Study On The Flutter Mode Transition And Nonlinear Self-excited Force Of Three-tower Suspension Bridge

With its unique advantages,the three tower suspension bridge is widely used in our country and even in the world,but the tower is very sensitive to wind load due to its lack of effective constraints.Therefore,compared with the traditional suspension bridge,the wind resistance performance of the three tower suspension bridge needs special attention.As a destructive form of wind-induced vibration of bridges,flutter has always been the focus of wind engineering research.In the wind tunnel test of the aeroelastic model of the whole bridge of Ma’anshan Yangtze River Bridge,a kind of flutter mode evolution phenomenon including but not limited to soft flutter,internal resonance,vibration mode conversion and many other nonlinear effects has been found by scholars.In order to further explore the internal evolution of flutter mode of three tower suspension bridges,this paper makes a series of research on this phenomenon:(1)In this paper,the research progress of bridge linear flutter theory,nonlinear flutter,internal resonance,structural vibration mode conversion and other aspects by scholars at home and abroad are described.The current situation and shortcomings of flutter research are summarized,and the research significance of flutter mode evolution is pointed out.(2)The soft flutter mechanism of the cross section is explained from the view of flow field.Based on the CFD software and free vibration method,structural dynamics solution,fluid structure coupling algorithm and dynamic grid technology,the soft flutter phenomenon of Ma’anshan Yangtze River Bridge section is simulated from the perspective of two-dimensional section.According to the streamline characteristics,surface wind pressure and other calculation results,the driving mechanism of vortex to section flutter is analyzed in detail.(3)According to the structural system and mechanical characteristics of the three tower suspension bridge,the nonlinear discrete mathematical model of Maanshan Yangtze River Bridge is established,a series of undetermined parameters in the discrete mathematical model are identified,and the comparison between the nonlinear discrete mathematical model and the finite element model is completed.(4)The evolution between the first vertical bending vibration mode and the first three torsional vibration modes of Ma’anshan Yangtze River Bridge is simulated without considering the external factors such as aerodynamic force and damping force.From the point of view of nonlinear dynamics,based on the two-dimensional section model,the Poincare section of torsional displacement and torsional velocity is used to explain the mechanism of modal evolution.(5)Based on the theory of Mathieu function,the evolution mechanism of different torsional modes of Ma’anshan Yangtze River Bridge is explained.The internal relationship between Mathieu equation and nonlinear discrete mathematical model of Maanshan Yangtze River Bridge is established,and the nonlinear mathematical model of Maanshan Yangtze River Bridge in the form of Mathieu equation is established.The process of energy transfer through vertical bending mode of different order torsional mode is explained by Mathieu eigenvalue curve.(6)Based on the original closed system model,the influence of aerodynamic force is considered.The Scanlan linear time-domain self excitation force model expressed by pulse function and the nonlinear self excitation force model based on the third-order Taylor series expansion are used to consider the aerodynamic force successively.The displacement time history curves obtained by considering aerodynamic forces are in good agreement with the results of wind tunnel experiments of Ma’anshan Yangtze River Bridge in terms of modal evolution trend.A summary of the phenomenon of modal evolution and suggestions for engineering application are given.