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Analyse expérimentale et numérique du comportement de véhicules terrestres en présence d'un vent latéral instationnaire

Abstract : 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
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Submitted on : Thursday, November 21, 2013 - 3:12:24 PM
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Raffaele Volpe. Analyse expérimentale et numérique du comportement de véhicules terrestres en présence d'un vent latéral instationnaire. Autre [cond-mat.other]. Université de Bourgogne, 2013. Français. ⟨NNT : 2013DIJOS006⟩. ⟨tel-00907635⟩



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