Nous nous intéressons au transport turbulent de particules de taille grande devant l’échelle de Kolmogorov. Cette situation se retrouve à la fois dans les écoulements naturels (comme le transport de sédiments) et dans les écoulements industriels (solutés solides dans un mélangeur par exemple). Pour aborder ce problème, nous étudions la dynamique de particules de taille proche de l’échelle intégrale, de densité égale ou légèrement différente de celle du fluide, dans un écoulement turbulent de von Kármán contra-rotatif, à l’aide d’un montage de suivi lagrangien rapide. L’étude de la dynamique rapide des particules montre une diminution forte des fluctuations selon la taille, mais aussi l’apparition d’un phénomène nouveau : à partir d’une certaine taille, les particules n’explorent plus l’écoulement de façon homogène. Cette exploration préférentielle est liée à la structure moyenne de l’écoulement de von Kármán, qui crée une force de piégeage. Cette force devient alors supérieure aux fluctuations des particules quand leur taille dépasse une taille critique. Une étude dans le régime laminaire, où l’écoulement moyen domine largement les fluctuations, a en effet mis en évidence un piégeage fortement accru. Les particules orbitent alors pendant des temps très longs autour des attracteurs stables des particules fluides de l’écoulement laminaire. Même en régime pleinement turbulent, le déplacement des particules entre ces zones s’effectue sur des durées longues, décorrélées des temps de la dynamique turbulente. Nous avons adapté les outils d’analyse pour caractériser cette dynamique et l’avons comparée à celle de particules isodenses dans un écoulement de von Kármán qui possède deux états asymétriques. Nous avons également élaboré un modèle qui reproduit ces caractéristiques dans les cas symétrique et asymétrique. Ces questions sont intimement liées au transfert de masse ou de chaleur entre une particule et l’écoulement. Nous avons donc aussi étudié la fusion de grosses billes de glace en turbulence développée, analysant l’influence de la taille des billes et de la vitesse de glissement sur le transfert thermique, à l’aide d’un montage d’ombroscopie afocale. Nous avons notamment montré que les grosses billes de glace fondent dans un régime ultime de convection forcée lorsqu’elles sont librement advectées par l’écoulement. / We are interested in the turbulent transport of particles whose size is bigger than the Kolmogorov length scale. This issue takes place as much in natural flows (such as sediment transport) as in industrial flows (solid solute in mixer for instance). To tackle this problem, we study the dynamics of particle with size close to the integral length scale, whose density can be neutral or slightly different from the one of the fluid, in a turbulent counter-rotating von Kármán flow, through a fast Lagrangian tracking setup. Studying the fast scale motions, we find out that the fluctuations decrease strongly with particle diameter, but we also discover a new phenomena: particles bigger than a certain size do not sample the flow homogeneously. This preferential sampling is link to the von Kármán mean structure, which applies a trapping force on the particles, overcoming their fluctuations as their size becomes bigger than a critical size. A study in the laminar flow regime, where the mean flow is much greater than the fluctuations, showed an strongly increased trapping effect. The particles indeed orbit for very long times around stable attractors of the fluid particles of the laminar flow. Even in turbulent regime, the motion of the particle between these areas occurs at long times intervals, in a decorrelated way of the turbulent motion. We adapted our analysis tools to characterize this dynamics, comparing it to the one of large neutrally-buoyant particles in a von Kármán flow which presents two asymmetric states. We also designed a model that can reproduce these characteristics in both symmetrical and asymmetrical cases. These issues are tightly linked to mass or heat transfer between a particle and the carrier flow. Therefore, we also studied the melting dynamics of large ice balls in fully developed turbulence, analyzing the impact of particles size and sliding velocity on the turbulent heat transfer, through an afocal shadowgraphy setup. We showed in particular that large freely advected ice balls melt in the ultimate regime of heat transfer.

