Mechanism of three-dimensional effects in a semi-active flapping hydrofoil of aspect ratio 4 under unsteady inflow at Re=50000

This study investigates the mechanism of three-dimensional effects in a semi-active flapping hydrofoil under unsteady inflow. The results show that the hydrofoil lift exhibits pronounced spanwise non-uniformity, decreasing gradually from the mid-span region toward the wingtip. Spanwise vorticity-transport analysis indicates that this lift loss is closely associated with the redistribution of vorticity from the wingtip toward the mid-span region. Under unsteady inflow, the instantaneous variations in the local Reynolds number enhance the overall vortex strength of the hydrofoil. However, the strengthened tip vortex further intensifies spanwise vorticity transport, with the maximum magnitude of the area-averaged spanwise velocity on the monitoring section at z/c = 2 reaching approximately 0.7 (Formula presented). This enhances the three-dimensional effects, enlarges the LEV-suppressed region, and promotes LEV detachment from the hydrofoil surface, thereby shortening its near-surface residence time. Quantitatively, the reduction in peak lift in the wingtip region relative to the mid-span region increases from 0.036 under steady inflow to 0.077 under unsteady inflow. In addition, higher-order harmonic inflows with up to three frequency components were tested. For the representative three-component harmonic inflow, the directly simulated mean total power coefficient deviates from the linear-superposition prediction by approximately 5.3%, indicating nonlinear interactions among different harmonic components.