Unsteady aerodynamic characteristics of a floating offshore wind turbine in propeller state

The operating state of floating offshore wind turbines can transition from windmill state for harnessing wind energy to a propeller state for driving airflow, due to oscillations caused by wind-induced thrust and wave-induced motions. Accurate assessment of the unsteady aerodynamic characteristics of floating offshore wind turbines is essential for comprehensive dynamic analysis. The aerodynamic characteristics of wind turbines in a windmill state have been thoroughly studied and are considered representative of floating offshore wind turbines. Nevertheless, the aerodynamic mechanisms of floating wind turbines in a propeller state remain unclear, due to the high unsteadiness and strong nonlinearity in the local flow field around the blades. This study investigates the unsteady aerodynamic characteristics of a floating offshore wind turbine in a propeller state by inducing surge-direction oscillations using a computational fluid dynamics approach. The three-dimensional unsteady flow fields around wind turbine blades undergoing surge and rotation motions in vortex ring and propeller states are thoroughly demonstrated and elucidated. The aerodynamic thrust and torque characteristics of the wind turbine in a propeller state are analyzed and compared to those in a windmill state. Frequency-dependent aerodynamic added mass and damping characteristics of the floating offshore wind turbine are investigated using forced oscillation approach. The findings indicate that at high oscillation frequencies, the wind turbine encounters negative aerodynamic thrust due to blade tip-vortex interaction and flow recirculation at the blade root. The maximum aerodynamic thrust significantly increases with higher oscillation frequencies, whereas the mean aerodynamic thrust remains nearly constant regardless of oscillation frequency. The negative aerodynamic torque is observed despite the small yet positive aerodynamic thrust, and a secondary torque cycle is found significant at high oscillation frequencies. In the propeller state, the wind turbine experiences reduced aerodynamic damping compared to the windmill state.