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arXiv · 2606.11126

Inertial effects on the mechanical efficiency of a semi-passive oscillating hydrofoil energy harvester

Zihan Zhang·Qimin Feng·Yuanhang Zhu·Qiang Zhong
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Abstract

Oscillating-foil-based energy harvesters have demonstrated strong potential for low-speed hydrokinetic energy extraction; however, the actuator-level mechanical energy balance associated with prescribed pitching motion remains poorly understood. The present work experimentally characterizes how foil mass ratio, pitching-axis location, and reduced frequency jointly govern the hydrodynamic and mechanical efficiencies of a semi-passive oscillating hydrofoil. Results show that rotational inertia redistributes actuator demand through phase-dependent torque exchange, while heave-pitch coupling can partially cancel this demand when favorably phased. Pitching-axis location modifies the phase and direction of the fluid torque through changes in the effective hydrodynamic moment arm. Reduced frequency governs the balance between enhanced unsteady loading and inertia-amplified actuator demand. Optimal performance is achieved within reduced frequency region of 0.125-0.16 using quarter-chord to one-third-chord pitching axes and relatively low foil mass ratios from about 0.5 to 2.0, yielding a peak mechanical efficiency of 33.96% -- which can diverge from the hydrodynamic efficiency by approximately 38.16% depending on configuration. Torque-loop analysis and PIV measurements show that this synchronization is a key mechanism governing the observed efficiency trends.

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Zihan Zhang, Qimin Feng, Yuanhang Zhu, Qiang Zhong. 2026-06-09. Inertial effects on the mechanical efficiency of a semi-passive oscillating hydrofoil energy harvester. https://arxiv.org/abs/2606.11126

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