Une voie prometteuse pour améliorer le rendement des centrales solaires à tour consiste à chauffer de l'air pressurisé à haute température afin d'alimenter un cycle thermodynamique de Brayton. Pour cela, il est indispensable de concevoir des récepteurs solaires performants,permettant de forts transferts de chaleur vers le fluide. Le développement de tels récepteurs passe par une compréhension fine de leurs écoulements internes. Il s'agit d'écoulements complexes, combinant de hauts niveaux de turbulence et un fort gradient de température entre la paroi irradiée par le flux solaire concentré et la paroi arrière isolée. On se propose dans ce travail de réaliser une étude amont numérique et expérimentale de ce type d'écoulements.D'une part, des mesures de vitesse par SPIV (vélocimétrie par images de particules stéréoscopique) sont effectuées dans une soufflerie de canal plan turbulent lisse dont la cellule de mesure est représentative d'un récepteur solaire surfacique. On observe en particulier l'influence d'un chauffage asymétrique sur les statistiques de la turbulence. Ces mesures sont d'autre part complétées par des simulations fines LES (simulation des grandes échelles)menées dans les conditions de la soufflerie. Pour finir, une simulation LES d'un canal plan texturé sur une paroi par une géométrie innovante est conduite. Cette architecture interne du récepteur combine des générateurs de tourbillon et des riblets afin d'intensifier les échanges de chaleur vers le fluide. / A promising line of research to increase the efficiency of solar tower power plants consists in heating pressurized air to high temperatures in order to fuel a Brayton thermodynamic cycle. This requires to design effective solar receivers that allow for intense heat transfers toward the fluid. To develop such receivers, an in-depth understanding of their internal flows is needed. These are complex flows, combining strong turbulence and strong temperature gradient between the concentrated sun irradiated wall and the back insulated wall.The aim of this work is to investigate numerically and experimentally such flows.On one hand, velocities are measured by SPIV (Stereoscopic Particle Image Velocimetry) in a turbulent channel flow wind tunnel whom measurement cell is similar to a surface solar receiver. The influence of an asymmetric heating on the turbulence statistics are especially investigated. These measurements are supplemented by Large Eddy Simulations run under the same conditions as the wind tunnel. Finally, a Large Eddy Simulation is run in a channel flow textured on one wall by an innovative geometry. This internal receiver design combines vortex generators and riblets in order to enhance the heat transfers.

Cette thèse porte sur la caractérisation numérique et la modélisation stochastique de l’accélération du fluide pour l’écoulement en canal à grand nombre de Reynolds. La motivation concerne l’observation et l’analyse des effets de l’intermittence liés aux interactions à longue portée à travers le canal. Dans la première partie, l’accélération est étudiée par simulation numérique directe pour trois différents nombres de Reynolds (180, 590 et 1000). La lognormalité de la norme de l’accélération est observée quelle que soit la distance à la paroi. Un profil universel de la norme de l’accélération est également recherché par analyse dimensionnelle. La seconde partie présente une modélisation stochastique de l’accélération basée sur la décomposition norme/orientation. Le modèle stochastique pour la norme s’appuie sur un processus de fragmentation afin de représenter les interactions à longue portée à travers le canal. Pour l’orientation, l’évolution vers l’isotropie lorsque la distance à la paroi augmente (observée par la DNS) est reproduite grâce à un modèle de marche aléatoire sur une sphère. Ces modèles ont été appliqués à l’approche LES-SSAM (Stochastic Subgrid Acceleration Model) introduite par Sabel’nikov, Chtab et Gorokhovski. Nos calculs montrent que les estimations de la vitesse moyenne, du spectre d’énergie, des contraintes de l’écoulement et de la non-gaussianité des statistiques de l’accélération peuvent être améliorées de façon significative par rapport à la LES classique. L’intérêt de l’approche LES-SSAM, donnant un accès vers la structure intermittente de sous-maille, est illustré dans la dernière partie, par l’étude du transport de particules inertielles ponctuelles par l’écoulement de canal. Cette étude commence par l’analyse par DNS de l’influence des structures de paroi sur la dynamique des particules / The main objective of this thesis is to observe numerically and to analyze the effects of intermittency in a high Reynolds number turbulent channel flow. To this end, the thesis is focused on characterization and stochastic modelling of the fluid acceleration in such a flow, with emphasis on long-range interactions across the channel. In the first part, the acceleration is studied using DNS for three Reynolds numbers (180, 590 et1000). It is observed that the norm of acceleration is log-normal whatever the wall distance is. The universal form of scaling law for the acceleration is proposed by dimensional analysis. In the second part, the acceleration is simulated stochastically, assuming the norm/orientation decomposition. The stochastic model for the norm is based on the fragmentation process in order to represent the long-range interactions across the channel. The orientation is simulated by random walk on a sphere in order to reproduce the relaxation towards isotropy with increasing the wall distance. This was observed preliminary in our DNS. These models were applied in the framework of LES-SSAM approach (Stochastic Subgrid AccelerationModel), which was introduced by Sabel’nikov, Chtab and Gorokhovski and assessed in the case of the box turbulence. Our computations showed that the mean velocity, the energy spectra, the viscous and turbulent stresses, as well as the non-gaussianity of acceleration statistics can be considerably improved in comparison with standard LES. The advantage of the LES-SSAM approach, which accounts for intermittency on subgrid scales, is demonstrated in the last part of this thesis. Here the transport of inertial point wise particles was studied by DNS and by LES-SSAM. The influence of wall structures on the particle’s dynamics is analyzed.

