Extreme CFD workshop - User contributions [en]

Ecfd:ecfd 9th edition

2026-02-04T07:15:41Z

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

{{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 $\vec{u}\cdot\vec{n}\,dS$ 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) ====
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 (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. 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.

Ecfd:ecfd 9th edition

2026-02-04T07:14:26Z

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

{{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 $\vec{u}\cdot\vec{n}\,dS$ 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) ====
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. Initial simulations have been performed and have produced promising results.
The next step will be to integrate mesh adaptation and the compressible wall model, and to compare numerical diagnostics with experimental measurements in order to validate both the modeling approach and 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 (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. 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.

Ecfd:ecfd 8th edition

2025-02-19T12:51:20Z

Bricteux: /* M6 - Dynamic mesh adaptation of turbulent flows (G. Balarac, L. Bricteux, P. Bénard, S. Mendez) */

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

== Description ==

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

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

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

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

== News ==

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

== Thematics / Mini-workshops ==

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

To come...

== Projects ==

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

* WP1: Improve inflow generation

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

==== 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) ====

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

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

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

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

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

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

Ecfd:ecfd 8th edition

2025-02-19T12:48:31Z

Bricteux: /* M6 - Dynamic mesh adaptation of turbulent flows (G. Balarac, L. Bricteux, P. Bénard, S. Mendez) */

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

== Description ==

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

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

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

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

== News ==

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

== Thematics / Mini-workshops ==

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

To come...

== Projects ==

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

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

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

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

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

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

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

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

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

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

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

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

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

==== M6 - Dynamic mesh adaptation of turbulent flows (G. Balarac, L. Bricteux, P. Bénard, S. Mendez) ====
This work focuses on developing dynamic mesh adaptation strategies for turbulent flows, particularly in cases where static (statistical-based) adaptation is not suitable. The objective is to create a dynamic mesh metric that can compute the required physical metric directly, eliminating the need for an explicitly prescribed mesh size. Here, we assessed th method based on QC6 developed at LEGI with encouraging results.
The approach has been validated with excellent results on hemisphere flow at Re=55k (a regime for which LDV experimental measurements are available).
The vortex ring collision test case which is a typical test case for which static adaptation is not relevant has also been ran with promising results showing that the adaptation performs well in vortical flow regions.
The adaptation strategy has already been used in the case of an elliptic jet flow Elliptic (aspect ratio 3) generating vortex rings at Re = 30,000. It was shown that the dynamically generated meshes follows properly the region of interests.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

* WP1: Improve inflow generation

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

==== 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) ====

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

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

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

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

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

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

Ecfd:ecfd 8th edition

2025-02-19T12:45:40Z

Bricteux: /* Mesh adaptation - A. Grenouilloux, ONERA & G. 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)====

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

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

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

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

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

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

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

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

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

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

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

==== M6 - Dynamic mesh adaptation of turbulent flows (G. Balarac, L. Bricteux, P. Bénard, S. Mendez) ====
This work focuses on developing dynamic mesh adaptation strategies for turbulent flows, particularly in cases where static (statistical-based) adaptation is not suitable. The objective is to create a dynamic mesh metric that can compute the required physical metric directly, eliminating the need for an explicitly prescribed mesh size. Here, we assessed th method based on QC6 developed at LEGI with encouraging results.
The approach has been validated with excellent results on hemisphere flow at Re=55k (a regime for which LDV experimental measurements are available).
The vortex ring collision test case which is a typical test case for which static adaptation is not relevant has also been ran with promising results showing that the adaptation performs well in vortical flow regions.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

* WP1: Improve inflow generation

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

==== 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) ====

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

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

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

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

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

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

Ecfd:ecfd 7th edition

2024-02-09T21:28:26Z

Bricteux: /* T5: Implementation of the RVMs-WALE model in YALES2 – L. Bricteux (UMONS), P. Benard (CORIA), Y. Bechane (CORIA) */

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

== Description ==

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

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

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



== Agenda ==

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

== Thematics / Mini-workshops ==

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

To come...

== Projects ==

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

We thank Ghislain Lartigue and Renaud Mercier for helpful discussions.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

==== P8: Wall liquid film numerical model - N. Gasnier (EM2C & Safran), J. Leparoux (Safran), J. Carmona (CORIA) ====

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Ecfd:ecfd 6th edition

2023-05-26T13:31:35Z

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

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

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

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

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

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



== Agenda ==

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

== Thematics / Mini-workshops ==

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

== Projects ==

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

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

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

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

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

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

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

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

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

* '''Sub-project 6: Development of a traction open boundary condition (TOBC) in Yales2 (J.B. Lagaert, G. Balarac)'''
In simulations, artificial boundaries need to be introduced due to the limited size of computational domains. At these boundaries, flow variables need to be calculated in a way that will not induce any perturbation of the interior solution.
During ECFD#6, open traction boundaries (TOBC) has been implemented in Yales2. Instead of enforced pressure value or normal derivative on boundaries, the traction is settled. The traction term gathers the deformation stress and pressure gradient.
The implementation has been validated on simple test-case : comparison has been performed between theoritical solutions and numerical solution computed with a Dirichlet TOBC. Future work will add a stabilization term for large backflow and traction models to estimate the values to enforce on outlet boundaries

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Ecfd:ecfd 6th edition

2023-05-26T11:14:29Z

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

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

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

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

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

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



== Agenda ==

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

== Thematics / Mini-workshops ==

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

== Projects ==

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

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

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

/Users/laurentbricteux/CALCUL/yales2runs/ics_3D_cylinder_100K/from_lucia/3DFIELD/VIEW1.png

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

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

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

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

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

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

* '''Sub-project 6: Development of a traction open boundary condition (TOBC) in Yales2 (J.B. Lagaert, G. Balarac)'''
In simulations, artificial boundaries need to be introduced due to the limited size of computational domains. At these boundaries, flow variables need to be calculated in a way that will not induce any perturbation of the interior solution.
During ECFD#6, open traction boundaries (TOBC) has been implemented in Yales2. Instead of enforced pressure value or normal derivative on boundaries, the traction is settled. The traction term gathers the deformation stress and pressure gradient.
The implementation has been validated on simple test-case : comparison has been performed between theoritical solutions and numerical solution computed with a Dirichlet TOBC. Future work will add a stabilization term for large backflow and traction models to estimate the values to enforce on outlet boundaries

