Research On Unsteady Aerodynamic Characteristics Of Horizontal Axis Wind Turbine Blades In Field Environment

Due to the influence of atmospheric turbulence,wind shear,gust,tower shadow and accommodative lag,the power of wind turbines operating in the field environment presents strong fluctuation characteristics.And as wind turbine energy conversion components,the unsteady aerodynamic characteristics of wind turbine will directly affect the power fluctuation of wind turbines.At present,there are theoretical,experimental and numerical simulation methods to study the unsteady aerodynamic characteristics of wind turbines.However the theoretical research is limited by many assumptions and modified models,wind tunnel experiment is difficult to reconstruct the field wind condition accurately,and has the limitation of similarity theory.So,they cannot accurately reflect the aerodynamic characteristics of the wind turbine in the field environment.In this thesis,numerical simulation and field experiments were used to study the surface pressure distribution characteristics of a 33 kW horizontal axis wind turbine blade,analyze the sensitivity of blade aerodynamics to the field wind conditions,and study the evolution mechanism of blade unsteady characteristic.The main research contents are as follows:1.Based on the support of "973" project,the comprehensive measurement methods of wind turbine inflow,wake flow and aerodynamic characteristic in field environment were studied,and a field experimental platform was built in Jingtai,Gansu Province.2.The unsteady aerodynamic characteristics of tip airfoil and root airfoil were numerically studied by using k-ω SST model under uniform inflow conditions.(1)The formation mechanism of unsteady aerodynamic forces was analyzed.The results showed that the lift and drag coefficient of tip airfoil varied periodically due to the combined effect of greater surface roughness and increased trailing edge thickness.While the large-scale vortex structure appeared in the flow field of root airfoil due to the combined action of large angle of attack(11.273°)and increased trailing edge thickness,which led to unsteady aerodynamic characteristics.(2)By numerically research technique,the flow field structure and aerodynamic distribution of the rotating wind turbine were researched,the pressure distribution,stall characteristics and yaw operation characteristics of the wind turbine blade were analyzed.The results showed that the leading edge pressure of the blade suction was sensitive to the variation of the incoming wind speed and azimuth angle.3.By combining AR linear filtering method and large eddy simulation(LES),a turbulent wind field simulation method was developed,the turbulent wind field with shear and fluctuation characteristics was then reconstructed to study the unsteady aerodynamic characteristics of airfoils.The results showed that,according to the characteristics of pressure probability distribution,airfoil surface could be divided into gaussian distribution area and non-gaussian distribution area,and the non-gaussian distribution area appeared in the leading edge of airfoil suction.The spectral characteristics of the pressure varied greatly in the low frequency band,and almost coincided in the high frequency band.This phenomenon indicates that the unsteady aerodynamic force of the airfoil is mainly derived from the low-frequency large-scale vortex in the incoming flow.In turbulent wind field,the power spectrum characteristics of thrust and power showed that the power spectrum had obvious peak value,and the peak value decreased with the increase of frequency.The corresponding peak frequency was consistent with the integral multiple of the blade passage frequency.4.In the field experiments,the pressure distribution characteristics of airfoils with different spanwise were measured under yaw operating conditions.The pressure dispersion,deflection amplitude,steep and slow degree and non-Gaussian characteristics of seven airfoils were compared and analyzed by means of mean,standard deviation,skewness,kurtosis and probability distribution statistics.The sensitivity of airfoil pressures with different spanwise directions to the field wind conditions was also studied.The results showed that the leading edge was the most sensitive to the field wind conditions.The dynamic variation of lift and drag of each airfoil was studied and the results were compared with CFD simulation.The influence of three-dimensional rotating effect on lift and drag was studied.The stall delay,tip effect and dynamic characteristics of the rotating blade ware also verified.The fluctuation of pressure coefficient indicated that there was a ridge-like low pressure zone in the leading edge of suction,which indicated the variable-speed and variable-angle-of-attack coupling characteristics of each airfoil when the blade is yawing.5.FFT transform was carried out on the pressure fluctuation of each airfoil to analyze the variation of the pressure power spectrum peak along the chord direction,the relationship between amplitude and frequency,and to study the pulsation source of the fluctuating pressure at each point on the blade surface of the horizontal axis wind turbine.The results showed that,(1)when the frequency was equal to 1 times of the wind turbine running frequency,peak values occurred in all pressure power spectra and the leading edge peak was highest.The power spectrum peak of the trailing edge pressure of the airfoil increased gradually with the span gradually approaching the blade root.This explained that the pressure fluctuation was influenced by the wind shear and yaw operation of the blade,which were related to the speed of the blade during the blade rotation.(2)The slope of medium frequency power spectrum conformed to Kolmogorov power law with exponent of-5/3,which indicated that the flow characteristics of wind turbine blades presented energy-containing region,inertia sub-region and dissipation region.When the frequency was equal to 1 and 2 times of the wind turbine rotation frequency,the peak value of lift-drag power spectrum of airfoil appeared,and the peak value of lift coefficient was much larger than that of drag coefficient,which indicated that the lift-drag coefficient of airfoil changed periodically with the integral times of the wind turbine rotation frequency.