Research Of Aeroelastic Damping Of Swept-blade Based On Eigenvalue Method

The back-swept wind turbine blade is a new type of intelligent blade with adaptive and self-unloading functions.Due to the bending of the stacked line of the back-swept blade,the elastic axis of the blade is separated from the torsional axis,which enhances the coupling effect of the blade’s bending and torsion.The nonlinear deformation and three-dimensional flow of the blade are more significant,which makes the blade more susceptible to occur fluttering instability.In this research,the modal characteristics of large-scale wind turbine back-swept blades under rotation,flexible deformation and wind load and the mechanism of bending and torsional chatter will be studied by using eigenvalue method.The analysis model of the coupling between aerodynamic and nonlinear nonlinear vibration and bending and torsional flutter of the swept blade was established and the corresponding algorithm was implemented.The research work has important theoretical significance and engineering application value for the optimal design and safe and stable operation of the back-swept blade.In order to exactly describe the nonlinear deformation characteristics of flexible blades,the super-element method is used to discretize the flexible blades into a number of rigid bodies.And the multi-body system is formed by connecting the motion pair and the force element,and then the blade’s nonlinear dynamic equation is established by the Newton-Euler equation.Considering the significant three-dimensional flow of the swept blade,the aeroelastic coupling dynamic equations of the swept blade is constructed by super-element method and the lift line theory.Then Base on the multi-body recursive modeling method and the variational principle,aeroelastic coulping equations are linearized to build the aeroelastic eigenvalue equation.The Beddoes-Leishman(B-L)dynamic stall model is used to accurately describe the aerodynamic parameters of the blade in the linearied aeroelastic model.The eignvalue equation is constructed according to the aeroelastic state equation.The aeroelastic frequency and the aeroelastic damping ratio are calculated by Solving the eigenvalues of eignvalue equation.The critical conditions of flutter of the swept blades were studied by using the aeroelastic damping ratio as judgment.Two kind of back-swept blades which base on NREL 5 MW wind turbine blade are designed.The steady state deformation of the blade under certain tip speed ratio(TSR)and various rotation ratio conditions are calculated to reveal the effect of blade swept back on the structural coupling deformation.The effects of rigid rotation and steady state deformation on blade’s natural frequency are analysed.Finally,combined with the linearized aerodynamic model,the aeroelastic frequency and aeroelastic damping ratio of the swept blade are calculated to analyse the influence of the blade swept back on the flutter characteristics.The results show that the blade swept back reduces the flagwise deformation and increse the torsional deformation of the blade under extreme wind conditions,but the limit speed of fluttering instability of blade are also reduces.