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Ecfd:ecfd 6th edition

2023-05-26T11:14:28Z

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

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

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

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

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

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



== Agenda ==

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

== Thematics / Mini-workshops ==

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

== Projects ==

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

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

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

/Users/laurentbricteux/CALCUL/yales2runs/ics_3D_cylinder_100K/from_lucia/3DFIELD/VIEW1.png

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

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

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

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

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

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

* '''Sub-project 6: Development of a traction open boundary condition (TOBC) in Yales2 (J.B. Lagaert, G. Balarac)'''
In simulations, artificial boundaries need to be introduced due to the limited size of computational domains. At these boundaries, flow variables need to be calculated in a way that will not induce any perturbation of the interior solution.
During ECFD#6, open traction boundaries (TOBC) has been implemented in Yales2. Instead of enforced pressure value or normal derivative on boundaries, the traction is settled. The traction term gathers the deformation stress and pressure gradient.
The implementation has been validated on simple test-case : comparison has been performed between theoritical solutions and numerical solution computed with a Dirichlet TOBC. Future work will add a stabilization term for large backflow and traction models to estimate the values to enforce on outlet boundaries

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Ecfd:ecfd 6th edition

2023-05-26T11:13:40Z

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

Ecfd:ecfd 6th edition

2023-05-26T11:09:43Z

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

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

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

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

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

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



== Agenda ==

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

== Thematics / Mini-workshops ==

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

== Projects ==

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

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

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

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

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

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

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

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

* '''Sub-project 6: Development of a traction open boundary condition (TOBC) in Yales2 (J.B. Lagaert, G. Balarac)'''
In simulations, artificial boundaries need to be introduced due to the limited size of computational domains. At these boundaries, flow variables need to be calculated in a way that will not induce any perturbation of the interior solution.
During ECFD#6, open traction boundaries (TOBC) has been implemented in Yales2. Instead of enforced pressure value or normal derivative on boundaries, the traction is settled. The traction term gathers the deformation stress and pressure gradient.
The implementation has been validated on simple test-case : comparison has been performed between theoritical solutions and numerical solution computed with a Dirichlet TOBC. Future work will add a stabilization term for large backflow and traction models to estimate the values to enforce on outlet boundaries

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Ecfd:ecfd 6th edition

2023-05-26T11:07:47Z

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

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

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

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

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

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



== Agenda ==

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

== Thematics / Mini-workshops ==

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

== Projects ==

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

=== Mesh adaptation - R. Letournel, Safran & V. Moureau, CORIA ===
'''Sub-project 1: Multi-level domain decomposition method (DDM) for coupled systems of differential-algebraic equations (A. Quirós Rodrígues, V. Le Chenadec)'''

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

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

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

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

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

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

* '''Sub-project 6: Development of a traction open boundary condition (TOBC) in Yales2 (J.B. Lagaert, G. Balarac)'''
In simulations, artificial boundaries need to be introduced due to the limited size of computational domains. At these boundaries, flow variables need to be calculated in a way that will not induce any perturbation of the interior solution.
During ECFD#6, open traction boundaries (TOBC) has been implemented in Yales2. Instead of enforced pressure value or normal derivative on boundaries, the traction is settled. The traction term gathers the deformation stress and pressure gradient.
The implementation has been validated on simple test-case : comparison has been performed between theoritical solutions and numerical solution computed with a Dirichlet TOBC. Future work will add a stabilization term for large backflow and traction models to estimate the values to enforce on outlet boundaries

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Ecfd:ecfd 6th edition

2023-05-26T11:07:11Z

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

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

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

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

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

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



== Agenda ==

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

== Thematics / Mini-workshops ==

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

== Projects ==

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

=== Mesh adaptation - R. Letournel, Safran & V. Moureau, CORIA ===
Sub-project 1: Multi-level domain decomposition method (DDM) for coupled systems of differential-algebraic equations (A. Quirós Rodrígues, V. Le Chenadec)

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

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

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

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

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

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

* '''Sub-project 6: Development of a traction open boundary condition (TOBC) in Yales2 (J.B. Lagaert, G. Balarac)'''
In simulations, artificial boundaries need to be introduced due to the limited size of computational domains. At these boundaries, flow variables need to be calculated in a way that will not induce any perturbation of the interior solution.
During ECFD#6, open traction boundaries (TOBC) has been implemented in Yales2. Instead of enforced pressure value or normal derivative on boundaries, the traction is settled. The traction term gathers the deformation stress and pressure gradient.
The implementation has been validated on simple test-case : comparison has been performed between theoritical solutions and numerical solution computed with a Dirichlet TOBC. Future work will add a stabilization term for large backflow and traction models to estimate the values to enforce on outlet boundaries

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

File:Ecfd3 final project6.pdf

2020-02-07T19:02:22Z

Bricteux:

File:Ecfd3 final project22.pdf

2020-02-07T19:00:53Z

Bricteux:

Ecfd:ecfd 3rd edition

2020-01-31T10:57:08Z

Bricteux: /* Project #22: Numerical prediction of wind turbine wakes using AMR */

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

== Sponsors ==

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

== Participants ==

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

[[File:lecfd3_participants_photo.jpg|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 ===

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The objective of this project was to further develop the cold plasma solver in YALES2 in order to accurately model silane nanodusty discharges. The electron temperature equation has been implemented successfully and validated against a reference plasma code. In a second step, a detailed electron kinetics has been implemented in YALES2 in order to couple the electron temperature with the charged species mass fractions. The user can now define a list of reactions whose rates depend on the electron temperature. These improvements open the path to the simulation of nanoparticle production in silane discharges using a Lagrangian description for the nanoparticles.

