Numerical investigation of angle of attack profile on propulsion performance of an oscillating foil

Effects of effective angle of attack (AOA) profile on an oscillating foil thrust performance are studied using a computational method. The foil is subject to a combined pitching/plunging motion with effective AOA satisfying a harmonic cosine function. To achieve this, either the pitching or plunging motion is modified from the conventional harmonic sinusoids. Investigations are performed over a series of Strouhal numbers (St), three maximum effective angles of attack and three different phase angles between pitching and plunging. It is shown that the degradation of thrust force and efficiency with sinusoidal pitching/plunging oscillation, at higher St, is effectively alleviated or removed when the AOA is imposed as a cosine profile. The improvement is more significant for the phase angle being different from 90°. A better performance is obtained with the imposed modification on pitching motion. The stronger reversed Von Karman vortex wake associated with leading-edge vortex development is observed with the modified motions, which is believed to induce the improved thrust performance.