Structure of tip vortex and wake of unsteady wings

Experiments were carried out for plunging wings in order to investigate the unsteady features of the tip vortex and wake. Wave-like motion of the wake in the mid-span plane as well as the periodic variations of the tip vortex and wake in a crossflow plane were captured in flow visualization and velocity measurements. The amplitude and phase of these oscillations vary with wing oscillation frequency and amplitude. Both the tip vortex amplitude and the wake oscillation amplitude have poor correlation with Strouhal number, but the latter is always smaller at low Strouhal numbers. With increasing Strouhal number, wake oscillations become smaller as a reverse Karman street develops in the mid-span plane, while multiple vortical structures appear in the crossflow plane due to the interconnected vortex loops. These results have implications for vortex interactions with downstream wings.