[[media:ecfd3_final_project17.pdf | Final presentation of project #17]]

=== Project #18: L’Evaporo O Maıtre ===

[[media:ecfd3_final_project18.pdf | Final presentation of project #18]]

=== Project #19: The Clone Wars ===
''H. Maldonado Colman (EM2C), C. Nguyen Van (EM2C - Safran-Tech), R. Mercier (Safran-Tech), B. Fiorina (EM2C)''

The aim of this project was to increase the computation performance using virtual chemistry approach in the YALES2 solver.

In order to reach this goal three test cases where identified:

- 1D laminar premixed flame (methane / air combustion with carbon monoxide prediction)

- 2D laminar premixed bunsen flame (methane / air combustion with carbon monoxide prediction)

- 3D two phase and turbulent flame (nheptane / air combustion with nitrogen monoxide prediction)

Several ways were explored:

- Profiling of reactive simulations when using Virtual Chemistry

- Effect of redundant species transport

- Effect of the size and the numbers of jacobian matrix to compute and solve

- Effect of the correction functions smoothing

Conclusions of the study are:

- String trimming and concatenation heavily affect computing performances

- Redundant species transport and source terms computations has a minor impact on performances

[[media:ecfd3_final_project19.pdf | Final presentation of project #19]]

=== Project #20: Stiff complex fluid simulation with YALES2 ===
''Sam Whitmore, Yves Dubief, M2CE, University of Vermont''

The objective was to simulate (1) ionized gases and (2) polymer solutions in flows using YALES2. Both problems are challenging owing to their stiff thermodynamics (1) or polymer dynamics (2). Significant gains were achieved in the implementation of the respective models thanks to the stiff integrator library CVODE. The plasma flow demonstrated an increase in time step of two orders of magnitude compared to previous implementation of the plasma chemistry in the variable density solver. Polymer models are notoriously prone to numerical instability. Again the use of CVODE showed equivalent if not superior stability of the solution at a fraction of the cost of commonly employed algorithms designed to address the stiffness of the problem.

[[media:ecfd3_final_project20.pdf | Final presentation of project #20]]

=== Project #21: AVBP Dense Gases ===

''Paolo ERRANTE (LMFA), Alexis GIAUQUE (LMFA), Christophe CORRE (LMFA)''

The simulation of dense gas flows using AVBP currently relies on the Martin-Hou Equation of State (EoS) to obtain the thermodynamic variables in each grid cell from the local value of density (or specific volume v) and internal energy derived from the conservative variables. The project develops an alternative approach where thermodynamic quantities in each cell are derived from a (given) set of tabulated thermodynamic states (Look-up Table or LuT). In order to preserve flow physics, the interpolation process in the LuT tables must be performed in a consistent way (a simple bilinear interpolation on v and T for each thermodynamic variable is not sufficient). Describing Helmholtz free energy f(v,T) with a bi-quintic Hermitian polynomial function in each cell of the LuT allows to ensure a consistent interpolation process (since all thermodynamic variables are obtained by differentiating the polynomial function). During the workshop the functions needed to perform the consistent interpolation have been implemented in the real gas module. Short-term perspectives are completing the implementation, validating the development on some test-cases previously computed using MAH EoS and optimizing the implementation (in particular the strategy used to identify the position in the LuT of each local grid state).

[[media:ecfd3_final_project21.pdf | Final presentation of project #21]]

=== Project #22: Numerical prediction of wind turbine wakes using AMR ===
S. Zeol (UMONS)i, P. Bénard (CORIA), G. Balarac (LEGI), L. Bricteux (UMONS)

The project considered here demonstrates the feasibility of the use of an adaptive mesh refinement method in the Eulerian finite volume code YALES2 for wind turbine wakes prediction.
The objective is to determinate the more effective methodology to adapt the mesh based on appropriate criterion.
We consider two methods : one for statistically steady flows based and one for purely unsteady flows (e.g. turbine with yaw, wind turbine with strong turbulence, inducing wake meandering)
Preliminary results were obtained on a testcase for which wind tunnel data are available: the NTNU blind test 1.
The static adaptation method applied on this case produced promising results and should eventually reduce the computational cost of this kind of simulations.
The dynamic adaptation method has been elaborated and some tests were performed to find the best adaptivity parameters.
The next step is to fully validate the methodology and consider then a more challenging test case with yaw adaptation.

Ecfd:ecfd 3rd edition

2020-01-31T10:55:37Z

Bricteux: /* Project #22: Numerical prediction of wind turbine wakes using AMR */

{{DISPLAYTITLE: ECFD workshop, 3rd edition, 2020}}
__NOTOC__

== Sponsors ==

[[File:ecfd3_sponsors.png|center|frameless|800px]]

== Participants ==

[[File:ecfd3_participants.png|center|frameless|1000px]]

[[File:lecfd3_participants_photo.jpg|center|frameless|1000px]]

== Flyer ==

* [[media:ecfd3_flyer.pdf | Flyer]]

== Presentations ==

* [[media:ecfd3_intro.pdf | Introduction workshop]]
* [[media:ecfd3_intro_genci.pdf | Introduction GENCI]]
* [[media:ecfd3_avbp_roadmap_HPC.pdf | Roadmap AVBP (HPC)]]
* [[media:ecfd3_yales2_roadmap.pdf | Roadmap YALES2]]

== Booklet ==

* [[media:ecfd3_booklet_template.zip | Template]]

== Project achievements ==

=== Project #1: Hackathon GENCI/ATOS/AMD/CERFACS on AVBP ===

''C. Piechurski (GENCI), S. Jauré (ATOS), B. Pajot (ATOS), P.-A. Harraud (AMD), P. Mohanamuraly (CERFACS), G. Staffelbach (CERFACS), J. Legaux (CERFACS)''

We ported the AVBP solver to the AMD Rome system available at GENCI -TGCC ( IRENE Joliot Curie).
Characterisation of the application on the architecture showed a 1/3 performance dependency to bandwidth and 2/3 to compute.
Strong scaling performance up to 130k cores was measured with openmpi and provided an acceleration of 75% without optimisations.
Weak scaling up to 32k MPI ranks suggests that decimation of the processes by a factor 2 improves computational efficiency by up to 30%.
This suggests a trade off between mpi imbalance and decimation is possible if imbalance is higher than 30% to improve time to solution.

