The configuration effect of flapping foils for energy harvesting

Abstract

The configuration of multiple flapping foils is studied via numerical simulations. We comprehensively consider the effects of the streamwise distance, vertical spacing, and phase difference on the energy harvesting performance of flapping foils. We divide flapping foil configurations into three categories: tandem, unaligned, and parallel. The foil tandem configuration is optimal for multiple foils if using the existing efficiency formula. However, tandem configurations expand the diffusion range of the wake turbulence. Wake diffusion has a critical effect on multiple foil configurations, and the utilization of the vertical spacing has been neglected. Here, the effective angle of attack and the effective velocity are proposed based on multiple flapping foils, which can well predict various rules of the lift coefficient and guide studies on the optimal configuration of multiple foils. The optimal phase difference for the simple parallel configuration system is 135°, and the energy harvesting efficiency of parallel foils decreases with increasing vertical spacing. The stepwise configuration maximizes the utilization of the vertical spacing, making it optimal for practical engineering applications. The stepwise configuration not only recycles the wake of the upstream foils but also utilizes it to enhance energy harvesting from flapping turbines. In the multiple foil configurations, the energy harvesting efficiency of the downstream foil over the upstream foil is achieved for the first time using a stepwise configuration.