Unsteady aeroelastic performance of the 15 MW floating offshore wind turbine under surge condition

For the floating offshore wind turbines (FOWTs), aeroelastic issues are more pronounced and complex due to the combined effects of blade deflections and platform motions, particularly for ultra-large wind turbines with blade lengths exceeding 100 m. The present study focuses on the unsteady aeroelastic performance of the 15 MW FOWT under surge condition. The ways in which the blade deflections affect the wind turbine performance are analyzed, and the hysteresis induced by the unsteady aeroelastic factors and its effect on the aerodynamic stability of the FOWT are discussed. Besides, the effects of the surge amplitude and frequency are investigated. The results show that, the torsional deflection and the flapwise vibration of the blade may affect the aerodynamic loads by reducing the angle of attack and the local normal velocity at the blade sections, and the edgewise deflection may bring the bend-twist coupling effect like the swept blade and thus affect the blade torsional deflection. Obvious hysteresis in the load responses is observed under surge condition, primarily due to the trailing vortex induction, the airfoil unsteady aerodynamics and the mass inertia of the blade. The hysteresis has a negative effect on the aerodynamic stability of the FOWT. Moreover, surge frequency is found to have a more pronounced effect on aerodynamic stability of the FOWT than surge amplitude, and has significant effects on the phase lag in the load responses and blade vibration velocity.