Underbody flow control for base drag reduction of a real car model

A 1:5 scale realistic car model of the original Twingo GT but presenting a flat underbody and no exhaust line is tested in a wind tunnel at Reynolds numbers $Re=2.15\times 10^5$ to $4.3\times 10^5$. A range of underbody flow characteristic velocities $U_b=\left(0.5-0.72\right)U_{\infty }$ ($U_{\infty }$ the free-stream velocity) is obtained by two techniques: flow obstruction and low momentum injection. Force balance, pressure measurements and Particle Image Velocimetry are used to characterize the aerodynamics of the model while changing the underbody flow velocity. A very sharp transition in the wake is found at a critical underbody velocity $U_b=0.65U_{\infty }$. It corresponds to a sudden wake reversal with a bistable behavior between 2 equilibrium states, N or P depending on the vertical base pressure gradient or the wake orientation. The drag of the N state is larger than that of the P state. The control of the wake state by reducing the underbody flow velocity leads to beneficial increase of the base suction of approximately 20% when selecting the P state compared to the N state. The low momentum injection technique reduces drag by 10% but is ineffective at yaws 5° and 10°, while the obstruction technique consistently increases base suction but induces additional drag.