Impact of the ground effect on the energy extraction properties of a flapping wing

Abstract

The energy harvesting device with flapping foil has the characteristic of environmental adaptability, particularly in shallow water. This study applies a transient numerical calculation method based on a dynamic grid technique in an absolute coordinate system to analyze the flow fields and pressure distributions of a flapping airfoil near a ground wall. The effects of Reynolds number (Re), average ground clearance, and effective angle of attack on the power extraction efficiency of the flapping airfoil are investigated with respect to variations in the oscillation frequency. The simulation results indicate that the ground effect can greatly increase the power extraction efficiency of the flapping airfoil, and that the highest efficiency is obtained at an optimum average ground clearance. This ground effect is asymmetric over the course of flapping airfoil motion toward and away from the ground wall, and the efficiency is most increased as the airfoil moves far from or closely to the ground wall. The formation of the corresponding divergent or convergent tunnel effects alters the flow fields and pressure distributions around the airfoil near the leading edge, and, in particular, decreases the pressure on the suction side, which results in an overall increase in the heaving force and a correspondingly increased power harvesting capability. With or without the ground effect, the efficiency of the flapping airfoil firstly increases and then decreases with increasing oscillation frequency or effective angle of attack at constant Re and average ground clearance values. Moreover, the positive impact of the ground wall effect on the power extraction efficiency of the airfoil becomes increasingly obvious when the oscillation frequency and the angle of attack are increased simultaneously. Finally, the oscillation frequency obtaining the highest power extraction efficiency in conjunction with the ground effect decreases as the flow fields transition from laminar to turbulent flow with increasing Re.