Reducing the wave induced loading of tidal turbine blades through the use of a flexible blade

A study into the loading effects of wave-current interactions on a blade section of a horizontal axis tidal turbine was performed. Wave-current interactions were calculated based on 2^nd order linear wave theory and a numerical model for estimating loads on both rigid and flexible blade sections is presented, based on a quasi-steady analysis. Results from this model are compared with load measurements on two constant cross-section hydrofoils, one rigid and one flexible, in combined waves and currents in order to assess whether a flexible blade can lead to lower load fluctuations. Particle image velocimetry was used to investigate the flow field surrounding the hydrofoils throughout a wave period in order to better understand the underlying hydrodynamics. The flow experienced by each hydrofoil is found to be highly unsteady with hysteresis effects resulting in different loading profiles than the quasi-steady analysis predicts. The experimental results indicate that the oscillating pressure field, associated with an oscillating free surface, significantly changes the hydrodynamic behaviour of the hydrofoils. The flexible blade was found to reduce the magnitude of load fluctuations in addition to achieving higher lift to drag ratio as compared to its rigid counterpart.