Research on energy harvesting characteristics of a flapping foil with trailing edge jet flap

Abstract

Flapping foils offer a potential means for extracting hydrokinetic energy from ocean and river flows by utilizing a combined pitching and heaving motion of the airfoil. In this paper, the effectiveness of trailing edge active flow control in enhancing the energy harvesting performance of the conventional flapping foil is examined through numerical simulations. In this approach, an injections slot is opened at the trailing edge of the flapping foil, from which a high velocity jet is discharged into ambient water. Since the water jet is issued vertically downwards perpendicular to the airfoil surface, it operates like a Gurney flap and thus is known as jet flap in aeronautics. Parametric studies show that the optimal injection position should be as far as possible from the leading edge of the flapping foil. In addition, it is also found when the width of the exit slot of jet equals to 2.6 % of the airfoil chord and the momentum coefficient of jet is 0.02, the flapping foil with the jet flap can increase the maximum net energy harvesting efficiency obtained after deducting the energy consumption for the flow control from 39.22 % to 47.6 % compared to its baseline counterpart, indicating the effectiveness of this method. Furthermore, the dynamic mode decomposition (DMD) method is also adopted to reveal the spatial-temporal features of the coherent structures in the wake flow behind the proposed flapping foil in an attempt to gain a deep understanding of the underlying flow mechanism involved in promoting its energy harvesting performance.