| With the large consumption of traditional fossil energy and the rapid development of modern society and economy,wind power,as an emerging clean energy industry,has received strong supports from various countries’ policies and is booming and flourishing.Wind turbines are the basis and key links for converting wind energy into electrical energy.Limitations of the natural environment and technological innovations have driven the single wind turbine capacity to increase.Increasing the size of modern wind turbines becomes a trend.Increasing blades and the tower lengths is often accompanied by their increased structural elasticities,which is more likely to cause elastic deformations.The complexity of the inflows and the instability of the structural elastic deformations affect the aerodynamic performances and loads of the wind turbine adversely.Therefore,the thesis is based on the FAST software platform,taking the NREL 5MW onshore horizontal-axis wind turbine as the research object,using a modified blade element momentum(BEM)theory and geometrically exact beam theory(GEBT)coupled method for aeroelastic research of the blades,considering the coupling effects of elastic deformations of tower and other structural components,to study the aeroelastic deformations of the blades and tower and their effects on aerodynamic performances and loads of the whole wind turbine under inflows.The main research work is as follows:(1)The accuracy of the numerical calculation method for aeroelastic coupling implemented by the FAST code was verified.The aerodynamic and structural dynamic coupling calculation method which using AeroDyn+BeamDyn can reflect the nonlinearity and large deformation characteristics of the 5MW wind turbine blades compared with AeroDyn+ElastoDyn accurately,and it also describes the bendingtorsion coupling characteristics of the blades.Consequently,the calculation method is suitable for aeroelastic analysis and research of large wind turbines.(2)For uniform inflows,we studied the periodic change trends of the aeroelastic performances for elastic and rigid wind turbines at rated uniform wind speed first.The calculation results show that the aerodynamic performances of the wind turbine are affected by the blade passing through the front of the tower three times during a period with three minimum fluctuations;the elastic deformations of the blade and tower will cause the aerodynamic performances of the wind turbine to decline,and the thrust、torque and aerodynamic power of the elastic wind turbine are reduced by 3.0%,2.7% and 3.0% respectively compared to rigid turbine.The blade flapwise deformation and bending moment,elastic torsion angle and torque all deviate from the sine or cosine variation rules at 180° azimuth angle and decrease significantly;the blade edgewise deformation and bending moments change with the sine or cosine curves.The fore-aft and side-to-side bending deformations of the tower change steadily during an operating cycle.The fore-aft bending moment and torque are also affected by the blade passing through the tower,and three fluctuation peaks appear in the revolution cycle.Next,we studied aeroelastic performances of a wind turbine for different wind speeds which below and above rated speed.The results show that under uniform wind inflows,the main elastic deformation modes of the wind turbine are blade flapwise deformation and tower fore-aft bending.As the wind speed increases,the elastic deformations of the turbine increase before rated wind speed.After rated wind speed,the bending deformations of blade and tower decrease with the increases of the wind speed.And the maximum elastic deformation appears at rated wind speed.Compared with the rigid turbine,the wind turbine which considering the structural elasticity of blades and tower,has a reduced thrust coefficient,torque coefficient and power coefficient near the rated wind speed region.(3)The shear wind inflows that change exponentially with height are established,and the aeroelastic performances of the wind turbine under wind speeds with different shear indexes are studied.The results show that aerodynamic performances of the elastic turbine are also lower than that of the rigid turbine under the shear wind inflows.And with the increase of the shear index,the periodic mean values of the thrust,torque and aerodynamic power of the wind turbine decrease.The blade tip elastic deformation has an azimuth lag of about 20°,the amplitude of the periodic fluctuations of the flapwise and edgewise deformations increase with the increases of the shear index;the stress loads on the blade root are basically consistent with the periodic change trend of the blade tip elastic deformations.However,the sudden change of the blade tip elastic twist at the azimuth angle of 180° is not same as the change trend of the root torque.The period averages of both the bending deformations and the stress loads of the tower increase with the increases of the shear index.(4)Based on the Von Karman velocity spectrum model,turbulent wind fields with different intensities were established.The fluctuating velocity power spectrums of the simulated wind field in three directions at the hub height coincided with the objective function spectrums,so that the reliability of the simulated turbulent wind speed distribution was verified.The turbulent wind fields with four strengths of 5%,10%,15%,and 20% established in this thesis are used as the inflow conditions of the wind turbine,and the aeroelastic performances of the wind turbine when considering the structural elasticity were studied.The calculation results of the 200~300s region in the total effective simulation time of 300 s were selected to analyze the aeroelastic performances of the wind turbine under different turbulent inflows.The results show that with the intensity of turbulence increases,the mean values of the aerodynamic performances、output power、elastic deformations and loads of the wind turbine decrease,and the fluctuation ranges increase.However,the average values of yaw torque on the tower top and base increase with the increase of turbulence intensity.In the period of 200 ~ 220 s,the instantaneous wind speeds in the downwind direction are higher than the rated wind speed under four different turbulence intensities.The rotor thrust、 blade tip flapwise deflection and bending moment and the tower fore-aft deflection all decrease synchronously.The blade root torque decreases from the positive direction,and changes from positive to negative at high turbulence intensity,which indicates that under large instantaneous wind speed,the moderate increase of the pitch and elastic torsion angle of the blade can effectively reduce the blade、rotor and tower normal loads relative to the rotation plane,and stabilize the output power of the turbine at the same time.In addition,the loads on the tower are closely related to the aerodynamic forces of the three-blade wind turbine.The yaw torque moment at the top of the tower is related to the unbalanced aerodynamic forces which distribute on the left and right half rotor planes,the tilting moment is related to the unbalanced aerodynamic forces from the up and down half rotor planes.The overturning bending moment at the bottom of the tower is related to the axial thrust of the rotor.The research method and analysis results of the thesis can provide a reference for the aerodynamic-structural integrated optimization design of large horizontal-axis wind turbine blade.The use of the open source software tool FAST can provide a basis for the reliable calculation of wind turbine loads when considering aeroelastic deformations in engineering practice. |
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