Extreme CFD workshop - User contributions [en]

Ecfd:ecfd 9th edition

2026-02-02T08:34:19Z

Houtin: /* Projects */

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

== Description ==

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


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

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



== News ==

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

== Thematics / Mini-workshops ==

To be announced...

== Projects ==

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

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

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

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

==== T9 - LES-based aero-servo-elastic simulation of wind turbines - Etienne MULLER (CORIA & SGRE), Pierre BENARD (CORIA), Félix HOUTIN-MONGROLLE (SGRE), Bastien Duboc (SGRE), Hakim HAMDANI (GDTech) ====
The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine as well as the structural deformation when Yales2 is coupled to an external library/code. In the past years two coupling library have been developed, one to BHawC (SGRE certification tool) and one to OpenFast (NREL open access/open source). To improve the user and developer experience a generalization of the two coupling is conducted in this project.

'''WP1/2''': Update the existing coupling libraries for OpenFast and BHawC coupling with ALM of YALES2.

'''WP3''': Generalize/uniformalize the coupling DLL and the calls to it in YALES2.

'''WP4''': Enable both coupling to work with both Actuator lines and Actuator Disks.

'''WP5''': Implementation and integration of the Risoe Dynamic stall model. Following ECFD6 T5 (Turbulence flow project 5).

'''WP6''': Miscellaneous related to actuator line covered through this ECFD9.

Ecfd:ecfd 8th edition

2025-02-10T17:03:15Z

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

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

== Description ==

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

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

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

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

== News ==

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

== Thematics / Mini-workshops ==

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

To come...

== Projects ==

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

==== T2 - Dynamic Smagorinsky in Dorothy ====

==== T3 - Turbulence injection strategy for compressible flows ====

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

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

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

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

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

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

* WP1: Improve inflow generation
Anand: pressure controller

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

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

==== T6 - FSI model in Dorothy ====

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Ecfd:ecfd 7th edition

2024-02-06T09:46:59Z

Houtin: /* Turbulence - P. Benard, CORIA & L. Bricteux, UMONS */

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

== Description ==

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

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

[[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.

=== 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, IJNMF 2020, Bernard et. al''

==== 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, 2022

==== 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.

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

==== 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 ====
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.

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

==== 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.

=== 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.

==== 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.

==== 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 ===

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

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.

Ecfd:ecfd 7th edition

2024-02-06T09:46:49Z

Houtin: /* Turbulence - P. Benard, CORIA & L. Bricteux, UMONS */

Ecfd:ecfd 7th edition

2024-02-06T09:46:18Z

Houtin: /* Turbulence - P. Benard, CORIA & L. Bricteux, UMONS */

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

== Description ==

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

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

[[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.

=== 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, IJNMF 2020, Bernard et. al''

==== 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, 2022

==== 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.

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

==== T4: Atmospheric solver ====
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.

==== 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.

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

==== 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.

=== 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.

==== 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.

==== 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 ===

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

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.

Ecfd:ecfd 6th edition

2024-02-06T09:44:13Z

Houtin: /* Mesh adaptation - R. Letournel, Safran & V. Moureau, CORIA */

{{DISPLAYTITLE: ECFD workshop, 6th edition, 2022}}

== Description ==
{| align="right" style="text-align:center;" cellpadding="2"
| [[File:Logo_ECFD6.png | center | thumb | 350px | ECFD6 workshop logo.]]
|}
* Event from '''23th of January to 3rd of February 2023'''
* Location: [https://www.hotelclubdelaplage.com Hôtel Club de la Plage], Merville-Franceville, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* More than 60 participants from academics, HPC center/experts and industry.

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

[[File:Banniere_ECFD6.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_6th_edition]]

[[File:Banniere_ECFD6_sponso.png|text-bottom|600px]]



== Agenda ==

[[File:ECFD6_program.png|text-bottom|600px]]

== Thematics / Mini-workshops ==

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

== Projects ==

=== Hackathon - G. Staffelbach, CERFACS & P. Begou, LEGI ===

=== Mesh adaptation - R. Letournel, Safran & V. Moureau, CORIA ===
*'''Sub-project 1: Mesh adaptation for vortical flows (MAVERICK) for coupled systems of differential-algebraic equations (L. Bricteux, G. Balarac, S. Zeoli, G. Lartigue)'''

This project is in the continuation of what has been investigated in the previous ECFD workshops.
It aims to develop mesh adaptation strategies for DNS of vortical flows. Turbulent flows are vortical flows. It is thus of primary importance to capture the vorticity dynamics properly if one wants to obtain physically realistic results. As the vorticity field is compact, mesh adaptation allows to refine the mesh only where flow physics happens, in the same way a vortex particle-mesh method would do.
The main findings gathered during ECFD6 are:
1/ Gmsh can produce high quality base mesh for AMR with Yales2, yet the proper choice of options is not trivial.
2/ Initial mesh should be as smooth as possible (low hgrad), which is really challenging for wall resolved turbulent wall bounded flows.
3/ Refinement sensor based on vorticity gradients (palenstrophy) seems to works fairly wel.l
4/ two levels strategy provides encouraging results

*'''Sub-project 2: ALE and remeshing for surface mesh based on 1D deformations (F. Houtin-Mongrolle, E. Muller, B. Duboc)'''
During operation and maintenance, wind turbines are subject to particular inflow that may trigger significant 3D aerodynamic effects (Vortex, Induced Vibrations, over-speed, flutter…) that deform the blades. Typical Actuator line methods (ALM) are not enough to capture these types of Fluid-structure interactions. This project aimed to go from an ALM way of modeling the blades to a body-fitted mesh with ALE/re-meshing, depending on the blade deformations. The work is divided into three work packages. Work package 1: impose displacements from a 1D discretized line (6 DOF) into the blade surface. Work package 2: trigger re-meshing if the displacement of the cells reaches a bad quality (high skewness, negative node volume). The last work package is to be able to retrieve the forces and moments from the 2D surface to the initial 1D discretized line.

=== Numerics - S. Mendez, IMAG & M. Bernard, LEGI ===

* '''Sub-project 1: Multi-level domain decomposition method (DDM) for coupled systems of differential-algebraic equations (A. Quirós Rodrígues, V. Le Chenadec)'''
The numerical approximation of multi-physics problems gives rise to complex linear systems, the solution of which leverages preconditioning techniques such as multi-grid or domain decomposition methods. This project aimed at coupling two Julia packages that being actively developed: a two-dimensional Navier-Stokes solver for free-surface and two-phase flows (Flower.jl) on the other, and a Domain Decomposition package for Cartesian grids (DDM.jl). The decomposed matrix-vector product was optimised to reduce the overhead associated with halo exchanges. The implementation of a deflated Conjugate Gradient as well as one- and two-level Additive Schwartz Method were also completed and shown to significant reduce the number of iterations for inverting monolithic systems (i.e. without resorting to operator splitting), shown to be independent of the number of subdomains for constant property flows. Future work will focus on a further optimisation of the implementation for vectorisation and multi-threading, and extension of the deflation to generalised coarse spaces to support highly discontinuous transport properties (GenEO).

