High-fidelity modelling of lift-based wave energy converters in a numerical wave tank

Abstract

This work presents the development of a numerical model for the investigation of lift-based wave energy converter (lift-WEC) hydrodynamics. The developed model, based on the RANS equation, is validated regarding its ability to replicate foil-flow interaction in various conditions based on experimental references from literature. Limitations of the numerical model are discussed, such as the overprediction of stall angles at low Reynolds numbers and the possibility of artificial rotor ventilation near the free surface. Within the range of relevant operating conditions for a lift-WEC, the model is found to replicate experimental results with good accuracy. Subsequently, a particularity of the cyclorotor WEC concept is investigated: Load measurements in calm water showed positive tangential forces, apparently indicating the generation of thrust in the absence of external energy sources. Numerical simulations reveal that this is the result of a significant azimuthal shift of the hydrodynamic load centre for non-zero pitch and that the consideration of foil pitch moments is required in order to obtain accurate shaft torque predictions. The hydrodynamic coefficients of the cyclorotor foils are compared with coefficients obtained for symmetrical foils in straight flight. It is shown that the assumption of similarity, which is often applied when modelling these devices in semi-analytical tools, is only valid for small angles of attack.