Effects of Rotation Direction on Blade Dynamics of Cross-Flow Turbines Under a Free Surface with Waves

This research computationally explores how surface waves interact with a cross-flow turbine (CFT) in a confined channel. CFTs extract energy from fast-moving water currents with an axis of rotation perpendicular to the freestream flow. Previous studies model CFTs with uniform freestream and walls at the channel boundaries. However, realistic operation commonly places CFTs in unsteady flows, often due to free surface effects. Using a volume of fluids approach to model the air-water interface, a free surface with small-amplitude waves is implemented above a uniform current. A horizontal CFT is placed in the channel below the free surface and operated at a moderate tip-speed ratio in both clockwise and counterclockwise directions. Resulting effects on power generation, unsteady forces, and blade-level flow dynamics are explored. It is expected that blade position during the strongest interactions with fluctuating velocities due to the surface waves will most strongly predict wave-turbine interaction effects. For a clockwise turbine, the strongest free surface interaction occurs in the latter portion of the blade power stroke, where previous research has shown blade acceleration to improve attachment of flow to the blade. For a counterclockwise turbine, the interaction occurs at the start of the power stroke, where velocity fluctuations are more likely to encourage earlier development or shedding of vortices along the blade.