Energy-harvesting behavior and configuration effect of two semi-active flapping foils

This study investigates the energy-harvesting behavior and configuration effects of two semi-active flapping foils through numerical simulations, considering both tandem and unaligned configurations, as well as the influence of Reynolds number and pitch amplitude. The tandem configuration shows the greatest efficiency improvement, maximizing the utilization of the upstream foil's wake. In contrast, the unaligned configuration allows the downstream foil to surpass the energy-harvesting efficiency of the upstream foil, which is advantageous for clustered systems. The phase difference impacts wake structures, with the downstream foil recapturing the wake, though its efficiency is limited by the lower wake speed compared to the incoming flow velocity. The Reynolds number effect does not appear to influence the peak efficiency ratio, suggesting that the efficiency relationship between the foils remains consistent across different Reynolds numbers, despite potential differences in the optimal phase difference. Furthermore, reducing the pitch amplitude of the upstream foil optimally enhances the efficiency of the downstream foil, a factor that has often been overlooked. Lowering the pitch amplitude increases the wake speed, providing the downstream foil with a larger energy utilization space. This study provides valuable insights into the configuration design of flapping turbines, with the optimal two-foil system achieving an impressive efficiency of up to 46.3%. In the unaligned configuration, the lower the pitch amplitude of the upstream foil, the higher the efficiency ratio, which can reach up to similar to 1.8. With different pitch amplitudes, the efficiency ratio consistently exceeds 1, indicating that the downstream foil benefits from the wake of the upstream foil. Combining more foils might further enhance efficiency in the unaligned configuration, as the higher efficiency in the tandem configuration is due to the repeated utilization of the sweep area by the two-foil system.