Fluid-structure interaction of a fully passive flapping foil for flow energy extraction

Abstract

We experimentally investigated the fluid-structure interaction (FSI) and energy extraction performance of a novel flapping-foil based flow-energy harvester. Different from most of existing concepts, this device can extract energy from flows through a foil's fully passive flapping motion, i.e., the foil's heaving and pitching motions are induced by the flow without using any actuator. The foil's dynamics and energy extraction performance were studied under various flow and operating conditions. It was found that, when operating at the Reynolds number near 105, the device has a cut-in speed of 0.45 m/s and can generate a mean power of about 1 W in a water flow of 0.55 m/s, leading to a power conversion efficiency of 32.5%. Detailed FSI studies revealed that both the heaving and pitching motions can make positive contributions to energy harvesting. The heaving velocity reaches its extremes at the end of each pure heaving phase, whereas the heaving force reaches its extremes at around the end of each stroke reversal, leading a phase difference of nearly 90°. As such, the heaving power is positive in the pure heaving phases but negative in the stroke reversal phases. Both the pitching velocity and pitching moment peak at the end of stroke reversals, hence generating a significant peak in the pitching power. It was also found that, compared to the foil's effective angle of attack, the leading-edge vortices produced during flapping have a very limited impact on the foil's dynamics. A parametric study revealed that, as the foil's pivot axis is moved towards the trailing edge, both the mean heaving power and the mean pitching power increase. As a result, the total power increases significantly. The same trend was also observed when the foil's pitching amplitude increases from 30° to 60°. On the contrary, it seems that there exists an optimal water speed between 0.46 and 0.69 m/s, at which the power conversion efficiency is maximum.