Numerical study on the energy-harvesting performance of a semi-activated coupled-pitching hydrofoil in shear flows

A two-dimensional numerical model of a semi-activated coupled-pitching hydrofoil was established using the computational fluid dynamics software ANSYS Fluent. The kinematic response, hydrodynamic characteristics, and energy-harvesting performance of the hydrofoil in shear flow were investigated. The coupled-pitching hydrofoil achieved a stable limited-cycle passive-pitching motion and efficient energy harvesting in shear flows under various shear rates, pitching amplitudes, and reduced frequencies. The flow structures, pressure distributions, and hydrodynamic torque were analyzed to reveal the mechanism of the stable passive-pitching motion of the coupled-pitching hydrofoil in shear flow. The stable passive-pitching motion was dominated by the hydrodynamic torque, which was equivalent during the upward and downward strokes. Shear flow has a certain degree of influence on energy-harvesting efficiency. The energy-harvesting efficiency decreased with increasing shear rate. When the activated-pitching amplitude is small, the effect of shear rate on energy-harvesting performance was limited. The effect of shear rate was enhanced as the activated-pitching amplitude increased. The average relative difference in the energy-harvesting efficiency in the uniform and shear flows was 15 %, when the activated-pitching amplitude was 70° and the shear rate was 1.0. The peak value of the efficiency can reach 19.8 % when the shear rate was 1.0. The coupled-pitching hydrofoil is adaptable to shear flows. The results of this study provide valuable information for the application of coupled-pitching hydrofoils under real tidal flow conditions.