Extreme CFD workshop - User contributions [en]

Ecfd:ecfd 9th edition

2026-02-04T10:29:49Z

Mbernard: /* N1 - Improving ICS robustness and accuracy - M. Bernard (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI) & T. Berthelon (LEGI) */

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

== Description ==

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


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

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



== News ==

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

== Thematics / Mini-workshops ==

To be announced...

== Projects ==

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

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

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

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

==== U1 - Yales2 Trame Editor, toward a fully featured graphical user interface for YALES2 ====

Participants: L. Korzeczek(GDTECH), T-P. Luu (GDTECH), S. Meynet (GDTECH), M. Cailler (SAFRAN), R. Letournel (SAFRAN), G. Lartigue (CORIA)

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

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

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

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

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

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

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

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

==== T2 - Vorticity model for discharge-induced flow dynamics - S. Wang (EM2C), T. Kebir (EM2C), E. Roger (EM2C), Y. Bechane (CORIA) & V. Moureau (CORIA) ====

==== T3 - Discharge movement model for breakdown prediction - S. Wang (EM2C), T. Kebir (EM2C), E. Roger (EM2C), Y. Bechane (CORIA) & V. Moureau (CORIA) ====

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

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

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

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

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

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

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

==== T9 - LES-based aero-servo-elastic simulation of wind turbines - E. Muller (CORIA & SGRE), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE), B. Duboc (SGRE) & H. Hamdani (GDTech) ====
The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine as well as the structural deformation when YALES2 is coupled to an external library/code. In the past years two coupling libraries have been developed, one to BHawC (SGRE certification tool) and one to OpenFast (NREL open access/open source tool). To improve the user and developer experience, a generalization and uniformization of the two coupling has been conducted in this project. Extensive tests and validations were performed to guarantee the non-regression.

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

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

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

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

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

==== C3 - LES of the thermal degradation of a composite material - A. Grenouilloux (ONERA), K. Bioche (CORIA), N. Dellinger (ONERA) and R. Letournel (SafranTech) ====
In order to certify new composite materials for aerospace applications, it is essential to understand their degradation dynamics under severe thermal loads. The ONERA FIRE test bed was designed for this purpose. This burner generates a premixed air propane flame that reproduces a thermal flux consistent with certification standards near the impinging region. During tests, a strong emission of pyrolysis gases and a secondary diffusion flame are observed, and these gases can self ignite in regions not directly exposed to the primary flame. The project aimed to improve the modeling of this burner using Large-Eddy Simulation and reduce the overall computational cost. A reduced kinetic mechanism was derived with the Brookesia library, enabling the modeling of both premixed and diffusion flames to take into account appropriate chemistry at the front face. Used in FIRE simulations, this mechanism achieved a CPU speed-up of a factor of two compared with the previous scheme. A second reduced mechanism was generated to target auto ignition of pyrolysis gas mixtures that can occur at the rear face, and a dedicated test case was designed. Recent developments in the CWIPI interface allow for mesh adaptation during coupling between YALES2 and MoDeTheC solvers.

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

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

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

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

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

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

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

Ecfd:ecfd 9th edition

2026-02-03T13:52:34Z

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

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

== Description ==

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


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

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



== News ==

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

== Thematics / Mini-workshops ==

To be announced...

== Projects ==

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

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

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

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

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

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

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

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

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

==== T2 - Vorticity model for discharge-induced flow dynamics - S. Wang (EM2C), T. Kebir (EM2C), E. Roger (EM2C), Y. Bechane (CORIA) & V. Moureau (CORIA) ====

==== T3 - Discharge movement model for breakdown prediction - S. Wang (EM2C), T. Kebir (EM2C), E. Roger (EM2C), Y. Bechane (CORIA) & V. Moureau (CORIA) ====

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

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

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

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

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

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

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

==== T9 - LES-based aero-servo-elastic simulation of wind turbines - E. Muller (CORIA & SGRE), P. Benard (CORIA), F. Houtin-Mongrolle (SGRE), B. Duboc (SGRE) & H. Hamdani (GDTech) ====
The YALES2 library includes an advanced modular implementation of the Actuator Line Method (ALM). This approach remains state-of-the-art when performing an LES-based analysis of a wind turbine wake. The method also provides an accurate assessment of the aerodynamic loads applied on the turbine as well as the structural deformation when YALES2 is coupled to an external library/code. In the past years two coupling libraries have been developed, one to BHawC (SGRE certification tool) and one to OpenFast (NREL open access/open source tool). To improve the user and developer experience, a generalization and uniformization of the two coupling has been conducted in this project. Extensive tests and validations were performed to guarantee the non-regression.

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

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

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

==== T10 – Numerical simulation of engine rotors - L. Bricteux (UMONS), G. Balarac (LEGI), Y. Bechane (CORIA) & P. Benard (CORIA) ====

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

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

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

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

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

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

Ecfd:ecfd 8th edition

2025-02-10T13:09:41Z

Mbernard: /* Numerics - M. Bernard, LEGI & G. Lartigue, CORIA */

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

== Description ==

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

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

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

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

== News ==

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

== Thematics / Mini-workshops ==

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

To come...

== Projects ==

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

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

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

==== 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 ====

=== 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 solver nucleate boiling had difficulties to run at very small contact angles (with angles below 30°). A modified version of the reinitialization has been implemented during the ECFD. It has been tested successfully on the spray solver where no mass transfer occurs, improving the quality of the interface position without significant increase in the computational time. 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.

==== 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) ====

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

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.

Ecfd:ecfd 8th edition

2025-02-10T13:07:56Z

Mbernard: /* Numerics - M. Bernard, LEGI & G. Lartigue, CORIA */

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

== Description ==

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

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

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

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

== News ==

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

== Thematics / Mini-workshops ==

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

To come...

== Projects ==

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

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

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

==== 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.

==== 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.

=== 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 solver nucleate boiling had difficulties to run at very small contact angles (with angles below 30°). A modified version of the reinitialization has been implemented during the ECFD. It has been tested successfully on the spray solver where no mass transfer occurs, improving the quality of the interface position without significant increase in the computational time. 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.

==== 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) ====

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

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.

Ecfd:ecfd 8th edition

2025-02-10T10:56:12Z

Mbernard: /* Numerics - M. Bernard, LEGI & G. Lartigue, CORIA */

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

== Description ==

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

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

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

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

== News ==

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

== Thematics / Mini-workshops ==

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

To come...

== Projects ==

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

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

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

==== 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), M. Bernard (LEGI) ====

==== N5 - Local timestep. T. Berthelon (LEGI), M. Bernard (LEGI), G. Balarac (LEGI) ====

==== 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.

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

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

==== 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)

==== 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)

==== 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) ====

==== 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

2025-02-10T10:55:18Z

Mbernard: /* Numerics - M. Bernard, LEGI & G. Lartigue, CORIA */

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

== Description ==

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

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

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

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

== News ==

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

== Thematics / Mini-workshops ==

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

To come...

== Projects ==

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

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

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

==== 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 - Local timestep. T. Berthelon (LEGI), M. Bernard (LEGI), G. Balarac (LEGI) ====

==== N6 - Determination of timestep in semi-implicit solver. T. Berthelon (LEGI), G. Balarac (LEGI), M. Bernard (LEGI) ====

==== 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.

