On the performance of a high-order multiscale DG approach to LES at increasing Reynolds number

The variational multiscale (VMS) approach based on a high-order discontinuous Galerkin (DG) method is used to perform LES of the sub-critical flow past a circular cylinder at Reynolds 3 900, 20 000 and 140 000. The effect of the numerical flux function on the quality of the LES solutions is also studied in the context of very coarse discretizations of the TGV configuration at Re = 20 000. The potential of using p-adaption in combination with DG-VMS is illustrated for the cylinder flow at Re = 140 000 by considering a non-uniform distribution of the polynomial degree based on a recently developed error estimation strategy. The results from these tests demonstrate the robustness of the DG-VMS approach with increasing Reynolds number on a highly curved geometrical configuration

Modélisation statistique des écoulements turbulents en convection forcée, mixte et naturelle

L'objectif général de la thèse est d'améliorer la modélisation numérique RANS des flux thermiques turbulents notamment en proposant un modèle fonctionnant dans les trois régimes de convection thermique (forcée, mixte et naturelle).Pour ce faire, un état des lieux, non exhaustif, des modèles des flux thermiques utilisant les approches algébriques et à équations de transport, est effectué. Puis, le modèle EB-RSM (Elliptic Blending-Reynolds Stress Model) étant utilisé pour modéliser la turbulence, le principe de la pondération elliptique est appliqué aux modèles des flux thermiques turbulents algébriques EB-GGDH (EB-General Gradient Diffusion Hypothesis), EB-AFM (EB-Algebraic Flux Model) et à équations de transport EB-DFM (EB-Differential Flux Model). Une attention particulière a été apportée aux échelles de temps et de longueur utilisées pour ces modèles. Il en résulte qu'utiliser une échelle de longueur thermique différente de l'échelle de longueur dynamique et une échelle de temps mixte dans le terme de flottabilité de l'équation de la dissipation turbulente est préférable.Pour valider les formulations retenues, nous avons effectué des tests pour des fluides usuels (nombre de Prandtl de l ordre de 1) dans les trois régimes de convection à l'aide de l'outil de calcul Code_Saturne sur des configurations académiques, semi-académiques et industrielles.Des résultats satisfaisants ont été obtenus en associant l'EB-RSM et le GGDH en convection forcée ou mixte et l'EB-RSM aux modèles EB-DFM et AFM en convection naturelle.The PhD main objective is to improve the turbulent heat flux RANS modelling especially by proposing a model working in the three thermal convection regime (forced, mixed and natural).In order to achieve this, a non-exhaustive state of art of heat flux model, using algebraic approach and transport equations, is done. Then, as EB-RSM model (Elliptic Blending-Reynolds Stress Model) is used to model turbulence, elliptic blending approach is apply to algebraic turbulent heat flux model EB-GGDH (EB-General Gradient Diffusion Hypothesis), EB-AFM (EB-Algebraic Flux Model) and transport equation model EB-DFM (EB-Differential Flux Model). Special attention was paid to time and length scales used with these models. It follows that using a thermal length scale different from dynamic length scale and a mixed time scale in the buoyant term of turbulent dissipation equation is better.To validate these models, some test were done for common fluids (Prandtl number in the order of 1) in the three convection regimes with the tool Code_Saturne on academic, mid-academic and industrial cases.Good results were obtained combining EB-RSM with GGDH in forced or mixed convection and EB-RSM with EB-DFM or AFM in natural convection.POITIERS-SCD-Bib. électronique (861949901) / SudocSudocFranceF

Design and overview of the Origins of Alzheimer’s Disease Across the Life course (ORACLE) study

Brain development and deterioration across the lifespan are integral to the etiology of late-life neurodegenerative disease. Factors that influence the health of the adult brain remain to be elucidated and include risk factors, protective factors, and factors related to cognitive and brain reserve. To address this knowledge gap we designed a life-course study on brain health, which received funding through the EU ERC Programme under the name Origins of Alzheimer’s Disease Across the Life course (ORACLE) Study. The ORACLE Study is embedded within Generation R, a prospective population-based cohort study of children and their parents, and links this with the Rotterdam Study, a population-based study in middle-aged and elderly persons. The studies are based in Rotterdam, the Netherlands. Generation R focuses on child health from fetal life until adolescence with repeated in-person examinations, but has also included data collection on the children’s parents. The ORACLE Study aims to extend the parental data collection in nearly 2000 parents with extensive measures on brain health, including neuroimaging, cognitive testing and motor testing. Additionally, questionnaires on migraine, depressive symptoms, sleep, and neurological family history were completed. These data allow for the investigation of longitudinal influences on adult brain health as well as intergenerational designs involving children and parents. As a secondary focus, the sampling is enriched by mothers (n = 356) that suffered from hypertensive disorders during pregnancy in order to study brain health in this high-risk population. This article provides an overview of the rationale and the design of the ORACLE Study

