Effect of Surface Air Entrapment in Small-Scale Flapping Fin Propulsion*

Propulsive performance of flapping propulsion decays with decreasing Reynolds number (Re). Introducing apparent slip at the fin surface may serve to facilitate vortex formation at low Re and improve performance. This work investigates the effects of air entrapment on the fin to provide slip flow and its effects on vortex formation from a small-scale fin propulsion. The air entrapment fin uses a constant air supply to a perforated surface to keep air at the fin surface and relax the no-slip condition. Utilizing a water tunnel, Particle Image Velocimetry (PIV) measurements of the flow around a pitching and heaving 2D flat plate fin were made at Re=300. The circulation of the vortices and time-averaged thrust were determined from these measurements for a range of flapping parameters. Using this method, the results for a fin with air entrapment will be analyzed and compared with the results from a solid, control fin.