Energy-input-based Experimental Investigations On The Flutter Mechanism Of Long-span Bridge

A number of long-span bridges have been constructed around the world,and the span of bridge will be more broaden in the future.The safety of long-span bridge with an increasing span is more significantly affected by wind as a dominant load.Flutter,a kind of wind-induced vibrations,is the most terrible phenomena among all varieties of wind disasters of bridge.The structure of bridge would be pull down thoroughly when flutter once occurs,which causes enormous damage and loss.As a result,it makes a huge significance to study the mechanism of flutter.Flutter is usually classified as two types: divergent flutter and limit cycle flutter(LCF).In the present research,the mechanism of divergent flutter has been well understood,which is induced by negative aerodynamic damping.However,the mechanism of LCF is unclear yet and the study still has a long way to go.Some new nonlinear theories will be required urgently to interpret the phenomenon of LCF.In this thesis,a series of experimental investigations are conducted to explore the flutter mechanism of long-span bridge.A spring-suspended sectional model with a 10:1 rectangular(width to depth)section is applied in the test.Different streamlined and blunt aerodynamics configurations are implemented by setting up different wind fairings at the inflow side and the wake side of the sectional model respectively.And then different types of flutter are induced in the wind tunnel test.The nonlinearity of structural damping and stiffness of the sectional model are explored at first.In the next step,based on the results of vibrations and forces measurement,the effects of aerodynamics configuration on the flutter behavior,the dynamic properties and the work acted by aerodynamic are analyzed when the divergent flutter and LCF occur.Finally,the mechanism of two types of flutter is discussed sufficiently in terms of energy input during flutter process.The detailed content of this thesis is listed as follows:I.Explored the damping and stiffness nonlinearity of the section model vibration system in wind tunnel test,and pointed out that it is essential to introduce the nonlinear damping into flutter analysis and then the energy dissipation of vibration system is able to be considered more accurately.II.Analyze the vibration results of flutter test of the sectional model with different aerodynamic configurations,including the varying pattern between flutter amplitude and wind speed,the time-frequency characteristics of vibration,the performance of phase space and the features of bending-torsional coupling.Explore the effects of aerodynamic configuration on flutter behavior and propose that the type of flutter is dependent on the configuration of the inflow side of model section.Point out the features of bending-torsional coupling is prominent for divergent flutter while torsional oscillation dominates LCF.III.Measure the Aerodynamic force on sectional model with double force balances when flutter occurs.Analyze the time-frequency characteristics of the aerodynamic force,and both find higher harmonic components of the aerodynamic force for divergent flutter and LCF.Interpret that the coupling frequency of flutter between the vertical and the torsional natural frequencies is caused by the specific phase between aerodynamic forces and inertia force.IV.Analyze the characteristics of word done by aerodynamic force.Find that for divergent flutter the work done is positive by vertical aerodynamic force while negative by torsional aerodynamic force,which is contrary for LCF.Explore effects of the work done by the higher harmonic components of aerodynamic force and phase difference between displacement and aerodynamic force on energy input of flutter.Put forward a phase-driven theory to explain the mechanism of flutter and point out that difference of the phase relationship between displacement and aerodynamic force led to the different type of flutter.