WAKE STRUCTURE AND DRAG OF VEHICLES WITH ROUNDED REAR EDGES

The wake structure at the rear of road vehicle is known to be of prime importance in aerodynamics performances [3]: about 30% of the total pressure drag derives from the rear end of the vehicle. While production vehicle present significant curvature at the rear end, most of fundamental aerodynamic analyses were carried out around simplified car models presenting sharp edges at the rear. Since recently, very few papers addressed the question of rear edges curvature in aerodynamics performances. Thacker et al. Showed that rounding the edge between the end of the roof and the rear slant suppressed the separation over the rear window and resulted in 10% drag reduction. Fuller et al.Studied the effect on spatial stability and intensity of the pillar vortex when rounding the side rear pillars. For both of these works, the analysis was focused on the flow behaviour over the rear window: the impact of the rear end rounding on the near wake topology was not discussed. The current study aims to understand how the use of rounded pillars with respect to sharp edges modifies the flow field (over the body surface and in the near wake) and hence the global drag. Moreover, an “academic” and an “industrial” model will be characterized to discuss the applicability of simplified models to simulate properly the sensitivity of pillars rounding

Aerodynamic performances of rounded fastback vehicle

Experimental and numerical analyzes were performed to investigate the aerodynamic performances of a realistic vehicle with a different afterbody rounding. This afterbody rounding resulted in a reduction to drag and lift at a yaw angle of zero, while the crosswind performances were degraded. Rounding the side pillars generated moderate changes to the drag and also caused important lift reductions. A minor effect on the drag force was found to result from the opposite drag effects on the slanted and vertical surfaces. The vorticity distribution in the near wake was also analyzed to understand the flow field modifications due to the afterbody rounding. Crosswind sensitivity was investigated to complete the analysis of the aerodynamic performances of the rounded edges models. Additional tests were conducted with geometry modifications as spoilers and underbody diffusers

Contribution à l'amélioration de la simulation numérique du décrochage dynamique sur une éolienne à axe vertical

De meilleurs outils numériques de simulation du comportement et des performances des éoliennes à axe vertical sont devenus essentiels au développement de solutions de production décentralisée d’électricité. Le rapport de solidité élevée de l’éolienne à axe vertical considérée ici engendre un fonctionnement à faible vitesse réduite et la présence de tourbillons de décrochage dynamique pour lesquels la simulation numérique est difficilement accessible à la modélisation. On utilise un modèle tourbillonnaire bidimensionnel basé sur des surfaces portantes pour représenter les éléments de pale et des particules lagrangiennes porteuses de tourbillon pour reproduire le sillage. Le phénomène de décrochage dynamique y est incorporé par un modèle global dont il faut ajuster le comportement. Dans ce but, des essais en soufflerie sur une maquette d’éolienne Darrieus tripale ont été réalisés. L’objectif est d’analyser la dynamique du décollement de la couche limite pour des vitesses réduites de 1,0 à 2,0. Des mesures instationnaires de répartition de pression à mi-envergure des pales ont été effectuées à l’aide de capteurs de pression déportés aux extrémités des pales. Les variations de la pression donnent des indications sur les effets des lâchers tourbillonnaires, leur position azimutale et leur localisation sur la pale. Des mesures de vitesse par image de particules (PIV) ont été réalisées simultanément aux mesures de pression pour déterminer le champ de vitesse dans le sillage proche de la maquette, à mi-envergure des pales, à une distance de un à deux rayons en aval de l’axe de rotation. Ces informations sur la dynamique tourbillonnaire permettent l’amélioration de la modélisation numérique des effets induits sur les pales

Drag and crosswind sensitivity of rounded fastback vehicle

Experimental and numerical analyzes were performed to investigate the aerodynamic performances of a realistic vehicle with a different afterbody rounding. This afterbody rounding resulted in a reduction to drag and lift at a yaw angle of zero, while the crosswind performances were degraded. Rounding the side pillars generated moderate changes to the drag and also caused important lift reductions. A minor effect on the drag force was found to result from the opposite drag effects on the slanted and vertical surfaces. The vorticity distribution in the near wake was also analyzed to understand the flow field modifications due to the afterbody rounding. Crosswind sensitivity was investigated to complete the analysis of the aerodynamic performances of the rounded edges models

Influence of afterbody rounding on the near wake of fastback vehicle

Experimental and numerical analyzes were performed to investigate the aerodynamic performances of a realistic vehicle with different afterbody rounding. Rounding the side pillars generated moderate changes in drag and important lift reductions. The minor effect on the drag force was found to result from opposite drag effects on the slanted and vertical surfaces. The vorticity distribution in the near wake was also analyzed to understand the flow field modifications due to afterbody rounding

6es Rencontres FORMIST - 2006

Actes des 6èmes rencontres FORMIST qui se sont déroulées le 15 juin 2006 à l\u27ENSSIB. Tour d\u27horizon des usages des nouvelles technologies de l\u27information par les étudiants : pratiques documentaires, outils bibliographiques, aides et formation à la recherche d\u27information, réseaux sociaux. Synthèses sur des outils particuliers : cartes heuristiques, wikis, flux RSS, blogs, etc