Currently Openmpi offers the best perfofrmance, intelmpi is still a bit unstable.

During the Hackathon we also introduced colour based cache blocking using ColPack in the code in order to use OpenMP without critical sections.
On a 2x18 core Skylake processor the new implementation offered similar speedup using full threading versus full MPI with the best trade off being 4 MPI and 9 threads per MPI.
On AMD Rome, Full threading did not offer much acceleration and needs to be inversigated but 8 MPI and 16 threads per MPI seem quite promising.

[[media:ecfd3_final_project1.pdf | Final presentation of project #1]]

=== Project #2: Hackathon GENCI/ATOS/AMD/CORIA on YALES2 ===
''C. Piechurski (GENCI), S. Jauré (ATOS), P.-A. Harraud (AMD), P. Mohanamuraly (CERFACS), G.Lartigue (CORIA), F. Gava (CORIA), K. Bioche (CORIA), P. Begou (LEGI)''

[[media:ecfd3_final_project2.pdf | Final presentation of project #2]]

=== Project #3: Implementation of a secondary atomization model in YALES2 ===

''C. G. Guillamon (Safran Tech), L .Voivenel (Safran Tech), R. Mercier (Safran Tech)''

In Lagrangian simulations, droplets are transported following a ballistic motion in an eulerian mesh. For non-reactive environments, droplets might undergo secondary atomization due to the aerodynamic interaction. In this work, we implement in YALES2 a breakup model known as Taylor-Analogy Breakup (TAB). This model is based on the analogy between a droplet and a second-order mechanical system, hence making possible to determine the breakup behaviour by means of Newton's second law.

Another model, the stochastic breakup model by Gorokhovski, is also suggested for future work and will be implemented in YALES2.

[[media:ecfd3_final_project3.pdf | Final presentation of project #3]]

=== Project #4: Conservative Heat Transfers in the the Accurate Conservative Level-Set framework ===

François Pecquery (ARIANE GROUP), Mélody Cailler (SAFRAN TECH), Romain Janodet (SAFRAN TECH/CORIA) and Vincent Moureau (CORIA)

Objectives of the project was to introduce conservative heat transfers in the Accurate Conservative Level-Set framework to be able to describe heat transfers and liquid dynamics in an accurate, robust and conservative manner. A Multi-Phase Transport solver is introduced relying on the conserving and level-set coherent transport of the temperature. The solution is to use the fluxes of a phase indicator that may be sharp, contrarily to the level-set. The new solver was used on a simplified test case where a liquid droplet is transported in a temperature stratified environment. Results show promising capabilities of the new framework. Next work include improvement of the transport equation stability, and of the jump condition at the interface.

[[media:ecfd3_final_project4.pdf | Final presentation of project #4]]

=== Project #5: Jet-in-crossflow par une méthode d’interface diffuse ===

''T. Laroche, N. Odier, B. Cuenot (CERFACS). In collaboration with M. Pelletier, T. Schmitt, S. Ducruix (EM2C)''

In the context of fuel injection in an aircraft engine, liquid fuel is injected through a swirler, and sheared by a high-speed oxyder which destabilizes the liquid interface. This interaction induces liquid ligaments, which break up into large droplets (primary atomization), and then themselves break into small droplets (secondary atomization)
This project deals with the implementation of a diffuse-interface method in the massively parallel solver AVBP to represent the liquid interface destabilization during primary atomization for compressible applications. This methodology is found to be very efficient, however a control of the interface diffusion is mandatory as soon as convective effects are added. During this workshop, the methodology proposed by Chiodi and Desjardins ( ''A reformulation of the conservative level set reinitialization equation for accurate and robust simulation of complex multiphase flows'', JCP 2017) to control the interface thickness has been implemented in AVBP, and is currently under validation on a periodic liquid jet with surface tension effects.

[[media:ecfd3_final_project5.pdf | Final presentation of project #5]]

=== Project #6: Accurate numerical predicti􏴇on of vorti􏴇cal flows using AMR ===

[[media:ecfd3_final_project6.pdf | Final presentation of project #6]]
We try to demonstrate that Eulerian method YALES2 using AMR can do a very good job to capture complex vortical flows at moderate Re=10k
Here we use an AMR strategy based on vorticity. We investigate the problem of vortex ring collision. We have a gain of 1000 on the numbers of elements compared
to a non adaptative approach. We are able to capture the transition from a very simple laminar flow to a complex turbulent flow.

=== Project #7: Modélisation de parois pour la simulation des grandes échelles ===

[[media:ecfd3_final_project7.pdf | Final presentation of project #7]]

=== Project #8: Accurate numerical simulation of contact lines with dynamic mesh adaptation ===
''S. Pertant (LEGI), G. Ghigliotti (LEGI), G. Balarac (LEGI)''

The main objective of this project was to develop a methodology to simulate contact lines on unstructured meshes. We especially wanted to get rid of mesh influence on contact line movement when the flow is driven by surface tension and the contact line close to its equilibrium position. A slight modification in the Ghost Fluid Method to apply the pressure jump has been tested and seems promising. The pressure gradient at contact line is indeed less sensitive to mesh elements for high density ratios. Furthermore, dynamic mesh adaptation has been used to simulate a 2D vapour bubble lying on a wall. Due to gravity, the two contact lines are receding until their merging and the bubble departure. The mesh remains fine to capture the contact line dynamics. As a future work, we plan to perform mesh adaptation on 3D contact line cases and to include additional physics such as contact angle imposition (already implemented but not used yet with mesh adaptation).