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La chaîne himalayenne constitue le paradigme actuel des chaînes de collision. Cependant, les processus de formation de cette chaîne sont toujours en discussion. Bien que fondamentales pour comprendre la formation de la chaine, les klippes de Haut Himalaya Cristallin (HHC) sont paradoxalement assez peu intégrées dans les différents modèles. Dans la klippe de Jaljala (Centre – Ouest Népal) la combinaison d’études structurales pétrographiques et géochronologiques (40Ar/39Ar) ont permis de caractériser près du front de l’Himalaya la géométrie et la cinématique du Main Central Thrust (MCT) et d'une zone de cisaillement top vers le nord : la zone de cisaillement de Jaljala, failles qui encadrent le HHC. Les résultats montrent que le MCT et la zone de cisaillement de Jaljala ont été replissés et que le que la zone de cisaillement de Jaljala est proche du MCT au nord de la klippe. Une faille normale intra – séquences téthysiennes (TH) a été découverte, faille interprétée comme étant la zone de cisaillement de Jaljala sur le flanc sud de la klippe. Les données pétrographiques montrent une augmentation progressive de la température entre 350 et 550 °C au travers du MCT dans le Haut Himalaya Cristallin alors qu’elle atteint plus de 650 °C au Nord dans les zones internes. Les pseudosections montrent que ce pic de température est atteint après un échauffement isobare à desvaleurs de pression variant entre 7 à 9 kbars. Les âges 40Ar/39Ar sur micas montrent trois populations : environ 20, environ 40 et environ 100 Ma dans le HHC et dans les séquences téthysiennes. Deux hypothèses peuvent être proposées : soit l’exhumation est marquée par les âges à 40 Ma ce qui représente une date relativement ancienne pour l’exhumation du Haut Himalaya Cristallin au front de la chaîne, soit elle est datée à 20 Ma ce qui représente des âges plus communs d’exhumation sur le MCT et sous le STD (South Tibetan Detachment). La nature des roches observées, leurs déformations ainsi que les corrélations avec les résultats des autres klippes montrent que la zone de cisaillement de Jaljala ne peut être connecté au STD des zones internes. Le MCT et le STD ne peuvent se rejoindre en profondeur au front de la klippe ce qui exclut le modèle de prisme tectonique. Enfin la continuité des pressions et températures des zones internes avec les roches de la klippe va à l’encontre du modèle de fluage de croûte chenalisée puisqu’il n’y a pas de fusion partielle dans la klippe de Jaljala. Les structures, les conditions métamorphiques et les âges seraient plutôt compatibles avec la formation d’un duplex de Haut Himalaya Cristallin dont la géométrie est cependant mal contrainte et qui nécessiterait de présenter un système de plat – rampe frontal pour transférer les écailles les plus internes sur le front de la chaîne et ainsi former les klippes comme la klippe de Jaljala qui seront ensuite isolées de la zone interne par la formation d’un duplex Moyen Himalaya. / The Himalayan belt is the actual paradigm of collision mountain belt. However, formation model remains still under discussion. Even fundamental to understand the belt formation, the High Himalaya Cristalline (HHC) klippen are poorly integrated to the different existing models. In the Jaljala klippe (Western Central Nepal) a combination of structural, petrographic and geochronological (40Ar/39Ar) studies have allowed to caracterise close to the Himalaya front, the Main Central Thrust (MCT) and a top - to - the North shear zone : the Jaljala shear zone geometry and kinematics, faults that bordered the HHC. Results show that the MCT and the Jaljala shear zone have been refolded and the Jaljala shear zone is close to the MCT in the North of the klippe. An intra téthyan sequences (TH) have been discovered and interpreted as the Jaljala shear zone in the southern flank of the klippe. Petrographic datas show a progressive augmentation of temperature between 350 and 550 ° C cross to the MCT in High Himalaya Cristalline instead of 650 °C