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Platform motions induced by waves pose a challenge to accurately predict the aerodynamic performance of floating offshore wind turbines (FOWTs). In view of this, the power performance and wake structure of FOWTs under platform pitch, surge, and their combined motions were investigated in this paper, using the computational fluid dynamics software, STAR-CCM+, with overset meshing and rigid body motion techniques. First, the simulation cases in single and same-phase combined motions with different amplitudes and frequencies were performed. Afterward, the approach of calculating the phase difference between pitch and surge motions was proposed to investigate the influence of the combined motion with phase difference on the aerodynamic performance. Results show that the increment of amplitude and frequency augments the mean power output and aggravates the power fluctuation in single and same-phase combined motions. The intensity of power variation under combined motion with a phase difference is weakened at 0.1 Hz compared to the single motion, while enhanced at 0.2 Hz, showing a different influence law on the aerodynamic performance. In addition, this paper established the power fluctuation table based on real sea states of Shidao in China, providing a certain reference for the controller design in this sea area.

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Accurate prediction of the dynamic responses of the floating offshore wind turbine (FOWT) under the blade pitch motion is quite challenging because of the strong nonlinear effects. In this study, a fully coupled and highly elaborated model was established based on the computational fluid dynamics (CFD), with the dynamic fluid body interaction (DFBI) method. The multi-stage movements consisting of the six degrees of freedom (DOF) motions of the platform, the rotation of the rotor, and the blade pitch motion, were defined by the superposition motion technologies. The blade pitch control module was created through the user-defined function (UDF) to regulate the blade pitch motion. Then several coupled dynamic simulations of the full-configuration DeepCwind floating wind turbine system were performed in power production, shutdown and startup cases. The simulation results in the power production case indicate that the blade pitch motion decreases the generated aerodynamic loads and amplifies the response amplitude of the platform as negative damping is introduced in the FOWT system. The simulation results in the shutdown and startup cases indicate that the extreme motion responses are enlarged, and the mooring line tension oscillates dramatically when it is in high-tension states. In addition, the nonlinear interference effects in the unsteady flow fields, such as the shedding vortices broken by the blade pitch motion, are visualized and investigated in detail.

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