Extreme CFD workshop - User contributions [en]

Ecfd:ecfd 8th edition

2025-02-10T14:55:17Z

Ghigliot5g: /* TP1 - Towards very small contact angles in Nucleate boiling */

{{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
(Felix)

* Rotating Actuator Disk Model (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)

==== 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)

==== 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 8th edition

2025-02-10T14:50:38Z

Ghigliot5g:

{{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
(Felix)

* Rotating Actuator Disk Model (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 very small contact angle (angle formed by the two-phase interface on the wall, at values lower than 30°) when simulating nucleate boiling. A similar limitation was applying to the spray (SPS) solver. A modified version of the 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 has been tested successfully on the spray solver (where no phase change occurs). Then, this new reinitialisation has been tested for nucleate boiling with great improvements. Now simulations of nucleate boiling at very small contact angle (10°) can be 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.

==== 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)

==== 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)

==== 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 5th edition

2022-01-28T00:37:45Z

Ghigliot5g: /* Multi-phase flows - M. Cailler, SAFRAN TECH */

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

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

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

== News ==

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

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

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

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

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

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

== Agenda ==

[[File:ECFD5_program.png|text-bottom|600px]]

== Thematics / Mini-workshops ==

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

=== Combustion - K. Bioche, VUB ===

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

=== Multi-phase flows - M. Cailler, SAFRAN TECH ===

D3: convergence of the interface curvature computation.
The computation of interface curvature in a levelset framework is based on the classic formula as divergence of the gradient of the levelset function. This function being computed at 2nd order, one obtains a O(0) curvature, meaning that the error does not decrease with mesh refinement.
We have implemented in YALES2 a strategy proposed by Emmanuel Maître and collaborators in a finite element method based on the regularization (filtering) of the levelset gradient and curvature.
This strategy has been tested for the simple test case of a static circular interface.
We used two types of filters (simple gather-scatter or bilaplacian as developed by Lola Guedot (PhD thesis 2015)) on different mesh types (split quadrilaterals, isotropic triangular mesh, unstructured triangular mesh).
The results are encouraging since a O(1) convergence is obtained in all cases.
Further work is needed to tune the filter properties (amplitude and size) for different spatial resolutions and levelset "narrow band" width.

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

=== Turbulent flows - P. Bénard, CORIA ===
* '''Sub-project 1: Optimization of the actuator set for several wind turbines in YALES2 (F. Houtin Mongrolle, S. Gremmo, E. Muller & B. Duboc)'''

=== Compressible - L. Bricteux, UMONS ===

=== User experience - J. Leparoux, SAFRAN TECH ===

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

File:Ecfd3 final project12.pdf

2020-02-03T19:45:52Z

Ghigliot5g:

Ecfd:ecfd 3rd edition

2020-01-31T10:46:39Z

Ghigliot5g: /* Project #12: Stability of a semi-implicit compressible cavitation solver */

{{DISPLAYTITLE: ECFD workshop, 3rd edition, 2020}}

== Sponsors ==

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

== Participants ==

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

== Flyer ==

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

== Presentations ==

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

== Booklet ==

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

== Project achievements ==

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

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

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

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

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

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

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

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

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

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

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

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

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

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

François Pecquery (ARIANE GROUP), Mélody Cailler (SAFRAN TECH), Romain Janodet (SAFRAN TECH/CORIA) and Vincent Moureau (CORIA)

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

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

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

''T. Laroche, N. Odier, B. Cuenot (CERFACS). In collaboration with M. Pelletier, T. Schmitt, S. Ducruix (EM2C)''

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

''H. Garg (LEGI), G. Ghigliotti (LEGI) and G. Balarac (LEGI)''

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

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

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

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

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

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

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

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

=== Project #14: High Order Framework ===
''M. Bernard (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI), V. Moureau (CORIA)''

Aim of this project is to extend the high order framework (HOF) in Yales2.
As a reminder, the HOF permits to reconstruct a point-wise quantity from the volume-averaged one, arising from classical Finite-Volume schemes, and thus to improve spatial accuracy of numerical schemes.

