Energy-extraction performance and efficiency-enhancing mechanism of a gate-assisted flapping-foil system

This study presents a gate-assisted flapping-foil system to enhance energy-extraction performance. A two-dimensional (2D) numerical model is established to investigate the energy-extraction performance and efficiency-enhancing mechanism under different flapping frequencies and clearance spacings between the gate and the foil. The intention is to maximize the energy-extraction performance by determining the appropriate gate size and flapping frequency of the foil. The results show that the energy-extraction efficiency exhibits a single peak pattern with the flapping frequency, and there exists an optimal frequency corresponding to the maximum efficiency. The optimal frequency and maximum energy-extraction performance increased with narrowing of the clearance spacing. Compared to the wall-free case, a narrow clearance spacing significantly enhances the energy-extraction efficiency by 116.01%. The lift variation due to the confinement effect is the primary contributor. The two walls promote flow separation and the rolling-up of wall vortices by redirecting and accelerating the vortex flow, leading to the merging and pairing of the vortices. The pressure differences across the foil are amplified, especially when the foil strokes near the wall, which is beneficial for lift generation and energy harvesting. This study enhances the understanding of the energy-extraction mechanism and provides inspiration for innovative energy-harvesting systems.