Aerodynamic drag modification induced by free-stream turbulence effects on a simplified road vehicle

We report an extensive experimental investigation into the effects of inflow turbulence on a simplified road vehicle, the so-called square back Ahmed body. Variations reaching up to +16% and −17% of the drag coefficient are observed for free-stream turbulence representative of open-road conditions [J. W. Saunders and R. B. Mansour, “On-road and wind tunnel turbulence and its measurement using a four-hole dynamic probe ahead of several cars,” SAE Trans. 109, 477 (2000)]. Regular turbulence grids are mounted upstream the Ahmed body. The turbulence intensity and the integral length scale of turbulence are varied using different mesh, bar sizes, and solidity. The boundary layer developing around the body together with the structure of the wake is strongly altered by free-stream turbulence where both the length of the recirculation and the shear layer characteristics are modified. A weakly non-parallel stability analysis of the shear layers together with a momentum budget, both bounding the recirculation region, shows that coherent structures, traced through the Reynolds stresses and streamwise turbulent fluctuations, are the key mechanisms that control drag. Subsequently, the analysis of the shear layer together with the stability analysis demonstrate that the mean vertical shear is the key component that controls the Reynolds stresses and thereby the drag experienced by the vehicle. These findings raise the question of the importance of free-stream turbulence when considering studies dedicated to car aerodynamics and subsequent control strategies, most of which neglect the influence of inflow conditions. This issue is also of major importance for guiding the design of the next generation of control strategies for drag reduction.