During the ECFD workshop #3, a dedicated solver has been created, the high order solver (hos), duplicated from the incompressible solver (ics).
We started activating the HOF ingredients previously developed, starting from velocity field advancement.
Development is still in progress, but the static Taylor-Green vortices test-case has been investigated in order to see the early improvement.

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

=== Project #15: Validation of a fluid structure interaction case with the coupling ALE/SMS ===
''T. Fabbri (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI), V. Moureau (CORIA)''

The objective of this project was the validation of the Turek(2006) benchmark for fluid structure case.
The Structural Mechanics Solver (SMS) was already existing before the workshop, as the coupling with the Arbitrary-Lagrangian Eulerian solver.
However, the results were not in agreement with the case. The data compared here are the flexible part tip displacement, but also the drag and the lift integrated
on the cylinder and the flexible part.
The pure structure test cases were validated, but the forces computed for the pure fluid test cases were not satisfying.
The work of this week was then to improve the viscous shear computation, which implies the wall normal gradient computation.

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

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

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

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

=== Project #17: IMPLEMENTATION OF A COLD PLASMA MODEL IN YALES2 ===

''J.-M. Orlac'h (EM2C), G. Lartigue (CORIA), B. Fiorina (EM2C)''

The objective of this project was to further develop the cold plasma solver in YALES2 in order to accurately model silane nanodusty discharges. The electron temperature equation has been implemented successfully and validated against a reference plasma code. In a second step, a detailed electron kinetics has been implemented in YALES2 in order to couple the electron temperature with the charged species mass fractions. The user can now define a list of reactions whose rates depend on the electron temperature. These improvements open the path to the simulation of nanoparticle production in silane discharges using a Lagrangian description for the nanoparticles.

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

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

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

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

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

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

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

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

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

Several ways were explored:

- Profiling of reactive simulations when using Virtual Chemistry

- Effect of redundant species transport

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

- Effect of the correction functions smoothing

Conclusions of the study are:

- String trimming and concatenation heavily affect computing performances

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

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

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

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

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

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

''Paolo ERRANTE (LMFA), Alexis GIAUQUE (LMFA), Christophe CORRE (LMFA)''

The simulation of dense gas flows using AVBP currently relies on the Martin-Hou Equation of State (EoS) to obtain the thermodynamic variables in each grid cell from the local value of density (or specific volume v) and internal energy derived from the conservative variables. The project develops an alternative approach where thermodynamic quantities in each cell are derived from a (given) set of tabulated thermodynamic states (Look-up Table or LuT). In order to preserve flow physics, the interpolation process in the LuT tables must be performed in a consistent way (a simple bilinear interpolation on v and T for each thermodynamic variable is not sufficient). Describing Helmholtz free energy f(v,T) with a bi-quintic Hermitian polynomial function in each cell of the LuT allows to ensure a consistent interpolation process (since all thermodynamic variables are obtained by differentiating the polynomial function). During the workshop the functions needed to perform the consistent interpolation have been implemented in the real gas module. Short-term perspectives are completing the implementation, validating the development on some test-cases previously computed using MAH EoS and optimizing the implementation (in particular the strategy used to identify the position in the LuT of each local grid state).

