Study On The Bend-twist Coupling Effect Of Wind Turbine Blades Based On Eccentric O-beam

Aero-elastic cutting method is a passive load reduction technology for blades.Through the coupling design of bending and torsion of the material or structure,the section of the blade can be twisted when bending deformation occurs,and the aerodynamic Angle of attack can be changed,thus reducing the aerodynamic load.In this paper,the finite element analysis method is mainly used to study the torsional coupling problem(including aeroelasticity)of an O-beam and an improved design structure under bending load.The main work and research contents are as follows:1.The second-order finite element calculation is realized by self-programming,and the accuracy is verified.The displacement matrix,strain matrix,gaussian integral method and the formation of stiffness matrix are introduced in detail by using 20-node second order hexahedral element.By calculating and analyzing the bending of an L beam,the accuracy of finite element program is verified.2.The stiffness,natural frequency and bending-torsional coupling effect of symmetric O-beam and inner eccentric O-beam are analyzed by using self-programmed program.The total material and theoretical bending stiffness of the beam are equal to that of the symmetric O-beam by using the inner circle eccentricity,which is also proved by the calculation results.Because of the asymmetry of the structure of the eccentric O-beam,when the bending deformation occurs,the section is accompanied by torsional deformation(symmetric O-beam has no torsion),and this deformation is related to the eccentricity and the thickness of the beam.It is also found that eccentricity causes a decrease in torsional mode frequency.3.The effect of asynchronous eccentricity on the bottom and top two inner rings of O beam is studied.The bending and torsional deformation,frequency modal characteristics and bending-torsional coupling effect of different O-beams under static load conditions are introduced in detail.The bending stiffness and bending-torsional coupling coefficient of three O-beams with different eccentricity forms and different wall thicknesses are compared and analyzed.4.Three dimensional structural dynamics algorithm and one dimensional Yesin Grossman aerodynamics method are used to establish an aeroelastic analysis program.Using NACA64318 airfoil skin and four kinds of beam structures(symmetric O-beam and three kinds of eccentric O-beam),the wind-induced vibration phenomena of the four geometrical models at 12m/s,15m/s and 18m/s,respectively,are studied.By analyzing the time curve of the further-end blade element’s ups and downs displacement,pitching Angle,lift force and torque,the ups and downs displacement amplitude of the eccentric O-beam(compared with the non-eccentric state)is reduced by 25%,50% and 70% respectively(in the flutter state),which has a good effect of reducing the amplitude of wind-induced vibration.The self-programmed algorithm program can save the time of repeated modeling and multi-parameter comparative analysis,and the physical problems affected by multiparameter can be calculated and compared comprehensively.At the same time,the study proves that this O-type eccentric girder structure can achieve better bending-torsion coupling effect and realize passive load reduction of the blade.Further research on the influence of composite materials(composite structure)can be carried out on this basis,which has reference significance for load reduction technology of large wind turbine blades.