Experimental and numerical studies of flow characteristics around a ground vehicle

Flow around ground vehicles is fully three-dimensional and turbulent. Large-scale separation and circulation regions, a complex wake flow, long trailing vortices and the interaction of boundary layer flows can be given as typical characteristics of the flow around ground vehicles. A Thorough understanding of the structure of the flow around ground vehicles is essential for the designer when developing a new road vehicle because the shape of vehicles has a strong influence on the aerodynamic drag, fuel consumption, noise production and road handling. The simplified vehicle shape employed by Ahmed et al. (1984) retains some main features of the flow around real cars including fully three-dimensional large regions of separation elongated downstream of the vehicle, large coherent vortex structures and unsteadiness.Thus, the aim of the present study is to investigate characteristics of flow structures around ground vehicles with the help of Ahmed body using both experimental and numerical methods. Therefore, 1/4 scale Ahmed body having 25o slant angle was employed. The Reynolds number based on the body heights, H and the free stream velocity, U was ReH=1.48x104. Investigations were conducted in two parts. In the first part of the study, particle image velocimetry (PIV) technique was used for the measurement of the instantaneous velocity fields around the Ahmed body. In addition, measurements were performed at several planes which were selected to highlight the aerodynamic characteristics of the body. In the second part of the study, Large Eddy Simulation (LES) method was used to resolve the flow structures around the Ahmed body, numerically. Most of the flow features around the Ahmed body such as the formation of trailing vortices, separation and re-circulatory flows were well predicted. Finally the instantaneous and time-averaged velocity vectors maps, vorticity contours, streamline topology and turbulence characteristics of the flow fields were presented and discussed in details.