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

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

==== 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)

==== 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)

==== 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) ====

==== 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

2025-02-10T10:28:10Z

Mbernard: /* N2 - Treatment of Inlet/Outlet conditions in High-Order solver. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) */

{{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) ====

==== 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 ===

==== 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.

==== 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.

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

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

==== 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)

==== 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)

==== 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 ====

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) ====

==== 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

2025-02-10T10:11:46Z

Mbernard: /* N3 - Conservative mesh-to-mesh interpolation. M. Bernard (LEGI), Ghislain Lartigue (CORIA), Guillaume Balarac (LEGI) */

Ecfd:ecfd 8th edition

2025-02-10T09:57:49Z

Mbernard: /* Numerics - M. Bernard, LEGI & G. Lartigue, CORIA */

Ecfd:ecfd 7th edition

2024-02-05T11:07:01Z

Mbernard: /* WIP N1: High-Order schemes for Navier-Stokes - M. Bernard & G. Balarac (LEGI) & G. Lartigue (Total Eneriges) */

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

== Description ==

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

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

[[File:ecfd7.png|600px|link=https://ecfd.coria-cfd.fr/index.php/Ecfd:ecfd_6th_edition]]

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



== Agenda ==

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

== Thematics / Mini-workshops ==

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

To come...

== Projects ==

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

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

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

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

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

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

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

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

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

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

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

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

==== N6: Electrodeformation of red blood cells, extension to 3D and improved accuracy at membrane - A. Spadotto & S. Mendez (IMAG), M. Bernard (LEGI) ====
The Leaky Dielectric Model is a popular framework to describe electric stresses over micro-scale membranes. We have adopted it to simulate the effect of a DC electric field on a red blood cell using the YALES2BIO solver. The goal of the project is to reproduce the electric charging process of the membrane, as well as the resulting stresses, which may yield to electrodeformation of the cell. From the point of view of the implementation, the grid is represented by a 2D surface mesh embedded in a 3D eulerian grid. The need to make variables stored on the surface interact with quantities stored on the Eulerian grid calls for a proper bidirectional 2D-membrane/3D-grid dynamic connectivity. The advancement of theis task during this ECFD has led to the first 3D simulation of a charging fixed spherical shell. Moreover, the estimation of grid variables on elements cut by the membrane has been improved thanks to a High-Order extrapolation. The latter has been successfully tested on 2D configurations. The project opens the way for a series of validation tests. In particular, future work will demand treatment of instabilities emerging in symmetrical configurations.

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

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

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

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

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

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

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

Ecfd:ecfd 7th edition

2024-02-02T13:32:44Z

Mbernard: /* Numerics - S. Mendez, IMAG & G. Balarac, LEGI */

Ecfd:ecfd 6th edition

2023-02-05T18:56:42Z

Mbernard: /* Numerics - S. Mendez, IMAG & M. Bernard, LEGI */

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

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

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

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

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



== Agenda ==

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

== Thematics / Mini-workshops ==

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

== Projects ==

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

=== Mesh adaptation - R. Letournel, Safran & V. Moureau, CORIA ===

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

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

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

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

* '''Sub-project 4: Force coupling method (FCM) for particulate flows (C. Raveleau, S. Mendez)'''

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

* '''Sub-project 6: Development of a traction open boundary condition (TOBC) in Yales2 (J.B. Lagaert, Guillaume Balarac)'''

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

'''

* '''Sub-project 8: DOROTHY optimization (M. Roperch, H. Mulakaloori, G. Pinon, P. Bénard)'''

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

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

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

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

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

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

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

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

* '''Sub-project 4: Wall law for Immersed Boundaries & Rough surfaces (M. Cailler, A. Cuffaro, P. Benez, S. Meynet)'''

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

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

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

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

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

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

Ecfd:ecfd 6th edition

2023-02-04T19:10:32Z

Mbernard: /* Numerics - S. Mendez, IMAG & M. Bernard, LEGI */

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

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

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

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

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



== Agenda ==

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

== Thematics / Mini-workshops ==

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

== Projects ==

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

=== Mesh adaptation - R. Letournel, Safran & V. Moureau, CORIA ===

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

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

* '''Sub-project 2: Ghost fluid method (GFM) for Electrodeformation (A. Spadotto , S. Mendez)'''

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

* '''Sub-project 4: Force coupling method (FCM) for particulate flows (C. Raveleau, S. Mendez)'''

* '''Sub-project 5: Breaking limitations of the linearized implicit time advancement (T. Berthelon, G. Lartigue, G. Balarac)'''

* '''Sub-project 6: Development of a traction open boundary condition (TOBC) in Yales2 (J.B. Lagaert, Guillaume Balarac)'''

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

'''

* '''Sub-project 8: DOROTHY optimization (M. Roperch, H. Mulakaloori, G. Pinon, P. Bénard)'''

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

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

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

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

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

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

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

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

* '''Sub-project 4: Wall law for Immersed Boundaries & Rough surfaces (M. Cailler, A. Cuffaro, P. Benez, S. Meynet)'''

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

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

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

Ecfd:ecfd 6th edition

2023-02-03T06:50:07Z

Mbernard: /* Numerics - S. Mendez, IMAG & M. Bernard, LEGI */

Ecfd:ecfd 6th edition

2023-02-03T06:24:13Z

Mbernard: /* Numerics - S. Mendez, IMAG & M. Bernard, LEGI */

Ecfd:ecfd 6th edition

2023-02-03T06:06:14Z

Mbernard: /* Turbulence - P. Benard, CORIA & G. Balarac, LEGI */

Ecfd:ecfd 6th edition

2023-02-03T06:04:09Z

Mbernard: /* Numerics - S. Mendez, IMAG & A. Pushkarev, GE Hydro */

Ecfd:ecfd 5th edition

2022-02-01T09:09:38Z

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

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

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

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

== News ==

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

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

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

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

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

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

* 28/04/2022: '''The ECFD5 event is now finished!''' It was again a successful scientific week. See you next year for the '''ECFD6'''! [https://www.linkedin.com/posts/l%C3%A9a-voivenel-642ab7186_avbp-yales2-yales2-activity-6892778892801716224-3zxn| LinkedIn post]

[[File:ECFD5_PhotoGroupe.jpeg|text-bottom|600px]]

== Agenda ==

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

== Thematics / Mini-workshops ==

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

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

* '''Sub-project 1: H2/air jet-in-cross-flow numerical simulations (R. Le Dortz, E. Riber, Q. Douasbin)'''

The use of hydrogen as an aviation fuel requires new combustion chamber design. Among strategies to prevent flame flashback and low flame residence time, the micromix injection system is further studied by ENABLEH2. This systems corresponds to a multitude of H2/air jet-in-cross-flow configurations. A 3D numerical simulation with realistic thermodynamics and kinetics is now tractable thanks to massively parralel computing. This week saw the completion of the first steps towards the establishment of a complete simulation. (I) The non-reactive air injection in the combustion chamber. (II) The cross-injection of H2 without ignition. (III) The ignition of this mixture modeled with the skeletal kinetic mechanism of Boivin (H2, H, O2, OH, O, H2O, HO2, H2O2, N2). Further work will be realised concerning mesh refinement, modelling of NOx and porting of the computation on GPU.