in the internal zones. Pseudosections show this temperature peak is achieved after an isobaric warming at pressure varying between 7 and 9 bars. 40Ar/39Ar ages on micas show three ages populations : about 20, about 40 and about 100 Myrs in the HHC and in Tethyan sequences. Two hypothesis can be proposed : on the one hand, the exhumation can be testified by 40 Myrs ages which represent an ancient age for the High Himalaya Crystalline in the front belt, on the other hand, it is dated at 20 Myrs which represent more commons ages for exhumation on MCT and under STD (South Tibetan Detachment). Rock lithology and their deformations and correlations with results for other klippen show that the STD in the Jaljala klippe cannot be connected width the STD in internal zones. The MCT and the STD cannot converge in depth at the front that excluded the tectonic wedge model. Finally, the pressures and temperature continuities in internal zones and with the klippe rocks excluded the channel flow model because partial melting is absent in the Jaljala klippe. Structures, metamorphic conditions and ages are more compatible with High Himalaya Crystalline duplex formation whose geometry is still poorly constrained and which necessitate a frontal flat - ramp system to transfer crustal nape on the front of the belt and then to form klippe as the Jaljala klippe that will then isolated from internal zones by Lesser Himalaya duplex formation.

Un mascaret est une vague remontant contre le courant d’un fleuve lorsque la marée se propage dans un estuaire. À son passage, le mascaret induit une forte turbulence et un fort mélange dont les effets sur la vie de l’estuaire sont encore mal quantifiés. Ici, le phénomène est étudié expérimentalement et numériquement en utilisant un modèle d’onde positive se propageant contre un courant permanent.L’étude en laboratoire a permis de mesurer les variations de la surface libre, de la vitesse de l’écoulement ainsi que des échelles de turbulence. Lors de son passage, des fluctuations importantes de la surface libre et de la vitesse de l’écoulement sont observées, ainsi que des variations des échelles de turbulences. Des structures turbulentes semblent se former près du fond sous le front de l’onde et montent dans la colonne d’eau après le passage du front.La simulation numérique fut réalisée à partir de données expérimentales d’onde positive ondulée sur fond lisse. Une validation des méthodes numériques a été réalisée pour différente configuration. Les résultats des simulations d’onde positives donnent une cartographie détaillée de l’écoulement dans tout le canal. De plus, la simulation a permis d’identifier une inversion de la vitesse près des parois lors du passage des crêtes des ondes générant dans certaines configurations des structures turbulentes. / Tidal bores are surge waves propagating upstream rivers as the tide rushes into estuaries. They induce large turbulences and mixing of the river and estuary flow of which effects remain scarcely studied. Herein, tidal bores are investigated experimentally and numerically with an idealised model of positive surges propagating upstream an initially steady flow. The experimental work estimated flow changes and typical turbulent length scale evolution induced by undular bores with and without breaking roller. The bore passage was associated with large free surface and flow velocity fluctuations, together with some variations of the integral turbulent scales. Coherent turbulent structures appeared in the wake of leading wave near the bed and moved upward into the water column during the bore propagation. The numerical simulations were based on previous experimental work on undular bores. Some test cases were realised to verify the accuracy of the numerical methods. The results gave access to the detailed flow evolution during the bore propagation. Large velocity reversals were observed close to the no-slip boundaries. In some configurations, coherent turbulent structures appeared against the walls in the wake of the bore front.

