Modelling of stationary three-dimensional separated air flows around an Ahmed reference model

Abstract

Abstract:When analysing the topology of turbulent flow systems for alternative vehicle configurations, conventional wind-tunnel tests may not be sufficient. Vehicle development teams investigating this kind of physical phenomenon are increasingly interested in numerical simulation, but reliable implementation of computational techniques in automotive aerodynamics requires validation to confirm that such methods are capable of accurately reproducing experimentally proven physical behaviour. We address this issue by comparing the results of experimental wind-tunnel tests with simulated values for aerodynamic drag coefficient and vortex wake flows at different angles of car rear window inclination, using an Ahmed [1] reference model. -INTRODUCTIONTo meet targets for shorter lead-times and lower costs in modern automobile design processes, aerodynamics specialists are constantly seeking new solutions capable of providing prompt and accurate guidance in design practice. One such solution is numerical simulation, which provides a valuable complement to results obtained from experimental wind tunnel tests. However, if computational techniques are to become a viable proposition in automotive aerodynamics, they must demonstrate their ability to accurately reproduce the elementary phenomena observed on simple geometric forms in the wind tunnel. With this objective in mind, we discuss computational work on an Ahmed reference model (Ahmed, Ramm and Faltin, 1984 [1]), following the work of Baxendale, Graysmith, Howell and Haynes (1994 [2]). The Ahmed model was chosen for its geometrical simplicity, and because of the body of documented experimental results available for this model in automotive aerodynamics applications. Specifically, we compare computed results against experimental findings for different angles of rear window inclination, and offer qualitative analysis, chiefly concerning aerodynamic drag coefficient and the behaviour of vortex wake flows. -EXPERIMENTAL RESULTSThe Ahmed reference model is shown in figure 1. The angle α represents the angle of inclination of the rear window with respect to the horizontal.

Simulation du coup de vent latéral Attentes du secteur automobile et état de l'art

“…Plus récemment, des expérimentations numériques ont été effectuées sur le corps de Ahmed en faisant varier l'inclinaison de la lunette arrière [16]. Réalisé à partir de contraintes industrielles, le maillage volumique constitué de prismes et de tétraèdres ne comprend que 5.10 5 éléments.…”

“…Cette géométrie permet de fixer la position des décollements sur l'arrière du véhicule et de s'affranchir localement des limites structurelles des modèles de turbulence associés aux approches de type RANS. Le maillage volumique réalisé à partir de contraintes industrielles est constitué de prismes et de tétraèdres [16]. Il peut comprendre jusqu'à 2 .…”

“…Le nombre de Reynolds de l'écoulement vaut 1,6.10 6 . La métho-dologie s'appuie sur J'utilisation d'un code industriel de type RANS et s'inscrit en prolongement des travaux effectués par Gilliéron et al [16] et Chometon et al [10]. Le modèle de turbulence utilisé est du type k -E avec loi de paroi logarithmique.…”

Modification of Flow Structure Over a Van Model by Suction Flow Control to Reduce Aerodynamics Drag

“…The Ahmed vehicle model is a very simplified bluffbody which is frequently employed as a benchmark in vehicle aerodynamics. It has been used in several experiments [3][4][5][6][7] and numerical studies [8][9][10][11][12][13]. In another computational study, a passenger van is modeled with a modified form of Ahmed's body by changing the orientation of the flow from its original form (modified/reversed Ahmed Body).…”

Passive Control Around the Two-Dimensional Square Back Ahmed Body Using Porous Devices

“…In practise, the flow control is obtained when the wall pressure distribution is successfully modified on the back and on the rear window, using various adapted devices which change locally the geometry. Control experiments in wind tunnel on reduced or real ground vehicles are performed [28] and measurements of the wall static pressures and of the aerodynamic torque [29] allow to quantify the effect of the control [20]. However, due to the design constraints, the real gain is rather weak and so new control techniques have to be developed.…”