The importance of rear pillar geometry on fastback wake structures

Abstract

The wake of a fastback type passenger vehicle is characterised by trailing vortices from the rear pillars of the vehicle. These vortices strongly influence all the aerodynamic coefficients.Working at model scale, using two configurations of the Davis model with different rear pillar radii, (sharp edged and 10mm radius) the flow fields over the rear half of the models were investigated using balance measurements, flow visualisations, surface pressure and PIV (Particle Image Velocimetry) measurements.For a small geometry change between the two models, the changes to the aerodynamic loads and wake flow structures were unexpectedly large with significant differences to the strength and location of the trailing vortices in both the time averaged and unsteady results. The square edged model produced a flow field similar to that found on an Ahmed model with a sub-critical backlight angle. The round edged model produced a flow structure dominated by trailing vortices that mix with the wake behind the base of the model and is weaker. This flow structure was more unsteady than that of the square edged model. Consequently, although both models can be described as having a wake dominated by trailing vortices, there are significant differences to both the steady state and unsteady flow fields that have not been described previously. This also shows that the fastback wake structure described by Ahmed is not definitive.