Numerical study of asymmetric pitching amplitude effects on energy extraction performance of a semi-activated hydrofoil in shear flows

Abstract

As a representative tidal current energy device, the operational stability and energy extraction performance of hydrofoils are significantly influenced by shear flow. Inspired by natural models, including insects, birds, and fish, the amplitude asymmetric pitching has been proposed as a strategy to mitigate the adverse impacts of the shear flow. A 2-dimensional (2D) numerical model of a semi-activated hydrofoil was developed using ANSYS-Fluent computational fluid dynamics software to assess the feasibility of amplitude asymmetric pitching. Hydrofoils employing the asymmetric and symmetric pitching motions under uniform and shear flows were studied. The hydrodynamic response and energy extraction performance of the hydrofoil were compared at various shear rates, pitching amplitudes, and asymmetric ratios. The results indicate that amplitude asymmetric pitching enhances both the operational stability and energy extraction performance of the hydrofoil under shear flow conditions. The appropriate asymmetric ratio for stable operation and energy extraction varies depending on the pitching amplitude and shear rate. Appropriate asymmetric ratios were determined using the trial-and-error method. Additionally, flow field comparisons between the asymmetric and symmetric pitching motions elucidated the underlying mechanisms. The amplitude asymmetric pitching does not completely eliminate the adverse effects of shear flow on the hydrofoil's energy extraction performance. The power coefficient of the hydrofoil with an appropriate asymmetric ratio decreases with increasing shear rate.