Dissecting the flow physics of wave-induced flapping foil propulsion

The wave glider is an autonomous, unmanned surface vehicle that converts wave energy into propulsion using oscillating hydrofoils. To understand the dynamics and optimize the parameters of the wave glider propulsion system, we conducted a computational study of flow-induced pitch oscillations of a sinusoidally heaving foil at a chord-based Reynolds number of 10000. The objective is to generate maximum thrust under different conditions of spring stiffness and elastic axis location. A sharp-interface immersed boundary method is used to simulate two-dimensional incompressible flow, and this is coupled with the equations for a sinusoidally oscillating foil supported at the elastic axis with a linear torsional spring. The variation in spring stiffness leads to different amplitude of pitch motion and thrust coefficient and we show the utility of 'maps' of energy exchange between the flow and the airfoil system, as a way to understand and predict this behavior. Next, the Force Partitioning Method (FPM) is employed, and loads induced by individual vortices are quantified to explain the behavior of energy maps. We then examine the flow physics of multi- foil systems. We find that the trailing foil can be placed at an optimum location such that the wake-induced flow of the leading foil enhances the leading-edge vortex (LEV) of the trailing foil, leading to improvement of thrust by the trailing foil by around 80%.