Research On Wake And Aerodynamic Load Characteristics Of Horizontal-axis Wind Turbines Based On Autoregressive Linear Filtering

When a wind turbine is operating in a wind farm,it is subjected to complex inflow characteristics such as wind shear and turbulence,which has an important impact on the aerodynamic and wake characteristics of wind turbine.Under different inflow conditions,the wind turbines exhibit different wake characteristics.Studying the wake characteristics and load characteristics of wind turbine under different inflow conditions is of great significance to the microsite selection of wind farms,wind turbine power prediction and load design.Therefore,in this paper,autoregressive linear filtering method is used as large eddy simulation turbulent inflow condition to generate a China’s class B turbulent wind field.Based on the turbulent inflow method,a 33 kW wind turbine is numerically simulated to study wake evolution law and aerodynamic characteristics of wind turbin in shear inflow and Class B turbulent fields.The main research contents are as follows:(1)Based on the Autoregressive Linear Filtering method(AR method),the Kaimal spectrum and the exponential rate wind profile distribution law is used to obtain a series random data sequences that meets the characteristics of the target flow field.The random data sequences are injected into the computing domain as turbulent inflow condition.Compared the results of the turbulent field obtained by the AR method and the precursor method.The results show that compared with precursor method,the AR method can effectively improve the simulation accuracy and calculation efficiency in simulating turbulent wind field and it is easier to control the generation of the target turbulent wind field.(2)Under shear flow conditions,the frequency of velocity changes is equal to the rotation frequency of the wind turbin at 0 azimuth of blade tip positions of the turbin wake.As for the time-averaged axial velocity distribution in the wake region,the velocity profile presents the shape of "W" in the range of 1-7D.As the wake continues to develop backward,the "W" shape disappears and the "S" shape distribution appears.Due to the shear effect,the axial velocity profile is asymmetrically distributed at the height of the hub center.For the axial velocity at the height of the hub,the maximum loss occurs at the 7D position,and the maximum loss rate is 24.8%.For the turbulent kinetic energy at various positions downstream of wheel,the turbulent kinetic energy has the largest change range at the 1D position,indicating that this position is most affected by the wind wheel.As the distance from the wind wheel increases,the turbulent kinetic energy change range gradually decreases.The analysis of the instantaneous velocity contour shows that within the range of 1D-3D,the wake area is regular.As it develops backward,the wake area diffuses significantly and the affected area becomes irregular in downstream of wheel.From the results of wavelet analysis,regular turbulent energy changes appear at small-scale locations in range of 1-9D downstream,and as it develop backward,the regularity disappear.Overall,as the distance from the wind wheel increases,the small-scale turbulent structure gradually evolves into the large-scale turbulent structure.(3)Under the China’s Class B turbulent wind field,the average axial velocity profile of the wake is distributed in a "W" shape in the range of 1-5D downstream of the wind turbin."W" shape disappears as wake evolution.For the axial velocity at the height of the hub,the trend of velocity change is decrease first and then increase.At the 3D position,the axial velocity is the smallest,the speed loss is the largest,and the maximum loss rate is 14.5%.For the instantaneous velocity contour,the wake loss is obvious at 1D position,and the boundary with the outflow area is obvious.The mixing speed of the wake area with the mainstream area is accelerated in 3D later.The wake and the mainstream area have basically been mixed at 11 D position and wake velocity has basically recovered.Along the axis of the wind turbin,the turbulent kinetic energy increases in near wake area of the wind turbin.The turbulent kinetic energy gradually decreases and the turbulent kinetic energy returns to the inflow state in 3D later.For the power spectrum of the axial pulsation speed,compared with the 1D position in front of the wind wheel,the energy of the axial pulsation speed power spectrum at the downstream position of the wind wheel 1D-5D is significantly increased due to the influence of the wind wheel.At the downstream 1D position,due to the existence of small-scale turbulent vortices at this position,the inertial subregion is narrower and the dissipation region is advanced compared to the-1D location.At the 3D and 5D positions,the range of the inertial sub-region increases,and the position of the dissipation zone moves backward.At the 5D position,the frequency of entering the dissipation phase is 1.5 Hz.Wavelet analysis shows that small-scale and high-frequency turbulent structure appears at 0.3D position.Highfrequency turbulence and low-frequency turbulence coexist.With the backward development,the small-scale turbulent structure disappears,and the large-scale turbulent structure is the main turbulent structure.At the 11 D position,the turbulent structure is similar to the turbulent structure of-1D position.(4)Compared with the shear flow conditions,the axial velocity recovery of the wind turbine wake under the China’s Class B turbulent wind field is significantly faster,and the maximum loss rate is relatively small.(5)In the China’s Class B turbulent wind field,the aerodynamic load of the wind turbine exists peak value at the position of an integer multiple of the blade passing frequency,and the peak value decreases as the frequency increases.Compared with the shear flow,the energy of the torque and thrust of the wind turbin is significantly increased under the Class B turbulent field,and the torque distribution range is increased.For the pressure and streamline distribution around blade in the shear flow and Class B turbulent wind field,the pressure difference is large near the suction surface of the blade leading edge under the two conditions,indicating that the blade front suction is more sensitive to the turbulent characteristics of the incoming flow.Compared with the shear flow,the separation point of the blade boundary layer in the Class B turbulent wind field is closer to the trailing edge of the blade,and the boundary layer separation position moves backward,and the separation vortex scale also decreases relatively.This indicates that turbulence causes the blade boundary layer more resistant to backpressure gradients,leading to delay of the separation position of the boundary layer.(6)Compared with shear flow,the fluctuation range of blade load is significantly increased in Class B turbulence field.According to the spectrum analysis,the energy of blade load under turbulent flow is higher.