| The horizontal axis wind turbines(HAWTs)operating in the atmospheric boundary layer are seriously affected by the flow exsiting in the boundary layer.As a result,their aerodynamic characteristics are highly unsteady.Especially,with the increase of the rotor diameter and installation height of the HAWT,the influence of the flow in the boundary layer on their aerodynamic characteristics is becoming more and more obvious.The non-uniformity and fluctuating characteristics of the inflow wind not only affect the distribution of the aerodynamic loads of the wind turbine,but also lead to complicated wake structure,which will affect the inflow conditions of the downstream wind turbine.Therefore,in order to clarify the influence of the inflow conditions on the aerodynamic loads and flow field characteristics of HAWT,the accuracy of the numerical calculation model and method used in this thesis was verified.Subsequently,considering the turbulence intensity,wind shear and low-level jet(LLJ)characteristics in the atmospheric boundary layer,the rotor aerodynamic loads and flow field characteristics of HAWT under complex inflow conditions were studied.The main researches in this work include:(1)Based on the inverse Fourier transform method,the uniform inflow wind fields with different turbulence intensity were established by IEC Von Karman spectrum model.The influence of turbulence intensity on the rotor aerodynamic loads of HAWT was studied.The results show that under neutral atmospheric conditions,when the turbulence intensity varies between 5% and 20%,the increase of inflow turbulence intensity is beneficial to increase the rotor torque,but it also causes the unbalanced rotor aerodynamic force and moment(lateral force,longitudinal force,tilt moment and yaw moment)to increase.(2)Based on the Reynolds time-averaged CFD method,the variations of the blade surface pressure and rotor aerodynamic force and moment were studied under shear inflow conditions.The results show that when the wind shear exponent changes,the surface pressure near the leading edge of the blade suction surface is more sensitive to the variation of the inflow wind speed and azimuth angle.There is phase lag phenomenon in the surface pressure with azimuth angle,and the closer to the blade root,the more obvious the lag phenomenon.The phase lag phenomenon can also be observed from changes in rotor torque and thrust with azimuth angle,and the larger the wind shear exponent,the more obvious the phase lag phenomenon.(3)Based on the plane wall jet theory and Von Karman spectra model with user-defined scaling,a fluctuating wind speed model named Jet_Shear was established,and a method for establishing the LLJ inflow fluctuating wind field was proposed.Under the LLJ inflow fluctuating wind fields with different heights,different intensities and different stabilities established by the Jet_Shear model and Great Plains Low-Level Jet(GP_LLJ)spectral model,the feasibility of the method of establishing the LLJ inflow fluctuating wind field by the Jet_Shear model was verified by comparing the rotor aerodynamic forces and moments.The results of the study under stable atmospheric conditions show that the LLJ height,LLJ intensity and LLJ stability have a significant impact on the rotor aerodynamic forces and moments when the LLJ is located at the heights of the rotor swept area.(4)Under the shear and LLJ inflow conditions,the influence of the single-point wind speed at the hub height method and the equivalent wind speed method on the calculation result of the inflow wind power was compared.The results show that the method based on the equivalent wind speed can significantly improve the accuracy of the calculation of the inflow wind power.Under the shear inflow fluctuating wind field established by the IEC Von Karman spectrum model and the LLJ fluctuating wind field established by the Jet_Shear model,the influence of the single-point wind speed at the hub height method and equivalent wind speed method on the rotor power output was compared.The results show that when the equivalent wind speed method is used to predicte the rotor output power,there is no obvious advantage under shear inflow conditions,but the accuracy of the prediction is significantly improved under LLJ inflow conditions.(5)Based on the Reynolds time-averaged CFD method,the wake characteristics of HAWT were studied under uniform and shear inflow conditions.The results show that the self-similarity of the axial velocity deficit is related to the inflow condition and tip speed ratio.Under the uniform inflow conditions,with the increase of the tip speed ratio,the curve of the axial velocity deficit is more likely to reach self-similarity state at the axial position closer to the rotor.And in addition to the edge of the wake region,the curve of the axial velocity deficit coincides with the Gaussian distribution curve,that is,the Gaussian self-similarity is satisfied.Under shear inflow conditions,the curve of the axial velocity deficit in the vertical plane does not satisfy the Gaussian self-similarity,and the deviation of the axial velocity deficit curve from the Gaussian distribution curve increases with the increase of wind shear exponent.Under the uniform and shear inflow conditions,as the tip speed ratio increases,the expansion effect of the wake is more obvious,and the pitch of the helix formed by the tip vortex gradually decreases.Under the shear inflow conditions,the shear effect of the inflow wind and the shear layer in the wake region cause the tip vortex undergo a Kelvin-Helmholtz instability.(6)Based on the Von Karman spectra model with user-defined scaling,the uniform,shear and LLJ inflow wind fields with the same turbulence length scale and standard deviation were established,respectively.The FAST.Farm open source code developed by NREL was used to study the influence of the inflow conditions on the wake meandering characteristics of HAWT.The results show that when the turbulence length scale and the turbulence standard deviation of the inflow wind are the same,the variation of the average wind speed distribution has no obvious influence on the position of the wake center.The farther away from the rotor,the greater the fluctuation of the position of the wake center in the horizontal and vertical plane. |
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