Experimental Investigation into the Time-Varying Propulsive Performance and Unsteady Wakes of Bio-Inspired Pitching Panels

Stereoscopic particle image velocimetry and time-resolved force and torque measurements are used to investigate the unsteady, three-dimensional wakes and propulsive performances of bio-inspired pitching panels with varying trailing edge geometries. The planform shapes of the five panels were selected as rudimentary approximations of the shapes of caudal fins and cetacean flukes found in nature. All panels were sinusoidally pitched about their leading edges through five different angular pitching amplitudes within a constant free stream flow, generating five unique Strouhal numbers for each specific panel geometry. Time-averaged propulsive performance results reveal that panels of high aspect ratio and a forked trailing edge fail to generate positive mean thrust under any condition tested, an unanticipated result given biological observations of high-performance swimming animals. The time-varying propulsive performance of all panel geometries was explored through investigations of the time-varying thrust and time-varying lateral force. Time-varying force measurements demonstrate that the specific panel geometry and Strouhal number have little influence on the qualitative behaviors of these forces, but they have a significant effect upon the magnitude of these forces. Thrust maxima tend to develop as the panel retreats away from a motion extreme, and the values of these maxima increase with Strouhal number and trailing edge convexity. Thrust minima always occur slightly after the panel crosses over the wake centerline, and time-averaged thrust becomes positive when the thrust maxima exceed the drag maxima. Maximum magnitude lateral forces also increase with trailing edge convexity and Strouhal number. Peak magnitude lateral forces always occur shortly after thrust maxima, and the delay between these two events generally increases with Strouhal number. Measurements of spanwise vorticity indicate that the time-varying thrust rises as vorticity builds up on the surface panel, and peak thrust is associated with the release of a primary trailing edge vortex. Thrust degrades further as previously generated spanwise vorticity is shed and enters the wake.