Energy extraction of oscillating foil with an active deflecting trailing-edge flap

Abstract

The method of oscillating-foil energy extraction can be used to extract kinetic energy from the surrounding flow by a combined pitching and heaving motion of the foil. In order to improve the efficiency of energy extraction, a slotted foil with an active deflecting trailing-edge flap—inspired by the structure of the tail edge of bird wings—is designed. In this study, the unsteady Reynolds-averaged Navier–Stokes equation is solved to investigate the energy extraction performance of an oscillating foil at a Reynolds number of 5.0 × 105. In the numerical simulation, the dynamic overset mesh technology is used in order to ensure the accuracy and convergence of numerical solution. The effect of the deflecting motion of trailing-edge flaps on the efficiency of energy extraction is studied at a range of oscillating frequencies. In addition, the flow control mechanisms of slots on the oscillating foil are revealed by comparing the flow fields of the slotted foil and the NACA0015 foil. The result shows that active deflecting trailing-edge flaps can improve the efficiency of energy extraction over a wide range of oscillation frequencies. The active deflection of trailing-edge flaps increases the energy extraction efficiency of oscillating foils by 21.1% relative to conventional foils under a specific operating condition of oscillating frequency f* = 0.18. A detailed analysis of the flow fields indicates that the slot on the foil can suppress flow separation, while it has a negative effect on the attachment of leading-edge vortices. The deflecting trailing-edge flap enhances the heaving force. Therefore, the energy output and the efficiency of the oscillating foil are enhanced especially at the operating conditions of the oscillating foil without leading-edge vortex shedding.