The hysteretic behavior of Wells turbines is a well-recognized phenomenon. As it appears at nondimensional frequencies orders of magnitude lower than the ones studied in rapidly pitching airfoils and wings, the cause is likely to be different. Some authors found its origin in the interaction between secondary flow structures and trailing edge vortices. In this work, a detailed numerical analysis of the performance of a Wells turbine submitted to a sinusoidal bidirectional flow is presented. Computational results are compared with experimental data available from literature and suggest a new explanation of the phenomenon

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Sea wave energy is one of the main renewable energy resources. Its exploitation is relatively simple and determines a minimum impact on the environment. The system that is most often used for wave energy harvesting is composed of an oscillating water column device together with a Wells turbine. When designing the Wells turbine, its interaction with the oscillating water column system must be taken into account, if the energy collected is to be maximized. The most important interaction phenomenon is the so called hysteresis effect, i.e. the time delay between the piston-like motion of the air water interface and the torque developed by the turbine. This work presents a detailed analysis of the flow within an oscillating water column system, focusing on the differences in performance and in secondary flow structures between acceleration and deceleration, and between the inflow and outflow phases. This analysis demonstrates how the hysteresis between acceleration and deceleration is caused uniquely by compressibility effects within the oscillating water column system, while differences in the flow parameters and secondary structures near the rotor are negligible, if equivalent flow conditions are compared. The effects of the oscillating water column system configuration on the performance are also highlighted.