Optimizing the Performance of Flapping Foil Wave-Assisted Propulsion Systems: Insights from Computational Fluid-Structure Interaction Models

The wave glider is one example of a watercraft that converts wave energy into propulsion using a series of oscillating hydrofoils. The trailing foils in the wave glider can utilize the wake of the leading foil, improving thrust generation. Additionally, constraining the pitching motion of the foils with torsional spring or angle limiter can enhance the performance of these flapping foils. To understand the dynamics and optimize the performance of the wave glider system, we conduct a computational study of flow-induced pitch oscillations of sinusoidally heaving foils at a chord-based Reynolds number of 10,000. We first examine the flow physics of multi-foil systems. We find that the trailing foil can be optimally placed to enhance its leading-edge vortex (LEV) by utilizing the wake of the leading foil, resulting in a thrust improvement of around 80%. We then investigate the effect of constraining the pitching motion of the foil using torsional spring and angle limiter in different sea-states. Our results indicate that angle limiters are a simple and effective method for generating thrust under varying sea conditions. Additionally, the LEV-based model is used to uncover the role of kinematics of the foil on thrust generation.