* '''Sub-project 2: LES calculation of the MICADO test rig with multicomponent jetA1 (S. Puggelli, T. Lesaffre, E. Riber, B. Cuenot)'''

The EU-funded project ALTERNATE has the goal of exploring the possibility for a wider utilisation of aviation sustainable fuels. A part of the project deals with the assessment of the effect of SAFs on soot production: using the experimental information obtained at ONERA in high-pressure conditions on the MICADO test rig, the effect of Alcohol to Jet (ATJ-SPK) fuel on soot levels are assessed and compared with standard jet A1 emissions. During the project, STech and CERFACS are working jointly on the numerical modelling of soot emissions for jet-A1 and ATJ-SPK combustion in AVBP. Starting from the numerical setup under-development for jet-A1, the worshop permitted to: (I) Switch from a 2-step kinetic mechanism to a complex 29 species, 233 reacs and 15 QSS mechanism. This transition was efficiently conducted with the tool Multi Table Generator. (II) At this stage, an assessment of the effects of the flame sensor on the calculation results was carried out, indicating the consistent behaviour of a recently developped sensor w.r.t classical tools. (III) Switch towards a multicomponent formulation of jet-A1 and assessment of the effect of such advanced approach with respect to the single-component formulation previously employed. Further work will be realised to manage the stiffness of employed kinetics and to compare jet-A1 and ATJ-SPK fuels from a chemical point of view.

* '''Sub-project 3: Euler-Lagrange Multigrid Simulation (T. Lesaffre, O. Vermorel, E. Riber, B. Cuenot)'''

In Lagrange simulations, the point-source approach is based on a ponctual approximation of the particule and requires this last to be smaller than the mesh. The very fine meshes required to represent the Eulerian phase of Euler-Lagrange two-phase flow simulations can lead to a non-validity of the point-source hypothesis. This project aimed at implementing, in the AVBP solver, the simultaneous management and coupling of several simulations. During this week, the Eulerian and Lagrangian phase were successfuly computed on two different meshes and coupled via the CWIPI library. The good behaviour of this framework was assessed on a 1D Evaporation of kerosene droplets in an air stream test case. Encouraging preliminary performance results were obtained on a 3D injection case and require further work.

* '''Sub-project 4: Devolatilization modelling for biomass combustion (K. Bioche, L. Bricteux)'''

Biomass combustion simulations require the modelling of numerous physical phenomena: particle drying, devolatilization, gas-phase combustion, chars oxidation. Besides, the valorisation chains for biomass include fluidized bed reactors, fixed bed reactors and pulverized fuel burners. The Granular Flow Solver of YALES2 offers a good framework for the simulation of fluidized bed reactors and is functionnaly coupled with the reactive gas-phase solver of the same code. This week permitted to partically implement the modelling of devolatilization in this solver. A single-step kinetic scheme is considered for the particle mass evolution equation while the particle diameter evolves during the process. Further work is necessary to account for the thermal and mass couplings with the fluid phase.

* '''Sub-project 5: Thickened-Flame LES model in a Lattice-Boltzmann Method framework (P. Boivin, S. Zhao, M. Le Boursicaud)'''

The TFLES framework of the hybrid Lattice-Boltzmann sover ProLB was extended to account for recent sensor methods. During this week, a smooth flame sensor based on the curvature of the norm of the advancement variable gradient was developped. Also for filtering operations, the lattice requires to access data over three neighboring layers. A precise and continuous thickening factor was obtained with such method.

* '''Sub-project 6: NOx modeling applied to KIAI combustion chamber (J. Obando, P. Bénard, V. Moureau)'''

This project treated of the implementation of NOx modeling into simulations of the KIAI combustion chamber, experimentaly studied at CORIA lab. During this week, various NOx modeling strategies were listed. Associated kinetic mechanisms, among which analytical chemisty, were employed for 1D flame simulations in YALES2 solver. Further work include the use of such methods on the 3D computational case.

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

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

* '''Sub-project 1: Hybrid E-E/E-L two-phase flow method (M. Cailler, F. Pecquery, I. El Yamani, V. Moureau)'''

The High-Fidelity approach based on ACLS & DMA allows a reliable description of interface dynamics. For design exploration, low-CPU methods with controlled level of fidelity are required. An interesting approach to reduce CPU cost relies on an hybrid approach based on an Eulerian representation of the gas and a Lagrangian description for the liquid phase. Objective of the ECFD5 was to explore the capability to reconstruct the interface normal of a liquid droplet made of particles on an Eulerian grid. First, a level-set based strategy relying on Geometric Multiple Markers Projection (Janodet et al., 2022) has been first tested showing good capabilities providing that the iso-surface distance equal 0 is well captured on the mesh. An alternative strategy based on the liquid volume fraction has been tested. This information was then used to build a velocity correction that is used to transport particles and ensures regularized particle positions. This preliminary benchmark work will be pursued on a liquid jet propagation problem.

* '''Sub-project 2: Jet atomization with a diffuse interface mathod (N. Odier, B. Péden, J. Carmona, P. Boivin)'''

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

* '''Sub-project 3: Convergence of the interface curvature computation (G. Ghigliotti, J. Carmona, G. Balarac, G. Lartigue)'''

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

* '''Sub-project 4: Conservative two-fluid momentum transport (F. Pecquery, C. Merlin, M. Cailler, J. Carmona, V. Moureau)'''

The aim of this project was to investigate the applicability of the conservative two-fluid transport framework developed for passive scalars to the momentum conservation equation. First a complete two-fluid framework for the incompressible two-phase Navier-Stokes equations was proposed. Then the algorithms for discontinuous scalars (data extension in the other phase, reinitialization and transport) have been extended to discontinuous vectors. Moreover, some improvement of the data-structure were implemented to further generalize the framework and improve user-experience. To eventually conclude on the applicability of the two-fluid momentum approach, the proposed interfacial momentum flux model and correction step will be implemented in YALES2.

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

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

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

* '''Sub-project 4: Avoiding the acoustic timestep restriction in two-phase flow simulations (V. Boniou, J. Paris, A. Vié, T. Schmitt, C. Tenaud, Y. Béchane)'''
In the context of simulating compressible two-phase flows with phase change, the CFL constraint related to the acoustic part of an explicit solver, especially in the liquid phase, can be prohibitive for evaporation studies implying large timescale separation between evaporation and acoustic wave propagation.
The acoustic CFL constraint can be either relaxed by considering an implicit treatment of the acoustic part or avoided by deriving the low Mach limit of the multifluid model.

During this week, the first steps of this vast work have been initiated on single-phase flows by implementing the variable-density low Mach solver and the IMEX method applied on the Euler equations in TITAN.

The IMEX method was shown to provide degraded solutions in acoustic-driven cases, compared to conventional Riemann solutions. However, in transport-dominated cases, the increase of the transport CFL leads to a better numerical precision. A 2D cylindrical shock has been simulated with an acoustic CFL of 40 to assess the ability to run multidimensional compressible computations with large time step. The method was also extented to 4eq and 2nd order spatial accuracy.

Finally, the extension of the variable density solver to two-phase flows was performed using a VOF interface capturing method, considering a dilatable gas phase along with an incompressible liquid phase. Particular attention has been drawn to preserving liquid volume by treating the VOF transport accordingly with an adapted velocity field.