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

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

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

Ecfd:ecfd 3rd edition

2020-01-31T10:38:42Z

Ghigliot5g: /* Project #12: Stability of a semi-implicit compressible cavitation solver */

{{DISPLAYTITLE: ECFD workshop, 3rd edition, 2020}}

== Sponsors ==

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

== Participants ==

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

== Flyer ==

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

== Presentations ==

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

== Booklet ==

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

== Project achievements ==

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

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

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

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

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

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

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

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

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

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

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

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

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

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

François Pecquery (ARIANE GROUP), Mélody Cailler (SAFRAN TECH), Romain Janodet (SAFRAN TECH/CORIA) and Vincent Moureau (CORIA)

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

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

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

''T. Laroche, N. Odier, B. Cuenot (CERFACS). In collaboration with M. Pelletier, T. Schmitt, S. Ducruix (EM2C)''

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

''H. Garg (LEGI) and G. Ghigliotti (LEGI)''

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

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

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

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

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

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

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

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

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

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

=== Project #15: Validation of a fluid structure interaction case with the coupling ALE/SMS ===
''T. Fabbri (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI), V. Moureau (CORIA)''

The objective of this project was the validation of the Turek(2006) benchmark for fluid structure case.
The Structural Mechanics Solver (SMS) was already existing before the workshop, as the coupling with the Arbitrary-Lagrangian Eulerian solver.
However, the results were not in agreement with the case. The data compared here are the flexible part tip displacement, but also the drag and the lift integrated
on the cylinder and the flexible part.
The pure structure test cases were validated, but the forces computed for the pure fluid test cases were not satisfying.
The work of this week was then to improve the viscous shear computation, which implies the wall normal gradient computation.

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

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

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

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

=== Project #17: IMPLEMENTATION OF A COLD PLASMA MODEL IN YALES2 ===

''J.-M. Orlac'h (EM2C), G. Lartigue (CORIA), B. Fiorina (EM2C)''

The objective of this project was to further develop the cold plasma solver in YALES2 in order to accurately model silane nanodusty discharges. The electron temperature equation has been implemented successfully and validated against a reference plasma code. In a second step, a detailed electron kinetics has been implemented in YALES2 in order to couple the electron temperature with the charged species mass fractions. The user can now define a list of reactions whose rates depend on the electron temperature. These improvements open the path to the simulation of nanoparticle production in silane discharges using a Lagrangian description for the nanoparticles.

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

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

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

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

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

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

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

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

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

Several ways were explored:

- Profiling of reactive simulations when using Virtual Chemistry

- Effect of redundant species transport

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

- Effect of the correction functions smoothing

Conclusions of the study are:

- String trimming and concatenation heavily affect computing performances

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

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

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

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

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

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

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

La simulation d'écoulements de gaz denses dans AVBP s'appuie actuellement sur l'équation d'état de Martin-Hou pour obtenir les grandeurs thermodynamique dans chaque cellule du maillage à partir de la masse volumique (ou du volume spécifique v) et de l'énergie interne déduites des variables conservatives. Le projet développe une approche alternative où les grandeurs thermodynamiques locales sont calculées à partir de propriétés thermodynamiques tabulées. Pour préserver la physique de l'écoulement, l'interpolation dans les tables doit être réalisée de façon consistante (une simple interpolation bilinéaire pour chaque grandeur thermodynamique ne suffit pas). La représentation de l'énergie libre f(v,T) par un polynôme hermitien bi-quintique dans chaque cellule de la table permet d'assurer une interpolation consistante (puisque toutes les grandeurs thermodynamique sont obtenues en dérivant ce polynôme). Le workshop a permis de mettre en place les fonctions nécessaires à cette interpolation dans le module "real gas". Les perspectives à court terme sont la finalisation de cette implémentation, sa validation sur des cas-tests simples précédemment calculés avec équation d'état et l'optimisation de l'implémentation (en particulier la stratégie d'identification de la position dans la LuT pour chaque état local associé à une cellule du maillage

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

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

Ecfd:ecfd 3rd edition

2020-01-31T10:37:17Z

Ghigliot5g: /* Project #12: Stability of a semi-implicit compressible cavitation solver */

