Wake transitions and steady z-instability of an Ahmed body in varying flow conditions

The paper investigates experimentally the flow past a flat-back, taller than wide Ahmed body having rectangular base aspect ratio H/W = 1.11 in the context of ground vehicle aerodynamics. Parametric studies at Reynolds number 2.1 x 10(5) explore the sensitivity of the aerodynamic force and body pressure distribution with respect to varying flow conditions defined from variable ground clearance C (taken at mid-distance from the front and rear axles of the body), pitch angle alpha, and yaw angle beta (equivalent to a crosswind). Two-dimensional parametric maps in the parametric spaces (beta, C) and (beta, alpha) are obtained for parameter ranges covering real road vehicle driving conditions. The study of the base pressure gradient reveals non-trivial and sharp transitions identified as significant changes of near wake orientation in both parametric spaces. All unsteady transitions correspond to fluctuation crises of the vertical gradient component only. These transitions are summarized in phase diagram representations. Both phase diagrams in (beta, C) and (beta, alpha) parametric spaces can be unified at large yaw in the (beta, C-r) space, where the rear clearance C-r separating the lower edge of the base from the ground is a simple function of the pitch alpha and the clearance C. The impacts of the wake transitions are clearly identified in the base drag, drag, lift and side force coefficients. The contribution of the steady near wake z-instability (Grandemange et al., Phys. Fluids, vol. 25, 2013a, pp. 95-103) is assessed by repeating the sensitivity analysis with a rear cavity. As reported previously, the rear cavity suppresses the steady instability by symmetrizing the wake. A domain for the existence of the instability is finally proposed in the attitudes map (beta, alpha) defined from regions where the mean lateral force coefficients are significantly decreased by the presence of the rear cavity. In addition, it is found that the steady instability forces the wake to be less unsteady for all attitudes that do not correspond to unsteady transitions.