Free-surface effects on the energy-harvesting performance of a fully passive flapping foil

Xinchun Hua (Shanghai Branch of Changjiang Institute of Survey, Planning, Design and Research Co., Ltd.)
Feilin Wang (Department of Power Mechanical Engineering，University of Shanghai for Science and Technology)
Yang Chen (Shanghai Branch of Changjiang Institute of Survey, Planning, Design and Research Co., Ltd.)
Bing Zhu (Department of Power Mechanical Engineering，University of Shanghai for Science and Technology)

International Journal of Renewable Energy Development
Accepted Articles

Fully passive flapping-foil energy harvesters offer structural simplicity and require no external actuation, but their self-excited motion and power extraction may be strongly affected by a deformable free surface during near-surface operation. To clarify this influence, a two-dimensional numerical investigation is conducted on a fully passive NACA0015 flapping foil at a Reynolds number of 500,000 and a Froude number of 0.1581. The Volume of Fluid method and an overset-mesh technique are employed to resolve free-surface deformation and foil motion, respectively. The Reynolds number, foil geometry, pitching-axis location, and structural parameters are kept constant, while only the ratio of submergence depth to foil chord length is varied. The energy-harvesting efficiency exhibits a non-monotonic dependence on submergence depth. Among the examined cases, the maximum efficiency of 45.98% is obtained at a submergence-depth-to-chord ratio of 2.8, representing an improvement of approximately 8.16% over the unconfined baseline efficiency of 42.51%. When this ratio is 1.8 or less, strong free-surface deformation and wave radiation disrupt wake coherence and load–motion phase coordination, producing negative heaving-power intervals and substantially suppressing the positive pitching-power contribution. When the ratio reaches 3.5 or greater, the free-surface influence becomes weak and the efficiency approaches the unconfined baseline. The dominant Strouhal number of the heaving and pitching motions remains approximately 0.1, while pressure cross-correlation analysis shows that free-surface proximity attenuates pressure-fluctuation coherence without altering the leading-edge-vortex-governed propagation mechanism. These results demonstrate that moderate free-surface interaction can enhance energy extraction, whereas excessive interface proximity causes pronounced performance deterioration, providing a preliminary reference for selecting the deployment depth of fully passive flapping-foil energy harvesters.