* '''Sub-project 2: Ghost fluid method (GFM) for Electrodeformation (A. Spadotto , S. Mendez)'''
According to the Leaky Dielectric Model, red blood cells (RBCs) are subject to a force which is proportional to the jump of the Maxwell tensor. This latter is a quantity scaling as the square of the electric field, which under the quasi-static hypothesis is defined as the gradient of the electrostatic potential. To work out the potential, an elliptic interface problem must be solved, taking into account the presence of the RBC membrane. The aim of the project was implementing the Ghost Fluid Method (GFM) to face the interface problem. Good results were obtained on unstructured meshes. Secondly, a gradient calculation was performed applying the Green-Gauss scheme, modified in the style of the GFM. Future work will focus on interpolation of the gradient field onto the membrane to get an estimation of the effort. Possibly, high-order schemes for the gradient calculation will be explored. In a second time, the effort calculation will be merged into an Immersed Boundary solver for the RBC dynamics.

* '''Sub-project 3: Optimization of the high order framework (HOF) for Navier-Stokes incompressible (M. Bernard, P. Bégou, G. Lartigue, G. Balarac)'''
Over the past years, a framework has been developed to improve the spatial accuracy of numerical schemes on distorted meshes.
However, even if the solution is more precise, the computational cost of the overall resolution of Navier-Stokes equations is large.
As a consequence, HOF becomes profitable only on thin meshes thanks to a better spatial convergence order.
The code has been analized with different analysis tools (MAQAO, Gprof, Scalasca).
The main time consuming routines have been identified and improved.
Moreover, some algorithms have been refactors such that the resolution of Navier-Stokes equations has been speed-up by a factor 2.5.

* '''Sub-project 4: Force coupling method (FCM) for particulate flows (C. Raveleau, S. Mendez)'''
The Force Coupling Method (FCM) allows the representation of particles in flows from 0 to finite Reynolds number based on a regularization of the multipole expansion for Stokes flows, providing enough particle resolution to match the exact Stokes flow away from the particle. This is done by using a Gaussian function to spread the singular forces over a domain whose size is derived from physical quantities of interest such as the settling velocity of a particle under gravity in Stokes flow and the particle radius. During the workshop, the first term of the multipole expansion (monopole) and the antisymetric part of the second term (antisymetric dipole) were implemented and validated against test cases from the litterature. The monopole was tested in the case of a particle held fixed against an incoming flow and the dipole was tested by applying a torque on a particle in a still fluid. In both cases, the velocity profiles matched the results from the litterature and approximated well the Stokes solution at a distance of about 1.5 radii from the particle center. It was also compared to the conservative immersed boundary method (CLIB solver) implemented in YALES2 for the monopole test case. Both methods give good results, although the FCM is able to predict the expected solution with a coarser mesh than CLIB. Cases where the particle moves are under investigation.

* '''Sub-project 5: Breaking limitations of the linearized implicit time advancement (T. Berthelon, G. Lartigue, G. Balarac)'''
The explicit time advancement classically used in ics solver is limited by CFL constraint. In order to get ride of this constraint, an implicit time advancement method, based on the linearization of the convective term, has been recently developed.
However, the method is limited by difficulties to solve linear system, with the BiCGSTAB2 algorithm, during the prediction step. The objective of this project was to understand these limitations. The correction of a bug on the boundary conditions (viscosity imposed at zero) was identified. In addition, the spatial scheme and the presence of a buffer zone at the end of the domain showed a great influence on the convergence of the prediction. The perspectives for a more robust and efficient use of this temporal integration consist in working on the spatial schemes and on the pre-conditioning.

* '''Sub-project 6: Development of a traction open boundary condition (TOBC) in Yales2 (J.B. Lagaert, G. Balarac)'''
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#6, open traction boundaries (TOBC) has been implemented in Yales2. Instead of enforced pressure value or normal derivative on boundaries, the traction is settled. The traction term gathers the deformation stress and pressure gradient.
The implementation has been validated on simple test-case : comparison has been performed between theoritical solutions and numerical solution computed with a Dirichlet TOBC. Future work will add a stabilization term for large backflow and traction models to estimate the values to enforce on outlet boundaries

* '''Sub-project 7: Development of new spatial differential operators in Yales2 (M. Bernard, G. Lartigue)
It exists different philosophies for computing differential operators on distorted meshes.
In a HPC context, the 2 main approaches are the Green-Gauss operators and the Least-Squares operators.
During ECFD#6, 2 new types of "non-compact" Hessian operators have been implemented by computing successively the gradient operator, eather with Green-Gauss gradient, or with Least-Squares gradient.
Those operators lead to good convergence order, even on distorted mehes.
However, their application on low-resolution signals lead to large error magnitude due to their extended stencil.
Another pair of gradient & hessian Least-Squares operators have been implemented, leading to 2nd and 1st order accuracy for the gradient and hessian respectively.
Those operators have very interesting characteristics as their stencil is restricted to the direct neighbors only and their computational cost remains low.

* '''Sub-project 8: DOROTHY optimization (M. Roperch, H. Mulakaloori, G. Pinon, P. Bénard, G. Lartigue)'''
DOROTHY is three-dimensional unsteady Lagrangian Vortex Particles Methods code. Because of the number of particles that increases during the numerical simulation, some routines begin to be very expensive. During this workshop, MAQAO has been used to identify the most time consuming routines and why. They have been factorized, merged and vectorized. At the end, these routines have been speed up by a factor 4 and the global program by a factor 2.8.

* '''Sub-project 9: Anamika, a tool to improve programming productivity (K. Mohana Muraly, G. Staffelbach)'''

=== Turbulence - P. Benard, CORIA & G. Balarac, LEGI ===

* '''Sub-project 1: Explore hybrid RANS/LES strategies (T. Berthelon, G. Balarac)'''

For complex industrial applications, LES can still lead to a too long restitution time. In other hand, statistical approaches can lead too a lack of accuracy. In this project, the potentiality of hybrid approaches combining both have been explored. Conventional hybrid RANS/LES approaches consider a unique solution field, with an unique transport equation and a clusre terme modeled using RANS or LES models depending of the regions. The main idea is to evaluate a strategy based on a separation between mean fields and fluctuations with distinct coupled transport equations. First elements of validation using YALES2 code are shown that it was possible to correct the prediction of a RANS models, by performing LES of the fluctuations. Next steps should be to consider disctinct meshes, or even computational domains for RANS and LES with this strategy.

* '''Sub-project 2: Flow Instabilities over Rotating curved Surfaces (S. Sawaf, M. Shadloo, A. Hadjadj, S. Moreau, S. Poncet)'''

For evaluating the effect of the clearance between the blade tip and the casing of axial ducted fans on noise emissions, LES offers excellent tool to capture the consitricted flow around the blade tip especially for small clearances where RANS fails because of unsteady flow conditions. LES simulation of the aerodynamics is the first step toward extracting accoustics data that helps to improve the design of axial ducted fans so they comply with the noise emission regulations in admistrative buildings. noise emmisions are estimated using analytical aeroacoustic models informed by data that are extracted from the LES simulations.

* '''Sub-project 3: Automatic statistical convergence metric (C. Papagiannis, G. Balarac, O. Le Maitre, P. Congedo)'''

Statistics accumulation can be an important part of the restitution time in unsteady simulations (DNS/LES). In this project, the goal was to estimate uncertainties on the "finite time statistics". For time correlated data, it can be shown that the variance of the mean estimator (i.e. the fluctuation of the estimation of the mean) is dependent of the correlation time. Modeling this correlation time based on the integral time scale of the turbulence appears as a first way to define a practical metric to evaluate the statistic convergence on-fly during simulations. Next step should be to explore procedures to accelerate the statistics accumulation step.