=== Turbulent flows - P. Bénard, CORIA ===
* '''Sub-project 1: Optimization of the actuator set for several wind turbines in YALES2 (F. Houtin Mongrolle, S. Gremmo, E. Muller, B. Duboc)'''
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified:
(i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges.
(ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications.
(iii) Adapt the YALES2-BHawC aero-servo-elastic coupling to make it consistent with the new ALM implementation.
During the workshop, the (i)-algorithm has been implemented with additional optimizations. Tests on a 30-turbine wind farm showed a 60% improvement compared to previous performances, which is very promising. The (ii)-feature still needs to be developed but should lead to the performances objective. The tests with the coupled code YALES2-BHawC were not conclusive yet and still require some work.

* '''Sub-project 2: Thermal effect in an atmospheric solver (U. Vigny, L. Voivenel, S. Zeoli, P. Benard)'''
Given the current environmental and energy challenges, maximising the wind farm electricity production is essential. Therefore, it becomes necessary to develop the most reliable and accurate prediction and simulation tools. Following this tenet, an atmospheric solver, which will take into account meteorological phenomena, should be developed. The preliminary work, going from bibliography study to road map was performed during the extreme cod workshop. Thus five parts have been identified:
(I) The YALES2 Variable Density Solver (VDS) will be used because of the need to take into account buoyancy effect including for big density differences.
(II) A wall law correction term, relative to atmospheric boundary layer will be added.
(III) The actuator line method used to simulate wind turbine will be extended to VDS, modifying the velocity source term to a momentum source term.
(IV) The Coriolis effects, depending on the latitude will be implemented.
(V) The wall heat flux, allowing to simulate diurnal and nocturnal cycles on various terrains, is more realistic than a target wall temperature.
From this work, future development are now clear and just waiting to be developed.

* '''Sub-project 3: Dynamic stall correction models for Horizontal Axis Wind Turbine (S. Gremmo, E. Muller, B. Duboc, F. Houtin-Mongrolle)'''
The Actuator Line Method implementation into YALES2 library suffers from the lack of correction models for some 3D unsteady effects. An important one, the dynamic stall, implies an unsteady modification of the aerodynamic loads, i.e. the polars, with the angle of attack. With the standard ALM model, static 2D polars are used. Adding a dynamic stall correction model allows to have an hysteresis loop on the evolution of the forces coefficients with the angle of attack. Several models exist in the literature: Oye, BeddoesLeishman, Sheng, Risoe, ONERA... During the workshop, the Oye model was selected, because of its simplicity, and implemented. First, new ingredients necessary to the model were added in the polars look-up table generator used to describe the blades. Then, the Oye correction was integrated into the YALES2 library. Finally, the model was tested on simple configurations. It is now essential to further validate the implementation.

* '''Sub-project4: Immersed Boundary solvers uniformisation into YALES2 (I. Tsetoglou, M. Cailler, P. Benez, S. Mendez, P. Benard)'''
A novel Conservative Lagrangian Immersed Boundary (CLIB) method [Tsetoglou et al., 2021] has been developed by CORIA & Safran Tech for wind turbines & gearbox lubrication simulations. Nevertheless, two different implementations of the same method were created that tends to diverge. This sub-project aims to develop a shared numerical kernel (CLIB) on which 2 solvers are based: CIB for aerodynamic applications (ICS + CLIB) and TPS for two-phase flows (SPS+CLIB). The newly developed kernel and solvers were tested and validated on test cases: 2D flows around static/rotating/oscillating disk, 3D flow around a rotating cylinder and 2D flows around counter-rotating ellipses. The newt steps involve the continuation of the validation of both solvers and its documentation, as well as the development of wall model for immersed bodies.

* '''Sub-project 5: TBLE wall model for LES with pressure gradient on a simple turbomachinery geometry (M. Cizeron, N. Odier, R. Vicquelin)'''
Wall modeling is often used in LES to alleviate the computational cost that would be required to resolve all the length scales up to the solid boundaries of the domain. The classical way of doing it is by using an algebraic model to provide the wall friction and heat flux, with a coupling to the LES solver at the first off-wall nodes. The wall model was designed from analyzing RANS equation with strong assumptions such as planar flow, equilibrium and no pressure gradient. These assumptions are often far from true in real applications, such as turbomachinery applications, where the use of a wall model is mandatory due to the size of the calculation. During this workshop, a wall model relying on the resolution of the Thin Boundary Layer Equations (TBLE) was studied, which had been implemented by EM2C. The addition of a pressure gradient to these equations has been conducted and tested, at first only for the 1D wall model solver, then on a 3D turbulent channel. It remains to be tested on a diffuser configuration with a real pressure gradient to quantify the effect of the new wall model. The influence of the point considered to do the coupling between the LES and the wall model (ie. its distance to the wall) has also been tested both for the TBLE and the original algebraic model, showing that coupling farther from the wall yields better results and reduces the so-called log-layer mismatch.

* '''Sub-project 6: Tools for rough wall modelling (A. Barge, S. Meynet)'''
Within the STREAM project framework, a roughness-resolved Large-Eddy Simulation (RRLES) database is being built. The aim of this latter is to be representative of rough channel flows, especially for additive-manufacturing heat exchangers. First RRLES have already been performed. From turbulence and rough wall stress statistics analysis of the results, a first stochastic model, which reproduces the statistical behavior of the wall stress vector, have been proposed. The modeled wall stress allows a better prediction of the pressure drop in a flat wall channel compared to the use of the mean value of the wall stress measured in RRLES alone. However, the near wall region is still mispredicted and the model is correlated in time but not in space. The aim of this ECFD5 was to develop tools to improve modelling and explore new ways. A roughness mapping tool for smooth surfaces have been implemented into YALES2 to get local surface height. This tool is based on an existing in-house surface roughness generator developed for the STREAM project. The idea is to use the map to generated space correlated fluctuations for the wall shear stress. Some bugs still remain to fully use this tool. In parallel, the modelling approach was extended to passive scalar, especially for temperature. To this end, new random tools as white noise, unit sphere random walk and Gaussian / Log-normal stochastic processes have been coded. Finally, the idea of using walls as velocity source terms emerged during this ECFD5. The principle is to mask a grid layer above the wall and to transport the rough map on this grid to estimate the roughness effects above the wall. Parametrizing and testing these tools remained to be done at the end of ECFD5.

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

* '''Sub-project 1: Multi combustion model chemtable generator (S. Dillon, R. Mercier)'''

Due to the high computational cost of LES of turbulent combustion with detailed chemistry, tabulated chemistry approaches remain a popular choice within the combustion community. This popularity of tabulated chemistry leads to research into the development of novel combustion models for which a platform to test these models is required. The objective of the workshop was to therefore begin developing an easy-to-use chemistry table generator within Python. Given that there exists numerous turbulent combustion models (F-TACLES Diffusion, F-TACLES Premixed, 2PFT, etc.), a generalised multi-model code was necessary. Along with multi-model capabilities, the code was required to function with multiple solvers for the creation of the flame database (REGATH and CANTERA). With these objectives in mind, the code “TabulatEd Chemistry GenERator for Aeronautical CombusTion” (TECERACT) was created. Advancements were made in all key areas using code already developed by Safran Tech. TECERACT includes a parallel flame database generator and current developments are focused on multi-model functionality and simplified mesh management within Python. Once finished, the TECERACT code will provide a user-friendly chemistry table generator capable of producing chemistry tables for LES simulations and its multi-model structure will serve as a platform for future turbulent combustion model development/testing.

