Abstract
Flapping wing structures have drawn increasing attention for harvesting energy from flowing fluids. This work proposes the application of an adaptive Gurney flap to the trailing end of a flapping wing foil, which actively adjusts the position of a single Gurney flap to be consistently employed on the foil pressure side. The transient two-dimensional numerical simulations are conducted to compare the energy harvesting efficiency of the proposed adaptive flapping wing structure with that of a conventional flapping wing structure and the one with fixed Gurney flaps. In addition, the effects of wing oscillation frequency, Gurney flap length, and the position of the flap relative to the trailing edge of the foil were also considered. The results demonstrate that the adaptive Gurney flap provides greater energy harvesting efficiency for the oscillating wing primarily by increasing the heave force through the synchronous switching of the flap from one side to the other within the period of motion. Compared with fixed Gurney flaps, the adaptive flap can accommodate greater flap lengths and operation over a wider oscillation frequency range. The results indicate that the optimum position of the Gurney flap is as near as possible to the trailing edge of the foil. The results also reveal two relatively high harvesting efficiency oscillation frequency regions. In the low frequency region, if the leading edge vortex detaches from the trailing edge just when the foil exchanges the heaving direction, the foil can gain much more energy due to the positive power extraction contributed by the pitching moment. In the high frequency region, a small steady attached vortex maintains a high lift force on the foil, and, thus, the enhanced heaving force harvests more power.
Highlights:
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Adaptive Gurney flap oscillating wing greatly improves energy extraction efficiency.
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It allows for considerable flexibility in the choice of flap length.
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It should be positioned as near as possible to the foil trailing edge.
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Two relative high efficiency frequencies were observed with different mechanisms.