The context of this thesis is the numerical simulation of turbulent flows at moderate Reynolds numbers and the improvement of the capabilities of an in-house 3D unsteady and incompressible flow solver called SFELES to simulate such flows.<p>In addition to this abstract, this thesis includes five other chapters.<p><p>The second chapter of this thesis presents the numerical methods implemented in the two CFD solvers used as part of this work, namely SFELES and PHASTA.<p><p>The third chapter concentrates on the implementation of a new library called FlexMG. This library allows the use of various types of iterative solvers preconditioned by algebraic multigrid methods, which require much less memory to solve linear systems than a direct sparse LU solver available in SFELES. Multigrid is an iterative procedure that relies on a series of increasingly coarser approximations of the original 'fine' problem. The underlying concept is the following: low wavenumber errors on fine grids become high wavenumber errors on coarser levels, which can be effectively removed by applying fixed-point methods on coarser levels.<p>Two families of algebraic multigrid preconditioners have been implemented in FlexMG, namely smooth aggregation-type and non-nested finite element-type. Unlike pure gridless multigrid, both of these families use the information contained in the initial fine mesh. A hierarchy of coarse meshes is also needed for the non-nested finite element-type multigrid so that our approaches can be considered as hybrid. Our aggregation-type multigrid is smoothed with either a constant or a linear least square fitting function, whereas the non-nested finite element-type multigrid is already smooth by construction. All these multigrid preconditioners are tested as stand-alone solvers or coupled with a GMRES (Generalized Minimal RESidual) method. After analyzing the accuracy of the solutions obtained with our solvers on a typical test case in fluid mechanics (unsteady flow past a circular cylinder at low Reynolds number), their performance in terms of convergence rate, computational speed and memory consumption is compared with the performance of a direct sparse LU solver as a reference. Finally, the importance of using smooth interpolation operators is also underlined in this work.<p><p>The fourth chapter is devoted to the study of subgrid scale models for the large eddy simulation (LES) of turbulent flows.<p>It is well known that turbulence features a cascade process by which kinetic energy is transferred from the large turbulent scales to the smaller ones. Below a certain size, the smallest structures are dissipated into heat because of the effect of the viscous term in the Navier-Stokes equations.<p>In the classical formulation of LES models, all the resolved scales are used to model the contribution of the unresolved scales. However, most of the energy exchanges between scales are local, which means that the energy of the unresolved scales derives mainly from the energy of the small resolved scales.<p>In this fourth chapter, constant-coefficient-based Smagorinsky and WALE models are considered under different formulations. This includes a classical version of both the Smagorinsky and WALE models and several scale-separation formulations, where the resolved velocity field is filtered in order to separate the small turbulent scales from the large ones. From this separation of turbulent scales, the strain rate tensor and/or the eddy viscosity of the subgrid scale model is computed from the small resolved scales only. One important advantage of these scale-separation models is that the dissipation they introduce through their subgrid scale stress tensor is better controlled compared to their classical version, where all the scales are taken into account without any filtering. More precisely, the filtering operator (based on a top hat filter in this work) allows the decomposition u' = u - ubar, where u is the resolved velocity field (large and small resolved scales), ubar is the filtered velocity field (large resolved scales) and u' is the small resolved scales field. <p>At last, two variational multiscale (VMS) methods are also considered.<p>The philosophy of the variational multiscale methods differs significantly from the philosophy of the scale-separation models. Concretely, the discrete Navier-Stokes equations have to be projected into two disjoint spaces so that a set of equations characterizes the evolution of the large resolved scales of the flow, whereas another set governs the small resolved scales. <p>Once the Navier-Stokes equations have been projected into these two spaces associated with the large and small scales respectively, the variational multiscale method consists in adding an eddy viscosity model to the small scales equations only, leaving the large scales equations unchanged. This projection is obvious in the case of a full spectral discretization of the Navier-Stokes equations, where the evolution of the large and small scales is governed by the equations associated with the low and high wavenumber modes respectively. This projection is more complex to achieve in the context of a finite element discretization. <p>For that purpose, two variational multiscale concepts are examined in this work.<p>The first projector is based on the construction of aggregates, whereas the second projector relies on the implementation of hierarchical linear basis functions.<p>In order to gain some experience in the field of LES modeling, some of the above-mentioned models were implemented first in another code called PHASTA and presented along with SFELES in the second chapter.