* '''Sub-project 2: Task-driven automatic run sequence (R. Mercier, J. Leparoux, M. Cailler, R. Letournel)'''

The principal objective of this project was to develop a new simulation workflow based on Task-driven approach. This approach could offer a higher flexibility to manage CFD simulations by decomposing the whole simulation on small sequences (run initialization, fuel injection, etc.)

This week showed the achievement of the first steps towards a complete task-driven simulation. (i) The whole simulation workflow was set-up (ii) Missing actions were implemented allowing to update simulation status on-the-fly (from non reactive to reactive simulation) (iii) State are now embedded from the restart solution allowing to restart from the last known state. Further work is still needed to obtain the full implementation of the AMC framework based on task-driven approach. Especially to automate the adaptation loop

* '''Sub-project 3: YALES2 Tools & Gitlab CI (J. Leparoux, A. Tstetoglou)'''

* '''Sub-project 4: Wind energy tools (E. Muller, S. Gremmo, F. Houtin-Mongrolle, B. Duboc)'''

Set-up phase of simulations involving several wind turbines is very demanding and error-prone, especially for simulations carried out with the coupled solver YALES2-BHawC*. This type of simulation can involve several dozen input files and then, an assistant tool is required.

The objectives of this projet were (i) develop a python tools to set all necessary ACTUATOR_SET objects (ii) provide basic visualization plots allowing to monitor simulations on-the-fly (by post-processing YALES2 temporals and BHawC results files).

''*BHawC : aero-servo-elastic solver used and developped at Siemens Gamesa Renewable Energy for wind turbine design and certification
''

=== Hackathon - G. Staffelbach, CERFACS ===
* '''Participants: I. d'Ast, J. Legaux, G. Staffelbach, P. Begou, G. Lartigue, V. Moureau, A. Toure, C. Laurie, S. Delamare, C. Andrieu, C. Jourdain'''
AMD GPU hardware is still relatively unknown in our CFD community. This hackathon was the opportunity to deep dive into the AMD dev environment to prepare the arrival of AdAstra at CINES.
Both YALES and AVBP have been ported to the AOMP framework using ROCm 4.5 on the GRID5000 Neowise system.
CPU execution posed no issues and we were able to focus on GPU Offloading using OpenMP.
On the YALES2 side, a mini-app encompassing the typical YALES2 structure hierarchy and loop execution was extracted from the code to evaluate different porting strategies and on the AVBP side the current OpenACC GPU offloading was translated to OpenMP focusing on the viscosity computation kernel.
We learnt that the current supported standard of OpenMP in ROCm 4.5 does not allow for direct offloading of reference values inside an derived type structure but is was possible to use aliases such as pointers or flat array copies to do the job. This should be solved with the support of OpenMP 5.0
Another troublesome issues, was the lack of support for offloading of array vector operations (ex : array(:) = 1.0 ) rendering the explicitation of the loops for these manadatory.

Some bugs remain and it is encouraged to use the latest compiler version when working on the porting ( the release of flang 14.0.1 saved us a lot of time as we had started with 14.0.0 ).
Offloading of the miniapp of YALES2 yielded a times 60 acceleration of the kernel whereas the offloading of the viscosity model in a full avbp simulation yielded an 7 times factor in performance when comparing on core to one GPU. These results are to be taken with a grain of salt but are really encouraging.

For the next steps, a porting strategy for both codes will be implemented (depending on the OpenMP 5 support ) and discussions are underway with CINES and other partners so as to offer the best experience to both code's communities on AdAstra at its release.

Ecfd:ecfd 5th edition

2022-02-01T08:53:02Z

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

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

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

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

== News ==

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

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

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

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

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

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

* 28/04/2022: '''The ECFD5 event is now finished!''' It was again a successful scientific week. See you next year for the '''ECFD6'''! [https://www.linkedin.com/posts/l%C3%A9a-voivenel-642ab7186_avbp-yales2-yales2-activity-6892778892801716224-3zxn| LinkedIn post]

[[File:ECFD5_PhotoGroupe.jpeg|text-bottom|600px]]

== Agenda ==

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

== Thematics / Mini-workshops ==

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

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

* '''Sub-project 1: H2/air jet-in-cross-flow numerical simulations (R. Le Dortz, E. Riber, Q. Douasbin)'''

The use of hydrogen as an aviation fuel requires new combustion chamber design. Among strategies to prevent flame flashback and low flame residence time, the micromix injection system is further studied by ENABLEH2. This systems corresponds to a multitude of H2/air jet-in-cross-flow configurations. A 3D numerical simulation with realistic thermodynamics and kinetics is now tractable thanks to massively parralel computing. This week saw the completion of the first steps towards the establishment of a complete simulation. (I) The non-reactive air injection in the combustion chamber. (II) The cross-injection of H2 without ignition. (III) The ignition of this mixture modeled with the skeletal kinetic mechanism of Boivin (H2, H, O2, OH, O, H2O, HO2, H2O2, N2). Further work will be realised concerning mesh refinement, modelling of NOx and porting of the computation on GPU.

* '''Sub-project 2: LES calculation of the MICADO test rig with multicomponent jetA1 (S. Puggelli, T. Lesaffre, E. Riber, B. Cuenot)'''

The EU-funded project ALTERNATE has the goal of exploring the possibility for a wider utilisation of aviation sustainable fuels. A part of the project deals with the assessment of the effect of SAFs on soot production: using the experimental information obtained at ONERA in high-pressure conditions on the MICADO test rig, the effect of Alcohol to Jet (ATJ-SPK) fuel on soot levels are assessed and compared with standard jet A1 emissions. During the project, STech and CERFACS are working jointly on the numerical modelling of soot emissions for jet-A1 and ATJ-SPK combustion in AVBP. Starting from the numerical setup under-development for jet-A1, the worshop permitted to: (I) Switch from a 2-step kinetic mechanism to a complex 29 species, 233 reacs and 15 QSS mechanism. This transition was efficiently conducted with the tool Multi Table Generator. (II) At this stage, an assessment of the effects of the flame sensor on the calculation results was carried out, indicating the consistent behaviour of a recently developped sensor w.r.t classical tools. (III) Switch towards a multicomponent formulation of jet-A1 and assessment of the effect of such advanced approach with respect to the single-component formulation previously employed. Further work will be realised to manage the stiffness of employed kinetics and to compare jet-A1 and ATJ-SPK fuels from a chemical point of view.

* '''Sub-project 3: Euler-Lagrange Multigrid Simulation (T. Lesaffre, O. Vermorel, E. Riber, B. Cuenot)'''

In Lagrange simulations, the point-source approach is based on a ponctual approximation of the particule and requires this last to be smaller than the mesh. The very fine meshes required to represent the Eulerian phase of Euler-Lagrange two-phase flow simulations can lead to a non-validity of the point-source hypothesis. This project aimed at implementing, in the AVBP solver, the simultaneous management and coupling of several simulations. During this week, the Eulerian and Lagrangian phase were successfuly computed on two different meshes and coupled via the CWIPI library. The good behaviour of this framework was assessed on a 1D Evaporation of kerosene droplets in an air stream test case. Encouraging preliminary performance results were obtained on a 3D injection case and require further work.

