Three-dimensional vortex structure using phase-averaged stereoscopic PIV for an oscillating airfoil

A quasi-steady-state approach to the prediction of fluid forces acting on an insect wing leads to errors that suggest that flight is impossible. However, because biological flight does occur, the effect of unsteady fluid forces must be important to flight. The propulsive forces of these animals are related to unsteady fluid forces accompanied with the movement of vortices. Therefore, unsteady fluid forces must also be considered when estimating the propulsive force of swimming the front crawl and the flat scull technique in synchronized swimming. The purpose of this study was to investigate the effect of a three-dimensional airfoil shape on three-dimensional vortex structure for a high reduced frequency. The vortex structure and its behavior for a discoid airfoil were investigated. Furthermore, the vortex structure of the discoid airfoil was compared with that of a rectangular. The vortical flow fields were measured in the wake of the discoid airfoil using a stereoscopic particle images velocimetry technique. The three-dimensional vortex structure was depicted by plotting iso-vorticity surface calculated from the reconstructed velocity data. Compared with the rectangular airfoil, a strong vortex exists over a prolonged period of time in the wake of the discoid airfoil.