Propulsion performance of a passively oscillating flexible foil close to wave surface

A flexible body can interact with an incident wave and extract wave energy to develop thrust. To investigate this passive propulsion mechanism, the unsteady motion of a two-dimensional passively oscillating flexible foil with a fixed leading edge was simulated in an incident wave. The fluid motion was defined on a fixed Eulerian grid and the foil motion was defined on a moving Lagragian grid, where the interaction between them was considered in the framework of a sharp immersed boundary (IB) method. The coupled level-set volume-of-fluid (CLSVOF) method was adopted to capture the wave surface. The effects of swimming speed, the Reynolds number, and the submergence depth of the foil on the passive propulsion performance were examined. It is found that the Strouhal (St) and Reynolds number have a strong impact on the time-averaged propulsion performance. The foil reaches a peak propulsive efficiency within a narrow range of St between 0.2 and 0.4. The Reynolds number effect indicates the importance of hydrodynamic resistance in determining the propulsion performance. Some of the unsteady propulsion mechanisms, including Leading Edge Vortex (LEV) and wake structures, were also investigated to understand the passive propulsion performance.