Experimental Study On Aeroelastic Response Of Flexible Wind Turbine Blades

The trend in the development of offshore horizontal axis wind turbine blades is towards slender,highly flexible,and low-mass designs.However,the stability of wind turbine blades in shutdown mode is threatened by extreme weather conditions such as typhoons and tornadoes,which frequently occur in offshore wind farms along the southeastern coast.It will cause serious geometric nonlinearity,aeroelasticity,resonance,instability,and other dynamic problems due to high wind speeds,turbulence,and multiple wind directions.Therefore,it is necessary to study the influence of parameters such as pitch angle,azimuthal position,wind speed,and turbulence intensity on the aeroelastic response of large-scale offshore wind turbines in shutdown mode,in order to provide reference for the aeroelastic stability design of long,flexible blades.This paper uses wind tunnel testing to measure the displacement and acceleration of flexible wind turbine blades under different pitch angles,azimuthal positions,wind speeds,and turbulence intensities,and to analyze the aeroelastic response and aerodynamic damping characteristics of the blades.The main research content includes using the NREL 5MW wind turbine blade as the research object,designing a multi-degree-of-freedom aeroelastic model with 1:30 scale based on the similarity theory of scaled models,and using the sudden load-unloading method to obtain the free decay acceleration response of each section of the blade under different initial displacements,thus obtaining the dynamic response parameters of the scaled model of the blade.The results show that the aeroelastic model of the flexible blade is designed reasonably,and can simulate the first three frequencies and modes of the wind turbine blade as well as the aeroelastic response under different wind speeds and directions.A flexible wind turbine blade aeroelastic response testing platform was constructed based on an acceleration measurement experimental system and a scaled model of a flexible wind turbine blade.Wind tunnel experiments were carried out on the scaled model to study the aeroelastic behavior.The root mean square(RMS)acceleration of each section was analyzed with respect to the wind direction angle,wind speed,and height.It was found that the most unfavorable pitch angles were between-120°and-105°,and between 45° and 105°,where the tip acceleration response increased significantly.The RMS tip acceleration at pitch angles of 0°,60°,90°,and 105° was analyzed as a function of wind speed,and it was found that when the wind speed approached the excitation wind speed,the blade exhibited large amplitude lock-in vibrations in the flapping direction,with vortex-induced vibration having a significant effect on blade flapping direction.By using fast Fourier transform analysis on the acceleration frequency response,it was found that when the characteristic length of the blade was small(pitch angles of θ=90° and 105°),the frequency of vortex-induced vibration was more likely to lock in with the natural frequency.In the pitch angle range of 105° to 120°,the blade deflection was significantly affected by the pitch angle,causing significant geometric deformation.Near a pitch angle of 180°,high wind speeds had little effect on the RMS deflection of the tip,but had a significant effect on the maximum deflection.When the test wind speed exceeded the cut-out wind speed,the blade exhibited strong nonlinear characteristics,particularly in the wind vibration-sensitive region.The aerodynamic damping ratio of the model under the influence of wind speed and pitch angle was identified using the random decrement method and the autoregressive moving average model.It was found that the aerodynamic damping ratio of the blade when operating in the forward direction was lower than that at other pitch angles at most win.In this study,two turbulent wind fields with mean turbulence intensities of 11.8%and 20.0%were modulated to create three different wind fields for subsequent experiments.The power spectral density functions of the grid flow field for the three scenarios were compared with the corresponding empirical wind spectra,specifically the Kramers spectrum.The acceleration frequency spectra in the flapping direction of the blade at 90° and 105° pitch angles were analyzed,and it was found that the vortex-induced vibrations in the flapping direction of the blade were greatly reduced as the turbulence intensity increased.No obvious vortex-induced vibration phenomena were observed from the blade acceleration response,and vibration was dominated by the second-order natural frequency of the blade.Acceleration tests were performed on the scaled flexible wind turbine blade models under different turbulence intensities.The aeroelastic acceleration response characteristics of the flexible wind turbine blade under different turbulence intensities were studied and compared with the results of uniform flow tests,revealing that the turbulence intensity had a more significant impact on the blade’s aerodynamic instability under high angles of attack.The effect of turbulence intensity on the damping ratio of the blade under different pitch angles and wind speeds was also analyzed.It was found that high turbulence intensity at the leading edge position significantly reduced the damping ratio of the blade,which had a significant impact on the blade fatigue load.The findings of this study contribute to a deeper understanding of the aeroelastic coupling characteristics of flexible wind turbine blades and the factors influencing damping in typhoonprone regions,and provide guidance for the design of large-scale wind turbines with long,flexible blades in coastal and offshore areas.