Effect of angle of attack profiles in flapping foil propulsion

Propulsive performance of a harmonically heaving and pitching foil is degraded by a breakdown in the angle of attack profile, at high nondimensional frequencies. We build here upon the method employed by Read et al. (J. Fluid Structure 17 (2003) 163) to invert the key kinematic nonlinearity, and compare directly the performance obtained with four specific angle of attack profiles. These profiles are: (i) that due to simple harmonic motion in both heave and pitch motion, (ii) a square wave, (iii) a symmetric sawtooth wave, and (iv) a cosine function. The cosine angle of attack achieves a significant improvement over the other three cases, in the sense of high thrust values with reasonable efficiency. The highest thrust coefficients, however, are generally found in the sawtooth profile. Flow visualization confirms the partial or complete recovery of a reverse von Karman wake by all three controlled profiles, concurrent with the enhanced force and efficiency at high frequencies. (C) 2003 Elsevier Ltd. All rights reserved.