Salient three-dimensional features of the turbulent wake of a simplified square-back vehicle

In this paper, the unsteady wake of a simplified square-back vehicle, with and without wheels, is investigated using large-scale tomographic particle image velocimetry, at a Reynolds number of (based on the model height). In the no-wheel case, the time-averaged wake features a balanced toroidal shape, with a good level of symmetry in both vertical and lateral directions. However, analysis of the wake dynamics shows this widely accepted result to be a poor model of the wake structure. Application of proper orthogonal decomposition to the unsteady data reveals the existence of the widely reported bi-stable behaviour, consisting of random switches between two lateral symmetry-breaking states. For the first time, the three-dimensional topology of each state is fully characterised and the changes in wake topology during the switches between bi-stable states are also described. Each symmetry-breaking state is shown to feature a characteristic 'hairpin vortex' structure that is the result of the merging of two horseshoe vortices, aligned with the vertical edges of the model base. The mutual interactions between these vortices are found to be at the origin of the bi-stable mode. The vertical symmetry is lost when wheels are added to the model, resulting in the formation of an upwash-dominated wake. The bi-stable behaviour is removed but considerable mobility in the near wake remains, in the form of a swinging motion of the rear recirculation.