Bi-stability in the turbulent wake past parallelepiped bodies with various aspect ratios and wall effects

Abstract

The turbulent wake past parallelepiped bodies with a rectangular blunt trailing edge of height H and width W is investigated in wall proximity: various aspect ratios H* = H/W ∈ [0.51, 1.63] and ground clearances C* = C/W ∈ [0, 1.00] are explored at a Reynolds number of 4.5 × 104 based on the body width W. Base pressure measurements and particle image velocimetry show that the close wake often undergoes antisymmetric instabilities that can be either in the lateral direction (parallel to the wall) or in the vertical direction (normal to the wall). The instantaneous wake presents preferred states with high degrees of asymmetry; in some configurations, topology shifts are observed after long time scales Tl ∼ 103W/U0 leading to bistable behaviors. The effect of ground proximity is thoroughly studied for H* = 0.74 corresponding to the reference Ahmed geometry and for a case with a height dimension larger than its width H* = 1/0.74 = 1.34. When C* > 0.08, it is found that the Ahmed body is bistable in the lateral direction while the wake past the second geometry develops an instability in the vertical direction. In both cases, the instabilities vanish for sufficiently small values of ground clearance (C* < 0.08) which are associated with the apparition of a detachment of the underbody flow on the ground. However, the wall proximity does not necessarily stabilize the wake in the plane of symmetry since the flow retrieves the bi-stability in the lateral direction for C* < 0.03 and H* < 0.65. A general criterion for the existence of the instabilities is deduced from the parametric study in the domain (H*, C*) together with symmetry considerations.