Effects of the downstream spatial configuration on the energy extraction performance of tandem/parallel combined oscillating hydrofoils

Unsteady 2D laminar flow simulations were performed on tandem oscillating hydrofoils to study the influence of the downstream spatial location on the power extraction efficiency of a tandem/parallel combined configuration. A sinusoidal plunging/pitching motion of NACA 0015 profile was used. Inspired by insect inter-wing spacing, different configurations of the downstream foil (y)-position were first studied at a fixed (L/c) horizontal inter-foil spacing and reduced frequencies within the range of 0.02 to 0.32. It was found that the energy extraction efficiency improved by around 23 % as compared to the conventional tandem configuration by changing the vertical inter-foil spacing. An interesting finding is that, at specified frequencies and inter-foil spacing, the power efficiency extracted from the downstream foil, associated with the new configuration, exceeds the upstream foil efficiency. The horizontal inter-foil spacing was then investigated for an optimal (y)-position of the downstream foil, and the results show that the optimal horizontal spacing was reduced compared to the conventional tandem configuration, resulting in a gain in space and about 15 % of increment in the power extraction efficiency. Therefore, a total of 38 % of power efficiency enhancement was obtained at f * = 0.24 by using the new configuration in which the horizontal inter-foil spacing was reduced five times compared to the conventional configuration. An appropriate inter-foil spacing/frequency synchronization resulted in a favorable foil-vortex interaction that improves the power extraction from the oscillating foil system. The frequency and the inter-foil spacing were found to be critical factors that determine whether the interaction is favorable or unfavorable.