Performance evaluation and enhancement of a semi-activated flapping hydrofoil in shear flows

Highlights: • A semi-active hydrofoil in the shear flows is firstly study numerically. • The hydrofoil is pushed to the low-velocity area and heave about an equilibrium point. • The energy harvesting efficiencies are significantly smaller than those in the uniform flow. • Two practical controlling strategies are utilized to improve the energy-harvesting performance. • The linear spring makes the hydrofoil to achieve the highest efficiency in the shear flow. -- Abstract: The semi-activated flapping hydrofoil is a promising device for harvesting tidal stream energy in shallow water with a relatively higher efficiency. A two-dimensional numerical model, based on the computational fluid dynamics software ANSYS-Fluent, was established and validated to investigate a semi-activated hydrofoil with activated pitching and induced heaving motions in the shear flows. It was found that the hydrofoil can reach a stable stage to passively heave about an equilibrium point at various shear rates. Since the hydrofoil is pushed toward the low-velocity areas, its energy-harvesting efficiencies whether or not considering the hydrodynamic torques are lower than that in the uniform flow. Two controlling strategies were used to specifically restrict the downward motion ranges, namely a stop block or a linear spring, which demonstrated the ability to significantly increase the two efficiencies under various controlling parameters. The highest efficiencies of 0.50 and 0.52 were achieved using a linear spring at the shear rate of 2.0.