La turbine basse-pression est un composant essentiel d'un turboréacteur car elle entraine la soufflante qui génère la plus grande partie de la poussée dans la configuration actuelle des turboréacteurs à double flux. Dans la perspective d'accroître son rendement en termes de consommation de carburant, il y a une recherche permanente dans la réduction du nombre d'aubage (c'est-à-dire la réduction de la masse) qui implique un chargement plus élevé par aube de rotor. Cet environnement est caractérisé par un écoulement dont le nombre de Reynolds est faible ainsi qu'une large diffusion le long de la partie aval de l'extrados. Par conséquent, l'écoulement le long de cette surface est potentiellement sujet à une séparation laminaire qui, suivant le statut de la bulle de recirculation, pourrait causer une diminution de la performance aérodynamique (sillages plus larges et plus profonds). La présente thèse de doctorat se concentre sur l'investigation du phénomène de la transition induite par séparation dans les écoulements de turbines basse-pression. L'accent est mis sur les prédictions numériques basées sur une approche CFD RANS utilisant le modèle innovant de transition γ-Reθt à deux équations de transport (la première équation pour l'intermittence numérique et la seconde équation pour le nombre de Reynolds dont la longueur caractéristique est l'épaisseur de quantité de mouvement au début de transition γ-Reθt). Neuf aubes différentes de rotor de turbine basse-pression constituent une base de données de référence et couvrent les plages de fonctionnement de différents nombres de Reynolds de sortie isentropique, de nombres de Mach de sortie isentropique, d'intensités de turbulence d'entrée, avec ou sans sillage provenant d'une rangée d'aubes amont et avec deux configurations de rugosité locale. Une première analyse de cette base de données met en évidence l'effet de la séparation sur le début de la transition et sur les performances. La définition d'une corrélation a été tentée et permet de lier le taux de diffusion d'un aubage au nombre de Reynolds de sortie isentropique à la condition de « Bursting ». Une méthodologie numérique fiable et robuste a été établie afin de prédire la transition dans le cas d'un écoulement amont uniforme. Les résultats sont en bon accord avec les mesures expérimentales même si il a été nécessaire d'adapter les conditions limites dans le but de prédire une séparation laminaire numériquement pour des aubages fortement chargés et fia fort taux de diffusion uniquement. La résolution des profils de vitesse de la couche limite permet d'obtenir une évaluation détaillée des paramètres de la topologie de l'écoulement. Cela fournit une information sur l'épaisseur de quantité de mouvement qui est le paramètre principal définissant les corrélations de transition. La technique « Chimère » des maillages recouvrants est utilisée pour faciliter la modélisation des moyens de contrôle passif pour déclencher la transition. C'est une technique appropriée pour l'implémentation de géométries simples ou plus élaborées. / The Low-Pressure Turbine is a critical component of an Aero-Engine as it drives the Fan which produces most of the thrust in the current turbofan configuration. In order to increase the eficiency in terms of fuel consumption, there is a continuous research for blade count reduction (i.e. mass reduction) which entails a higher loading per rotor blade. It is well-known that this environment is characterised by a low Reynolds number low condition associated with high diffusion along the aft region of the suction side. Consequently, the flow along this surface is prone to laminar separation which, depending on the status of the separation bubble, would lead to detrimental decrease in the aerodynamic performance (larger and deeper wake). The present PhD thesis focuses on the investigation of the separation-induced transition phenomenon occurring in a Low-Pressure Turbine environment. The emphasis is put on the numerical predictions based on a CFD RANS approach using the innovative γ-Reθt transition model based on transport equations for the numerical intermittency (γ) and the transition onset momentum thickness Reynolds number (γ-Reθt). Nine Low-Pressure Turbine rotor blades form a comprehensive experimental reference database and cover a significant range of different isentropic outlet Reynolds numbers, isentropic outlet Mach numbers, inlet turbulence intensity levels, with or without incoming wakes and with two local roughness configurations. A first analysis of this database stresses the effect of the separation on the transition onset and on the performance. A correlation definition is attempted and allows to link the diffusion rate of a blade to the isentropic outlet Reynolds number at bursting. A reliable and robust numerical methodology is established to predict the transition in the case of uniform upstream flow. The results are in good agreement with the experiments even though it was necessary to adapt the boundary conditions to predict the laminar separation numerically for highly-loaded and strong diffusion rate blades only. The resolution of the boundary layer velocity profiles allows to have an in-depth examination of the flow topology parameters. This gives proper information on the momentum thickness which is the main driving parameter of transition correlations. The Chimera technique for overlapping meshes is used to ease the modelling of passive control devices to trigger transition. It is a decent technique to implement standard geometries or more elaborate designs.

