Utilisation conjointe de lois de parois et modèles bas-Reynolds.

RÉSUMÉ Ce mémoire présente une méthode générique pour développer des lois de parois adaptatives consistantes spécifiques à un modèle de turbulence bas-Reynolds. Elles permettent d'améliorer la description et le traitement des couches limites en se raccordant exactement à la solution prédite par le modèle choisi, à n'importe quelle distance de la paroi, de la sous-couche visqueuse à la zone logarithmique. Le modèle de Launder-Sharma, utilisé ici, est adapté à une formulation logarithmique des variables de turbulence de façon à garantir la positivité de celles-ci sans limiteurs. Cette approche robuste est efficace numériquement et permet une résolution rapide. Le modèle bas-Reynolds est tout d'abord intégré jusqu'à la paroi sur un problème unidimensionnel à l'aide d'un maillage fin de façon à produire des tables pour la vitesse et les variables de turbulences adimensionnelles. Ces tables sont spécifiques au modèle et serviront à calculer les conditions limites imposées par les lois de parois. Une loi de parois à deux échelles de vitesses est proposée afin d'améliorer les prédictions en présence de points de stagnation, décollement et recollement. Les performances de ces lois de parois sont évaluées sur des cas unidimensionnels puis bidimensionnels.En 1D, l'universalité des lois de parois est examinée sur des écoulements de Couette et de Poiseuille, afin de déterminer le domaine de validité des nouvelles lois de parois. L'imposition de conditions limites consistantes calculées à partir des profils tabulés élimine les limitations classiques des lois standard. Il est possible de raccorder avec précision les lois de parois et le modèle bas-Reynolds jusque dans la sous-couche visqueuse. Cette approche donne, avec des lois de parois, la même solution que celle obtenue par intégration jusqu'à la paroi mais à un coût moindre. Les lois de parois adaptatives à deux échelles de vitesses sont ensuite utilisées pour résoudre un écoulement sur une plaque plane en présence d'un décollement induit par un gradient de pression adverse. En imposant des conditions limites consistantes, on obtient des solutions indépendantes du maillage. La modélisation est également améliorée dans les zones de décollement en permettant de raccorder les solutions à une très petite distance. Enfin, la méthode est validée sur un cas bidimensionnel complexe afin d'évaluer son applicabilité et comparer les prédictions aux résultats expérimentaux. Pouvant être raccordées à n'importe quelle distance, les lois de parois adaptatives sont simples et souples d'emploi.----------ABSTRACT The present document introduces a new generic method to develop consistent wall-functions specific to an arbitrary RANS low Reynolds turbulence model. Such an approach allows to match exactly the wall-function and the turbulence model at any distance from the viscous sublayer to the Logarithmic zone. The Launder-Sharma low-Reynolds turbulence model is adapted to a logarithmic formulation to guarantee the positivity of turbulence quantities without clipping. This, in turn, yields a robust and accurate solution procedure that is computationally efficient. The model is first integrated down to the wall on the planar Coutte flow, in order to generate reference tables for dimensionless profiles of velocity and turbulence variables. These tables will be used to provide consistent boundary conditions in the wall-function strategy. A two-velocity scale wall function is proposed to improve predictions near stagnation, detachment and reattachment points. Performances of the present method are investigated on 1D and 2D cases. The usual assumption of universality of the profile is investigated on 1D Couette and Poiseuille flows to determine the range of validity of the precomputed tables. Using both model-consistent wall function and a low Reynolds number model largely reduces the limitation of traditional wall functions related to the choice of the wall distance. Furthermore, it yields solutions as accurate as when integration is performed down to the wall for a much reduced computational cost. The performances of the newly developed wall function in the presence of pressure gradient are then studied on a flat plate with pressure driven separation. Imposing a correctly computed normal derivative for turbulence kinetic energy largely improves results and the universality of the profile while leading to wall distance independent results. The present method is then validated on a complex flow by comparison with experimental results. Adaptive two-velocity scale wall function proves to be particularly easy to use as they can be matched at any distance. They did not show particular numerical difficulties and are even less stiff than one-velocity scale wall-functions

Comparing goal-oriented RANS error estimates applied to high-lift configuration computations

