The unsteady aerodynamics of static and oscillating simple automotive bodies

A wind tunnel based investigation into the effects of unsteady yaw angles on the aerodynamics of a simple automotive body has been carried out to increase the understanding of the effects of unsteady onset conditions similar to those experienced in normal driving conditions. Detailed flow field measurements have been made using surface pressure tappings and PIV around a simple automotive model in steady state conditions and these have been compared to measurements made whilst the model was oscillating in the yaw plane. The oscillating motion was created by a motored crank which was used to produce consistent and repeated motion which produced a reduced frequency that indicated that a quasi-static response should be expected. The PIV data are used to compare the wake flow structures and the surface pressures are used to infer aerodynamic loads and investigate the development of the flow structures across the surfaces of the model. This includes a comprehensive comparison of the surface pressures on the sides of the model during a transient and quasi-static yaw angel oscillation. These results show differences between the two test conditions with the oscillating model results containing hysteresis and the greatest differences in the flow field occurring on the leeside of the model. Two configurations of the same model with different rear pillar geometries were used to isolate model specific effects. Square rear pillars create strong and stable trailing vortices which are less affected by the model motion whereas radiused rear pillars created weaker and less steady vortices that mixed with the quasi-2D wake behind the model base and were affected to a greater extent by the model motion. The unsteadiness in the trailing vortex separation feeds upstream into the A-pillar vortex demonstrating that small geometry changes at the rear can affect the entire flow field around the model.