{{DISPLAYTITLE: ECFD workshop, 3rd edition, 2020}}

== Sponsors ==

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

== Participants ==

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

== Flyer ==

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

== Presentations ==

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

== Booklet ==

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

== Project achievements ==

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

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

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

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

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

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

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

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

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

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

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

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

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

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

François Pecquery (ARIANE GROUP), Mélody Cailler (SAFRAN TECH), Romain Janodet (SAFRAN TECH/CORIA) and Vincent Moureau (CORIA)

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

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

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

''T. Laroche, N. Odier, B. Cuenot (CERFACS). In collaboration with M. Pelletier, T. Schmitt, S. Ducruix (EM2C)''

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

''Himani GARG (LEGI) and Giovanni Ghigliotti (LEGI)''

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

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

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

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

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

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

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

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

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

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

=== Project #15: Validation of a fluid structure interaction case with the coupling ALE/SMS ===
''T. Fabbri (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI), V. Moureau (CORIA)''

The objective of this project was the validation of the Turek(2006) benchmark for fluid structure case.
The Structural Mechanics Solver (SMS) was already existing before the workshop, as the coupling with the Arbitrary-Lagrangian Eulerian solver.
However, the results were not in agreement with the case. The data compared here are the flexible part tip displacement, but also the drag and the lift integrated
on the cylinder and the flexible part.
The pure structure test cases were validated, but the forces computed for the pure fluid test cases were not satisfying.
The work of this week was then to improve the viscous shear computation, which implies the wall normal gradient computation.

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

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

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

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

=== Project #17: IMPLEMENTATION OF A COLD PLASMA MODEL IN YALES2 ===

''J.-M. Orlac'h (EM2C), G. Lartigue (CORIA), B. Fiorina (EM2C)''

The objective of this project was to further develop the cold plasma solver in YALES2 in order to accurately model silane nanodusty discharges. The electron temperature equation has been implemented successfully and validated against a reference plasma code. In a second step, a detailed electron kinetics has been implemented in YALES2 in order to couple the electron temperature with the charged species mass fractions. The user can now define a list of reactions whose rates depend on the electron temperature. These improvements open the path to the simulation of nanoparticle production in silane discharges using a Lagrangian description for the nanoparticles.

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

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

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

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

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

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

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

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

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

Several ways were explored:

- Profiling of reactive simulations when using Virtual Chemistry

- Effect of redundant species transport

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

- Effect of the correction functions smoothing

Conclusions of the study are:

- String trimming and concatenation heavily affect computing performances

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

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

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

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

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

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

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

La simulation d'écoulements de gaz denses dans AVBP s'appuie actuellement sur l'équation d'état de Martin-Hou pour obtenir les grandeurs thermodynamique dans chaque cellule du maillage à partir de la masse volumique (ou du volume spécifique v) et de l'énergie interne déduites des variables conservatives. Le projet développe une approche alternative où les grandeurs thermodynamiques locales sont calculées à partir de propriétés thermodynamiques tabulées. Pour préserver la physique de l'écoulement, l'interpolation dans les tables doit être réalisée de façon consistante (une simple interpolation bilinéaire pour chaque grandeur thermodynamique ne suffit pas). La représentation de l'énergie libre f(v,T) par un polynôme hermitien bi-quintique dans chaque cellule de la table permet d'assurer une interpolation consistante (puisque toutes les grandeurs thermodynamique sont obtenues en dérivant ce polynôme). Le workshop a permis de mettre en place les fonctions nécessaires à cette interpolation dans le module "real gas". Les perspectives à court terme sont la finalisation de cette implémentation, sa validation sur des cas-tests simples précédemment calculés avec équation d'état et l'optimisation de l'implémentation (en particulier la stratégie d'identification de la position dans la LuT pour chaque état local associé à une cellule du maillage

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

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

Ecfd:ecfd 3rd edition

2020-01-31T10:35:46Z

Ghigliot5g: /* Project #12: Stability of a semi-implicit compressible cavitation solver */