* '''Sub-project 4: Aerodynamics of floating wind turbines (R. Amaral, F. Houtin-Mongrolle, E. Muller)'''

Floating wind turbines have the potential to unlock up to 80% of the wind energy located offshore making it a very strong candidate to mediate the energy transition. For this reason, many projects will soon come online with plenty of others entering the project phase. However, since now the foundation will experience translational and rotational movements, the rotor will experience changes in its local and global velocity fields whose effect is not well understood. This project intends to shed light on this matter, by making use of the high-fidelity large-eddy simulations (LES) and the actuator-line model (ALM) that are available in the YALES2 framework. This project will first cover floating wind turbines under user-prescribed motion to identify how the different degrees-of-freedom of the platform affect the rotor aerodynamics and in which proportion and will later move to more realistic conditions with sea-states described by a spectrum, turbulent wind and elastic turbine. During the ECFD workshop, we focused on finalizing some details of the prescribed motion implementation on the actuator-line model of YALES2 and well as preparing the implementation to be merged with the master branch.

* '''Sub-project 5: Implementation of the Risoe dynamic stall model for YALES2 (V. Maronnier, E. Muller, B. Duboc)'''

Dynamic loads must be predicted accurately to estimate the fatigue life of wind turbines operating in turbulent wind conditions. Dynamic stall has a huge impact on these loads. A semi-empirical Beddoes-Leishman type model is formulated to represent the unsteady lift, drag, and pitching moment. The so called RISOE dynamic stall model follows the initial formulation presented in (Hansen et al. 2004) with improvements described in (Pirrung et al. 2018). This model considers impulsive and circulatory terms in attached flow, and trailing edge separation in stall regions while in the Oye model only the lift coefficient was corrected in detached flow. This model will be implemented in the actuator line model in YALES2 and will be validated against experimental wind tunnel data for single airfoils (FFA-W3-241 and NACA0015). Simulations for a real wind turbine will be performed to estimate the impact of this model on loads into the coupling with the aeroelastic code BHawC.

Hansen, M.H., M. Gaunaa, and H.A. Madsen. “A Beddoes-Leishman Type Dynamic Stall Model in Sate-Space and Indicial Formulations.” Risoe, 2004
Pirrung, G. R., and M. Gaunaa. “Dynamic Stall Model Modifications to Improve the Modeling of Vertical Axis Wind Turbines.” DTU Wind Energy, June 2018

* '''Sub-project 6: YALES2-OpenFAST coupling (A. Parinam, A. Viré, D. Von Terzi, B. Duboc, F. Houtin-Mongrolle, P. Bénard)'''

* '''Sub-project 7: Wall law for Immersed Boundaries & Rough surfaces (M. Cailler, A. Cuffaro, P. Benez, S. Meynet)'''

Conservative Lagrangian Immersed Boundaries (CLIB) are now a useful way to take into account complex geometries in YALES2. During the workshop, a brand-new data-structure for modular and generic immersed-body has been developed. This data-structure paves the way for various new capabilities for IB methods: penalization mask shape optimization for improved velocity imposition, better control of near wall discretization based on a reliable evaluation of wall units, wall-modeling, etc... For this purpose the periodic hill test case has been considered. Simulations of this configuration has been performed by using body-fitted meshes, and CLIB for both smooth and rough surfaces. This will allow to assess the accuracy of the IB methods, and will constitute a database for IB models improvement, and the development of wall-modeling strategies.

* '''Sub-project 8: Atmospheric flow (U. Vigny, L. Voivenel, P. Benard, S. Zeoli)'''

Atmospheric flow such as Atmospheric Boundary Layer (ABL) and thermal stratification have an impact on wind turbines aerodynamic and wakes. Mostly at a wind farm scale, the change of wind turbine wake size and recovery can modify the global power production. During the workshop, the Coriolis force implementation has been validated through neutral case (where no thermal stratification i.e. no temperature gradient). It also allowed to validate the pressure forcing term, needed to drive the flow in a periodic box. YALES2 results showed a good agreement with other numerical and experimental results. Afterwards, the stable case (i.e. temperature gradient downwards) has been studied. A surface temperature as boundary condition has been developed. Yet, results are not as expected and further investigation is needed.

=== Two Phase Flow - C. Merlin, Ariane Group & M. Cailler, Safran Tech ===

* '''Sub-project 1: Convergent computation of interface curvature (G. Ghigliotti, M. Benard, G. Balarac, J. Carmona, R. Mercier, G. Lartigue)'''

Though Level-set distance evaluation through GPMM (Janodet et al., 2022) converges at order 2, the interface curvature convergence is as best 0 using the non-compact Goldman formulation.
Following progresses obtained during ECFD5, a strategy based on parabolic fit of the interface has been explored during the workshhop. This method aims at fitting a parabola through least squares using the interface markers stored in the interface vicinity. First the method was applied on a 2-D perfectly spherical droplet with exact projection of the marker on the circle. This results in a first order convergent curvature. Without projection of the markers, the fiting strategy allows a slight decrease of the error but no improve on the curvature convergence order in comparison with the standard non-compact formulation. As a persective, these results will be validated on dynamic and 3-D cases (MMG3D meshes). Also, the sensitivity on the number of markers and their redundancy will be investigated.

* '''Sub-project 2: Phase Change Solver : towards pressurization & moving bodies (F. Pecquery, C. Merlin, V. Moureau, I. El Yamani)'''

Main objective of this sub-project was to extend the capabilities of the Phase Change Solver towards pressurization and coupling with the immersed-boundary method. During the workshop, the new and generalized data structure, proposed by F. Pecquery, allowing the treatment of n-phases whose properties are described by an equation of state has been validated by setting-up an aqat (mph_data_registration). Moreover, a new age Phase Change Solver based on this data-structure was implemented by adapting all numerical ingredients of the temporal loop : going from boundary treatment to velocity, phase indicator, data-set advancement and pressure evaluation. Perspectives of this work include extension towards multi-species treatment and derivation of n-velocities momentum conserving framework.

* '''Sub-project 3: Towards unity ratio between particle size & grid size (I. El Yamani, M. Helal, N. Gasnier, M. Cailler, V. Moureau)'''

In a two-way coupled Eulerian-Lagrangian framework, dedidated numerical treatment is necessary as soon as the ratio between the particle size and the cell size is greater than unity. In this configuration, in the best case the evaluation of the undisturbed velocity and drag force is inacurate, and in the worst situation the high local Eulerian source-term may lead to gas velocity divergence. During the workshop, a gaussian-based regularization of the Eulerian source term was implemented and tested on various test cases. Use of this regularization shows improvement on the particle trajectory description, but still some errors are obtained for low particle Reynolds numbers. To improve the accuracy of the strategy the model for undisturbed gas velocity prediction proposed by (Balachandar, 2019) was tested. Unfortunately some difficulties were encoutered regarding its numerical implementation. Next steps will include a thorough validation of the method on isolated and interacting particles test cases and identification of model parameter for the gaussian-filter size.

