Numerical and experimental study of active control and passive design for energy harvesting flapping foils under multi-parameter coupling

This study investigates the unsteady flow characteristics and the mechanisms of active control and passive design for flapping foils under complex operating conditions. Numerical simulations are first conducted to analyze the transient nonlinear flow behavior of the flapping foil across varying flow velocities and inflow directions. The coupled effects of key active control parameters are systematically examined. A quantitative relation of (Formula presented) is identified under optimal conditions, while the combined influence of pitching amplitude and phase difference highlights the critical role of foil kinematics in determining energy harvesting efficiency. The numerical results further clarify the evolution of vortex shedding and its actively regulated impact on power extraction. Moreover, integrated PIV measurements and simulations examine flow behavior and energy harvesting under different Reynolds numbers, pitching amplitudes, and passive configurations. Results show energy dissipation at low velocities, efficiency improvement with increasing Reynolds number, and a non-monotonic effect of pitching amplitude peaking at (Formula presented)  = 78°. Passive parameters significantly influence flow separation and vortex shedding, with NACA 0015 and 1/3c pitching axis performing best. Direct comparison between experiments and simulations validates vortex dynamics evolution. This study provides a comprehensive understanding of multi-parameter coupling and practical guidance for optimizing flow control, foil kinematics, and passive design.