{{DISPLAYTITLE: ECFD workshop, 3rd edition, 2020}}

== Sponsors ==

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

== Participants ==

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

== Flyer ==

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

== Presentations ==

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

== Booklet ==

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

== Project achievements ==

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

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

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

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

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

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

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

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

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

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

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

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

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

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

François Pecquery (ARIANE GROUP), Mélody Cailler (SAFRAN TECH), Romain Janodet (SAFRAN TECH/CORIA) and Vincent Moureau (CORIA)

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

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

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

''T. Laroche, N. Odier, B. Cuenot (CERFACS). In collaboration with M. Pelletier, T. Schmitt, S. Ducruix (EM2C)''

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

"Himani GARG (LEGI) and Giovanni Ghigliotti (LEGI)"

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

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

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

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

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

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

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

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

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

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

=== Project #15: Validation of a fluid structure interaction case with the coupling ALE/SMS ===
''T. Fabbri (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI), V. Moureau (CORIA)''

The objective of this project was the validation of the Turek(2006) benchmark for fluid structure case.
The Structural Mechanics Solver (SMS) was already existing before the workshop, as the coupling with the Arbitrary-Lagrangian Eulerian solver.
However, the results were not in agreement with the case. The data compared here are the flexible part tip displacement, but also the drag and the lift integrated
on the cylinder and the flexible part.
The pure structure test cases were validated, but the forces computed for the pure fluid test cases were not satisfying.
The work of this week was then to improve the viscous shear computation, which implies the wall normal gradient computation.

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

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

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

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

=== Project #17: IMPLEMENTATION OF A COLD PLASMA MODEL IN YALES2 ===

''J.-M. Orlac'h (EM2C), G. Lartigue (CORIA), B. Fiorina (EM2C)''

The objective of this project was to further develop the cold plasma solver in YALES2 in order to accurately model silane nanodusty discharges. The electron temperature equation has been implemented successfully and validated against a reference plasma code. In a second step, a detailed electron kinetics has been implemented in YALES2 in order to couple the electron temperature with the charged species mass fractions. The user can now define a list of reactions whose rates depend on the electron temperature. These improvements open the path to the simulation of nanoparticle production in silane discharges using a Lagrangian description for the nanoparticles.

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

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

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

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

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

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

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

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

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

Several ways were explored:

- Profiling of reactive simulations when using Virtual Chemistry

- Effect of redundant species transport

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

- Effect of the correction functions smoothing

Conclusions of the study are:

- String trimming and concatenation heavily affect computing performances

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

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

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

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

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

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

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

La simulation d'écoulements de gaz denses dans AVBP s'appuie actuellement sur l'équation d'état de Martin-Hou pour obtenir les grandeurs thermodynamique dans chaque cellule du maillage à partir de la masse volumique (ou du volume spécifique v) et de l'énergie interne déduites des variables conservatives. Le projet développe une approche alternative où les grandeurs thermodynamiques locales sont calculées à partir de propriétés thermodynamiques tabulées. Pour préserver la physique de l'écoulement, l'interpolation dans les tables doit être réalisée de façon consistante (une simple interpolation bilinéaire pour chaque grandeur thermodynamique ne suffit pas). La représentation de l'énergie libre f(v,T) par un polynôme hermitien bi-quintique dans chaque cellule de la table permet d'assurer une interpolation consistante (puisque toutes les grandeurs thermodynamique sont obtenues en dérivant ce polynôme). Le workshop a permis de mettre en place les fonctions nécessaires à cette interpolation dans le module "real gas". Les perspectives à court terme sont la finalisation de cette implémentation, sa validation sur des cas-tests simples précédemment calculés avec équation d'état et l'optimisation de l'implémentation (en particulier la stratégie d'identification de la position dans la LuT pour chaque état local associé à une cellule du maillage

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

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

Ecfd:ecfd 3rd edition

2020-01-31T10:34:54Z

Ghigliot5g: /* Project #12: Stability of a semi-implicit compressible cavitation solver */

