Relatively few studies have been carried out on the turbulent wake structure of a finite circular cylinder and a stack partially immersed in a flat-plate turbulent boundary layer. There is a need to develop a better understanding of the wakes of these structures, since they have many important engineering applications. This thesis investigates the influence of the aspect ratio on the wake of a finite circular cylinder and the effects of the ratio of jet flow velocity to crossflow velocity (velocity ratio, R) on the wake of a stack in a cross-flow. <p>The wake characteristics of flows over a finite circular cylinder at four different aspect ratios (AR = 3, 5, 7 and 9) were investigated experimentally at a Reynolds number of ReD = 6104 using two-component thermal anemometry. Each cylinder was mounted normal to a ground plane and was either completely or partially immersed in a flat-plate turbulent boundary layer. The ratio of boundary layer thickness to the cylinder diameter was 3. <p>A similar turbulent wake structure (time-averaged velocity, turbulence intensity, and Reynolds shear stress distributions) was found for the cylinders with AR = 5, 7, and 9, while a distinctly different turbulent wake structure was found for the cylinder with AR = 3. This was consistent with the results of a previous study that focused on the time-averaged streamwise vortex structures in the wake. In addition, irrespective of the value of AR, high values were observed for the skewness and flatness factors around the free end of the cylinders, which may be attributed to the interaction of the tip vortex structures and downwash flow that dominates this region of the cylinder.<p>The wake characteristics of a stack of aspect ratio AR = 9 were investigated using both the seven-hole pressure probe and thermal anemometry. The seven-hole probe was used to measure the three components of the time-averaged velocity field, while the thermal anemometry was used to measure two components of the turbulent velocity field at various downstream locations from the stack. The stack was mounted normal to the ground plane and was partially immersed in a flat-plate turbulent boundary layer, for which the ratio of boundary layer thickness to the stack diameter was 4.5. In addition, measurements of the vortex shedding frequency were made with a single-component hot-wire probe. The cross-flow Reynolds number was ReD = 2.3 x 104, the jet Reynolds number ranged from Red = 7.6 x 103 to 4.7 x 104, and R was varied from 0 to 3. <p>In the stack study, three flow regimes were identified depending on the value of R: the downwash (R < 0.7), cross-wind-dominated (0.7 < R < 1.5), and jet-dominated (R ≥ 1.5) flow regimes. Each flow regime had a distinct structure for the time-averaged velocity and streamwise vorticity fields, and turbulence characteristics, as well as the variation of the Strouhal number and the power spectrum of the streamwise velocity fluctuations along the stack height. The turbulence structure is complex and changes in the streamwise and wall-normal directions within the near and intermediate stack and jet wakes. In the downwash and crosswind-dominated flow regimes, two pairs of counter-rotating streamwise vortex structures were identified within the stack wake. The tip-vortex pair and base-vortex pair were similar to those found in the wake of a finite circular cylinder, located close to the free end and the base of the stack (ground plane), respectively. In the jet-dominated flow regime, a third pair of streamwise vortex structures was observed, referred to as the jet-wake vortex pair, which occurred within the jet-wake region above the free end of the stack. The jet-wake vortex pair has the same orientation as the base vortex pair and is associated with the jet rise.

