Study on the near Wake Aerodynamic Characteristics of Floating Offshore Wind Turbine under Combined Surge and Pitch Motion

Floating offshore wind turbines (FOWTs) may experience six degree of freedom (DoF) movements under the influence of environmental conditions. Different combinations of platform movements with the same amplitude and frequency may have distinct influences on the aerodynamic characteristics of the wind turbine. In this study, a detailed, full-scale CFD model of NREL 5 MW wind turbine is developed to investigate the specific aerodynamic and near wake characteristics under the influence of surge, pitch, and coupled surge-pitch platform motion based on the OpenFOAM tool box. It is clearly noted that different platform movements led to varying relative velocities of the blade, which affected the aerodynamic performance of wind turbines such as thrust, torque, and angle of attack (AOA). On the other hand, when the wind turbine was subjected to combined surge-pitch motion with the same phase, the wake velocity field fluctuated greatly, and the velocity at the center of the wake even exceeded the free flow velocity. Moreover, the platform movement affected the gap between the shed vortices. When the wind turbine moved forward, the gap between the vortices increased, while when the wind turbine moved backward, the gap between the vortices decreased or even converged, resulting in vortex-vortex interaction.