ECFD workshop, 3rd edition, 2020

== Sponsors ==

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

== Participants ==

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

== Flyer ==

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

== Presentations ==

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

== Booklet ==

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

== Project achievements ==

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

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

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

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

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

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

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

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

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

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

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

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

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

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

François Pecquery (ARIANE GROUP), Mélody Cailler (SAFRAN TECH), Romain Janodet (SAFRAN TECH/CORIA) and Vincent Moureau (CORIA)

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

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

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

''T. Laroche, N. Odier, B. Cuenot (CERFACS). In collaboration with M. Pelletier, T. Schmitt, S. Ducruix (EM2C)''

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

"Himani GARG (LEGI) and Giovanni Ghigliotti (LEGI)"

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

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

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

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

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

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

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

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

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

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

=== Project #15: Validation of a fluid structure interaction case with the coupling ALE/SMS ===
''T. Fabbri (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI), V. Moureau (CORIA)''

The objective of this project was the validation of the Turek(2006) benchmark for fluid structure case.
The Structural Mechanics Solver (SMS) was already existing before the workshop, as the coupling with the Arbitrary-Lagrangian Eulerian solver.
However, the results were not in agreement with the case. The data compared here are the flexible part tip displacement, but also the drag and the lift integrated
on the cylinder and the flexible part.
The pure structure test cases were validated, but the forces computed for the pure fluid test cases were not satisfying.
The work of this week was then to improve the viscous shear computation, which implies the wall normal gradient computation.

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

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

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

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

=== Project #17: IMPLEMENTATION OF A COLD PLASMA MODEL IN YALES2 ===

''J.-M. Orlac'h (EM2C), G. Lartigue (CORIA), B. Fiorina (EM2C)''

The objective of this project was to further develop the cold plasma solver in YALES2 in order to accurately model silane nanodusty discharges. The electron temperature equation has been implemented successfully and validated against a reference plasma code. In a second step, a detailed electron kinetics has been implemented in YALES2 in order to couple the electron temperature with the charged species mass fractions. The user can now define a list of reactions whose rates depend on the electron temperature. These improvements open the path to the simulation of nanoparticle production in silane discharges using a Lagrangian description for the nanoparticles.

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

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

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

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

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

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

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

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

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

Several ways were explored:

- Profiling of reactive simulations when using Virtual Chemistry

- Effect of redundant species transport

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

- Effect of the correction functions smoothing

Conclusions of the study are:

- String trimming and concatenation heavily affect computing performances

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

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

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

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

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

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

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

La simulation d'écoulements de gaz denses dans AVBP s'appuie actuellement sur l'équation d'état de Martin-Hou pour obtenir les grandeurs thermodynamique dans chaque cellule du maillage à partir de la masse volumique (ou du volume spécifique v) et de l'énergie interne déduites des variables conservatives. Le projet développe une approche alternative où les grandeurs thermodynamiques locales sont calculées à partir de propriétés thermodynamiques tabulées. Pour préserver la physique de l'écoulement, l'interpolation dans les tables doit être réalisée de façon consistante (une simple interpolation bilinéaire pour chaque grandeur thermodynamique ne suffit pas). La représentation de l'énergie libre f(v,T) par un polynôme hermitien bi-quintique dans chaque cellule de la table permet d'assurer une interpolation consistante (puisque toutes les grandeurs thermodynamique sont obtenues en dérivant ce polynôme). Le workshop a permis de mettre en place les fonctions nécessaires à cette interpolation dans le module "real gas". Les perspectives à court terme sont la finalisation de cette implémentation, sa validation sur des cas-tests simples précédemment calculés avec équation d'état et l'optimisation de l'implémentation (en particulier la stratégie d'identification de la position dans la LuT pour chaque état local associé à une cellule du maillage

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

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

Ecfd:ecfd 3rd edition

2020-01-31T10:34:37Z

Ghigliot5g: /* Project #12: Stability of a semi-implicit compressible cavitation solver */