[[media:ecfd3_final_project8.pdf | Final presentation of project #8]]

=== Project #9: Remeshed particle method at high Schmidt and Reynolds number ===

''S. Santoso (LJK), J.-B. Lagaert (Math Orsay), G.Balarac (LEGI)''

We study the advection of a scalar function in turbulent flows with a multimesh method. The finite volume method is used to solve Navier-Stokes equations on an unstructured mesh (YALES2). The advection equation is solved with remeshed particle method on a cartesian mesh. In the context of parallel computing, we face a very unbalanced problem since a large number of particles are created in a very fine meshed zone. Our strategy to load-balance the problem is to give a weight to every element group which is equal to the density of particle.

[[media:ecfd3_final_project9.pdf | Final presentation of project #9]]

=== Project #10: Adaptive mesh refinement for turbulent premixed combustion ===
''W. Agostinelli, O. Dounia, , T. Jaravel, O. Vermorel

The objective of the project was to evaluate the potential of adaptive mesh refinement (AMR) for premixed combustion in unsteady systems. Three target cases were identified: a semi-vented deflagration with laminar to turbulent transition, a planar detonation wave, and a bluff-body stabilized burner subjected to thermoacoustic oscillations. The simulations were performed with AVBP and coupled to the AMR implementation of YALES2. Several metrics and remeshing criterions were developed to identify and correctly resolve both the combustion wave front and the turbulent flow. The comparison of numerical results with reference simulations showed that the main features of the physics could be recovered with a significant speed-up in term of computational cost.

[[media:ecfd3_final_project10.pdf | Final presentation of project #10]]

=== Project #11: Multiphysics coupling for wind turbine wake modeling ===

''F.Houtin-Mongrolle (CORIA), B. Duboc (SGRE), P. Benard (CORIA)''

The goal of this project was to evaluate the coupling of YALES2 (flow solver) and BHawC(Aero-Servo-Elastic solver).

[[media:ecfd3_final_project11.pdf | Final presentation of project #11]]

=== Project #12: Stability of a semi-implicit compressible cavitation solver ===

''H. Garg (LEGI), G. Ghigliotti (LEGI) and G. Balarac (LEGI)''

The compressible cavitation solver is used to simulate cavitation inception in an initially liquid flow behind an obstacle.
This solver is based on the implicit compressible solver, that has been modified to include a « barotropic » pressure-density relationship playing the role of an equation of state independent from the temperature.
While this strategy has proven to be effective for DNS simulations of the implosion of vapour bubbles, the simulation of cavitation inception in an initially liquid flow was leading to strong instabilities in the simulation shortly after the appearence of vapour.
The test case chosen is a flow behind a 2D cylinder.
The analysis of the results has shown that instabilities were correlated with very low (and even unphysically negative) values of the pressure, that were triggering negative density values leading to code instability.
Using limiters to ensure a positive pressure and a density within the range of the equation of state improved the stability and allowed to perform a preliminary simulation of a cavitating flow behind an obstacle.
Ultimately instabilities appear anyways, so that the will look to the spatial discretisation schemes.

[[media:ecfd3_final_project12.pdf | Final presentation of project #12]]

=== Project #13: Validations and comparisons of Diffuse / Sharp interface methods in a structured DNS solver (Titan) ===
''V. Boniou (EM2C), J.M. Dupays (EM2C), M. Pelletier (EM2C), T. Schmitt (EM2C), A. Vié (EM2C)

The project aimed at using academic test cases to compare the sharp (incompressible) and diffuse (compressible) models. In particular, the test case of an inviscid initially elliptical oscillating droplet has been carried out.
The solvers features are the following:

- incompressible VOF solver (sharp): Numerical Method: Projection Method, Interface reconstruction: VOF, Surface tension: CSF

- compressible multifluid solver (diffuse): Advection scheme: MUSCL + RK2 + minmod limiter, Surface tension: CSF.

The source term is integrated with operator-splitting, and the curvature computation relies on a 2nd-order differentiation of the liquid volume fraction, which is previously smooth by filtering.
This test case showed good agreement on the oscillation period, while exhibiting a slight numerical diffusion in the incompressible case and a strong numerical diffusion in the compressible case.
In the compressible case, the use of higher-order splitting (Strang [SIAM Num. An. 1968]) has been tested, yielding no noticeable improvement. Reduction of the number of filtering iterations on the liquid volume fraction provides a slight improvement, which may indicate that a better curvature computation could participate to reduce the numerical diffusion.

[[media:ecfd3_final_project13.pdf | Final presentation of project #13]]

=== Project #14: High Order Framework ===
''M. Bernard (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI), V. Moureau (CORIA)''

Aim of this project is to extend the high order framework (HOF) in Yales2.
As a reminder, the HOF permits to reconstruct a point-wise quantity from the volume-averaged one, arising from classical Finite-Volume schemes, and thus to improve spatial accuracy of numerical schemes.

During the ECFD workshop #3, a dedicated solver has been created, the high order solver (hos), duplicated from the incompressible solver (ics).
We started activating the HOF ingredients previously developed, starting from velocity field advancement.
Development is still in progress, but the static Taylor-Green vortices test-case has been investigated in order to see the early improvement.

[[media:ecfd3_final_project14.pdf | Final presentation of project #14]]

=== Project #15: Validation of a fluid structure interaction case with the coupling ALE/SMS ===
''T. Fabbri (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI), V. Moureau (CORIA)''

The objective of this project was the validation of the Turek(2006) benchmark for fluid structure case.
The Structural Mechanics Solver (SMS) was already existing before the workshop, as the coupling with the Arbitrary-Lagrangian Eulerian solver.
However, the results were not in agreement with the case. The data compared here are the flexible part tip displacement, but also the drag and the lift integrated
on the cylinder and the flexible part.
The pure structure test cases were validated, but the forces computed for the pure fluid test cases were not satisfying.
The work of this week was then to improve the viscous shear computation, which implies the wall normal gradient computation.

