Study on the hydrodynamic and wake characteristics of variable speed control of horizontal axis tidal turbine under surge motion

The surge motion of the floating platform can affect the hydrodynamic performance and wake characteristics of tidal turbines, and variable speed control may be able to improve this issue. In this paper, CFD methods are utilized to investigate the variation of power coefficient, thrust coefficient, and wake flow for fixed speed and variable speed-controlled (fixed tip speed ratio) tidal turbines at different surge amplitudes and periods. The results show that under the same conditions, the variable speed control can significantly increase the mean output power of the tidal turbine compared with the fixed speed, and increase the fluctuation amplitude of the power coefficient and thrust coefficient. There is an obvious wake interaction phenomenon in the surge motion, and the variable speed control can avoid blade stall in the surge motion and improve the hydrodynamic performance of the tidal turbine. The near-field wake velocity deficit for the variable speed control is severe, but the wake recovery is much faster, and the recovery of the far-field wake is essentially the same as for the fixed speed. In addition, the variable speed control is conducive to cutting the intensity of the wake vortex, accelerating the dissipation of the wake vortex, and promoting the recovery of the wake flow field. Taken together, under the same surge conditions, variable speed control improves the generation efficiency and promotes wake instability and vortex interactions, which have a positive effect on wake recovery.