ECFD workshop, 3rd edition, 2020

== Sponsors ==

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

== Participants ==

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

== Flyer ==

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

== Presentations ==

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

== Booklet ==

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

== Project achievements ==

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

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

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

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

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

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

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

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

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

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

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

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

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

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

François Pecquery (ARIANE GROUP), Mélody Cailler (SAFRAN TECH), Romain Janodet (SAFRAN TECH/CORIA) and Vincent Moureau (CORIA)

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

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

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

''T. Laroche, N. Odier, B. Cuenot (CERFACS). In collaboration with M. Pelletier, T. Schmitt, S. Ducruix (EM2C)''

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

=== Project #15: Validation of a fluid structure interaction case with the coupling ALE/SMS ===
''T. Fabbri (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI), V. Moureau (CORIA)''

The objective of this project was the validation of the Turek(2006) benchmark for fluid structure case.
The Structural Mechanics Solver (SMS) was already existing before the workshop, as the coupling with the Arbitrary-Lagrangian Eulerian solver.
However, the results were not in agreement with the case. The data compared here are the flexible part tip displacement, but also the drag and the lift integrated
on the cylinder and the flexible part.
The pure structure test cases were validated, but the forces computed for the pure fluid test cases were not satisfying.
The work of this week was then to improve the viscous shear computation, which implies the wall normal gradient computation.

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

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

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

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

=== Project #17: IMPLEMENTATION OF A COLD PLASMA MODEL IN YALES2 ===

''J.-M. Orlac'h (EM2C), G. Lartigue (CORIA), B. Fiorina (EM2C)''

The objective of this project was to further develop the cold plasma solver in YALES2 in order to accurately model silane nanodusty discharges. The electron temperature equation has been implemented successfully and validated against a reference plasma code. In a second step, a detailed electron kinetics has been implemented in YALES2 in order to couple the electron temperature with the charged species mass fractions. The user can now define a list of reactions whose rates depend on the electron temperature. These improvements open the path to the simulation of nanoparticle production in silane discharges using a Lagrangian description for the nanoparticles.

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

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

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

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

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

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

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

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

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

Several ways were explored:

- Profiling of reactive simulations when using Virtual Chemistry

- Effect of redundant species transport

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

- Effect of the correction functions smoothing

Conclusions of the study are:

- String trimming and concatenation heavily affect computing performances

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

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

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

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

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

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

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

La simulation d'écoulements de gaz denses dans AVBP s'appuie actuellement sur l'équation d'état de Martin-Hou pour obtenir les grandeurs thermodynamique dans chaque cellule du maillage à partir de la masse volumique (ou du volume spécifique v) et de l'énergie interne déduites des variables conservatives. Le projet développe une approche alternative où les grandeurs thermodynamiques locales sont calculées à partir de propriétés thermodynamiques tabulées. Pour préserver la physique de l'écoulement, l'interpolation dans les tables doit être réalisée de façon consistante (une simple interpolation bilinéaire pour chaque grandeur thermodynamique ne suffit pas). La représentation de l'énergie libre f(v,T) par un polynôme hermitien bi-quintique dans chaque cellule de la table permet d'assurer une interpolation consistante (puisque toutes les grandeurs thermodynamique sont obtenues en dérivant ce polynôme). Le workshop a permis de mettre en place les fonctions nécessaires à cette interpolation dans le module "real gas". Les perspectives à court terme sont la finalisation de cette implémentation, sa validation sur des cas-tests simples précédemment calculés avec équation d'état et l'optimisation de l'implémentation (en particulier la stratégie d'identification de la position dans la LuT pour chaque état local associé à une cellule du maillage

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

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

Ecfd:ecfd 3rd edition

2020-01-31T10:01:58Z

Ghigliot5g: /* Project #12: Stability of a semi-implicit compressible cavitation solver */