The effect of inflow turbulence on wake symmetry breaking behind a three-dimensional bluff body*

In recent years, more and more researches have been carried out on the bi-stable wake behind a square back Ahmed body. This bi-stability is a long time scale of the order of 1000H/U_∞ flow phenomenon. It represents the random shift of the recirculation region between two reflectional symmetry-breaking positions which is mutually symmetric with respect to the vertical central plane. However, previous studies on the bistable wake has focused on testing with uniform freestream conditions and neglected real-world road turbulence effects. In this work, the effect of inflow turbulence on the bi-stable wake behind a square back Ahmed body is numerically explored by IDDES. The turbulence intensity (I≈2%~10%) and the integral length scale of turbulence (L_x≈0.38H~2H) are varied using the Synthetic Eddy Method. The results shown that the boundary layer developing around the body together with the structure of the wake is strongly altered by the free-stream turbulence where both the length of the recirculation and the shear layer characteristics are modified, leading to a complex variation trend of the drag coefficient C_D. When the length scales of the fluctuations fully encompass the length scales of the relevant wake flow structures behind the body (L_x≈1H~2H), increasing the inflow turbulence intensity will gradually accelerate the switching of bi-stability wake. In addition, the drag coefficient C_D rises with the increase of the turbulence intensity. For the small integral length scale (L_x≈0.38H), a weakly effect on both the switching frequecy of bi-stability wake and C_D has been noted until the turbulence intensity is high enough to change the boundary layer at the rear edge and the shear layer dramatically. These findings are of major importance for guiding the design of the next generation of control strategies for drag reduction, most of which neglect the influence of inflow conditions.