Experimental Study On Dynamic Stress Characteristics Of Blade Under Non-Uniform Wind Direction

Wind in nature is always changing.Horizontal axis wind turbines often operate under dynamic wind direction changes or yaw to wind conditions.At this time,the blades are subjected to a variety of coupled loads,and the force is very complex.In this paper,S-airfoil small horizontal axis wind turbine blades with long transition section are taken as the research object to analyze the blade stress characteristics under the dynamic change of wind direction.Firstly,an experimental study was carried out to explore the inherent properties of the blade and find the position of the stress and strain test points.By allocated in the different position leaves exhibition to acceleration sensor for modal experiment,determine the natural frequencies and found for the first three order modal vibration mode as the main mode,waving,shimmy as the main vibration mode,and to determine the first three order modal corresponding vibration section line,combining with the center line of the blade aerodynamic,find the blade stress and strain experiment of observation.Secondly,the sensitivity of stress distribution at different measuring points to wind speed change,tip speed ratio change and wind direction change speed is qualitatively analyzed,and the location of stress concentration area at blade root and blade spread is found.By controlling the dynamic yaw test bench to simulate the constant wind direction and the constant wind speed change,the stress data of different positions in the chord direction of blade root and the astward direction of blade were obtained by using a multi-channel synchronous acquisition instrument.The results show that the maximum principal stress at the same position is larger than the maximum shear stress,and the maximum principal stress and shear stress decrease from leading edge to trailing edge along the chord length direction of the blade root.The maximum principal stress and shear stress decrease from blade root to blade tip along the blade spreading direction.The direction of principal stress is almost constant under the influence of different factors.It is most sensitive to the centrifugal load caused by the change of the tip velocity ratio,and most insensitive to the aerodynamic load caused by the wind speed.Finally,three wind direction step change models were built to quantitatively analyze the response characteristics of stress at different measuring points of blades to wind direction change under rated working conditions.According to relevant standards,three wind direction models are put forward,which are step acceleration change of wind direction,step deceleration change and step fluctuation change respectively.And with three kinds of wind model for test condition,the quantitative analysis of blade measurement under the rated conditions different measuring point stress on the mutation of response characteristic,found that the maximum principal strain,the maximum principal stress of the fitted curve in a variety of slope and the direction of the wind model wind direction change of angular velocity one-to-one correspondence,by investigating the average,maximum and minimum value and the standard deviation of 4 aspects,It is determined that when the wind direction changes,the maximum principal stress also changes to some extent.As for the yaw pair risk control system,it is found that the optimal yaw angular velocity is within the range of 0.5 °/s～2.5 °/s.