Study on the performance of a floating horizontal-axis tidal turbine with pitch motion under wave-current interaction

Waves can induce motion in the floating platforms that support tidal turbines, affecting their hydrodynamic loads. To study the non-constant hydrodynamic of floating tidal turbines in a wave condition, this paper investigates the effect of pitch motion on the power coefficient (C_P), thrust coefficient (C_T), and wake flow of a tidal turbine using computational fluid dynamics. A pitch motion experiment is designed to verify the validity of the numerical method. The results show that the C_P and C_T exhibit periodic fluctuations under pitch motion, with the fluctuation period being consistent with the pitch period. Waves do not change the overall fluctuation trend of the C_P and C_T, but they do complicate the fluctuations and increase the likelihood of blade fatigue damage. Pitch motion reduces the mean power, with large-amplitude pitch motions particularly likely to result in severe power loss. The low-velocity region of the wake under pitch motion exhibits significant periodic oscillations. The wake is more susceptible to the pitch period than the pitch amplitude, and small-period pitch motions force the wake to deform earlier, accelerating wake vortex dissipation and velocity recovery. Increasing the immersion depth reduces the effect of waves on tidal turbine performance, but is not conducive to wake recovery. In summary, the rational design of the immersion depth and limiting the movement amplitude of the floating platform have the potential to prolong the working life of tidal turbines and increase their power output.