Experimental study of wheel-vehicle aerodynamic interactions

International audienceThis work aims to provide a better understanding of the wheel-vehicle aerodynamic interaction, via experimental investigations on a 2/5-th scale vehicle equipped with 2/5-th scale rotating wheels. The findings demonstrate that the baseline configuration has a well-balanced wake, which means that the distribution of TKE in the symmetrical plane of the vehicle wake is symmetric in the upper and lower shear layers.This well-balanced wake can be easily modified by different wheel states or tire modifications, especially at the rear axle. This results from a global effect of the underbody momentum modifications. On the contrary, when the vehicle mean wake develops into a non-balanced topology, it is more robust towards underbody perturbations. By eliminating front wheels, the underbody flow momentum is vastly increased; by eliminating the underbody diffuser, the underbody flow momentum is significantly reduced. In these two circumstances, one can observe a robust downwash from the roof, independent of the wheel states or tire modifications. Besides, there is a more local effect of the wheels’ near wakes on the aerodynamic lift and drag of the vehicle. Low pressure regions in the underbody downstream the wheels have an effect on vehicle lift. The rear wheel wakes interact with vehicle wake, imposing pressure conditions on the vehicle base, therefore influencing the vehicle drag

Analyse expérimentale d'un modèle de tourbillon conique et de sa sensibilité à une turbulence amont

La recherche expérimentale, présentée dans cette thèse, a pour objectif l'étude de l'influence d'un écoulement amont turbulent ou non sur les tourbillons coniques (primaire et secondaire) engendrés à partir d'une configuration de montant de baie. Cette configuration d'écoulement est rencontrée sur les véhicules terrestres et l'interaction des tourbillons avec les vitres latérales du véhicule est le principal responsable du bruit aérodynamique transmis dans le véhicule. Une maquette du type dièdre a été conçue et testée en soufflerie afin de générer et d'étudier ces tourbillons coniques. Les propriétés spatiales et temporelles des champs de vitesse et de pression pariétale ont été étudiées en associant simultanément la mesure de la vitesse par PIV ou Stéréo-PIV à la mesure de pression pariétale fluctuante par capteurs déportés. Nous avons montré qu'un niveau modéré de turbulence amont (intensité : 4%) se traduit par une modification très importante de la dynamique du tourbillon et de la pression pariétale fluctuante associée (Cp'). Notre conjecture - basée sur l'étude des moments statistiques et des corrélations spatio-temporelles - est qu'en l'absence de turbulence externe, la contribution principale au Cp' est liée à l'empreinte du cœur tourbillonnaire et du décollement secondaire induit. Au contraire, en présence de turbulence amont, la forte augmentation du Cp' semble associée à une réponse globale de la structure tourbillonnaire et à la modification associée de l'aérodynamique autour du corps. L'inclusion de dispositifs géométriques simples de contrôle passif sur la maquette a fait objet d'essais supplémentaires. Nous avons montré que ces dispositifs permettent de modifier très significativement le développement de la structure tourbillonnaire et son empreinte en terme de pression pariétale fluctuante.The experimental research presented in this thesis aims to study the influence of an upstream flow with and without free stream turbulence (FST) on conical vortices (primary and secondary) generated from a A-pillar strut. This flow configuration is encountered on land vehicles and the interaction of the A-pillar vortices with the side windows is the main contributor to aerodynamic noise transmitted in the car. A model of dihedral type was designed and tested in a wind tunnel in order to generate these vortices. Spatial and temporal properties of velocity and wall pressure fields were studied using simultaneously PIV (or stereo-PIV) measurements and fluctuating wall pressure measurements using off-set sensors. It has been shown that moderate free-stream turbulence (intensity : 4%) leads to a very important modification of the shape of the A-pillar vortex and of the resulting wall pressure field (Cp'). Our conjecture - based on the study of statistical moments and spatio-temporal correlations - is that without FST, the main contribution to Cp' is linked to the local contribution of the 3D vortex core and induced separation. On the contrary, Cp', in presence of FST, seems to be associated with a global response of the 3D separation and induced aerodynamic field around the body. The difference observed with and without FST in the pressure and velocity fields have a significant impact on the Cp' and means that these contributions might be transmitted in very different ways by the car structure because the frequency and length scale range are very distinct. The inclusion of simple geometric devices on the model has been further tested. We have shown that these devices modify very significantly the development of the conical vortex and of its fluctuating pressure footprint.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Influence of afterbody rounding on the pressure distribution over a fastback vehicle

International audienceExperimental analyzes were performed to understand the drag evolution and the flow field modifications resulting from afterbody rounding on the Ahmed body, a simplified vehicle model with 25 degrees rear slant. Curvature effects were investigated using balance measurements, flow visualizations, wall pressure, and particle image velocimetry measurements. The rear end of the original well-known Ahmed body has sharp connections between the roof and the rear window as well as squared rear pillars. Similarly to previous studies, rounding the roof/backlight intersection was shown to reduce drag up to 16 %. Surprisingly, additional rear curvature associated with side pillar rounding did not further modify the drag. However, the zero net effect was found to result from opposite drag effects on the slanted and vertical surfaces and to hide strong local modifications on the flow field. The tridimensional organization and vorticity transport in the near wake were analyzed and connected to the observed local increase in the pressure drag on the base. Finally, these results were shown to be generic for a realistic rounded-end car shape