[[media:ecfd3_final_project15.pdf | Final presentation of project #15]]

=== Project #16: Development of a RANS solver in YALES2 ===
''G. Sahut (LEGI), G. Balarac (LEGI), V. Moureau (CORIA), G. Lartigue (CORIA), P. Bénard (CORIA), A. Grenouilloux (CORIA)''

While the accuracy of LES usually approaches the one of DNS, LES are still too time-consuming for daily use in industrial applications. In this context, we started the development of a RANS solver in YALES2. We are first only interested in the steady state of the solution. In order to remove the CFL constraint, we developed, implemented and validated an implicit projection method for the resolution of the Navier-Stokes equations without turbulence models. The method is based on the implicitation of the velocity predictor ; the Poisson equation and the correction step of the velocity are then solved and applied as in the explicit incompressible solver. We validated the method on a stationary 2D Poiseuille flow with periodic boundary conditions: the simulation runs fine for CFL and Fourier numbers which are inaccessible with the explicit incompressible solver. The advection-diffusion equation for scalars has also been implicited and will be used to add turbulence models to the new implicit incompressible solver developped during this Workshop. More complex boundary conditions will also be addressed in a near future.

[[media:ecfd3_final_project16.pdf | Final presentation of project #16]]

=== Project #17: IMPLEMENTATION OF A COLD PLASMA MODEL IN YALES2 ===

''J.-M. Orlac'h (EM2C), G. Lartigue (CORIA), B. Fiorina (EM2C)''

The objective of this project was to further develop the cold plasma solver in YALES2 in order to accurately model silane nanodusty discharges. The electron temperature equation has been implemented successfully and validated against a reference plasma code. In a second step, a detailed electron kinetics has been implemented in YALES2 in order to couple the electron temperature with the charged species mass fractions. The user can now define a list of reactions whose rates depend on the electron temperature. These improvements open the path to the simulation of nanoparticle production in silane discharges using a Lagrangian description for the nanoparticles.

[[media:ecfd3_final_project17.pdf | Final presentation of project #17]]

=== Project #18: L’Evaporo O Maıtre ===

[[media:ecfd3_final_project18.pdf | Final presentation of project #18]]

=== Project #19: The Clone Wars ===
''H. Maldonado Colman (EM2C), C. Nguyen Van (EM2C - Safran-Tech), R. Mercier (Safran-Tech), B. Fiorina (EM2C)''

The aim of this project was to increase the computation performance using virtual chemistry approach in the YALES2 solver.

In order to reach this goal three test cases where identified:

- 1D laminar premixed flame (methane / air combustion with carbon monoxide prediction)

- 2D laminar premixed bunsen flame (methane / air combustion with carbon monoxide prediction)

- 3D two phase and turbulent flame (nheptane / air combustion with nitrogen monoxide prediction)

Several ways were explored:

- Profiling of reactive simulations when using Virtual Chemistry

- Effect of redundant species transport

- Effect of the size and the numbers of jacobian matrix to compute and solve

- Effect of the correction functions smoothing

Conclusions of the study are:

- String trimming and concatenation heavily affect computing performances

- Redundant species transport and source terms computations has a minor impact on performances

[[media:ecfd3_final_project19.pdf | Final presentation of project #19]]

=== Project #20: Stiff complex fluid simulation with YALES2 ===
''Sam Whitmore, Yves Dubief, M2CE, University of Vermont''

The objective was to simulate (1) ionized gases and (2) polymer solutions in flows using YALES2. Both problems are challenging owing to their stiff thermodynamics (1) or polymer dynamics (2). Significant gains were achieved in the implementation of the respective models thanks to the stiff integrator library CVODE. The plasma flow demonstrated an increase in time step of two orders of magnitude compared to previous implementation of the plasma chemistry in the variable density solver. Polymer models are notoriously prone to numerical instability. Again the use of CVODE showed equivalent if not superior stability of the solution at a fraction of the cost of commonly employed algorithms designed to address the stiffness of the problem.

[[media:ecfd3_final_project20.pdf | Final presentation of project #20]]

=== Project #21: AVBP Dense Gases ===

''Paolo ERRANTE (LMFA), Alexis GIAUQUE (LMFA), Christophe CORRE (LMFA)''

The simulation of dense gas flows using AVBP currently relies on the Martin-Hou Equation of State (EoS) to obtain the thermodynamic variables in each grid cell from the local value of density (or specific volume v) and internal energy derived from the conservative variables. The project develops an alternative approach where thermodynamic quantities in each cell are derived from a (given) set of tabulated thermodynamic states (Look-up Table or LuT). In order to preserve flow physics, the interpolation process in the LuT tables must be performed in a consistent way (a simple bilinear interpolation on v and T for each thermodynamic variable is not sufficient). Describing Helmholtz free energy f(v,T) with a bi-quintic Hermitian polynomial function in each cell of the LuT allows to ensure a consistent interpolation process (since all thermodynamic variables are obtained by differentiating the polynomial function). During the workshop the functions needed to perform the consistent interpolation have been implemented in the real gas module. Short-term perspectives are completing the implementation, validating the development on some test-cases previously computed using MAH EoS and optimizing the implementation (in particular the strategy used to identify the position in the LuT of each local grid state).

[[media:ecfd3_final_project21.pdf | Final presentation of project #21]]

=== Project #22: Numerical prediction of wind turbine wakes using AMR ===
S. Zeoli, G. Balarac, P. Bénard, L. Bricteux

The project considered here demonstrates the feasibility of the use of an adaptive mesh refinement method in the Eulerian finite volume code YALES2 for wind turbine wakes prediction.
The objective is to determinate the more effective methodology to adapt the mesh based on appropriate criterion.
We consider two methods : one for statistically steady flows based and one for purely unsteady flows (e.g. turbine with yaw, wind turbine with strong turbulence, inducing wake meandering)
Preliminary results were obtained on a testcase for which wind tunnel data are available: the NTNU blind test 1.
The static adaptation method applied on this case produced promising results and should eventually reduce the computational cost of this kind of simulations.
The dynamic adaptation method has been elaborated and some tests were performed to find the best adaptivity parameters.
The next step is to fully validate the methodology and consider then a more challenging test case with yaw adaptation.