* '''Sub-project 4: Devolatilization modelling for biomass combustion (K. Bioche, L. Bricteux)'''

Biomass combustion simulations require the modelling of numerous physical phenomena: particle drying, devolatilization, gas-phase combustion, chars oxidation. Besides, the valorisation chains for biomass include fluidized bed reactors, fixed bed reactors and pulverized fuel burners. The Granular Flow Solver of YALES2 offers a good framework for the simulation of fluidized bed reactors and is functionnaly coupled with the reactive gas-phase solver of the same code. This week permitted to partically implement the modelling of devolatilization in this solver. A single-step kinetic scheme is considered for the particle mass evolution equation while the particle diameter evolves during the process. Further work is necessary to account for the thermal and mass couplings with the fluid phase.

* '''Sub-project 5: Thickened-Flame LES model in a Lattice-Boltzmann Method framework (P. Boivin, S. Zhao, M. Le Boursicaud)'''

The TFLES framework of the hybrid Lattice-Boltzmann sover ProLB was extended to account for recent sensor methods. During this week, a smooth flame sensor based on the curvature of the norm of the advancement variable gradient was developped. Also for filtering operations, the lattice requires to access data over three neighboring layers. A precise and continuous thickening factor was obtained with such method.

* '''Sub-project 6: NOx modeling applied to KIAI combustion chamber (J. Obando, P. Bénard, V. Moureau)'''

This project treated of the implementation of NOx modeling into simulations of the KIAI combustion chamber, experimentaly studied at CORIA lab. During this week, various NOx modeling strategies were listed. Associated kinetic mechanisms, among which analytical chemisty, were employed for 1D flame simulations in YALES2 solver. Further work include the use of such methods on the 3D computational case.

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

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

* '''Sub-project 1: Hybrid E-E/E-L two-phase flow method (M. Cailler, F. Pecquery, I. El Yamani, V. Moureau)'''

The High-Fidelity approach based on ACLS & DMA allows a reliable description of interface dynamics. For design exploration, low-CPU methods with controlled level of fidelity are required. An interesting approach to reduce CPU cost relies on an hybrid approach based on an Eulerian representation of the gas and a Lagrangian description for the liquid phase. Objective of the ECFD5 was to explore the capability to reconstruct the interface normal of a liquid droplet made of particles on an Eulerian grid. First, a level-set based strategy relying on Geometric Multiple Markers Projection (Janodet et al., 2022) has been first tested showing good capabilities providing that the iso-surface distance equal 0 is well captured on the mesh. An alternative strategy based on the liquid volume fraction has been tested. This information was then used to build a velocity correction that is used to transport particles and ensures regularized particle positions. This preliminary benchmark work will be pursued on a liquid jet propagation problem.

* '''Sub-project 2: Jet atomization with a diffuse interface mathod (N. Odier, B. Péden, J. Carmona, P. Boivin)'''

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

* '''Sub-project 3: Convergence of the interface curvature computation (G. Ghigliotti, J. Carmona, G. Balarac, G. Lartigue)'''

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

* '''Sub-project 4: Conservative two-fluid momentum transport (F. Pecquery, C. Merlin, M. Cailler, J. Carmona, V. Moureau)'''

The aim of this project was to investigate the applicability of the conservative two-fluid transport framework developed for passive scalars to the momentum conservation equation. First a complete two-fluid framework for the incompressible two-phase Navier-Stokes equations was proposed. Then the algorithms for discontinuous scalars (data extension in the other phase, reinitialization and transport) have been extended to discontinuous vectors. Moreover, some improvement of the data-structure were implemented to further generalize the framework and improve user-experience. To eventually conclude on the applicability of the two-fluid momentum approach, the proposed interfacial momentum flux model and correction step will be implemented in YALES2.

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

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

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

* '''Sub-project 4: Avoiding the acoustic timestep restriction in two-phase flow simulations (V. Boniou, J. Paris, A. Vié, T. Schmitt, C. Tenaud, Y. Béchane)'''
In the context of simulating compressible two-phase flows with phase change, the CFL constraint related to the acoustic part of an explicit solver, especially in the liquid phase, can be prohibitive for evaporation studies implying large timescale separation between evaporation and acoustic wave propagation.
The acoustic CFL constraint can be either relaxed by considering an implicit treatment of the acoustic part or avoided by deriving the low Mach limit of the multifluid model.

During this week, the first steps of this vast work have been initiated on single-phase flows by implementing the variable-density low Mach solver and the IMEX method applied on the Euler equations in TITAN.

The IMEX method was shown to provide degraded solutions in acoustic-driven cases, compared to conventional Riemann solutions. However, in transport-dominated cases, the increase of the transport CFL leads to a better numerical precision. A 2D cylindrical shock has been simulated with an acoustic CFL of 40 to assess the ability to run multidimensional compressible computations with large time step. The method was also extented to 4eq and 2nd order spatial accuracy.

Finally, the extension of the variable density solver to two-phase flows was performed using a VOF interface capturing method, considering a dilatable gas phase along with an incompressible liquid phase. Particular attention has been drawn to preserving liquid volume by treating the VOF transport accordingly with an adapted velocity field.

=== Turbulent flows - P. Bénard, CORIA ===
* '''Sub-project 1: Optimization of the actuator set for several wind turbines in YALES2 (F. Houtin Mongrolle, S. Gremmo, E. Muller, B. Duboc)'''
The implementation of the Actuator Line Method (ALM) into the YALES2 library leads to poor performances when many wind turbine rotors are set. Indeed, each rotor object is a derived type treated sequentially by all the processors participating to the computation. With 30 turbines in a computation, the return time is increased by 70% while the arithmetic intensity appears to be low. The objective of this sub-project is to improve the computation performances of the ALM already identified:
(i) Assign one MPI communicator by rotor object gathering the processors close to the turbine and set-up a master/slave processus by communicator. This will allow the simultaneous rotors computation and reduce the number of MPI exchanges.
(ii) Work on the domain decomposition to limit the number of processors attributed to each turbine. This would reduce or even eliminate MPI communications.
(iii) Adapt the YALES2-BHawC aero-servo-elastic coupling to make it consistent with the new ALM implementation.
During the workshop, the (i)-algorithm has been implemented with additional optimizations. Tests on a 30-turbine wind farm showed a 60% improvement compared to previous performances, which is very promising. The (ii)-feature still needs to be developed but should lead to the performances objective. The tests with the coupled code YALES2-BHawC were not conclusive yet and still require some work.

