Numerical investigation into the energy extraction characteristics of 3D self-induced oscillating foil

Abstract

The energy extraction performance of the flow energy convertor based on fully passive oscillating foil is investigated numerically. The foil undergoes combined pitching and swing motions. The pitching motion is driven by periodically varying moment generated by vortex shedding, then the resulting varying angle-of-attack and lift force drives the swing motion and the connected generator. The study focuses on the three-dimensional flow characteristics and their effects on the energy extraction performance. Numerical results demonstrate that the convertor with theoretically infinite foil can reach maximum power coefficient of 1.34 and efficiency of 0.4, but the performance of the convertor with limited-length foil is reduced significantly. The relationship between power coefficient and foil length is described with a linear equation. Longer foil can release the loss caused by 3D effect; the drop in the efficiency of the 3D foil could be limited to around 15% as the foil length increases to 8 times mid-arc length. The examination of the flow filed shows that the loss caused by 3D effect is mainly attributed to the tip leakage flow and no obvious non-conformal or disorder vortex shedding is observed. The oscillation of the foil can synchronize the vortex shedding and suppress 3D instability.