notebooks was first organized. Assistance was then provided to successfully port MoDeTheC's and YALES2's internal couplings to the new version. Some fixes were made in CWIPI along the way, and will be reported in a new patched version.

==== U5: Integration of YALES2 in PRESTO supervisor - A. Pushkarev (GE Vernova), G. Balarac (LEGI) ====

A Graphical User Interface (GUI) exists at GE Vernova (Hydro) provides a user-friendly solution to perform numerical simulations for typical geometries of hydroelectric turbine in varied operation regimes. Previously, we implemented an interface for YALES2 code as alternative of CFX solver for this GUI client. Actual project is dedicated to implementation of the automatic mesh generation process for the runner section of the turbine using only section profile files of geometry such as blade profiles, meridional channel section, guide vane profile, etc... The algorithm should be able to generate a new *.msh mesh file once geometry profiles are updated as well as to setup standard named sections of the numerical domain.

==== U6: Optimization of YALES2 compilation time - R. Mercier (Safran), G. Lartigue (Total Energy) ====

YALES2 Compilation time are very different depending on the compilation options but also depending on the machine where the code is built. In particular, compiling on cluster always takes more time than on local station because of the slower filesystems. On these machines, the compilation time scales with the number of .f90 files to compile. 1) During this project we proposed a detailed compilation timing system activated with Y2_COMPILE_TIMER = TRUE and plotted using the tool y2_compilation_gantt.py. 2) This new profiling tool allowed us to spot some very large modules including a lot a dependencies. To enhance the tasks parallelization, the expl_comp_numerics module has been spliced in several modules. An important gain in compilation time has been obtained. 3) Another optimization of the compilation time have been developed using an on-the-fly modification of the .f90 tree. The use of _h.f90 is dynamically removed and replaced by including equivalent .defs files allowing the divide by 2 the number of .f90 files to compile. Some demonstrations have been performed but this compilation mode still have to be cross-check with different compilation option to ensure its robustness.
At the end of the project, the sum of all optimization allowed us to divide by 2 the compilation time.

Ecfd:ecfd 5th edition

2022-02-01T23:59:24Z

Dillon: /* User experience - J. Leparoux, SAFRAN TECH */

{{DISPLAYTITLE: ECFD workshop, 5th edition, 2022}}

== Description ==
{| align="right" style="text-align:center;" cellpadding="2"
| [[File:Logo_ECFD5.png | center | thumb | 350px | ECFD5 workshop logo.]]
|}
* Event from '''23th to 28th of January 2022'''
* Location: [https://www.bonsejour-laplage.com/vacances-tout-compris Centre Bonséjour], Merville-Franceville, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points.
** mini-workshops. Potential workshops are listed below.
* Free of charge
* More than 50 participants from academics (CERFACS, CORIA, IMAG, LEGI, EM2C, UMONS, UVM, VUB, UCL, TUDelft), HPC center/experts (GENCI, AMD, CINES, CRIANN) and industry (Safran, Ariane Group, Siemens-Gamesa).

* Objectives
** Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
** Identify the technological barriers of exaflopic CFD via numerical experiments
** Identify industrial needs and challenges in high-performance computing
** Propose action plans to add to the development roadmaps of the CFD codes

== News ==

* 03/11/2021: First announcement of the '''5th Extreme CFD Workshop & Hackathon''' !

[[File:Banniere_ECFD5_sponso.png|text-bottom|600px]]

* 13/01/2022: After discussions with the participants, the '''5th Extreme CFD Workshop & Hackathon''' is maintained as an in-person event! It will be also possible to attend to the plenary sessions and participate remotely to the workshop.

* 14/01/2022: The [[#Agenda|ECFD5 program]] is online! The plenary sessions will be announced soon!

* 20/01/2022: The plenary sessions are now defined:
** P1 - 24/01/2022: GPU porting challenges and quantum computing, présentation machine Adastra by G. Staffelbach (CERFACS) + Presentation of the new cluster from CINES called Adastra by C. Andrieu (CINES)
** P2 - 25/01/2022: News, perspectives and future of GPU computing applied to CFD by A. Toure (AMD)
** P3 - 26/01/2022: Theory, applications and perspectives of the Lattice-Boltzmann Method by P. Boivin (M2P2)
** P4 - 27/01/2022: Concepts and notions of mesh adaptation by C. Dapogny (LJK)

* 23/04/2022: '''The ECFD5 event has now started!!''' [https://www.linkedin.com/feed/update/urn:li:activity:6891053385072594944| LinkedIn post]

* 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 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 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

2022-02-01T00:01:11Z

Dillon: /* User experience - J. Leparoux, SAFRAN TECH */