Revision as of 08:01, 4 February 2026

Description

• Event from 19th of January to 30th of January 2026
• Location: 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).

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)

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.