Behavior of the square-back Ahmed body global modes at low ground clearance

The present work aims to study the evolution of the wake-flow dynamic, including the shear-layers interactions phenomena and the global modes behavior of a square-back Ahmed body with a width to height aspect ratio, $w/h$, of 1.346. Various ground-clearance configurations around the critical case associated with the onset of the lateral bistability and corresponding to a critical ground clearance, $g_c/w=0.1$, are investigated. For this purpose, vertical and transversal planar particle image velocimetry (PIV) and concurrent base pressure and force measurements are performed. Results show that the instantaneous wake-flow interactions between the side- or under-body flow and the wake shear layers depend mainly on the under-body flow rate and that a particular attention is required for the experimental under-body flow conditions. Pressure-velocity correlations and instantaneous filtered PIV snapshots analysis highlight the preferential shear-layer vortex shedding associated with the two selected stable and bistable wake-flow configurations. Present investigations underline the transition process from a stable wake configuration $(g/w<0.1)$ to a bistable state flow configuration $(g/w>0.1)$. The antisymmetric periodic global mode is strongly affected around the critical bistable case at the origin of a substantial drag reduction. The oscillation modes varies between stable and bistable states, and then the corresponding Strouhal numbers for the horizontal (respectively, vertical) evolve from 0.16 (respectively, 0.27) to 0.13 (respectively, 0.18). With regard to the present space-time evolution of the wake-flow features, a dynamical interpretation of the different interactions mechanisms is finally proposed for each case with a particular interest to the low-ground-clearance configurations poorly described in the literature.