Stereo PIV measurements of vortex behavior produced by a pitch-oscillating discoid airfoil

Abstract

Flying animals and insects flap their wings using a combination of pitching and heaving oscillations. Unsteady fluid forces presumably play an important role in the flight mechanism, and also in the propulsive force of swimmers performing the front crawl. In particular, the crawl stroke invokes a unique S-shaped pull motion, in which the direction of circulation around the hand changes during a single stroke cycle. We intend to elucidate the relationship between unsteady fluid forces and the three-dimensional vortex structure in the case of high reduced frequency. In the present study, pitch-oscillating motion was carried out as a basic unsteady motion. The vortex structure and behavior of a discoid airfoil (simulating a swimmer's hand) were investigated during pitch-oscillating motion. Vortical flow fields were measured by a particle image velocimetry (PIV) technique. Vortical field was evaluated in a wind tunnel test, which allows the easy alteration of the complex parameters affecting unsteady phenomena. Throughout one cycle of the pitch-oscillating motion, vortices grew to a large size and were eventually released into the wake close to the airfoil. Fluid force increased as the vortex grew near the airfoil, peaking twice during the pitching cycle. Thereafter, the fluid force decreased as the vortex was shed and traveled in the downstream direction.