<p>Finally, the relevance of our models is assessed with the large eddy simulation of a fully developed turbulent channel flow at a low Reynolds number under statistical equilibrium. In addition to the analysis of the mean eddy viscosity computed for all our LES models, comparisons in terms of shear stress, root mean square velocity fluctuation and mean velocity are performed with a fully resolved direct numerical simulation as a reference.<p><p>The fifth chapter of the thesis focuses on the numerical simulation of the 3D turbulent flow over a re-entry Apollo-type capsule at low speed with SFELES. The Reynolds number based on the heat shield is set to Re=10^4 and the angle of attack is set to 180º, that is the heat shield facing the free stream. Only the final stage of the flight is considered in this work, before the splashdown or the landing, so that the incompressibility hypothesis in SFELES is still valid.<p>Two LES models are considered in this chapter, namely a classical and a scale-separation version of the WALE model. Although the capsule geometry is axisymmetric, the flow field in its wake is not and induces unsteady forces and moments acting on the capsule. The characterization of the phenomena occurring in the wake of the capsule and the determination of their main frequencies are essential to ensure the static and dynamic stability during the final stage of the flight. <p>Visualizations by means of 3D isosurfaces and 2D slices of the Q-criterion and the vorticity field confirm the presence of a large meandering recirculation zone characterized by a low Strouhal number, that is St≈0.15.<p>Due to the detachment of the flow at the shoulder of the capsule, a resulting annular shear layer appears. This shear layer is then affected by some Kelvin-Helmholtz instabilities and ends up rolling up, leading to the formation of vortex rings characterized by a high frequency. This vortex shedding depends on the Reynolds number so that a Strouhal number St≈3 is detected at Re=10^4.<p>Finally, the analysis of the force and moment coefficients reveals the existence of a lateral force perpendicular to the streamwise direction in the case of the scale-separation WALE model, which suggests that the wake of the capsule may have some <p>preferential orientations during the vortex shedding. In the case of the classical version of the WALE model, no lateral force has been observed so far so that the mean flow is thought to be still axisymmetric after 100 units of non-dimensional physical time.<p><p>Finally, the last chapter of this work recalls the main conclusions drawn from the previous chapters. / Doctorat en Sciences de l'ingénieur / info:eu-repo/semantics/nonPublished

Turbulent boundary layers approximating those found on the NASA Orion Multi-Purpose Crew Vehicle (MPCV) thermal protection system during atmospheric reentry from the International Space Station have been studied by direct numerical simulation, with the ultimate goal of reducing aerothermodynamic heating prediction uncertainty. Simulations were performed using a new, well-verified, openly available Fourier/B-spline pseudospectral code called Suzerain equipped with a ``slow growth'' spatiotemporal homogenization approximation recently developed by Topalian et al. A first study aimed to reduce turbulence-driven heating prediction uncertainty by providing high-quality data suitable for calibrating Reynolds-averaged Navier--Stokes turbulence models to address the atypical boundary layer characteristics found in such reentry problems. The two data sets generated were Ma[approximate symbol] 0.9 and 1.15 homogenized boundary layers possessing Re[subscript theta, approximate symbol] 382 and 531, respectively. Edge-to-wall temperature ratios, T[subscript e]/T[subscript w], were close to 4.15 and wall blowing velocities, v[subscript w, superscript plus symbol]= v[subscript w]/u[subscript tau], were about 8 x 10-3 . The favorable pressure gradients had Pohlhausen parameters between 25 and 42. Skin frictions coefficients around 6 x10-3 and Nusselt numbers under 22 were observed. Near-wall vorticity fluctuations show qualitatively different profiles than observed by Spalart (J. Fluid Mech. 187 (1988)) or Guarini et al. (J. Fluid Mech. 414 (2000)). Small or negative displacement effects are evident. Uncertainty estimates and Favre-averaged equation budgets are provided. A second study aimed to reduce transition-driven uncertainty by determining where on the thermal protection system surface the boundary layer could sustain turbulence. Local boundary layer conditions were extracted from a laminar flow solution over the MPCV which included the bow shock, aerothermochemistry, heat shield surface curvature, and ablation. That information, as a function of leeward distance from the stagnation point, was approximated by Re[subscript theta], Ma[subscript e], [mathematical equation], v[subscript w, superscript plus sign], and T[subscript e]/T[subscript w] along with perfect gas assumptions. Homogenized turbulent boundary layers were initialized at those local conditions and evolved until either stationarity, implying the conditions could sustain turbulence, or relaminarization, implying the conditions could not. Fully turbulent fields relaminarized subject to conditions 4.134 m and 3.199 m leeward of the stagnation point. However, different initial conditions produced long-lived fluctuations at leeward position 2.299 m. Locations more than 1.389 m leeward of the stagnation point are predicted to sustain turbulence in this scenario. / text