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

This thesis deals with the dynamics of a thin liquid film falling down a heated plate. The heating yields surface tension gradients that induce thermocapillary stresses on the free surface, thus affecting the stability and the evolution of the film. Accounting for the coherence of the flow due to viscosity, two main approaches that reduce the dimensionality of the original problem are usually considered depending on the flow rate (as measured by the Reynolds number): the `long wave' asymptotic expansion for small Reynolds numbers and the `integral boundary layer' approximation for moderate Reynolds numbers. The former suffers from singularities and the latter from incorrectness of the instability threshold for the occurrence of hydrodynamic waves. Thus, the aim of this thesis is twofold: in a first part, we define quantitatively the validity of the `long wave' evolution equation (Benney equation) for the film thickness h including the thermocapillary effect; and in a second part, we improve the `integral boundary layer' approach by combining a gradient expansion to a weighted residual method. <p>In the first part, we further investigate the Benney equation in its validity domain in the case of periodically inhomogeneous heating in the streamwise direction. It induces steady-state deformations of the free surface with increased transfer rate in regions where the film is thinner, and also in average. The inhomogeneities of the heating also modify the nature of travelling wave solutions at moderate temperature gradients and allows for suppressing wave motion at larger ones.<p>Moreover, large temperature gradients (for instance positive ones) in the streamwise direction produce large local film thickening that may in turn become unstable with respect to transverse disturbances such that the flow may organize in rivulet-like structures. The mechanism of such instability is elucidated via an energy analysis. The main features of the rivulet pattern are described experimentally and recovered by direct numerical simulations.<p>In the second part, various models are obtained, which are valid for larger Reynolds numbers than the Benney equation and account for second-order viscous and inertial effects. We then elaborate a strategy to select the optimal model in terms of linear stability properties and existence of nonlinear solutions (solitary waves), for the widest possible range of parameters. This model -- called reduced model -- is a system of three coupled evolution equations for the local film thickness h, the local flow rate q and the surface temperature Ts. Solutions of this model indicate that the interaction of the hydrodynamic and thermocapillary modes is non-trivial, especially in the region of large-amplitude solitary waves.<p>Finally, the three-dimensional evolution of the solutions of the reduced model in the presence of periodic forcing and noise compares favourably with available experimental data in isothermal conditions and with direct numerical simulations in non-isothermal conditions.<p><p>------------------------------------------------<p><p>Cette thèse analyse la dynamique d'un film mince s'écoulant le long d'une paroi chauffée. Le chauffage crée des gradients de tension superficielle qui induisent des tensions thermocapillaires à la surface libre, altérant ainsi la stabilité et l'évolution du film. Grâce à la cohérence de l'écoulement assurée par la viscosité, deux approches permettant de réduire la dimensionnalité du problème original sont habituellement considérées suivant le débit (mesuré par le nombre de Reynolds): l'approximation asymptotique dite `longues ondes' pour les faibles nombres de Reynolds et l'approximation `intégrale couche limite' pour les nombres de Reynolds modérés. Cependant, la première approximation souffre de singularités et la dernière de prédictions imprécises du seuil de stabilité des ondes hydrodynamiques à la surface du film. Le but de cette thèse est donc double: dans une première partie, il s'agit de déterminer, de manière quantitative, la validité de l'équation d'évolution `longues ondes' (ou équation de Benney) pour l'épaisseur du film h, en y incluant l'effet thermocapillaire; et dans une seconde partie, il s'agit d'améliorer l'approche `intégrale couche limite' en combinant un développement en gradients avec une méthode aux résidus pondérés.<p>Dans la première partie, nous étudions l'équation de Benney, dans son domaine de validité, dans le cas d'un chauffage inhomogène et périodique dans la direction de l'écoulement. Cela induit des déformations permanentes de la surface libre avec un accroissement du transfert de chaleur dans les régions où le film est plus mince, mais aussi en moyenne. Un chauffage inhomogène modifie également la nature des solutions d'ondes progressives pour des gradients de températures modérés et conduit même à leur suppression pour des gradients de températures plus importants. De plus, ceux-ci, lorsqu'ils sont par exemple positifs le long de l'écoulement, produisent des épaississements localisés du film qui peuvent à leur tour devenir instables par rapport à des perturbations suivant la direction transverse à l'écoulement. Ce dernier s'organise alors sous forme d'une structure en rivulets. Le mécanisme de cette instabilité est élucidé via une analyse énergétique des perturbations. Les principales caractéristiques des structures en rivulets sont décrites expérimentalement et retrouvées par l'intermédiaire de simulations numériques. <p>Dans la seconde partie, nous dérivons une famille de modèles valables pour des nombres de Reynolds plus grands que l'équation de Benney, qui prennent en compte les effets visqueux et inertiels du second ordre. Nous élaborons ensuite une stratégie pour sélectionner le modèle optimal en fonction de ses propriétés de stabilité linéaire et de l'existence de solutions non-linéaires (ondes solitaires), et ce pour la gamme de paramètres la plus large possible. Ce modèle -- appelé modèle réduit -- est un système de trois équations d'évolution couplées pour l'épaisseur locale de film h, le débit local q et la température de surface Ts. Les solutions de ce modèle indiquent que l'interaction des modes hydrodynamiques et thermocapillaires n'est pas triviale, spécialement dans le domaine des ondes solitaires de grande amplitude. Finalement, l'évolution tri-dimensionnelle des solutions du modèle réduit en présence d'un forçage périodique ou d'un bruit se compare favorablement aux données expérimentales disponibles en conditions isothermes, ainsi qu'aux simulations numériques directes en conditions non-isothermes<p> / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished

Les phénomènes de transport en milieux poreux sont étudiés depuis près de deux siècles, cependant les travaux concernant les milieux fortement poreux sont encore relativement peu nombreux. Les modèles couramment utilisés pour les poreux classiques (lits de grains par exemple) sont peu applicables pour les milieux fortement poreux (les mousses par exemple), un certain nombre d’études ont été entreprises pour combler ce manque. Néanmoins, les résultats expérimentaux et numériques caractérisant les pertes de charge dans les mousses sont fortement dispersés. Du fait des progrès de l’imagerie 3D, une tendance émergente est la détermination des paramètres des lois d’écoulement à partir de simulations directes sur des géométries reconstruites. Nous présentons ici l’utilisation d’une nouvelle approche cinétique pour résoudre localement les équations de Navier-Stokes et déterminer les propriétés d’écoulement (perméabilité, dispersion, ...). / A novel kinetic scheme satisfying an entropy condition is developed, tested and implemented for the simulation of practical problems. The construction of this new entropic scheme is presented. A classical hyperbolic system is approximated by a discrete velocity vector kinetic scheme (with the simplified BGK collisional operator), but applied to an inviscid compressible gas dynamics system with a small Mach number parameter, according to the approach of Carfora and Natalini (2008). The numerical viscosity is controlled, and tends to the physical viscosity of the Navier-Stokes system. The proposed numerical scheme is analyzed and formulated as an explicit finite volume flux vector splitting (FVS) scheme that is very easy to implement. It is close in spirit to Lattice Boltzmann schemes, but it has the advantage to satisfy a discrete entropy inequality under a CFL condition and a subcharacteristic stability condition involving a cell Reynolds number. The new scheme is proved to be second-order accurate in space. We show the efficiency of the method in terms of accuracy and robustness on a variety of classical benchmark tests. Some physical problems have been studied in order to show the usefulness of both schemes. The LB code was successfully used to determine the longitudinal dispersion of metallic foams, with the use of a novel indicator. The entropic code was used to determine the permeability tensor of various porous media, from the Fontainebleau sandstone (low porosity) to a redwood tree sample (high porosity). These results are pretty accurate. Finally, the entropic framework is applied to the advection-diffusion equation as a passive scalar.