* '''Sub-project 2: Thermal effect in an atmospheric solver (U. Vigny, L. Voivenel, S. Zeoli, P. Benard)'''
Given the current environmental and energy challenges, maximising the wind farm electricity production is essential. Therefore, it becomes necessary to develop the most reliable and accurate prediction and simulation tools. Following this tenet, an atmospheric solver, which will take into account meteorological phenomena, should be developed. The preliminary work, going from bibliography study to road map was performed during the extreme cod workshop. Thus five parts have been identified:
(I) The YALES2 Variable Density Solver (VDS) will be used because of the need to take into account buoyancy effect including for big density differences.
(II) A wall law correction term, relative to atmospheric boundary layer will be added.
(III) The actuator line method used to simulate wind turbine will be extended to VDS, modifying the velocity source term to a momentum source term.
(IV) The Coriolis effects, depending on the latitude will be implemented.
(V) The wall heat flux, allowing to simulate diurnal and nocturnal cycles on various terrains, is more realistic than a target wall temperature.
From this work, future development are now clear and just waiting to be developed.

* '''Sub-project 3: Dynamic stall correction models for Horizontal Axis Wind Turbine (S. Gremmo, E. Muller, B. Duboc, F. Houtin-Mongrolle)'''
The Actuator Line Method implementation into YALES2 library suffers from the lack of correction models for some 3D unsteady effects. An important one, the dynamic stall, implies an unsteady modification of the aerodynamic loads, i.e. the polars, with the angle of attack. With the standard ALM model, static 2D polars are used. Adding a dynamic stall correction model allows to have an hysteresis loop on the evolution of the forces coefficients with the angle of attack. Several models exist in the literature: Oye, BeddoesLeishman, Sheng, Risoe, ONERA... During the workshop, the Oye model was selected, because of its simplicity, and implemented. First, new ingredients necessary to the model were added in the polars look-up table generator used to describe the blades. Then, the Oye correction was integrated into the YALES2 library. Finally, the model was tested on simple configurations. It is now essential to further validate the implementation.

* '''Sub-project4: Immersed Boundary solvers uniformisation into YALES2 (I. Tsetoglou, M. Cailler, P. Benez, S. Mendez, P. Benard)'''
A novel Conservative Lagrangian Immersed Boundary (CLIB) method [Tsetoglou et al., 2021] has been developed by CORIA & Safran Tech for wind turbines & gearbox lubrication simulations. Nevertheless, two different implementations of the same method were created that tends to diverge. This sub-project aims to develop a shared numerical kernel (CLIB) on which 2 solvers are based: CIB for aerodynamic applications (ICS + CLIB) and TPS for two-phase flows (SPS+CLIB). The newly developed kernel and solvers were tested and validated on test cases: 2D flows around static/rotating/oscillating disk, 3D flow around a rotating cylinder and 2D flows around counter-rotating ellipses. The newt steps involve the continuation of the validation of both solvers and its documentation, as well as the development of wall model for immersed bodies.

* '''Sub-project 5: TBLE wall model for LES with pressure gradient on a simple turbomachinery geometry (M. Cizeron, N. Odier, R. Vicquelin)'''
Wall modeling is often used in LES to alleviate the computational cost that would be required to resolve all the length scales up to the solid boundaries of the domain. The classical way of doing it is by using an algebraic model to provide the wall friction and heat flux, with a coupling to the LES solver at the first off-wall nodes. The wall model was designed from analyzing RANS equation with strong assumptions such as planar flow, equilibrium and no pressure gradient. These assumptions are often far from true in real applications, such as turbomachinery applications, where the use of a wall model is mandatory due to the size of the calculation. During this workshop, a wall model relying on the resolution of the Thin Boundary Layer Equations (TBLE) was studied, which had been implemented by EM2C. The addition of a pressure gradient to these equations has been conducted and tested, at first only for the 1D wall model solver, then on a 3D turbulent channel. It remains to be tested on a diffuser configuration with a real pressure gradient to quantify the effect of the new wall model. The influence of the point considered to do the coupling between the LES and the wall model (ie. its distance to the wall) has also been tested both for the TBLE and the original algebraic model, showing that coupling farther from the wall yields better results and reduces the so-called log-layer mismatch.

* '''Sub-project 6: Tools for rough wall modelling (A. Barge, S. Meynet)'''
Within the STREAM project framework, a roughness-resolved Large-Eddy Simulation (RRLES) database is being built. The aim of this latter is to be representative of rough channel flows, especially for additive-manufacturing heat exchangers. First RRLES have already been performed. From turbulence and rough wall stress statistics analysis of the results, a first stochastic model, which reproduces the statistical behavior of the wall stress vector, have been proposed. The modeled wall stress allows a better prediction of the pressure drop in a flat wall channel compared to the use of the mean value of the wall stress measured in RRLES alone. However, the near wall region is still mispredicted and the model is correlated in time but not in space. The aim of this ECFD5 was to develop tools to improve modelling and explore new ways. A roughness mapping tool for smooth surfaces have been implemented into YALES2 to get local surface height. This tool is based on an existing in-house surface roughness generator developed for the STREAM project. The idea is to use the map to generated space correlated fluctuations for the wall shear stress. Some bugs still remain to fully use this tool. In parallel, the modelling approach was extended to passive scalar, especially for temperature. To this end, new random tools as white noise, unit sphere random walk and Gaussian / Log-normal stochastic processes have been coded. Finally, the idea of using walls as velocity source terms emerged during this ECFD5. The principle is to mask a grid layer above the wall and to transport the rough map on this grid to estimate the roughness effects above the wall. Parametrizing and testing these tools remained to be done at the end of ECFD5.

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

* '''Sub-project 1: Multi combustion model chemtable generator (S. Dillon, R. Mercier)'''

Due to the high computational cost of LES of turbulent combustion with detailed chemistry, tabulated chemistry approaches remain a popular choice within the combustion community. This popularity of tabulated chemistry leads to research into the development of novel combustion models for which a platform to test these models is required. The objective of the workshop was to therefore begin developing an easy-to-use chemistry table generator within Python. Given that there exists numerous turbulent combustion models (F-TACLES Diffusion, F-TACLES Premixed, 2PFT, etc.), a generalised multi-model code was necessary. Along with multi-model capabilities, the code was required to function with multiple solvers for the creation of the flame database (REGATH and CANTERA). With these objectives in mind, the code “TabulatEd Chemistry GenERator for Aeronautical CombusTion” (TECERACT) was created. Advancements were made in all key areas using code already developed by Safran Tech. TECERACT includes a parallel flame database generator and current developments are focused on multi-model functionality and simplified mesh management within Python. Once finished, the TECERACT code will provide a user-friendly chemistry table generator capable of producing chemistry tables for LES simulations and its multi-model structure will serve as a platform for future turbulent combustion model development/testing.

* '''Sub-project 2: Task-driven automatic run sequence (R. Mercier, J. Leparoux, M. Cailler, R. Letournel)'''

The principal objective of this project was to develop a new simulation workflow based on Task-driven approach. This approach could offer a higher flexibility to manage CFD simulations by decomposing the whole simulation on small sequences (run initialization, fuel injection, etc.)

This week showed the achievement of the first steps towards a complete task-driven simulation. (i) The whole simulation workflow was set-up (ii) Missing actions were implemented allowing to update simulation status on-the-fly (from non reactive to reactive simulation) (iii) State are now embedded from the restart solution allowing to restart from the last known state. Further work is still needed to obtain the full implementation of the AMC framework based on task-driven approach. Especially to automate the adaptation loop

* '''Sub-project 3: YALES2 Tools & Gitlab CI (J. Leparoux, A. Tstetoglou)'''

* '''Sub-project 4: Wind energy tools (E. Muller, S. Gremmo, F. Houtin-Mongrolle, B. Duboc)'''

Set-up phase of simulations involving several wind turbines is very demanding and error-prone, especially for simulations carried out with the coupled solver YALES2-BHawC*. This type of simulation can involve several dozen input files and then, an assistant tool is required.

