The Vortical Field Downstream of an Airfoil Oscillated from Rest for One Cycle

The flow fields generated from airfoils undergoing small amplitude, high frequency cyclic motions (e.g. pitching, heaving, combined pitching/heaving) continue to be a topic of interest for thrust generation at low Reynolds numbers. Past studies have typically investigated these flow fields once the transient start-up has died out and the downstream flow field forms into an unsteady, but cyclic, semi-infinite vortical wake (i.e. the “steady state” condition). In the current work the flow field downstream of a NACA0012 airfoil that experiences a single oscillation about the ¼-chord location is examined using PIV to investigate the starting dynamics. The prescribed motion starts impulsively from α=0° and varies between ±α_max following a sinusoidal motion trajectory ending back at α=0°. The resulting flow fields show that multiple vortices of both signs were formed due to the impulsive start, two changes in direction and the impulsive stop of the motion. The size and strength of the vortices were found to depend on both the maximum angle and frequency of motion. The dynamics of the vortices were also found to depend on both the maximum angle and the frequency of the motion. For low frequency conditions isolated vortices formed and convected with little interaction. As the frequency was increased vortices began to form in proximity to each other resulting in complex interaction dynamics including pairing, merging, and combining. Progressively weaker vortices continued to form after the motion was completed, with the flow field taking on order of two convective times to return to the starting α=0° wake condition.