Effects of freestream turbulence on energy harvesting of a single semi-passive oscillating hydrofoil

Rivers pose challenges for renewable energy device deployment due to shallow depths and variable, unsteady flow conditions. Vertically oscillating hydrofoil turbines appear well suited to accommodate changing flow conditions, produce high efficiencies, and reduce impact on wildlife. However, no studies exist on the impact of freestream turbulence on oscillating hydrofoil turbine performance. In this study, a semi-passive hydrofoil turbine is experimentally tested in a water channel, operating at chord-based Reynolds numbers ranging from 69000 to 91000. A passive turbulence grid was incorporated upstream to assess performance in a nearly uniform turbulence intensity profile with a turbulence intensity of approximately 5% and turbulent integral length scales on the order of 0.01 m. The velocity field just upstream of the hydrofoil was first characterized using hot-wire anemometry. Baffle boards were placed on top of the flow to mitigate free surface effects. Experiments with the hydrofoil device were then conducted with and without the turbulence grid over various flow speeds to obtain hydrofoil kinematics and force data. The results indicate that increased turbulence levels enhance power production by increasing heave velocities and vertical forces throughout one oscillation period. The most benefit was observed on the downstroke, where heave range/speed increased due to gravity and inertia. As a result, hydrodynamic efficiency increased by up to 12% with an observed maximum value of 52%.