A review on the asymmetry, steady instability and bistability in the wake of industrial ground vehicles

Since the first observation of steady symmetry breaking in the turbulent wakes of a square-back Ahmed body by Grandemange et al. (2012), there have been reports of similar effects with ground vehicles of more complex geometries with square-back style, including real vehicles. The article reviews these cases of industrial flows, either at small or full scale in wind tunnels or numerical simulations with and without road effects. A clear consensus appears of asymmetric recirculating flows in the vertical direction that can, with well chosen parametric variations, reverse to the opposite asymmetry and sometimes lead to spectacular vertical bistable dynamics. These global changes of the base flow impact the body drag and lift. The underlying universal property of these wakes is likely the same steady instability as that of the square-back Ahmed body. • Vehicles show opposite asymmetric mean wakes or states in the vertical direction. • The state’s asymmetry is associated with a pair of longitudinal vortices. • States reversal occur with small manipulations of attitudes or air intake. • Fine tuning can trigger bistable dynamics involving the two asymmetric states.