Confinement and 3D effects on the fully-passive oscillating-foil hydrokinetic turbine

In recent years, the fully-passive oscillating-foil turbine (OFT) has been extensively studied for unconfined and 2D flows, with efficiencies reaching up to 50% at high Reynolds number (Boudreau et al., 2020). Such high efficiencies were obtained for a specific set of structural parameters, which passively control the pitch and heave motions of the foil. However, in more realistic conditions where confinement and 3D effects are present, the fully-passive OFT has not yet been tested. In this study, we conduct 3D URANS numerical simulations in which the blade's span and the turbine's confinement level are varied while maintaining the same set of optimal structural parameters. As expected, the performances of finite-span turbines are lower than their 2D counterparts due to blade tips effects. To mitigate the impact on power extraction, a simple adjustment of the heaving damping coefficient modeling the turbine's generator, is proposed and tested. It is found that lowering the heaving damping coefficient increases the turbine's efficiency. An original procedure to quantify the required adjustment of the heave damping coefficient is proposed and validated.