Direct Numerical Simulation Of A Turbulent Channel Flow Across A Sudden Expansion: Effects Of Spanwise Rotation

International audienceThe effects of a spanwise rotation on the channel flow across a sudden expansion are investigated using DNS. Four rotation regimes are considered with the same Reynolds number Re = 5000 and ratio expansion Er = 1.5. Upstream from the expansion, inflow turbulent conditions are generated realistically for each rotation rate through a very simple and efficient technique of recycling without the need of any precursor calculation. As the rotation is increased, the flow becomes progressively asymmetric with stabilization/ destabilization effects on the cyclonic/anticyclonic sides respectively. These rotation effects, already present in the upstream channel consistently with previous studies, lead further downstream to a reduction/increase of the separation size behind the anticyclonic/cyclonic step. In the cyclonic separation, the free-shear layer created behind the step corner leads to the formation of large-scale spanwise vortices that are found more and more 2D as the rotation is increased. Conversely, in the anticyclonic region, the turbulent structures in the separated layer are more 3D and also more active to promote the reattachment. In the present flow configuration where Coriolis forces do not work while being passive on a purely 2D dynamics, the phenomenological model of absolute vortex stretching is useful to understand how the rotation influences the flow dynamics

Développement d'un code de calcul combinant des schémas de haute précision avec une méthode de frontières immergées pour la simulation des mouvements tourbillonnaires en aval d'un bord de fuite

La réalisation de simulations des mouvements tourbillonnaires en aval d'un bord de fuite reste une tâche difficile en mécanique des fluides. Des développements numériques ont été réalisés à partir d'un code de calcul résolvant les équations de Navier-Stokes incompressibles à l'aide de schémas aux différences finies de haute précision. L'originalité de l'approche numérique réside dans l'utilisation d'une méthode spectrale pour la résolution de l'équation de Poisson qui combinée à une méthode de frontières immergées permet de simuler des écoulements en présence de parois. Un effort particulier a été fait pour l'amélioration de la résolution de l'équation de Poisson afin d'utiliser un maillage inhomogène et une grille décalée pour le champ de pression. Deux écoulements de couche de mélange avec la prise en compte des parois ont été simulés avec un bord de fuite épais puis mince dans le but de déterminer l'influence de la géométrie de la plaque séparatrice sur la dynamique de l'écoulement.To carry out simulations of the vortex dynamics behind a trailing edge remains a difficult task in fluid mechanics. Numerical development has been performed with a computer code which solves the incompressible Navier-Stokes equations with high order compact finite difference schemes on a Cartesian grid. The specificity of this code is that the Poisson equation is solved in the spectral space with the modified spectral formalism. This code can be combined with an immersed boundary method in order to simulate flows with complex geometry. A particular work was made to improve the resolution of the Poisson equation in order to use a stretched mesh and a staggered grid for the pressure. Two mixing layers flows with a blunt and a bevelled trailing edge were performed in order to determinate the influence of the separating plate's shape on the vortex dynamics.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Influence d'une rotation d'entrainement sur les tourbillons cohérents dans un canal

International audienceWe study with the aid of direct-numerical simulations the influence of a solid-body rotation of spanwise axis upon turbulence in a channel. Rotation is shown to reduce the vortical activity in the cyclonic zone. In the anticyclonic region, hairpin vortices are reorganized by rotation, and become more and more coherent and inclined towards the wall as the rotation rate increases. One observes also in this region the formation of a constant mean-velocity gradient equal to the entrainment vorticity and, if the rotation rate is high enough, the disappearance of high and low-speed streaks at the wall

Large- and small-scale stirring of vorticity and a passive scalar in a 3-D temporal mixing layer