The objectives of this projet were (i) develop a python tools to set all necessary ACTUATOR_SET objects (ii) provide basic visualization plots allowing to monitor simulations on-the-fly (by post-processing YALES2 temporals and BHawC results files).

''*BHawC : aero-servo-elastic solver used and developped at Siemens Gamesa Renewable Energy for wind turbine design and certification
''

=== Hackathon - G. Staffelbach, CERFACS ===
* '''Participants: I. d'Ast, J. Legaux, G. Staffelbach, P. Begou, G. Lartigue, V. Moureau, A. Toure, C. Laurie, S. Delamare, C. Andrieu, C. Jourdain'''
AMD GPU hardware is still relatively unknown in our CFD community. This hackathon was the opportunity to deep dive into the AMD dev environment to prepare the arrival of AdAstra at CINES.
Both YALES and AVBP have been ported to the AOMP framework using ROCm 4.5 on the GRID5000 Neowise system.
CPU execution posed no issues and we were able to focus on GPU Offloading using OpenMP.
On the YALES2 side, a mini-app encompassing the typical YALES2 structure hierarchy and loop execution was extracted from the code to evaluate different porting strategies and on the AVBP side the current OpenACC GPU offloading was translated to OpenMP focusing on the viscosity computation kernel.
We learnt that the current supported standard of OpenMP in ROCm 4.5 does not allow for direct offloading of reference values inside an derived type structure but is was possible to use aliases such as pointers or flat array copies to do the job. This should be solved with the support of OpenMP 5.0
Another troublesome issues, was the lack of support for offloading of array vector operations (ex : array(:) = 1.0 ) rendering the explicitation of the loops for these manadatory.

Some bugs remain and it is encouraged to use the latest compiler version when working on the porting ( the release of flang 14.0.1 saved us a lot of time as we had started with 14.0.0 ).
Offloading of the miniapp of YALES2 yielded a times 60 acceleration of the kernel whereas the offloading of the viscosity model in a full avbp simulation yielded an 7 times factor in performance when comparing on core to one GPU. These results are to be taken with a grain of salt but are really encouraging.

For the next steps, a porting strategy for both codes will be implemented (depending on the OpenMP 5 support ) and discussions are underway with CINES and other partners so as to offer the best experience to both code's communities on AdAstra at its release.

Ecfd:ecfd 4th edition

2021-03-29T07:11:12Z

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

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

== Description ==
{| align="right" style="text-align:center;" cellpadding="2"
| [[File:logo_ecfd4.png | center | thumb | 300px | ECFD4 workshop logo.]]
|}
* Virtual event from '''22nd to 26th of March 2021'''
* Two types of sessions:
** common technical presentations: roadmaps, specific points.
** mini-workshops. Potential workshops are listed below.
* Free of charge
* More than 50 participants from academics (CERFACS, CORIA, IMAG, LEGI, UMONS, UVM, VUB), HPC center/experts (GENCI, IDRIS, NVIDIA, HPE) and industry (Safran, Ariane Group).

== News ==

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


== Objectives ==

* Bring together experts in high-performance computing, applied mathematics and multi-physics CFDs
* Identify the technological barriers of exaflopic CFD via numerical experiments
* Identify industrial needs and challenges in high-performance computing
* Propose action plans to add to the development roadmaps of the AVBP and YALES2 codes

== Agenda ==

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

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

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

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

==== Plénière 2 ====

Lundi 22/03/2021 9h20-11h20

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

''Responsables de projets''

==== Plénière 3 ====

Lundi 22/03/2021 11h20-12h00

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

''P.-F. Lavallée (IDRIS)''

==== Plénière 4 ====

Mardi 23/03/2021 9h00-10h00

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

''F. Courteille (NVIDIA)''

==== Plénière 5 ====

Mercredi 24/03/2021 13h00-14h00

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

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

==== Plénière 6 ====

Jeudi 25/03/2021 9h00-10h00

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

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

==== Plénière 7 ====

Vendredi 26/03/2021 9h00-10h00

'''Roadmaps YALES2 & AVBP'''

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

==== Plénière 8 ====

Vendredi 26/03/2021 15h00-17h00

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

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

== Thematics / Mini-workshops ==

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

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

=== Dynamic mesh adaptation - G. Balarac, LEGI ===
* anisotropic mesh adaptation
* adaptation criteria for anisotropic mesh adaptation
* adaptation of periodic domains

=== Multi-phase flows - V. Moureau, CORIA ===
* scalar transport in two-phase flows
* three-phase flows: contact angle

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

This project gathered four sub-projects related to Numerical Methods. Most of these activities are related to the use of high-order schemes presented in [1] in the context of Finite-Volumes Method.
* '''Sub-project 1 (N. Odier, B. Martin, G. Lartigue):''' The main objective of this sub-project was to implement a '''High-Order Finite-Volume method in the Cell-Vertex compressible code AVBP'''.
* '''Sub-project 2 (M. Bernard, G. Balarac, G. Lartigue):''' The main objective of this sub-project was to work on the use of '''High-Order Finite-Volume method to solve the Poisson Equation in the incompressible code YALES2'''.

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

* '''Sub-project 3 (G. Sahut, G. Balarac, G. Lartigue):''' The main objective of this sub-project was to implement a '''URANS method with a semi-implicit solver in YALES2'''.
* '''Sub-project 4 (G. Lartigue, V. Moureau):''' The main objective of this sub-project was to '''improve the precision and robustness of the Laplacian Operator in YALES2'''. There is two class of operators in YALES2: '''ROBUST''' (a.k.a. PAIR_BASED and IGNORE_SKEWNESS) and '''PRECISE''' (a.k.a. SIMPLEX). It has been shown that for operators with constant coefficients (as in ICS and VDS solvers), the '''PRECISE''' approach is unconditionally stable and must be used in all situations. However, in the SPS solver, the density variations across a pair of vertex can lead to a non-PSD operator. A major achievement of the workshop was to propose an hybrid operator that mixes both operators to achieve both precision and robustness. This operator will be implemented in a near future.
* '''Discussion (All):''' A two-hours discussion on Tuesday afternoon have been dedicated to the analysis of the paper [2]. This paper deals with an optimal way of mixing a robust low-order numerical scheme with high-order scheme. The major interest of this mixing technique is that it preserves the boundedness of the solution with a so-called convex-limiting. This is similar to WENO techniques but it relies on the resolution of the interface Riemann

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

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

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

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

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

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

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

=== GENCI Hackathon - G. Staffelbach, CERFACS ===
* Porting to GPU with OpenACC

File:Ecfd3 final project14.pdf

2020-01-31T10:49:33Z

Mbernard: Ending presentation of the High-Order-Framework project

Ending presentation of the High-Order-Framework project

Ecfd:ecfd 3rd edition

2020-01-31T10:43:28Z

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

{{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) 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: 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 ===

[[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]]