Numerical Study on the Energy-Harvesting Performance of Multiple Flapping Foils

Flapping foils, inspired by the wing motions of birds and the swimming mechanisms of aquatic animals, offer a promising alternative to traditional turbines for extracting renewable energy from ambient flows found in nature. This study employs an immersed boundary-lattice Boltzmann method (IB-LBM) to numerically investigate the energy extraction performance of multiple flapping foils at a Reynolds number of 1100. Two staggered foils are systematically studied to identify the optimum spatial arrangements needed to achieve high energy-harvesting performance. The results show that the wake of the fore-foil mainly contributes to the negative performance of the hind-foil due to the loss of streamwise flow velocity, and the interaction between the two foils can enhance the energy-harvesting performance of the system, but cannot fully alleviate the effects of flow velocity loss. Therefore, the staggered arrangements, which help the hind-foil shed the wake, are essential to improve the energy-harvesting performance of the hind-foil. Comparable performance for the hind-foil is achieved at a horizontal gap of 2.5c and vertical gap of 2.5c with c being the chord length of the foil. The scaled-up systems, including three-, five-, and seven-foil configurations, are examined with gaps of 2.5c (horizontal) and 2.5c (vertical), and the results show that such `V’-shaped arrangements of these foils can achieve high energy-harvesting performance, with an enhancement up to 10.7% when seven foils are used, by utilizing the high mean streamwise velocity at the boundary of the leader’s wake, confirming the versatility of the optimum staggered arrangements for flapping-foil arrays.