Unsteady Fluid-Structure Interactions in Semi-Passive Oscillating Hydrofoil Turbines

When generating power from riverine flows, oscillating foil turbines (OFTs) are an attractive solution for their high efficiency, depth adaptability, performance resistance to bio-fouling, and low environmental impact. We present new fluid-structure interaction simulations that model the unsteady flows of OFT systems that include an oscillating foil, springs, and a generator. The foil's pitch motion is fully prescribed and its heave translation is determined by solving the equations of motion of the foil-spring-generator system. Since the heave kinematics and, consequently, the angle of attack profile cannot be prescribed, the trajectory is controlled and optimized for maximum efficiency by adjusting structural and generator properties such as the pitching frequency to natural frequency ratio, and the linear and nonlinear generator damping coefficients. It is demonstrated that the frequency ratio can alter the heave waveform significantly, and damping is utilized to get the optimal balance of phase difference and heave amplitude. This study aims to uncover the key fluid-structure interactions and unsteady flow characteristics of semi-passive OFT systems.