Ecfd:ecfd 3rd edition

2020-01-31T10:02:48Z

Bricteux: /* Project #6: Accurate numerical predicti􏴇on of vorti􏴇cal flows using AMR */

ECFD workshop, 3rd edition, 2020

== Sponsors ==

[[File:ecfd3_sponsors.png|center|frameless|800px]]

== Participants ==

[[File:ecfd3_participants.png|center|frameless|1000px]]

== Flyer ==

* [[media:ecfd3_flyer.pdf | Flyer]]

== Presentations ==

* [[media:ecfd3_intro.pdf | Introduction workshop]]
* [[media:ecfd3_intro_genci.pdf | Introduction GENCI]]
* [[media:ecfd3_avbp_roadmap_HPC.pdf | Roadmap AVBP (HPC)]]
* [[media:ecfd3_yales2_roadmap.pdf | Roadmap YALES2]]

== Booklet ==

* [[media:ecfd3_booklet_template.zip | Template]]

== Project achievements ==

=== Project #1: Hackathon GENCI/ATOS/AMD/CERFACS on AVBP ===

''C. Piechurski (GENCI), S. Jauré (ATOS), B. Pajot (ATOS), P.-A. Harraud (AMD), P. Mohanamuraly (CERFACS), G. Staffelbach (CERFACS), J. Legaux (CERFACS)''

We ported the AVBP solver to the AMD Rome system available at GENCI -TGCC ( IRENE Joliot Curie).
Characterisation of the application on the architecture showed a 1/3 performance dependency to bandwidth and 2/3 to compute.
Strong scaling performance up to 130k cores was measured with openmpi and provided an acceleration of 75% without optimisations.
Weak scaling up to 32k MPI ranks suggests that decimation of the processes by a factor 2 improves computational efficiency by up to 30%.
This suggests a trade off between mpi imbalance and decimation is possible if imbalance is higher than 30% to improve time to solution.

Currently Openmpi offers the best perfofrmance, intelmpi is still a bit unstable.

During the Hackathon we also introduced colour based cache blocking using ColPack in the code in order to use OpenMP without critical sections.
On a 2x18 core Skylake processor the new implementation offered similar speedup using full threading versus full MPI with the best trade off being 4 MPI and 9 threads per MPI.
On AMD Rome, Full threading did not offer much acceleration and needs to be inversigated but 8 MPI and 16 threads per MPI seem quite promising.

[[media:ecfd3_final_project1.pdf | Final presentation of project #1]]

=== Project #2: Hackathon GENCI/ATOS/AMD/CORIA on YALES2 ===
''C. Piechurski (GENCI), S. Jauré (ATOS), P.-A. Harraud (AMD), P. Mohanamuraly (CERFACS), G.Lartigue (CORIA), F. Gava (CORIA), P. Begou (LEGI)''

[[media:ecfd3_final_project2.pdf | Final presentation of project #2]]

=== Project #3: Implementation of a secondary atomization model in YALES2 ===

''C. G. Guillamon (Safran Tech), L .Voivenel (Safran Tech), R. Mercier (Safran Tech)''

In Lagrangian simulations, droplets are transported following a ballistic motion in an eulerian mesh. For non-reactive environments, droplets might undergo secondary atomization due to the aerodynamic interaction. In this work, we implement in YALES2 a breakup model known as Taylor-Analogy Breakup (TAB). This model is based on the analogy between a droplet and a second-order mechanical system, hence making possible to determine the breakup behaviour by means of Newton's second law.

Another model, the stochastic breakup model by Gorokhovski, is also suggested for future work and will be implemented in YALES2.

[[media:ecfd3_final_project3.pdf | Final presentation of project #3]]

=== Project #4: Application to combustion and lubrication applications ===

[[media:ecfd3_final_project4.pdf | Final presentation of project #4]]

=== Project #5: Jet-in-crossflow par une méthode d’interface diffuse ===

[[media:ecfd3_final_project5.pdf | Final presentation of project #5]]

=== Project #6: Accurate numerical predicti􏴇on of vorti􏴇cal flows using AMR ===

[[media:ecfd3_final_project6.pdf | Final presentation of project #6]]
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: Implémentation du calcul de la distance à une interface liquide-gaz proche d’une paroi sur maillage non structuré 3D avec YALES2 ===

[[media:ecfd3_final_project8.pdf | Final presentation of project #8]]

=== Project #9: Remeshed particle method at high Schmidt and Reynolds number ===

''S. Santoso (LJK), J.-B. Lagaert (Math Orsay), G.Balarac (LEGI)''

We study the advection of a scalar function in turbulent flows with a multimesh method. The finite volume method is used to solve Navier-Stokes equations on an unstructured mesh (YALES2). The advection equation is solved with remeshed particle method on a cartesian mesh. In the context of parallel computing, we face a very unbalanced problem since a large number of particles are created in a very fine meshed zone. Our strategy to load-balance the problem is to give a weight to every element group which is equal to the density of particle.

[[media:ecfd3_final_project9.pdf | Final presentation of project #9]]

=== Project #10: Adaptive mesh refinement for turbulent premixed combustion ===
''W. Agostinelli, O. Dounia, , T. Jaravel, O. Vermorel

The objective of the project was to evaluate the potential of adaptive mesh refinement (AMR) for premixed combustion in unsteady systems. Three target cases were identified: a semi-vented deflagration with laminar to turbulent transition, a planar detonation wave, and a bluff-body stabilized burner subjected to thermoacoustic oscillations. The simulations were performed with AVBP and coupled to the AMR implementation of YALES2. Several metrics and remeshing criterions were developed to identify and correctly resolve both the combustion wave front and the turbulent flow. The comparison of numerical results with reference simulations showed that the main features of the physics could be recovered with a significant speed-up in term of computational cost.