Relatively few studies have been carried out on the turbulent wake structure of a finite circular cylinder and a stack partially immersed in a flat-plate turbulent boundary layer. There is a need to develop a better understanding of the wakes of these structures, since they have many important engineering applications. This thesis investigates the influence of the aspect ratio on the wake of a finite circular cylinder and the effects of the ratio of jet flow velocity to crossflow velocity (velocity ratio, R) on the wake of a stack in a cross-flow. <p>The wake characteristics of flows over a finite circular cylinder at four different aspect ratios (AR = 3, 5, 7 and 9) were investigated experimentally at a Reynolds number of ReD = 6104 using two-component thermal anemometry. Each cylinder was mounted normal to a ground plane and was either completely or partially immersed in a flat-plate turbulent boundary layer. The ratio of boundary layer thickness to the cylinder diameter was 3. <p>A similar turbulent wake structure (time-averaged velocity, turbulence intensity, and Reynolds shear stress distributions) was found for the cylinders with AR = 5, 7, and 9, while a distinctly different turbulent wake structure was found for the cylinder with AR = 3. This was consistent with the results of a previous study that focused on the time-averaged streamwise vortex structures in the wake. In addition, irrespective of the value of AR, high values were observed for the skewness and flatness factors around the free end of the cylinders, which may be attributed to the interaction of the tip vortex structures and downwash flow that dominates this region of the cylinder.<p>The wake characteristics of a stack of aspect ratio AR = 9 were investigated using both the seven-hole pressure probe and thermal anemometry. The seven-hole probe was used to measure the three components of the time-averaged velocity field, while the thermal anemometry was used to measure two components of the turbulent velocity field at various downstream locations from the stack. The stack was mounted normal to the ground plane and was partially immersed in a flat-plate turbulent boundary layer, for which the ratio of boundary layer thickness to the stack diameter was 4.5. In addition, measurements of the vortex shedding frequency were made with a single-component hot-wire probe. The cross-flow Reynolds number was ReD = 2.3 x 104, the jet Reynolds number ranged from Red = 7.6 x 103 to 4.7 x 104, and R was varied from 0 to 3. <p>In the stack study, three flow regimes were identified depending on the value of R: the downwash (R < 0.7), cross-wind-dominated (0.7 < R < 1.5), and jet-dominated (R ≥ 1.5) flow regimes. Each flow regime had a distinct structure for the time-averaged velocity and streamwise vorticity fields, and turbulence characteristics, as well as the variation of the Strouhal number and the power spectrum of the streamwise velocity fluctuations along the stack height. The turbulence structure is complex and changes in the streamwise and wall-normal directions within the near and intermediate stack and jet wakes. In the downwash and crosswind-dominated flow regimes, two pairs of counter-rotating streamwise vortex structures were identified within the stack wake. The tip-vortex pair and base-vortex pair were similar to those found in the wake of a finite circular cylinder, located close to the free end and the base of the stack (ground plane), respectively. In the jet-dominated flow regime, a third pair of streamwise vortex structures was observed, referred to as the jet-wake vortex pair, which occurred within the jet-wake region above the free end of the stack. The jet-wake vortex pair has the same orientation as the base vortex pair and is associated with the jet rise.

L’aérodynamique latérale des véhicules automobiles suscite de nos jours de plus en plus d’intérêt de la part des constructeurs. L’automobiliste est en effet soumis quotidiennement à de forts courants d’air latéraux, que ce soit lors du dépassement d’un autre véhicule, ou alorsen passant dans un couloir de vent du à la topographie du terrain (passage devant un espace entre deux immeubles par exemple). Les efforts aérodynamiques mis en jeu dans ces situations peuvent provoquer des mouvements non désirés du véhicule, pouvant avoir des conséquences dramatiques si le conducteur se laisse surprendre. Des études expérimentales reproduisant les effets d’un dérapage dynamique ont mis en évidence des phénomènes transitoires importants mettant à défaut les modèles stationnaires généralement pratiqués par les constructeurs pour qualifier le comportement de leurs véhicules en présence de dérapage. Les mécanismes responsables de ces phénomènes transitoires sont encore mal connus de la communauté scientifique. Ce travail propose d’approfondir ce sujet au travers de l’étude de l’aérodynamique d’un véhicule terrestre fixe soumis à un vent longitudinal et à une rafale de vent latéral. Le but principal est d’identifier les structures tourbillonnaires au moyen de mesures PIV et de calculs numériques des champs de vitesse autour d’une maquette automobile et de les corréler aux efforts aérodynamiques. Un accord entre l’ISAT, composante de l’Université de Bourgogne, et l’Institut Supérieurde l’Aéronautique et de l’Espace (ISAE) de Toulouse a permis de mener l’étude avec les ressources de cet établissement. Le moyen d’essai principal, créé par l’ISAE, est le banc« rafale latérale », constitué d’une soufflerie principale et d’une soufflerie secondaire, dont la sortie à volet déferlants (« Mexican Wave ») est inspirée de l’approche proposée par Ryan et Dominy (2000). L’analyse