An Investigation into Large Scale Unsteady Structures in the Wake of Real and Idealized Hatchback Car Models

There are many aspects of the unsteady flow around fastback passenger cars that remain to be understood. These include the source and nature of unsteady flow structures, the relevant time-scales, the effect of geometric parameters and the impact of the unsteadiness in terms of steady and unsteady forces on the vehicle. This paper investigates large scale unsteady structures in the wake of the Ahmed form and of a scale model of a real car shape using two wind tunnels and model scales between 12.5% and 40%. The unsteadiness demonstrated only low coherence and weak periodicity and the Strouhal number of a given structure varied from tunnel to tunnel indicating a high sensitivity to external influences. Nevertheless, a novel visualization technique, used to display the results of time-accurate pressure probe measurements, was able to reveal structures involving both symmetric and anti-symmetric oscillations in the strength of the rear-pillar vortices. The unsteady flow structures and levels of fluctuation were very similar for different geometries, indicating that parameters such as radii on the vehicle surface may have surprisingly little impact on wake unsteadiness.