[[media:ecfd3_final_project10.pdf | Final presentation of project #10]]

=== Project #11: Multiphysics coupling for wind turbine wake modeling ===

''F.Houtin-Mongrolle (CORIA), B. Duboc (SGRE), P. Benard (CORIA)''

The goal of this project was to evaluate the coupling of YALES2 (flow solver) and BHawC(Aero-Servo-Elastic solver).

[[media:ecfd3_final_project11.pdf | Final presentation of project #11]]

=== Project #12: Stability of a semi-implicit compressible cavitation solver ===
The compressible cavitation solver is used to simulate cavitation inception in an initially liquid flow behind an obstacle.
This solver is based on the implicit compressible solver, that has been modified to include a « barotropic » pressure-density relationship playing the role of an equation of state independent from the temperature.
While this strategy has proven to be effective for DNS simulations of the implosion of vapour bubbles, the simulation of cavitation inception in an initially liquid flow was leading to strong instabilities in the simulation shortly after the appearence of vapour.
The test case chosen is a flow behind a 2D cylinder.
The analysis of the results has shown that instabilities were correlated with very low (and even unphysically negative) values of the pressure, that were triggering negative density values leading to code instability.
Using limiters to ensure a positive pressure and a density within the range of the equation of state improved the stability and allowed to perform a preliminary simulation of a cavitating flow behind an obstacle.
Ultimately instabilities appear anyways, so that the spatial discretisation is now under study, notably through upwinding.

[[media:ecfd3_final_project12.pdf | Final presentation of project #12]]

=== Project #13: Validations and comparisons of Diffuse / Sharp interface methods in a structured DNS solver (Titan) ===
''V. Boniou (EM2C), J.M. Dupays (EM2C), M. Pelletier (EM2C), T. Schmitt (EM2C), A. Vié (EM2C)

The project aimed at using academic test cases to compare the sharp (incompressible) and diffuse (compressible) models. In particular, the test case of an inviscid initially elliptical oscillating droplet has been carried out.
The solvers features are the following:

- incompressible VOF solver (sharp): Numerical Method: Projection Method, Interface reconstruction: VOF, Surface tension: CSF

- compressible multifluid solver (diffuse): Advection scheme: MUSCL + RK2 + minmod limiter, Surface tension: CSF.

The source term is integrated with operator-splitting, and the curvature computation relies on a 2nd-order differentiation of the liquid volume fraction, which is previously smooth by filtering.
This test case showed good agreement on the oscillation period, while exhibiting a slight numerical diffusion in the incompressible case and a strong numerical diffusion in the compressible case.
In the compressible case, the use of higher-order splitting (Strang [SIAM Num. An. 1968]) has been tested, yielding no noticeable improvement. Reduction of the number of filtering iterations on the liquid volume fraction provides a slight improvement, which may indicate that a better curvature computation could participate to reduce the numerical diffusion.

[[media:ecfd3_final_project13.pdf | Final presentation of project #13]]

=== Project #14: Méthode d'ordre élevé ===
''M. Bernard (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI), V. Moureau (CORIA)''

[[media:ecfd3_final_project14.pdf | Final presentation of project #14]]

=== Project #15: Utilisation d’éléments finis du second ordre dans le SMS ===
''T. Fabbri (LEGI), G. Lartigue (CORIA), G. Balarac (LEGI), V. Moureau (CORIA)''

[[media:ecfd3_final_project15.pdf | Final presentation of project #15]]

=== Project #16: Development of a RANS solver in YALES2 ===
''G. Sahut (LEGI), G. Balarac (LEGI), V. Moureau (CORIA), G. Lartigue (CORIA), P. Bénard (CORIA), A. Grenouilloux (CORIA)''

While the accuracy of LES usually approaches the one of DNS, LES are still too time-consuming for daily use in industrial applications. In this context, we started the development of a RANS solver in YALES2. We are first only interested in the steady state of the solution. In order to remove the CFL constraint, we developed, implemented and validated an implicit projection method for the resolution of the Navier-Stokes equations without turbulence models. The method is based on the implicitation of the velocity predictor ; the Poisson equation and the correction step of the velocity are then solved and applied as in the explicit incompressible solver. We validated the method on a stationary 2D Poiseuille flow with periodic boundary conditions: the simulation runs fine for CFL and Fourier numbers which are inaccessible with the explicit incompressible solver. The advection-diffusion equation for scalars has also been implicited and will be used to add turbulence models to the new implicit incompressible solver developped during this Workshop. More complex boundary conditions will also be addressed in a near future.

[[media:ecfd3_final_project16.pdf | Final presentation of project #16]]

=== Project #17: COUPLING OF A FLUID PLASMA SOLVER WITH A LAGRANGIAN SOLVER FOR THE MODELING OF DUSTY ===

[[media:ecfd3_final_project17.pdf | Final presentation of project #17]]

=== Project #18: L’Evaporo O Maıtre ===

[[media:ecfd3_final_project18.pdf | Final presentation of project #18]]

=== Project #19: The Clone Wars ===

[[media:ecfd3_final_project19.pdf | Final presentation of project #19]]

=== Project #20: Stiff complex fluid simulation with YALES2 ===
''Sam Whitmore, Yves Dubief, M2CE, University of Vermont''

The objective was to simulate (1) ionized gases and (2) polymer solutions in flows using YALES2. Both problems are challenging owing to their stiff thermodynamics (1) or polymer dynamics (2). Significant gains were achieved in the implementation of the respective models thanks to the stiff integrator library CVODE. The plasma flow demonstrated an increase in time step of two orders of magnitude compared to previous implementation of the plasma chemistry in the variable density solver. Polymer models are notoriously prone to numerical instability. Again the use of CVODE showed equivalent if not superior stability of the solution at a fraction of the cost of commonly employed algorithms designed to address the stiffness of the problem.

[[media:ecfd3_final_project20.pdf | Final presentation of project #20]]

=== Project #21: AVBP Dense Gases ===

[[media:ecfd3_final_project21.pdf | Final presentation of project #21]]

=== Project #22: Numerical prediction of wind turbine wakes using AMR ===

[[media:ecfd3_final_project22.pdf | Final presentation of project #22]]