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

* '''Sub-project 1: Multi-regime F-TACLES (S. Dillon, R. Mercier, B. Fiorina)'''

Filtered tabulated chemistry for large eddy simulations is currently a common tool to model premixed flames or diffusion flames. Tabulation using 1D counterflow flames, as a function of the mixture fraction and progress variable, was previously tested on laminar and turbulent cases. It resulted in difficulties to describe the outer mixing zone and yield a very stiff evolution of SGS source terms in the phase space. The model was modified to include the mixture fraction scalar dissipation rate as a table dimension. This solves previous limitations, but using 1D counterflow flames yields empty table zones, making the method numerically infeasible. Tabulation using both 1D counterflow flames and 1D partially premixed flames gives well-built tables, and was tested on 1D flames for various strain rates.

* '''Sub-project 2: Limiter model for turbulence combustion interaction in MILD combustion (E. Stendardo, L. Bricteux, K. Bioche)'''

MILD combustion yields intense turbulence and widespread reaction zones, requiring expensive mesh refinement over large areas. Practical mesh won’t be fine enough, leading to sub-grid heterogeneousness and effects of sub-grid turbulent fluctuations. A generic limiter type combustion model was implemented to solve for turbulence combustion interaction. This family of models includes Partially Stirred Reactor, Quasi Laminar Finite Rate and Laminar Finite Rate models. In these models, the source term is multiplied by a limiter factor and the residence time in inner cell reactive structure can be modelled. This implementation will permit testing the various limiter formulations in the future.

* '''Sub-project 3: Evaluate spatial discretization schemes on scalar transport (K. Bioche, Y. Bechane, R. Mercier, G. Lartigue, V. Moureau, J. Carmona, M. Bernard, L. Voivenel)'''

Common practice in combustion solvers is to use centred spatial schemes. Such low-dissipation schemes can prove unstable when applied to under-resolved scalar transport in presence of strong gradients. This is typically the case for H2/air combustion. Initial low-resolution simulations require thus adapted numerical schemes. Various spatial schemes were evaluated on the scalar transport problem, including: 4th order, 3rd order, 2nd order, WENO3, high order schemes, MUSCL schemes with various limiters (overbee, superbee, sweby, van leer, minmod). Their application to various configurations was discussed to emphasize on their robustness and accuracy. Tests cases include: 1D scalar convection Jiang Shu test case, 2D scalar bump convection for convergence analysis, a 2D reactive Bunsen burner and finally the 3D Preccinsta burner.

* '''Sub-project 4: Phenomenological plasma model for reacting systems (S. Wang, Y. Bechane, B. Fiorina)'''

Plasma assisted combustion consists in stabilizing flames in near extinction conditions thanks to electric discharges. Stabilization of lean premixed flames with Nanosecond Repetitively Pulsed electric Discharges is a strategy to reduce NOx emissions. Full 3D simulations of plasma assisted combustion are extremely expensive, so that the use of a semi-empirical strategy to model NRPD is preferred in CFD solvers. During the workshop, Castela’s model was implemented in a variable density solver. This model was extended to an explicit compressible solver. The model of Blanchard was also implemented in both frameworks. A 2D pin-to-pin configuration was successfully simulated with both models and frameworks.

* '''Sub-project 5: Development and assessment of combustion in an explicit compressible solver (Y. Bechane, L. Voivenel, R. Mercier, K. Bioche)'''

The implementation of reactive physics in the Explicit Compressible Solver (ECS) of the YALES2 platform was undertaken. To this aim, reactive gases thermochemical functions were implemented. Specific schemes were developed to increase the temperature and species diffusion schemes from 2nd to 4th order. Finally, a 2D methane-air Bunsen flame was simulated with low order numerical schemes (RK1 and SLAU2).

* '''Sub-project 6: Clustering for finite rate chemistry using PCA (R. Mercier, A. Stock)'''
To reduce the cost of species source terms computation, a clustering method was adopted. It consists in detecting nodes with similar properties and compute chemical source terms only once for these. Still, considering each species in this process creates a high dimensional cluster, while replacing species by a user-set progress variable may not well describe species. The strategy adopted here relies on the application of a PCA on species. It can be viewed as an automated “progress variable” creation. The use of such strategy was shown to reduce the simulation cost of source term computation by a factor 6 on a simple 2D flame ball case.

=== User Experience - J. Leparoux, Safran Tech & A. Pushkarev, GE Renewable Energy===

* '''Sub-project 1: External coupling with CWIPI (R. Letournel, V. Moureau, C. Merlin, M. Cailler, P. Bégou, S. Meynet)'''
The coupling between different YALES2 solvers but also between YALES2 and an external code has been generalized. It relies on the CWIPI coupling library, which allows to interpolate the data exchanged on any mesh. By introducing a new dedicated data structure, several simultaneous couplings can be performed, on different boundaries or on volume domains. Keywords also allow to pre-process the data to be sent, by a spatial filtering or a temporal average in case of temporal desynchronization of the solvers (asynchronous coupling). Dedicated test cases have been added to the distribution.

* '''Sub-project 2: Automated Grid Convergence refactoring (J. Leparoux, M. Cailler, R. Letournel)'''
Several grid convergence algorithms (Grenouilloux et al. (2022), Puggelli et al. (2022)) have been recently proposed but no one was fully embeded in the YALES2 solver which made the use of automatic mesh convergence tedious for users. A new data structure was introduced allowing to perform more easily grid convergence without to modify the fortran file of the YALES2 case. To go further, data structures state, event and action have been refactored allowing now to instantiate easily a new object (state, event or action) without previous user declaration. The next step will use these structures to propose an agile sequence that can be easily adapted. Dedicated test cases have been updated or added to the distribution.

* '''Sub-project 3: Advanced Liquid spray post-processing (J. Carmona, J. Leparoux, N. Gasnier, C. Brunet, I. El Yamani)'''

* '''Sub-project 4: YALES2 as industrial solver for GE design optimization tools (A. Pushkarev, H. Lam, G. Balarac)'''
A Graphical User Interface (GUI) exists at GE Hydro, which provide a rapid and user-friendly solution to run Ansys CFD Tools for typical hydroelectric simulations from geometry files.
We started develop an interface for Yales2 code, so that any user would only need a few clicks on a laptop to launch the simulation on a cluster with Yales2 solver instead of Ansys solutions.
This required to (i) Create a mesh file from a geometry files. We were stuck at the export of the file in a *.msh format because Ansys does not support it in batch mode.
(ii) Launching Yales2 solver from the GUI interfaced by y2_workflow. This was done successfully. (iii) Typical post-processing done at GE Hydro should be reproduced for the Yales2 code. We implemented the calculation of the evolution of losses along the streamlines direction.
Future works should find a way to export a mesh file in batch mode and further develop post processing and inlet conditions in Yales2

* '''Sub-project 5: YALES2 History and Geography (T. Marzlin, A. Dauptain, P. Bénard)'''

* '''Sub-project 6: Improve the HT solver: refactoring of linear solver operators & Robin BC (C. Merlin, V. Moureau, R. Letournel)'''
First some linear solver functions were refactored with the introduction of a yales2 pointer to store C pointer and the use of function pointer. A Robin condition was also implemented in Heat Transfer Solver (HTS) to take into account convective flux. A first 2D test case was used to validate the implementation versus an analytical solution.

