Aero-hydrodynamic Coupling Analysis Of Spar Floating Offshore Wind Turbine

In recent years,with the development trend of large-scale and offshore wind turbines,floating offshore wind turbines represented by Spar floating wind turbines have been favored by researchers.Unlike fixed foundation wind turbines,floating wind turbines are always in six degrees of freedom movement and affected by the coupling of aerodynamic and hydrodynamic forces.Compared to offshore engineering structures,floating wind turbines have a slender structure and a bigger upper wind-receiving area,the coupling relationship between aerodynamics and hydrodynamics is closer.Therefore,there is an urgent need to develop aerodynamic-hydrodynamic coupling models suitable for floating wind turbines,which is also of great significance for the design and optimization of floating wind turbines.In this paper,the NREL 5MW wind turbine is taken as the research object.Based on the lift surface method combined free wake model,the unsteady aerodynamic characteristics of floating wind turbines with various degrees of freedom are analyzed in detail.The aero-hydrodynamic coupling model of Spar floating offshore wind turbine is established.The movement characteristics and work performance of the floating wind turbine under the combined action of wind waves were studied.The main tasks of this paper as follows:1.A Spar-type floating offshore wind turbine aero-hydrodynamic coupling model is constructed based on Lagrange’s second equation;the lift surface method combined free wake model is adopted as the aerodynamic prediction model,taking into account the calculation accuracy and efficiency;based on the Airy wave and improved Morison equation,the hydrodynamic force of the floating platform is calculated;the mooring force of the floating wind turbine is calculated based on the quasi-static assumption.2.The unsteady aerodynamic characteristics of floating wind turbine under sway or yaw motion are analyzed in detail.Studies have shown that under sway motion,the overall power and thrust of the floating wind turbine are reduced,and the changes in the attack angle and effective speed of the airfoil along the azimuth are opposite,resulting in a certain phase difference between the blade power and thrust,and the thrust of blade1 continues Overload;The yaw motion causes the overall power of the floating wind turbine to increase and the thrust to decrease.The minimum power(the maximum thrust)is close to the static yaw condition,and the power and thrust of the blade3 continue to be overloaded.3.Evaluate the induced effect of the distorted far-field wake on the floating wind turbine.The mean induced effect of the distorted wake on the wind turbine is weakened,but the induced effect on some blades was enhanced.4.The aerodynamic characteristics of the floating wind turbine under the sway and yaw combined motion is analyzed.It is found that the performance of the floating wind turbine is similar to the linear superposition of the performance of the floating wind turbine under single-degree-of-freedom motion under small amplitude motion.As the movement intensifies,the intensity of the shedding vortex increases rapidly,and the linear characteristic no longer holds.5.Analyzed the motion characteristics of the floating wind turbine under the condition of no-wave and wind,only wave load.The natural frequencies of the system’s six degrees of freedom is obtained,which is in good agreement with the FAST software.Therefor the other models except the aerodynamic model are verified.At a wave incident angle of0 °,the wind turbine performs pitch and roll motions with the same characteristic frequency as the wave frequency;at the incident angle of90 °,it performs sway and roll motions.6.Based on the established aerodynamic-hydrodynamic coupling model,the analysis of the motion characteristics and aerodynamic characteristics of floating wind turbine under the combined action of wind and wave has been performed.Studies have shown that considering the aerodynamic-hydrodynamic coupling,floating Wind turbine motion will be slightly enhanced,accompanied by significant yaw motion.The influence of wind wave angle is also discussed.