Investigation of unsteady vehicle aerodynamics under time-dependent flow conditions

Unsteady aerodynamic flow phenomena are investigated by rotating of a 50% scale generic notchback car around its vertical axis. The model is exposed to unsteady oncoming flow conditions similarly as created by crosswind gusts. Unsteady aerodynamic loads are ob- served to differ significantly from quasi-steady loads. In particular, the unsteady yaw moment exceeds the maximum value encountered during steady flow conditions affecting driving stability. Based on experimental and numerical simulations it is shown that the wake flow of a vehicle being subjected to constant crosswind is characterized by a region of high velocity at the leeward side and two counter-rotating vortices establishing above and below that region. This structure reacts with a time delay to a change of the oncoming flow. The delayed response of the wake flow causes phase shifted pressure fluctuations at the rear side of the vehicle and thus causes an unsteady increase of the yaw moment and decrease of side force and roll moment. Numerical and experimental results both capture the unsteady phenomena occurring at transient oncoming flow conditions. However, the quantitative estimation of the pressure amplitude at the rear side differs, which alters the ratio of unsteady to quasi-steady loads as well as their phase lag.