Ecfd:ecfd 6th edition

2024-02-06T09:43:39Z

Houtin: /* Mesh adaptation - R. Letournel, Safran & V. Moureau, CORIA */

{{DISPLAYTITLE: ECFD workshop, 6th edition, 2022}}

== Description ==
{| align="right" style="text-align:center;" cellpadding="2"
| [[File:Logo_ECFD6.png | center | thumb | 350px | ECFD6 workshop logo.]]
|}
* Event from '''23th of January to 3rd of February 2023'''
* Location: [https://www.hotelclubdelaplage.com Hôtel Club de la Plage], Merville-Franceville, near Caen (14)
* Two types of sessions:
** common technical presentations: roadmaps, specific points
** mini-workshops. Potential workshops are listed below
* Free of charge
* More than 60 participants from academics, HPC center/experts and industry.

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

[[File:Banniere_ECFD6.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_6th_edition]]

[[File:Banniere_ECFD6_sponso.png|text-bottom|600px]]



== Agenda ==

[[File:ECFD6_program.png|text-bottom|600px]]

== Thematics / Mini-workshops ==

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

== Projects ==

=== Hackathon - G. Staffelbach, CERFACS & P. Begou, LEGI ===

=== Mesh adaptation - R. Letournel, Safran & V. Moureau, CORIA ===
*'''Sub-project 1: Mesh adaptation for vortical flows (MAVERICK) for coupled systems of differential-algebraic equations (L. Bricteux, G. Balarac, S. Zeoli, G. Lartigue)'''

This project is in the continuation of what has been investigated in the previous ECFD workshops.
It aims to develop mesh adaptation strategies for DNS of vortical flows. Turbulent flows are vortical flows. It is thus of primary importance to capture the vorticity dynamics properly if one wants to obtain physically realistic results. As the vorticity field is compact, mesh adaptation allows to refine the mesh only where flow physics happens, in the same way a vortex particle-mesh method would do.
The main findings gathered during ECFD6 are:
1/ Gmsh can produce high quality base mesh for AMR with Yales2, yet the proper choice of options is not trivial.
2/ Initial mesh should be as smooth as possible (low hgrad), which is really challenging for wall resolved turbulent wall bounded flows.
3/ Refinement sensor based on vorticity gradients (palenstrophy) seems to works fairly wel.l
4/ two levels strategy provides encouraging results

*'''Sub-project 2: ALE and remeshing for surface mesh based on 1D deformations - F. Houtin-Mongrolle (SGRE), E. Muller (SGRE), B. Duboc (SGRE)'''
During operation and maintenance, wind turbines are subject to particular inflow that may trigger significant 3D aerodynamic effects (Vortex, Induced Vibrations, over-speed, flutter…) that deform the blades. Typical Actuator line methods (ALM) are not enough to capture these types of Fluid-structure interactions. This project aimed to go from an ALM way of modeling the blades to a body-fitted mesh with ALE/re-meshing, depending on the blade deformations. The work is divided into three work packages. Work package 1: impose displacements from a 1D discretized line (6 DOF) into the blade surface. Work package 2: trigger re-meshing if the displacement of the cells reaches a bad quality (high skewness, negative node volume). The last work package is to be able to retrieve the forces and moments from the 2D surface to the initial 1D discretized line.

=== Numerics - S. Mendez, IMAG & M. Bernard, LEGI ===

* '''Sub-project 1: Multi-level domain decomposition method (DDM) for coupled systems of differential-algebraic equations (A. Quirós Rodrígues, V. Le Chenadec)'''
The numerical approximation of multi-physics problems gives rise to complex linear systems, the solution of which leverages preconditioning techniques such as multi-grid or domain decomposition methods. This project aimed at coupling two Julia packages that being actively developed: a two-dimensional Navier-Stokes solver for free-surface and two-phase flows (Flower.jl) on the other, and a Domain Decomposition package for Cartesian grids (DDM.jl). The decomposed matrix-vector product was optimised to reduce the overhead associated with halo exchanges. The implementation of a deflated Conjugate Gradient as well as one- and two-level Additive Schwartz Method were also completed and shown to significant reduce the number of iterations for inverting monolithic systems (i.e. without resorting to operator splitting), shown to be independent of the number of subdomains for constant property flows. Future work will focus on a further optimisation of the implementation for vectorisation and multi-threading, and extension of the deflation to generalised coarse spaces to support highly discontinuous transport properties (GenEO).

* '''Sub-project 2: Ghost fluid method (GFM) for Electrodeformation (A. Spadotto , S. Mendez)'''
According to the Leaky Dielectric Model, red blood cells (RBCs) are subject to a force which is proportional to the jump of the Maxwell tensor. This latter is a quantity scaling as the square of the electric field, which under the quasi-static hypothesis is defined as the gradient of the electrostatic potential. To work out the potential, an elliptic interface problem must be solved, taking into account the presence of the RBC membrane. The aim of the project was implementing the Ghost Fluid Method (GFM) to face the interface problem. Good results were obtained on unstructured meshes. Secondly, a gradient calculation was performed applying the Green-Gauss scheme, modified in the style of the GFM. Future work will focus on interpolation of the gradient field onto the membrane to get an estimation of the effort. Possibly, high-order schemes for the gradient calculation will be explored. In a second time, the effort calculation will be merged into an Immersed Boundary solver for the RBC dynamics.

* '''Sub-project 3: Optimization of the high order framework (HOF) for Navier-Stokes incompressible (M. Bernard, P. Bégou, G. Lartigue, G. Balarac)'''
Over the past years, a framework has been developed to improve the spatial accuracy of numerical schemes on distorted meshes.
However, even if the solution is more precise, the computational cost of the overall resolution of Navier-Stokes equations is large.
As a consequence, HOF becomes profitable only on thin meshes thanks to a better spatial convergence order.
The code has been analized with different analysis tools (MAQAO, Gprof, Scalasca).
The main time consuming routines have been identified and improved.
Moreover, some algorithms have been refactors such that the resolution of Navier-Stokes equations has been speed-up by a factor 2.5.

* '''Sub-project 4: Force coupling method (FCM) for particulate flows (C. Raveleau, S. Mendez)'''
The Force Coupling Method (FCM) allows the representation of particles in flows from 0 to finite Reynolds number based on a regularization of the multipole expansion for Stokes flows, providing enough particle resolution to match the exact Stokes flow away from the particle. This is done by using a Gaussian function to spread the singular forces over a domain whose size is derived from physical quantities of interest such as the settling velocity of a particle under gravity in Stokes flow and the particle radius. During the workshop, the first term of the multipole expansion (monopole) and the antisymetric part of the second term (antisymetric dipole) were implemented and validated against test cases from the litterature. The monopole was tested in the case of a particle held fixed against an incoming flow and the dipole was tested by applying a torque on a particle in a still fluid. In both cases, the velocity profiles matched the results from the litterature and approximated well the Stokes solution at a distance of about 1.5 radii from the particle center. It was also compared to the conservative immersed boundary method (CLIB solver) implemented in YALES2 for the monopole test case. Both methods give good results, although the FCM is able to predict the expected solution with a coarser mesh than CLIB. Cases where the particle moves are under investigation.