expérimentale a été effectuée à l’aide de la PIV résolue en temps et stéréoscopique, et d’une balance dard instationnaire à cinq composantes. Un banc« numérique » identique a été constitué à l’aide du logiciel FLUENT©, pour des calculs 3D. De plus, un modèle 2D annexe, basé sur la méthode « meshless », a été développé pour de futures investigations, en raison de sa robustesse pour des problèmes à fortes discontinuités et sa bonne adaptabilité aux problèmes avec frontières mobiles.Une première phase de ce travail a consisté en la mise au point des bancs expérimental et numérique, avec génération d’un champ de dérapage homogène, de 21° dans la zone de mesure. L’évolution du dérapage en chaque point respecte bien la forme d’un créneau imposé par la rafale. Pour l’analyse des efforts, deux géométries de maquette ont été étudiées, à savoir un corps de Windsor à culot droit générant, pour un écoulement longitudinal, des structures de sillage bidimensionnelles, et son homologue à culot incliné de 25°, générant des tourbillons « cigare ». Des pics d’efforts ont été observés à l’arrivée de la rafale, tout comme la littérature le prédit. Pour ce qui est du coefficient du moment de lacet, les sursauts sont de 29 % et 19 % respectivement pour la maquette à culot droit et celle à culot incliné, par rapport aux valeurs stationnaires. Concernant le coefficient de force de dérive, ils sont de 10 % et 14 %, respectivement. Lors de nos essais, ces efforts se sont établis après 5.5 longueurs de maquette. Afin d’expliquer la différence de comportement entre les deux maquettes en termes d’efforts, l’évolution temporelle des tourbillons nommés, dans ce mémoire, ΓA, ΓB, ΓC et Γ1 à été discutée. Il en est ressorti une forte corrélation entre la circulation du tourbillon ΓA, le plusénergétique, naissant à l’avant du flanc sous le vent de la maquette, et les efforts latéraux, de sorte que ce tourbillon serait le meilleur témoin des phénomènes instationnaires mis en jeu dans l’étude de l’effet du vent latéral. [...] / The automotive manufacturers are nowadays more and more interested in crosswind aerodynamics. Indeed, the driver is subjected every day to strong side air flows, for example when overtaking another vehicle or when passing through a lateral wind wall, generated by terrain topography (as in the case of the passage near the empty space between two buildings).The aerodynamic efforts generated in these situations can lead to undesired lateral deviations,which can be dramatic if the driver is surprised. Different experimental studies, reproducing the effects of a dynamic yaw angle, pointed out the issues of the steady methods, commonly used to qualify the vehicle crosswind behaviour. Little is still known about the physics behind these unsteady phenomena. This is the main topic of this work, by studying the aerodynamics of a fixed vehicle subjected to both a longitudinal flow and a side wind gust. The goal is the identification of the near-vehicle vortex structures, by means of PIV measurements and CFD calculations, and their correlation with the evolution of the efforts. An agreement between the ISAT, a department of the University of Burgundy, and the ISAE of Toulouse, permitted to carry out this research with the resources of the latter laboratory. The work focuses on the use of the “rafale latérale” (side gust) test bench, made up with a main wind tunnel connected with an auxiliary one by means of a shutter system,whose opening is held by a “Mexican Wave” law. This approach is inspired by the work of Dominy and Ryan (2000). The experimental analysis was carried out by means of Time-Resolved and stereoscopic PIV, and by a five components unsteady balance as well. Anidentical test bench was numerically reproduced with the 3D CFD software FLUENT©.Moreover, an additional 2D CFD model, based on the meshless method, has been developed for future studies. This kind of approximation method has been chosen for its robustness innon-continuous problems and because of its adaptability when moving boundaries are needed.The first phase of this work consisted on wind tunnels set-up, both for the real test bench and for the CFD model. The yaw angle field is homogeneous, 21° in the measurement region. The yaw angle evolution, at given point, respects the step wise behaviour, imposed by the gust passage. As far as the efforts are concerned, two versions of the Windsor body car model were studied, that is a squareback geometry, generating, for longitudinal flows, 2D wakestructures, and a fastback geometry (rear window inclined by 25°), producing cone-liketrailing vortices. Force overshoots were seen after the gust arrival, as seen in literature. In particular, the yaw moment coefficient overshoots are 29% and 19% higher than the steady yaw angle tests, for the squareback and the fastback geometries, respectively. If the side forceis concerned, the entities of these overshoots are 10% and 14%, respectively. Our testspointed out that efforts establish after the vehicle has driven 5.5 times its length in thecrosswind. In order to explain the different behaviour of the two geometries, it is discussed about the unsteady evolution of the vortices called, in this report, ΓA, ΓB, ΓC et Γ1. A strong correlation between the side efforts and the circulation of the most energetic vortex, ΓA,having its origin in the front leeward side of the vehicle. The ΓA vortex is so the best index for the study of the crosswind unsteady phenomena. The coupled analysis between vortex structures and efforts confirmed the presence of a higher side force for the squareback geometry. The inverted effect has been observed for the yaw moment