Experimental investigation of unsteady vehicle aerodynamics under time-dependent flow conditions - Part2

Unsteady aerodynamic flow phenomena are investigated in a wind tunnel by oscillating a realistic 50% scale model around the vertical axis. Thus the model is exposed to time-dependent flow conditions at realistic Reynolds and Strouhal numbers. Using this setup unsteady aerodynamic loads are observed to differ significantly from quasi steady loads. In particular, the unsteady yaw moment exceeds the quasi steady approximation significantly. On the other hand, side force and roll moment are over predicted by quasi steady approximation but exhibit a significant time delay. Part 2 of this study proves that a delayed and enhanced response of the surface pressures at the rear side of the vehicle is responsible for the differences between unsteady and quasi steady loads. The pressure changes at the vehicle front, however, are shown to have similar amplitudes and almost no phase shift compared to quasi steady flow conditions. The difference between unsteady and quasi steady yaw moment proves to be independent of oscillation amplitudes between 2deg and 4deg. It is assumed that the intensity of the unsteady flow phenomena is determined by the interaction of the time scale of the model rotation and the time scale of the delayed wake flow, described by the Strouhal number. The largest magnification factors for the yaw moment are found at 140kph and 2Hz for the notchback geometry, which results in a Strouhal number of Sr=0.12. It is finally shown that the yaw moment overshoot is less pronounced for a fastback and especially for a fullback geometry, which is explained by smaller unsteady pressure variations at the rear side of the fullback.