Behaviors of two semi-passive oscillating hydrofoils with a tandem configuration

Abstract

Two hydrofoils with a tandem spatial configuration can share the same flow window. The wake vortex interaction and blockage effect have noticeable impacts on the response of two hydrofoils for a semi-passive system. In this study, the responses of two oscillating hydrofoils with a tandem arrangement are examined using a numerical approach. The results indicate that the downstream hydrofoil has an inferior energy harvesting performance because it operates in the upstream hydrofoil wake. Several special cases were observed. As a result of a negative interaction in the wake vortices, (i) two hydrofoils were shown to have different heaving equilibrium positions and thus no longer share the flow window; and (ii) the downstream hydrofoil was demonstrated to have a larger heaving amplitude but a lower energy harvesting performance. At a very high oscillation frequency, the downstream hydrofoil tends to incur an irregular response owing to the intense vortex formation and shedding, as well as a constant interaction of the wake vortices. The downstream hydrofoil achieves an unexpectedly better energy harvesting performance due to a favorable interaction of the wake vortices. However, the entire system can’t achieve high efficiency due to the poor energy harvesting performance of the upstream hydrofoil. When two semi-passive oscillating hydrofoils have a tandem configuration, negative wake vortex interactions should be avoided instead of seeking a favorable interaction.