* '''Sub-project 5: Breaking limitations of the linearized implicit time advancement (T. Berthelon, G. Lartigue, G. Balarac)'''
The explicit time advancement classically used in ics solver is limited by CFL constraint. In order to get ride of this constraint, an implicit time advancement method, based on the linearization of the convective term, has been recently developed.
However, the method is limited by difficulties to solve linear system, with the BiCGSTAB2 algorithm, during the prediction step. The objective of this project was to understand these limitations. The correction of a bug on the boundary conditions (viscosity imposed at zero) was identified. In addition, the spatial scheme and the presence of a buffer zone at the end of the domain showed a great influence on the convergence of the prediction. The perspectives for a more robust and efficient use of this temporal integration consist in working on the spatial schemes and on the pre-conditioning.

* '''Sub-project 6: Development of a traction open boundary condition (TOBC) in Yales2 (J.B. Lagaert, G. Balarac)'''
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#6, open traction boundaries (TOBC) has been implemented in Yales2. Instead of enforced pressure value or normal derivative on boundaries, the traction is settled. The traction term gathers the deformation stress and pressure gradient.
The implementation has been validated on simple test-case : comparison has been performed between theoritical solutions and numerical solution computed with a Dirichlet TOBC. Future work will add a stabilization term for large backflow and traction models to estimate the values to enforce on outlet boundaries

* '''Sub-project 7: Development of new spatial differential operators in Yales2 (M. Bernard, G. Lartigue)
It exists different philosophies for computing differential operators on distorted meshes.
In a HPC context, the 2 main approaches are the Green-Gauss operators and the Least-Squares operators.
During ECFD#6, 2 new types of "non-compact" Hessian operators have been implemented by computing successively the gradient operator, eather with Green-Gauss gradient, or with Least-Squares gradient.
Those operators lead to good convergence order, even on distorted mehes.
However, their application on low-resolution signals lead to large error magnitude due to their extended stencil.
Another pair of gradient & hessian Least-Squares operators have been implemented, leading to 2nd and 1st order accuracy for the gradient and hessian respectively.
Those operators have very interesting characteristics as their stencil is restricted to the direct neighbors only and their computational cost remains low.

* '''Sub-project 8: DOROTHY optimization (M. Roperch, H. Mulakaloori, G. Pinon, P. Bénard, G. Lartigue)'''
DOROTHY is three-dimensional unsteady Lagrangian Vortex Particles Methods code. Because of the number of particles that increases during the numerical simulation, some routines begin to be very expensive. During this workshop, MAQAO has been used to identify the most time consuming routines and why. They have been factorized, merged and vectorized. At the end, these routines have been speed up by a factor 4 and the global program by a factor 2.8.

* '''Sub-project 9: Anamika, a tool to improve programming productivity (K. Mohana Muraly, G. Staffelbach)'''

=== Turbulence - P. Benard, CORIA & G. Balarac, LEGI ===

* '''Sub-project 1: Explore hybrid RANS/LES strategies (T. Berthelon, G. Balarac)'''

For complex industrial applications, LES can still lead to a too long restitution time. In other hand, statistical approaches can lead too a lack of accuracy. In this project, the potentiality of hybrid approaches combining both have been explored. Conventional hybrid RANS/LES approaches consider a unique solution field, with an unique transport equation and a clusre terme modeled using RANS or LES models depending of the regions. The main idea is to evaluate a strategy based on a separation between mean fields and fluctuations with distinct coupled transport equations. First elements of validation using YALES2 code are shown that it was possible to correct the prediction of a RANS models, by performing LES of the fluctuations. Next steps should be to consider disctinct meshes, or even computational domains for RANS and LES with this strategy.

* '''Sub-project 2: Flow Instabilities over Rotating curved Surfaces (S. Sawaf, M. Shadloo, A. Hadjadj, S. Moreau, S. Poncet)'''

For evaluating the effect of the clearance between the blade tip and the casing of axial ducted fans on noise emissions, LES offers excellent tool to capture the consitricted flow around the blade tip especially for small clearances where RANS fails because of unsteady flow conditions. LES simulation of the aerodynamics is the first step toward extracting accoustics data that helps to improve the design of axial ducted fans so they comply with the noise emission regulations in admistrative buildings. noise emmisions are estimated using analytical aeroacoustic models informed by data that are extracted from the LES simulations.

* '''Sub-project 3: Automatic statistical convergence metric (C. Papagiannis, G. Balarac, O. Le Maitre, P. Congedo)'''

Statistics accumulation can be an important part of the restitution time in unsteady simulations (DNS/LES). In this project, the goal was to estimate uncertainties on the "finite time statistics". For time correlated data, it can be shown that the variance of the mean estimator (i.e. the fluctuation of the estimation of the mean) is dependent of the correlation time. Modeling this correlation time based on the integral time scale of the turbulence appears as a first way to define a practical metric to evaluate the statistic convergence on-fly during simulations. Next step should be to explore procedures to accelerate the statistics accumulation step.

* '''Sub-project 4: Aerodynamics of floating wind turbines (R. Amaral, F. Houtin-Mongrolle, E. Muller)'''

Floating wind turbines have the potential to unlock up to 80% of the wind energy located offshore making it a very strong candidate to mediate the energy transition. For this reason, many projects will soon come online with plenty of others entering the project phase. However, since now the foundation will experience translational and rotational movements, the rotor will experience changes in its local and global velocity fields whose effect is not well understood. This project intends to shed light on this matter, by making use of the high-fidelity large-eddy simulations (LES) and the actuator-line model (ALM) that are available in the YALES2 framework. This project will first cover floating wind turbines under user-prescribed motion to identify how the different degrees-of-freedom of the platform affect the rotor aerodynamics and in which proportion and will later move to more realistic conditions with sea-states described by a spectrum, turbulent wind and elastic turbine. During the ECFD workshop, we focused on finalizing some details of the prescribed motion implementation on the actuator-line model of YALES2 and well as preparing the implementation to be merged with the master branch.

* '''Sub-project 5: Implementation of the Risoe dynamic stall model for YALES2 (V. Maronnier, E. Muller, B. Duboc)'''

Dynamic loads must be predicted accurately to estimate the fatigue life of wind turbines operating in turbulent wind conditions. Dynamic stall has a huge impact on these loads. A semi-empirical Beddoes-Leishman type model is formulated to represent the unsteady lift, drag, and pitching moment. The so called RISOE dynamic stall model follows the initial formulation presented in (Hansen et al. 2004) with improvements described in (Pirrung et al. 2018). This model considers impulsive and circulatory terms in attached flow, and trailing edge separation in stall regions while in the Oye model only the lift coefficient was corrected in detached flow. This model will be implemented in the actuator line model in YALES2 and will be validated against experimental wind tunnel data for single airfoils (FFA-W3-241 and NACA0015). Simulations for a real wind turbine will be performed to estimate the impact of this model on loads into the coupling with the aeroelastic code BHawC.

