Numerical And Experimental Study On Nonlinear Characteristics Of Post Flutter Of Typical Bridge Decks

The aeroelasticity of bridge decks has significant nonlinear characteristics under large-amplitude vibration,and the post flutter may be featured by limit cycle oscillation(LCO)rather than linear divergence.In this study,the research status of the nonlinear flutter of bridge decks is first reviewed.Then,the aerodynamic nonlinearity of a rectangular section with aspect ratio B/D=5(B and D is the width and depth of the rectangular section,respectively)is studied,as well as the damping and stiffness nonlinearities of a section model test system.The numerical prediction for the post-flutter behavior of the rectangular section is further performed.Finally,the post-flutter characteristics of a truss stiffening girder are comprehensively investigated via section model tests.The main research works are shown as follows:(1)The aerodynamic forces of the 5:1 rectangular section are obtained through forced-vibration simulation with the computational fluid dynamics(CFD)method.The nonlinear characteristics of self-excited forces are analyzed from various viewpoints,i.e.,flutter derivatives,high-order components,and hysteresis curves.The frequency spectrum characteristics of the post-flutter LCO displacement and the energy contributions of the self-excited forces are analyzed.Furthermore,the feasibility of describing the nonlinear self-excited force by the amplitude-dependent aerodynamic parameters is discussed.The results show that the flutter derivatives of the rectangular section have significant amplitude dependence,and the self-excited forces contain the high-order components under large amplitude.Besides,the post-flutter LCO displacement can be approximately regarded as simple harmonic vibration.(2)For the section model test system of bridge decks,the traditional linearization method of its structural parameters is first reviewed.Then,free-decay tests of the rectangular section model are carried out in still air,and the nonlinear structural parameters are identified using the equivalent linearization method.On this basis,the effects of the system’s structural nonlinearity on the identification of flutter derivatives and the characteristics of post flutter are studied.The results show that the section model test system has significant damping nonlinearity and slight stiffness nonlinearity under large-amplitude vibration.If the structural nonlinearity of the section model tested system is ignored,the results of the flutter derivative identification would contain larger errors.The nonlinear damping of the system may cause the post-flutter LCO.(3)The prediction of the post-flutter behavior of the 5:1 rectangular section with two-degree-of-freedom(2DOF)is performed via a numerical method.For this method,a hybrid strategy of time-frequency domain is introduced to obtain the time-history solution of self-excited vibration,and the amplitude-dependent aerodynamic parameters are employed to describe the nonlinear aerodynamic forces.Moreover,damping and stiffness nonlinearities of the section model test system are additionally taken into account.In addition,the numerical approach is extended to the continuum model to estimate the three-dimensional(3D)effects of the nonlinear aerodynamic forces on the post-flutter behavior.The wind tunnel test of the rectangular section model is also performed for verification.The numerical results show that the system may be featured by LCO due to the aerodynamic and/or damping nonlinearities.The LCO amplitude of the continuum model is slightly larger than that of the section model.(4)Based on Kaizhou Lake Bridge in Guizhou,the post-flutter characteristics of a truss stiffening girder are investigated via section model tests.The post-flutter response is analyzed from different aspects,i.e.,steady-state amplitude,frequency spectrum,and vibration form.Moreover,the mechanism of the self-limited vibration is discussed in terms of nonlinear damping.The results show that the post-flutter LCO with obvious bending-torsion coupling occurs in the truss stiffening girder at all tested attack angles.The structural damping nonlinearity is a favorable factor for the stability of LCO,while the aerodynamic damping nonlinearity has different effects at various attack angles.