Hydrodynamic analysis of flapping-foil thrusters operating beneath the free surface and in waves

Oscillating wings are investigated as unsteady thrusters, augmenting ship's overall propulsion in waves. Flapping propulsor's heave is induced by ship's motions, while pitching motion is set by an active control mechanism. For the detailed investigation of the free-surface effects, a two-dimensional panel method is developed for the hydrodynamic analysis of the flapping hydrofoil. The instantaneous angle of attack is influenced by foil's oscillatory motion and the incident waves. We consider moderate submergence and speed, permitting us to approximately neglect effects of breaking waves and cavitation, and linearize the free-surface boundary conditions and the trailing vortex wake dynamics. Numerical calculations are presented concerning the performance of the developed BEM over a range of motion parameters and compared against other methods and experimental data. Our analysis indicates that significant efficiency is achieved under optimal operating conditions and the free surface effects cannot be neglected. In the presence of waves the thrust coefficient is observed to raise well above its value in infinite domain, with maximum gain reaching 20%, for appropriate selection of the parameters. The present method could serve as a useful tool for the assessment, preliminary design and control of the studied system, extracting energy from sea waves for marine propulsion.