Hansen, M.H., M. Gaunaa, and H.A. Madsen. “A Beddoes-Leishman Type Dynamic Stall Model in Sate-Space and Indicial Formulations.” Risoe, 2004
Pirrung, G. R., and M. Gaunaa. “Dynamic Stall Model Modifications to Improve the Modeling of Vertical Axis Wind Turbines.” DTU Wind Energy, June 2018

* '''Sub-project 6: YALES2-OpenFAST coupling (A. Parinam, A. Viré, D. Von Terzi, B. Duboc, F. Houtin-Mongrolle, P. Bénard)'''

* '''Sub-project 7: Wall law for Immersed Boundaries & Rough surfaces (M. Cailler, A. Cuffaro, P. Benez, S. Meynet)'''

Conservative Lagrangian Immersed Boundaries (CLIB) are now a useful way to take into account complex geometries in YALES2. During the workshop, a brand-new data-structure for modular and generic immersed-body has been developed. This data-structure paves the way for various new capabilities for IB methods: penalization mask shape optimization for improved velocity imposition, better control of near wall discretization based on a reliable evaluation of wall units, wall-modeling, etc... For this purpose the periodic hill test case has been considered. Simulations of this configuration has been performed by using body-fitted meshes, and CLIB for both smooth and rough surfaces. This will allow to assess the accuracy of the IB methods, and will constitute a database for IB models improvement, and the development of wall-modeling strategies.

* '''Sub-project 8: Atmospheric flow (U. Vigny, L. Voivenel, P. Benard, S. Zeoli)'''

Atmospheric flow such as Atmospheric Boundary Layer (ABL) and thermal stratification have an impact on wind turbines aerodynamic and wakes. Mostly at a wind farm scale, the change of wind turbine wake size and recovery can modify the global power production. During the workshop, the Coriolis force implementation has been validated through neutral case (where no thermal stratification i.e. no temperature gradient). It also allowed to validate the pressure forcing term, needed to drive the flow in a periodic box. YALES2 results showed a good agreement with other numerical and experimental results. Afterwards, the stable case (i.e. temperature gradient downwards) has been studied. A surface temperature as boundary condition has been developed. Yet, results are not as expected and further investigation is needed.

=== Two Phase Flow - C. Merlin, Ariane Group & M. Cailler, Safran Tech ===

* '''Sub-project 1: Convergent computation of interface curvature (G. Ghigliotti, M. Benard, G. Balarac, J. Carmona, R. Mercier, G. Lartigue)'''

Though Level-set distance evaluation through GPMM (Janodet et al., 2022) converges at order 2, the interface curvature convergence is as best 0 using the non-compact Goldman formulation.
Following progresses obtained during ECFD5, a strategy based on parabolic fit of the interface has been explored during the workshhop. This method aims at fitting a parabola through least squares using the interface markers stored in the interface vicinity. First the method was applied on a 2-D perfectly spherical droplet with exact projection of the marker on the circle. This results in a first order convergent curvature. Without projection of the markers, the fiting strategy allows a slight decrease of the error but no improve on the curvature convergence order in comparison with the standard non-compact formulation. As a persective, these results will be validated on dynamic and 3-D cases (MMG3D meshes). Also, the sensitivity on the number of markers and their redundancy will be investigated.

* '''Sub-project 2: Phase Change Solver : towards pressurization & moving bodies (F. Pecquery, C. Merlin, V. Moureau, I. El Yamani)'''

Main objective of this sub-project was to extend the capabilities of the Phase Change Solver towards pressurization and coupling with the immersed-boundary method. During the workshop, the new and generalized data structure, proposed by F. Pecquery, allowing the treatment of n-phases whose properties are described by an equation of state has been validated by setting-up an aqat (mph_data_registration). Moreover, a new age Phase Change Solver based on this data-structure was implemented by adapting all numerical ingredients of the temporal loop : going from boundary treatment to velocity, phase indicator, data-set advancement and pressure evaluation. Perspectives of this work include extension towards multi-species treatment and derivation of n-velocities momentum conserving framework.

* '''Sub-project 3: Towards unity ratio between particle size & grid size (I. El Yamani, M. Helal, N. Gasnier, M. Cailler, V. Moureau)'''

In a two-way coupled Eulerian-Lagrangian framework, dedidated numerical treatment is necessary as soon as the ratio between the particle size and the cell size is greater than unity. In this configuration, in the best case the evaluation of the undisturbed velocity and drag force is inacurate, and in the worst situation the high local Eulerian source-term may lead to gas velocity divergence. During the workshop, a gaussian-based regularization of the Eulerian source term was implemented and tested on various test cases. Use of this regularization shows improvement on the particle trajectory description, but still some errors are obtained for low particle Reynolds numbers. To improve the accuracy of the strategy the model for undisturbed gas velocity prediction proposed by (Balachandar, 2019) was tested. Unfortunately some difficulties were encoutered regarding its numerical implementation. Next steps will include a thorough validation of the method on isolated and interacting particles test cases and identification of model parameter for the gaussian-filter size.

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

* '''Sub-project 1: Multi-regime F-TACLES (S. Dillon, R. Mercier, B. Fiorina)'''

Filtered tabulated chemistry for large eddy simulations is currently a common tool to model premixed flames or diffusion flames. Tabulation using 1D counterflow flames, as a function of the mixture fraction and progress variable, was previously tested on laminar and turbulent cases. It resulted in difficulties to describe the outer mixing zone and yield a very stiff evolution of SGS source terms in the phase space. The model was modified to include the mixture fraction scalar dissipation rate as a table dimension. This solves previous limitations, but using 1D counterflow flames yields empty table zones, making the method numerically infeasible. Tabulation using both 1D counterflow flames and 1D partially premixed flames gives well-built tables, and was tested on 1D flames for various strain rates.

* '''Sub-project 2: Limiter model for turbulence combustion interaction in MILD combustion (E. Stendardo, L. Bricteux, K. Bioche)'''

MILD combustion yields intense turbulence and widespread reaction zones, requiring expensive mesh refinement over large areas. Practical mesh won’t be fine enough, leading to sub-grid heterogeneousness and effects of sub-grid turbulent fluctuations. A generic limiter type combustion model was implemented to solve for turbulence combustion interaction. This family of models includes Partially Stirred Reactor, Quasi Laminar Finite Rate and Laminar Finite Rate models. In these models, the source term is multiplied by a limiter factor and the residence time in inner cell reactive structure can be modelled. This implementation will permit testing the various limiter formulations in the future.

* '''Sub-project 3: Evaluate spatial discretization schemes on scalar transport (K. Bioche, Y. Bechane, R. Mercier, G. Lartigue, V. Moureau, J. Carmona, M. Bernard, L. Voivenel)'''

Common practice in combustion solvers is to use centred spatial schemes. Such low-dissipation schemes can prove unstable when applied to under-resolved scalar transport in presence of strong gradients. This is typically the case for H2/air combustion. Initial low-resolution simulations require thus adapted numerical schemes. Various spatial schemes were evaluated on the scalar transport problem, including: 4th order, 3rd order, 2nd order, WENO3, high order schemes, MUSCL schemes with various limiters (overbee, superbee, sweby, van leer, minmod). Their application to various configurations was discussed to emphasize on their robustness and accuracy. Tests cases include: 1D scalar convection Jiang Shu test case, 2D scalar bump convection for convergence analysis, a 2D reactive Bunsen burner and finally the 3D Preccinsta burner.

* '''Sub-project 4: Phenomenological plasma model for reacting systems (S. Wang, Y. Bechane, B. Fiorina)'''

Plasma assisted combustion consists in stabilizing flames in near extinction conditions thanks to electric discharges. Stabilization of lean premixed flames with Nanosecond Repetitively Pulsed electric Discharges is a strategy to reduce NOx emissions. Full 3D simulations of plasma assisted combustion are extremely expensive, so that the use of a semi-empirical strategy to model NRPD is preferred in CFD solvers. During the workshop, Castela’s model was implemented in a variable density solver. This model was extended to an explicit compressible solver. The model of Blanchard was also implemented in both frameworks. A 2D pin-to-pin configuration was successfully simulated with both models and frameworks.

