Research On Stress Stiffening Characteristics Of Large Wind Turbine Blades

At the first plenary meeting of the new State Council,Premier Li Qiang proposed to further promote environmental pollution prevention and control,continuously improve the quality of the ecological environment,and actively and steadily promote carbon peak and carbon neutralization.Achieving carbon peak and carbon neutralization is a major strategic decision made by the Party Central Committee and the State Council.It is of great significance to ensure energy security and efficiency,promote economic transformation and upgrading,accelerate the construction of ecological civilization,lead the response to climate change,and achieve the “two centenary” goals.In the process of promoting carbon peak,carbon neutralization and energy green,clean and low-carbon transformation,wind power will be the main supporting force.In the process of rapid development of wind power industry,the trend of large-scale development of wind turbines is becoming more and more obvious.In the process of the operation of wind turbines,the influence of stress stiffening effect caused by aerodynamic,gravity,centrifugal force and other loads on the aerodynamic and structural performance of wind turbine blades is more and more significant.In this paper,NREL 5MW wind turbine is taken as the research object,and the stress stiffening characteristics generated during the operation of wind turbine blades are studied and analyzed in detail by combining theoretical analysis with numerical simulation.The main contents and conclusions are as follows:1.Taking the NREL 5MW wind turbine as the research object,the aerodynamic performance of wind turbine is numerically simulated based on the CFD method,and the aerodynamic load of the wind turbine blade is calculated and analyzed.Secondly,combined with the basic principle and analysis process of the finite element method,the finite element software ANSYS Mechanical APDL is used to realize the composite laminate structure design and finite element modeling of the wind turbine blade.The structural static characteristics and stability analysis and research of wind turbine blades are completed based on the one-way fluid structure coupling method,the rationality and structural performance of the designed finite element model of the composite wind turbine blade are evaluated.The results show that the aerodynamic loads on the blades increase with the increase of wind speed.The results of static analysis show that the structural deformation of the wind turbine blade meets the requirements of the structure design standard of wind turbine blades,and the equivalent stress at the tip of the wind turbine blade is relatively small.Along the chord direction of the blade,compared with the stress at the position of the main beam,the stress at the leading edge and the trailing edge is smaller;the overall stress of the webs in contact with the blade skin is larger,and the overall stress of the middle part of the web and the web near the root and tip of the wind turbine blade is smaller.After considering gravity in the static analysis of the wind turbine blade,the maximum value of the surface stress and the two peaks of the average surface stress in the rotation period of wind turbine blades appear at the horizontal positions.The minimum value of the total displacement of the blade tip,the surface stress and the average surface stress of the wind turbine blade occur near the 180 ° azimuth during the rotation of the wind turbine blade.The results of the modal analysis and resonance check of the wind turbine blade show that the blade has no resonance risk within the operating range of wind turbine speed.The buckling analysis results show that the composite blade designed in this paper has high buckling resistance,and the stability meets the design requirements of wind turbine blades.2.Based on the finite element method and Euler-Bernoulli beam theory and through the analysis of the stiffness matrix of the beam element,the causes and influencing factors of the stiffening or softening effect in the rotation process of the wind turbine blade are clarified with the adoption of the simplified wind turbine blade model and the one-way fluid structure coupling method is used to analyze the prestressed modal of the wind turbine blade based on the more reliable finite element model of the composite wind turbine blade.The effects of aerodynamic prestress,gravity and centrifugal force on the modal characteristics of the wind turbine blade are studied and the degree of stiffening is evaluated.The results show that under the influence of aerodynamic prestress the blade has a “softening” effect.The first fiveorder modal frequencies gradually decrease with the increase of wind speed,that is,the increase of aerodynamic prestress.The effect of aerodynamic prestress on torsional vibration mode is related to the magnitude of aerodynamic prestress.When both aerodynamic prestress and centrifugal force are considered,the “stiffening” or“softening” effect of wind turbine blades is related to the vibration form of blades and the size of wind speed.The prestress has the greatest influence on the first-order modal frequency of the wind turbine blade.Considering the combined action of aerodynamic prestress,centrifugal force and gravity,the degree of stress stiffening of the first-order modal frequency of the wind turbine blade is the largest at vertical downward position of the blade tip.3.Aiming at the aerodynamic characteristics and structural mechanical properties of composite wind turbine blades under extreme operating gusts,the two-way fluid structure coupling numerical simulation analysis of the dynamic response characteristics of the NREL 5MW wind turbine composite blade under extreme operating gusts are studied by using Fluent and Transient Structural modules of ANSYS Workbench platform.The main conclusions are as follows: the aerodynamic loads of the wind turbine are greatly affected by the extreme operating gust,under the extreme operating gust,the torque and axial thrust of the wind turbine respond greatly with the change of wind speed,and the peak value of the response is earlier than that of the gust.In the rapid decrease stage of the wind speed of extreme operating gusts,the stall degree of wind turbine blades decreases gradually,and the torque and axial thrust of wind turbine fluctuate to a certain extent due to the influence of flow instability around the blade,and the fluctuation time of axial thrust is longer than that of the torque of wind turbine.Affected by the extreme operating gust,the displacements of the wind turbine blade in the flapwise and edgewise direction with the change of wind speed appears two peaks before and after the peak of wind speed and the two obvious response times of the two peaks of the displacement in the flapwise direction of the blade tip appear when the axial thrust reaches the peak and the fluctuation of the axial thrust is the most obvious.Compared with the uniform-stream stage,the average equivalent stress on the blade surface increases by 22.57% during the extreme operating gust,and the overall stress of the blade increases greatly.The laminated structure design can be further optimized to improve the bearing characteristics of the wind turbine blade,and the appropriate pitch strategy can be adopted to reduce the impact of extreme loads on the stable operation of the wind turbine.