Extreme CFD workshop - User contributions [en]

Ecfd:ecfd 6th edition

2023-02-07T23:28:57Z

Letournel: /* User Experience - J. Leparoux, Safran Tech & A. Pushkarev, GE Renewable Energy */

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

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

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

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

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



== Agenda ==

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

== Thematics / Mini-workshops ==

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

== Projects ==

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

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

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

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

* '''Sub-project 2: Ghost fluid method (GFM) for Electrodeformation (A. Spadotto , S. Mendez)'''
According to the Leaky Dielectric Model, red blood cells (RBCs) are subject to a force which is proportional to the jump of 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, Guillaume Balarac)'''

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

'''

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

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

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

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

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

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

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

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

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

* '''Sub-project 4: 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.

=== 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)'''

* '''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)'''

* '''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)'''

Ecfd:ecfd 5th edition

2022-02-02T01:23:02Z

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

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

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

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

== News ==

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

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

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

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

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

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

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

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

== Agenda ==

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

== Thematics / Mini-workshops ==

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

* '''Sub-project 5 : Numerical schemes for anisotropic meshes (R. Letournel, G. Lartigue, G. Balarac, M. Bernard)'''
Many efforts are devoted to the development of mesh adaptation based on anisotropic meshes in Yales2. For many configurations (e.g. shocks, mixing layers, near wall flows), a significant gain in terms of computational cost can be expected through the use of such meshes. The aim of this project was to characterize Yales2 numerical schemes on anisotropic meshes in comparison with isotropic meshes.

The strategy was at first to test different Laplacian operators on analytic functions (linear, quatratic, bump in one or two directions) in direct sense (apply the Laplacian operator) and indirect (solve the Poisson equation), to verify the order of convergence of the operators. Different meshes were considered: Isomesh, Quad2tri, MMG meshes, with different stretching (corresponding to anisotropic ratios). This exhaustive study showed that only the SIMPLEX operator remains of order 2 for the Poisson solver, while the IGNORE_SKEWNESS and PAIR_BASED operators do not converge.

In a second step, the performance of the incompressible solver on a 2D Poiseuille flow have been evaluated for increasingly stretched meshes. The coarsening in the preferential direction of the flow allows to decrease the convective time step and thus to reduce the RTTS (Return Time To Solution), while keeping the same error. These conclusions are valid for anisotropic ratios of the order of 10, beyond which the time step no longer decreases and further studies are needed to understand the limitations in these cases.

* '''Sub-project 6 : Poisson solver comparison for a two-phase flow CFD code (J. C. Brändle de Motta, A. Poux, T. Ménard)'''
Poisson solvers known to be highly sensitive to discontinuities and represent the main computational costs of incompressible codes.
Archer is a mature two-phase flow CFD code. Its historical Poisson solver is currently based on a Conjugate gradient with multigrid preconditioner (MGCG).
The aim of this project was to couple Archer and the PETSc library, assess its performances and challenge the historical solver of Archer.

The coupling with PETSc has been implemented during the workshop and the comparison with MGCG has been done up to 2 thousands cores in single and two phase turbulent flow.
Multiple PETSc configuration has been tested, and show that MGCG is very competitive (one order of magnitude faster than PETSc)

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

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

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

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

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

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

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

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

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

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

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

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

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

Y2 smart archive is a useful tool which gives the user more freedom when it comes storing their data. Y2 smart archive allows for automatic smart archiving and has the capability of taking user-specified maximum number of tar files and/or the maximum tar file size. The python tool conserves the original tree structure of the folder and has the ability to automatically detect the tree depth where the tar files are to be saved. During the project, y2 smart archive was tested on a windows computer. This implementation verified the ability to archive save a whole Windows computer with automatic handling of multi-platform symlinks. In the case where files may be skipped, log files are stored for reference. Current developments of the tool include automatic background launching, which will result in a reduction of required user-input hours.

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

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

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

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

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

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

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