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The aerodynamic characteristics of a vehicle have a significant role on its dynamics performances, fuel consumption, level of produced noise etc. In this sense the automotive engineers design vehicle having some much elaborated exterior shape, the main goal being the reduction of drag. But, in many cases, the geometry of the underbody was neglected from this point of view, especially in the cases of the off-road vehicles, which have recently become more versatile, as seen in Sport Utility Vehicles (SUV). In this paper, using a SUV as example, the contribution of the underbody geometry on total drag is studied using the facilities offered by a professional CFD code. The results show that solutions for the optimization of the body of vehicles from an aerodynamic point of view can be obtained even in a very early design stage, accelerating the design process.

In this paper, using the facilities offered by the ANSYS CFX, CFD code, the authors investigate numerically the flow around the Ahmed body for the rear slanted upper surface of 35°, fitted with a simple underbody diffuser, without endplates, in order to find the influence of the later one on the main aerodynamic characteristics, drag and lift. Relative motion between body and ground is simulated. The study is performed for different geometrical configurations, length and the angle of the diffuser being the parameters which are varied in ranges which are relevant for hatchback passenger cars. Later, based on a theoretical approach, a coefficient of the equivalent hydraulic resistance of the diffuser is computed, which helps to evaluate the drag due to underbody diffuser.

Being a continuous subject of research, the authors present in this paper new aspects and results concerning the relevance of diffusers in vehicle aerodynamics. In the first stage of investigation, a generic car model with wheels was considered, starting from the Ahmed body, and the effect of adding wheels to the body was studied. Latter, the influence of diffuser length and angle, within ranges relevant to hatchback passenger cars, on drag and lift was studied for the Ahmed body with wheels. The results show that the addition of wheels and wheelhouses results in increasing of drag and lift due to the vortices which originate from the wheels. Concerning the underbody diffuser, the results show that both drag and lift of the bluff bodies with wheels are reduced with increasing diffuser length for moderate values of angle, up to six degrees for the cases studied.

Because the aerodynamic loads, which are acting on the high-speed vehicles, play a significant part concerning the dynamic behaviour of the latter, the aerodynamics is one of the most important design considerations for cars such as Indy or Formula 1. The goal of this study is to investigate the contributions of the main structural components of a racing car on total drag using the facilities offered by a professional CFD code. The results show solutions for the optimization of the body of vehicles from an aerodynamic point of view, even in a very early design stage, the design process being accelerated.