Abstract
The effect of an upstream bluff body on energy harvesting performance of a heaving and pitching hydrofoil is investigated numerically using a two-dimensional immersed boundary method at Re = 1000. The presence of the upstream body changes flow structure around the hydrofoil and enhances efficiency significantly by two mechanisms. Mutual interaction of the vortex shed from the upstream body and the leading-edge vortex of the hydrofoil precipitates the separation of the leading-edge vortex from the hydrofoil and its streamwise transport. The incoming flow deflected by the upstream body changes the effective angle of attack for the hydrofoil. These phenomena significantly increase heaving force and pitching moment during stroke reversal, and major contribution to efficiency enhancement is from the change in pitching moment. 30% increase in efficiency, relative to a hydrofoil without an upstream body, can be achieved for same kinematics. However, the upstream body may be disadvantageous in some configurations. If the hydrofoil is placed closely to the body in transverse direction, the leading-edge vortex formation is suppressed after stroke reversal. When flapping frequency does not match with vortex shedding frequency of the upstream body, non-periodic flow structure formed around the hydrofoil can cause efficiency drop and irregular power generation.
Highlights:
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Effect of an upstream bluffbody on the energy harvesting performance of a pitching and heaving hydrofoilis investigated.
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The upstream body can improve the efficiency of the hydrofoil by about 30%.
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Mutual interaction of the vortices shed from upstream body and hydrofoil is the main mechanism for efficiency improvement.
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The change in the effective angle of attack for the hydrofoil is another important mechanism for efficiency improvement.
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The upstream body placed in an improper position can cause efficiency drop and irregular power generation.