Influence of inter-foil spacing on energy extraction of tandem oscillating hydrofoils

The tandem configuration of two hydrofoils is an effective way to improve the power of the energy-extraction system. In such a configuration, the inter-foil spacing between two hydrofoils is one of the important configuration parameters. The effect of the inter-foil spacing on the energy-extraction power is investigated by the STAR CCM + based on an overset mesh. First, a fixed motion phase shift between two foils of - 180 degrees is proposed with adjustment of the inter-foil spacing. Then, the global phase shift is established; for keeping it constant, the motion phase shift is correspondingly changed with adjustment of the inter-foil spacing. Based on the systematical studies of the energy-extraction power of the tandem foils with spacing, it is found that in the tandem foils system, the energy extraction of the upstream foil is changed tiny. With increasing the spacing between two foils, the energy-extraction power of the downstream foil shows a very obvious periodic fluctuation when the motion phase shift is constant. Due to the loss of wake velocity, the global phase using the freestream velocity causes errors in describing the downstream foil-vortex interaction, so the wake phase model using the average wake velocity is a better choice. For the small effective angle of attack hydrofoils with only shear layer vortices, the wake energy does not change with distance, so the maximum efficiency of tandem foils with different inter-foil spacings is almost the same. But when the leading edge vortex shedding occurs, the energy extraction of the tandem foils is significantly affected by the inter-foil spacing due to the change of the wake energy.