* '''Sub-project 5: Development and assessment of combustion in an explicit compressible solver (Y. Bechane, L. Voivenel, R. Mercier, K. Bioche)'''

The implementation of reactive physics in the Explicit Compressible Solver (ECS) of the YALES2 platform was undertaken. To this aim, reactive gases thermochemical functions were implemented. Specific schemes were developed to increase the temperature and species diffusion schemes from 2nd to 4th order. Finally, a 2D methane-air Bunsen flame was simulated with low order numerical schemes (RK1 and SLAU2).

* '''Sub-project 6: Clustering for finite rate chemistry using PCA (R. Mercier, A. Stock)'''
To reduce the cost of species source terms computation, a clustering method was adopted. It consists in detecting nodes with similar properties and compute chemical source terms only once for these. Still, considering each species in this process creates a high dimensional cluster, while replacing species by a user-set progress variable may not well describe species. The strategy adopted here relies on the application of a PCA on species. It can be viewed as an automated “progress variable” creation. The use of such strategy was shown to reduce the simulation cost of source term computation by a factor 6 on a simple 2D flame ball case.

=== User Experience - J. Leparoux, Safran Tech & A. Pushkarev, GE Renewable Energy===

* '''Sub-project 1: External coupling with CWIPI (R. Letournel, V. Moureau, C. Merlin, M. Cailler, P. Bégou, S. Meynet)'''
The coupling between different YALES2 solvers but also between YALES2 and an external code has been generalized. It relies on the CWIPI coupling library, which allows to interpolate the data exchanged on any mesh. By introducing a new dedicated data structure, several simultaneous couplings can be performed, on different boundaries or on volume domains. Keywords also allow to pre-process the data to be sent, by a spatial filtering or a temporal average in case of temporal desynchronization of the solvers (asynchronous coupling). Dedicated test cases have been added to the distribution.

* '''Sub-project 2: Automated Grid Convergence refactoring (J. Leparoux, M. Cailler, R. Letournel)'''
Several grid convergence algorithms (Grenouilloux et al. (2022), Puggelli et al. (2022)) have been recently proposed but no one was fully embeded in the YALES2 solver which made the use of automatic mesh convergence tedious for users. A new data structure was introduced allowing to perform more easily grid convergence without to modify the fortran file of the YALES2 case. To go further, data structures state, event and action have been refactored allowing now to instantiate easily a new object (state, event or action) without previous user declaration. The next step will use these structures to propose an agile sequence that can be easily adapted. Dedicated test cases have been updated or added to the distribution.

* '''Sub-project 3: Advanced Liquid spray post-processing (J. Carmona, J. Leparoux, N. Gasnier, C. Brunet, I. El Yamani)'''

* '''Sub-project 4: YALES2 as industrial solver for GE design optimization tools (A. Pushkarev, H. Lam, G. Balarac)'''
A Graphical User Interface (GUI) exists at GE Hydro, which provide a rapid and user-friendly solution to run Ansys CFD Tools for typical hydroelectric simulations from geometry files.
We started develop an interface for Yales2 code, so that any user would only need a few clicks on a laptop to launch the simulation on a cluster with Yales2 solver instead of Ansys solutions.
This required to (i) Create a mesh file from a geometry files. We were stuck at the export of the file in a *.msh format because Ansys does not support it in batch mode.
(ii) Launching Yales2 solver from the GUI interfaced by y2_workflow. This was done successfully. (iii) Typical post-processing done at GE Hydro should be reproduced for the Yales2 code. We implemented the calculation of the evolution of losses along the streamlines direction.
Future works should find a way to export a mesh file in batch mode and further develop post processing and inlet conditions in Yales2

* '''Sub-project 5: YALES2 History and Geography (T. Marzlin, A. Dauptain, P. Bénard)'''

* '''Sub-project 6: Improve the HT solver: refactoring of linear solver operators & Robin BC (C. Merlin, V. Moureau, R. Letournel)'''
First some linear solver functions were refactored with the introduction of a yales2 pointer to store C pointer and the use of function pointer. A Robin condition was also implemented in Heat Transfer Solver (HTS) to take into account convective flux. A first 2D test case was used to validate the implementation versus an analytical solution.

File:Ecfd3 final project11.pdf

2020-01-31T08:20:47Z

Houtin:

Ecfd:ecfd 3rd edition

2020-01-31T08:19:39Z

Houtin: /* Project #11: Multiphysics coupling for wind turbine wake modeling */

ECFD workshop, 3rd edition, 2020

== Sponsors ==

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

== Participants ==

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

== Flyer ==

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

== Presentations ==

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

== Project achievements ==

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

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

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

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

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

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

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

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

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

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

''C. G. Guillamon. L .Voivenel, R. Mercier''

=== Project #4: Application to combustion and lubrication applications ===

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

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

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

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

[[media:ecfd3_final_project6.pdf | Final presentation of project #6]]

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

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

=== Project #8: Implémentation du calcul de la distance à une interface liquide-gaz proche d’une paroi sur maillage non structuré 3D avec YALES2 ===

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

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

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

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

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

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

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

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

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

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

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

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

=== Project #12: Stability of a semi-implicit compressible cavitation solver ===

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

=== Project #13: DNS of droplet dynamics and evaporation : comparison between structured and unstructured solvers ===

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

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

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

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

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

=== Project #16: Development of a RANS solver in YALES2 ===

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

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

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

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

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

=== Project #19: The Clone Wars ===

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

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

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

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

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

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

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

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

Ecfd:ecfd 3rd edition

2020-01-31T08:16:51Z

Houtin: /* Project #11: Multiphysics coupling for wind turbine wake modeling */

ECFD workshop, 3rd edition, 2020

== Sponsors ==

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

== Participants ==

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

== Flyer ==

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

== Presentations ==

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

== Project achievements ==

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

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

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

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

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

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

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

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

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

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

=== Project #4: Application to combustion and lubrication applications ===

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

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

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

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

[[media:ecfd3_final_project6.pdf | Final presentation of project #6]]

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

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

=== Project #8: Implémentation du calcul de la distance à une interface liquide-gaz proche d’une paroi sur maillage non structuré 3D avec YALES2 ===

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

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

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

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

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

=== Project #10: Remaillage dynamique pour la combustion turbulente prémélangée ===
''W. Agostinelli, O. Dounia, , T. Jaravel, O. Vermorel

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

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

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

''F.Houtin-Mongrolle (CORIA), B. Duboc (SGRE), P. Benard (CORIA)''
The goal of this project was to evaluate the coupling of YALES2 (flow solver) and BHawC(Aero-Servo-Elastic solver).

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

=== Project #12: Stability of a semi-implicit compressible cavitation solver ===

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

=== Project #13: DNS of droplet dynamics and evaporation : comparison between structured and unstructured solvers ===

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

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

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

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

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

=== Project #16: Development of a RANS solver in YALES2 ===

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

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

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

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

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

=== Project #19: The Clone Wars ===

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

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

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

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

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