Investigation on energy extraction performance of an oscillating foil with modified flapping motion

A modified flapping motion for an oscillating foil is proposed for energy extraction and the energy extraction performance of this proposed motion is numerically studied. Computations are performed at Re = 104 with a 2-D elliptical foil with thickness of 10% chord applied. The investigation is undertaken over a wide range of kinematic parameters (reduced frequency k, pitching amplitude θ0). The results reveal that the power extracted from the oncoming flow mainly comes from the plunging motion and the pitching contribution is quite limited. Detailed examination of motion parameters indicates that at a fixed θ0 the output power increases with k at first and then decreases with the further increasing k. A similar trend for the variation of output power with θ0 at a fixed k is also observed. By affecting the effective angle of attack profile, the motion parameters notably influence the development of leading edge vortices and variation of lift force. A mapping of power-extraction efficiency for the oscillating foil in the frequency and pitching amplitude domain is presented. It is found that high k together with low θ0 is beneficial to energy extraction. For the best energy extraction performance, relatively high k and low θ0 should be chosen.