International audienceWith the aid of a three-dimensional direct-numerical simulation (using pseudospectral methods) at high resolution (1283 grid points in a cubic box containing four fundamental longitudinal wavelengths), the origin and topology of the longitudinal vortex filaments which appear in the temporally growing mixing layer are investigated. The basic velocity field is a hyperbolic-tangent profile U tanh 2y/δi, with a Reynolds number Uδi/ν=100. The basic flow is forced initially by two small Gaussian random perturbations: a three-dimensional one, of kinetic energy ε3D U2, and a two-dimensional one, of kinetic energy ε2D U2. For ε2D=10−4 and ε3D=10−5, quasi-two-dimensional large coherent Kelvin–Helmholtz rolls form and merge. Because of unstable oblique modes, they oscillate slightly as in the translative instability proposed by Pierrehumbert and Widnall [J. Fluid Mech. 114, 59 (1982)], but not exactly in phase. Between these big rollers, weak vortex lines are stretched. For ε2D=0 and ε3D=10−4 the oblique modes observed in the previous case yield local roll-ups and pairings occurring in certains regions of the span only, giving rise to a large-scale vortex lattice similar to that observed by Nygaard and Glezer in forced experiments [Phys. Fluids A 2, 461 (1990)]. This is numerical evidence of Pierrehumbert and Widnall’s helical-pairing instability. Furthermore, a thin and intense streamwise hairpin vortex appears, stretched in between the large vortices of the lattice. A low Reynolds number counterpart of the mixing transition occurs during the second pairing, characterized by short k−5/3 subranges in the spectra, intense vorticity extrema, and intermittent distributions of local helicity

Modélisation hybride RANS/LES temporelle des écoulements turbulents

In industrial situation, the computation of unsteady and three-dimensional characteristics of complex flows, is often necessary. Large-eddy simulation demands prohibitive computational costs, peculiarly near walls. One of the objectives of hybrid methods, is to optimize the computational cost, by simulating certain areas of flows in RANS mode, and in LES mode elsewhere. The latter centres generally around spatial filtering of the studied variables, while RANS corresponds to a temporal average, in most flows. The PITM approach (Partially Integrated Transport Model), developed in homogeneous turbulence, is a theoretically justified hybrid method. Its transposition to the temporal context was done previously, showing that, under certain hypotheses, both spatial and temporal versions are formally identical. The PITM method has however difficulties in controlling the resolution level. This thesis suggests a dynamic approach to fix that point. Secondly, the temporal version of PITM, the T-PITM, is compared to DES (Detached Eddy Simulation), a popular but empirical hybrid method. It is shown that both methods produce similar results, providing DES an indirect theoretical justification. The underlying RANS model is the elliptic blending Reynolds-stress model (EB-RSM), accounting for wall effects, using neither damping functions nor wall laws.En situation industrielle, le calcul des caractéristiques instationnaires et tridimensionnelles d'écoulements complexes, est souvent nécessaire. La simulation des grandes échelles requiert un coût de calcul prohibitif surtout près de parois. L'un des objectifs des méthodes hybrides est d'optimiser le coût de calcul, en simulant certaines zones d'un écoulement en mode RANS et d'autres en mode LES. Cette dernière s'articule en général autour du filtrage spatial, alors que dans la plupart des écoulements, l'opérateur RANS correspond à une moyenne temporelle. L'approche PITM (Partially Integrated Transport Model ), conçue en turbulence homogène, est une méthode hybride justifiée théoriquement. Sa transposition au contexte temporel (turbulence stationnaire) a déjà été explorée précédemment, montrant que, sous certaines hypothèses, les versions spatiale et temporelle sont formellement identiques. La méthode PITM présente toutefois une certaine difficulté à piloter le niveau de résolution. La présente thèse propose une approche dynamique pour corriger ce point. Dans un second temps, la version temporelle du PITM, le T-PITM, est comparée à a DES (Detached Eddy Simulation), une méthode hybride populaire, mais empirique. Il est montre que les deux méthodes produisent des résultats similaires, conférant une justification théorique par procuration, à la DES. Le modèle RANS sous-jacent est la pondération elliptique, permettant la prise en compte des effets induits par une paroi, sans utiliser de fonctions d'amortissement ni de lois de paroi.POITIERS-BU Sciences (861942102) / SudocPOITIERS-UFR Sc-Phys.Ingénieur (860622201) / SudocSudocFranceF