Unsteady Experimental Characterization of the Natural Wake of a Squareback Ahmed Model

The increasing demand for such vehicles as station wagons or minivans has focused aerodynamic research for drag reduction on squareback models of which, the one proposed by Ahmed et al. [1] is one of the most used. The presented study aims at finely characterize the natural wake of a 1:1 squareback Ahmed body with experimental measurements, in order to draw possible strategies for drag reduction. Our analysis focused both on the detection of the mean wake topology, by means of 2D PIV as well as parietal pressure measurements, and on the study of the spectral fluctuations of velocity and pressures. The experimental results exhibit a left/right bistable behavior for the wake, highly sensitive to the incoming wind yaw angle, so that, most generally, the mean aerodynamic field shows an asymmetric behavior due to the predominance of one of the two stable positions. The characterization of the mean wake by averaging the fields on the whole acquisition time can then be biased and show an asymmetrical wake. Then, as far as the symmetric average field is of interest, it is relevant to study separately each topology using conditional averaging, and then build a symmetrical field by averaging the two bistable phases. The analysis shows that the well-known symmetrical wake and the corresponding rear pressure field derive from a random rotation of the toric recirculation bubble, which can induce instantaneous pressure losses of about 25%. The bistability does not affect the spectral energy distribution. Our results show a spectral activity mostly at low frequencies. In particular, we found in measurements along the vertical symmetry plane a peak at St_H = 0.07, indicating the presence of the “bubble pumping” phenomenon, whereas pressure measurements at vehicle mid-height indicate the existence of vortex shedding.