| Due to the random change characteristics of wind direction and the hysteresis effect of wind turbine yaw adjustment,wind turbines operating in an outdoor environment are often in a yaw state.And wind turbines are subjected to strong unsteady aerodynamic loads,which can easily cause excessive structural fatigue and damage,which will seriously affect service life.This paper first takes the airfoils at the 30%R,63.3%R and 95%R(R is the radius of the rotor)section of the NREL PhaseⅥ wind turbine blades as the research object,and theoretically calculates the threesection wing under the 30°yaw condition.The change rule of the angle of attack and the inflow velocity is established,and the corresponding change model is established.Secondly,based on the LES method,the dynamic aerodynamic characteristics of the pitch airfoil with uniform flow and variable speed flow are studied.Finally,the dynamic aerodynamic characteristics of the Phase Ⅵ rotor in a uniform incoming wind field with a yaw angle of 30°are studied.The main conclusions are:(1)When the wind wheel is operating under yaw conditions,the angle of attack and the inflow velocity of the airfoil in each section of the blade show an approximate sine and cosine change law,that is,the airfoil is pitching.And the transient change law of the inflow velocity and the angle of attack differ by 180 °.The average angle of attack,pitch amplitude and reduced frequency of the blade root airfoil in pitching motion are the largest,and the average value and fluctuation amplitude of the blade tip airfoil inflow velocity are the largest.(2)During the airfoil pitching process,flow separation occurs and dynamic stall occurs.The dynamic stall of the blade root airfoil is relatively large,and the dynamic stall of the tip airfoil is relatively small.Separation bubbles appear at the airfoil up side,and the dynamic change process of the separation bubbles,lead to a small range of violent oscillations in the front of the airfoil up side,which enhance the instability of the flow.The pressure at each point on the airfoil surface presents a high degree of unsteady characteristics,especially the pressure fluctuations on the suction surface have a large sudden change characteristic,and the airfoil the pressure diversification range near front of airfoil upside is the largest,and the maximum pressure fluctuation range of the airfoil leading edge at 95%R is 3.7 times and 1.5 times the maximum pressure fluctuation range of the airfoil leading edge at 30%R and 63.3%R,respectively.(3)the hysteresis loop area of each pitching airfoil under variable-speed flow has increased 2.1 times,8.8 times and 23 times,respectively,the stall occurs earlier.Flow separation on the airfoil surface is more intense,and the flow recovery is faster,indicating that the airfoil coupled with variable-speed and variable attack angle is more prone to dynamic stall.The closer to the blade tip,the more significant the influence of the change of the incoming wind speed on the aerodynamic characteristics of the pitch airfoil.The airfoil pressure fluctuates the area with the largest range is still near the leading edge,and the pressure fluctuation range of the tip airfoil leading edge is2.6 and 1.8 times that of the root airfoil and the middle airfoil respectively.Compared with the uniform incoming flow,the pressure fluctuation range of each airfoil leading edge is increased,indicating that the coupling of variable attack angle and variable angle of attack has increased the unsteady aerodynamic characteristics of the airfoil surface.(4)For a wind wheel operating under yaw conditions,the aerodynamic force of the blades and the aerodynamic force of the airfoil(stationary,pitching)have a large deviation.Due to the influence of the yaw angle and the three-dimensional rotation effect,the induction effect of the wind wheel on the airflow is increased,causing the flow hysteresis effect,which makes the distribution of the blade surface pressure vary with the azimuth angle.There is a phase shift phenomenon in the change.This phenomenon is more pronounced at the blade root.the pressure diversification range is larger near the front of the blade.Compared with the root of the blade,the pressure fluctuation range of the leading edge of the leaf has increased by about 5 times,and the pressure fluctuation range of the leading edge of the blade tip has increased to 6.8of that of the leading edge of the blade.It indicates that the leading edge of the blade tip is more sensitive to unsteady flow,and the unsteady position near the leading edge of the tip is greater.That is,the area near the leading edge of the tip has a greater contribution to the power fluctuation,which can be used for research the tip flow control method,optimize the blade design,reduce the dynamic load of the blade,smooth the power fluctuation,and provide a certain reference for improving the quality of wind power. |
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