This work is concerned with posibility of measuring a wind flow by Von Karman vortex sheed structure. The bluff body is situated in the way of air flow propagation and consequentally vortexes will be appeared. Important part of speedmeter design is measurment chamber must allow to vortex sheed propagation. The transient and the reciever are situated vertically to propagation of flow.The Ultrasonic carrier is transmitted and modulated by freqency of vortex sheeding in measurment chamber.Demodulator uses PLL to “focusing“ detection of the ultrasonic beam. This can be indicated like lock and unlock phase loop. From known value of sheed frequency can be directly calculated speed of flow.

This diploma work considers conceptual design of aircraft in accordance in FAR 103 regulation. On the base of statistical dates has been made basic geometrical and aero-dynamical concept of a single occupant aircraft. For conception have been chosen optimal driving force and bases calculations of the load pressure and dimensions are included.

Drag polars for three Micro Air Vehicles (MAVs) were measured at Reynolds numbers of 70,000, 50,000, 30,000, and 10,000 and compared to predictions generated using the classical approach. The MAVs tested had different configurations and aspect ratios varying from 1.2 to 1.6 and ratios of wetted surface area to planform area from 2.6 to 3.9. A force balance was used to measure the lift and drag on the MAVs at angles of attack ranging from -5 degrees (or -10 degrees) to 10 degrees. The force balance allowed the MAVs to rotate in the pitching axis. The MAV angle of attack was set using an elevator installed on the MAV and controlled using a standard radio control used by RC plane enthusiasts. Uncertainty analysis performed on the data showed the uncertainty for high Reynolds numbers was dominated by velocity uncertainty, and uncertainty for the lower Reynolds numbers was dominated by uncertainty in the force measurements. Agreement between measured and predicted drag polars was good with the measured drag never being more than two times the predicted drag. For the majority of the tests, the drag coefficients followed the expected Reynolds number trend: increasing with decreasing Reynolds number.

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

Splitter plates are passive flow control devices for reducing drag and suppressing vortex shedding from bluff bodies. Most studies of splitter plates involve the flow around an “infinite” circular cylinder, however, in the present study the flow around a surface-mounted finite-height circular cylinder, with a wake-mounted splitter plate, was studied experimentally in a low-speed wind tunnel using a force balance and single-component hot-wire anemometry. Four circular cylinders of aspect ratios AR = 9, 7, 5 and 3 were tested for a Reynolds number range of Re = 1.9×10^4 to 8.2×10^4. The splitter plates had lengths, relative to the cylinder diameter, of L/D = 1, 1.5, 2, 3, 5 and 7, thicknesses ranging from T/D = 0.10 and 0.15, and were the same height as the cylinder being tested. The cylinders were partially immersed in a flat-plate turbulent boundary layer, where the range of boundary layer thickness relative to the cylinder diameter was δ/D = 1.4 to 1.5. Measurements were made of the mean drag force coefficient, the Strouhal number at the mid-height position, and the Strouhal number and power spectra along the cylinder height. For all four finite circular cylinders, the splitter plates were effective at reducing the magnitude of the Strouhal number, and weakening or even suppressing vortex shedding, depending on the specific combination of AR and L/D. Compared to the case of an infinite circular cylinder, the splitter plate is less effective at reducing the mean drag force coefficient of a finite circular cylinder. The largest drag reduction was obtained for the cylinder of AR = 9 and splitter plates of L/D = 1 to 3, while negligible drag reduction occurred for the shorter cylinders.