International audienceThis presentation discusses an anisotropic adaptive strategy in the context of the 3rd AIAA high-lift prediction workshop. If anisotropic mesh adaptation has proven its reliability for inviscid flows [1, 2], additional challenges remain to be solved to have the full gain of adaptivity, including early asymptotic (spatial second) order convergence, early capturing of the scales of then physical phenomena,. .. Several (fundamental) modifications are needed in the classical adaptive loop to address complex viscous effects. This includes the way error estimates are evaluated, how viscous solutions are interpolated between (anisotropic) meshes, and finally the generation of the boundary layer mesh to comply with the metric size prescription. Addressing fully each of this component remains a challenge on itself. In this presentation, we propose two error estimates to address the RANS equations, this implies in particular to treat appropriately the considered turbulence model. Here, we only consider the one equation Spalart-Allmaras turbulence model

Comparing anisotropic adaptive strategies on the 2nd AIAA sonic boom workshop geometry

International audienceThe recent release of the 2nd AIAA sonic boom geometry offers the opportunity to review the classical anisotropic adaptive strategies for complex geometries. The design of Mach-aligned tailored grids is also a great challenge to see how adaptivity can compete with user-defined tailored grids. Two classical adaptive strategies, multi-scale and goal-oriented, are compared with the results obtained on tailored grids. For the flow solver, we discuss several low-dissipation numerical schemes of order 4th and 6th with respect to regular 2nd order scheme both on inviscid and RANS flow conditions. We finally perform a non-linear error analysis to assess the convergence of the sequence of adaptive meshes with respect to tailored grids. All results and discussions are based on the C25D baseline geometry

Comparing anisotropic adaptive strategies on the second AIAA sonic boom workshop geometry

International audienceThe recent release of the 2nd AIAA sonic boom geometry offers the opportunity to review the classical anisotropic adaptive strategies for complex geometries. The design of Mach-aligned tailored grids is also a great challenge to see how adaptivity can compete with user-defined tailored grids. Two classical adaptive strategies, multi-scale and goal-oriented, are compared with the results obtained on tailored grids. For the flow solver, we discuss several low-dissipation numerical schemes of order 4th and 6th with respect to regular 2nd order scheme both on inviscid and RANS flow models. We finally perform a non linear error analysis to assess the convergence of the sequence of adaptive meshes with respect to tailored grids. All results and discussions are based on the C25D baseline geometry provided for the 2nd AIAA Sonic Boom Prediction workshop

Grapevine fingerprinting using microsatellite repeats

Five different microsatellite loci have been amplified through PCR (Polymerase Chain Reaction) in order to find polymorphisms useful for identification of 16 cvs and 42 clones or populations of Vitis vinifera L. No genetic variability was detected among clones and populations. Ail the cultivars have been identified, except those derived from bud mutations involving the colour of the berries, as the Pinot group (P. noir, P. blanc, P. gris), or those thought to be closely correlated or synonymous, such as 'Refosco di Faedis' and 'Refoscone'. One locus (VVS2) showed a high polymorphism with 12 alleles detected on high resolution agarose gel

Nonlinear corrector for RANS equations

International audienceThe scope of this paper is to present a nonlinear error estimation and correction for Navier-Stokes and RANS equations. This correction is obtained by deducing a source term from the evaluation of the residual of the solution interpolated on the h/2 mesh. To avoid the generation of the h/2 mesh (which is prohibitive for realistic applications), the residual at each vertex is computed by local refinement only in the neighborhood of the considered vertex. It successfully improves solution predictions and yields a sharp estimate of the numerical error

Verification of Unstructured Grid Adaptation Components

Adaptive unstructured grid techniques have made limited impact on production analysis workflows where the control of discretization error is critical to obtaining reliable simulation results. Recent progress has matured a number of independent implementations of flow solvers, error estimation methods, and anisotropic grid adaptation mechanics. Known differences and previously unknown differences in grid adaptation components and their integrated processes are identified here for study. Unstructured grid adaptation tools are verified using analytic functions and the Code Comparison Principle. Three analytic functions with different smoothness properties are adapted to show the impact of smoothness on implementation differences. A scalar advection-diffusion problem with an analytic solution that models a boundary layer is adapted to test individual grid adaptation components. Laminar flow over a delta wing and turbulent flow over an ONERA M6 wing are verified with multiple, independent grid adaptation procedures to show consistent convergence to fine-grid forces and a moment. The scalar problems illustrate known differences in a grid adaptation component implementation and a previously unknown interaction between components. The wing adaptation cases in the current study document a clear improvement to existing grid adaptation procedures. The stage is set for the infusion of verified grid adaptation into production fluid flow simulations