Research On Wind-induced Vibration Mechanism And Control Of Suspenders In The Wake Region Of Bridge Tower

Followed the rapid economic growth in China,the bridge,as a significant transportation connection form,has witnessed its dramatic changes in development and its number across the country exceeding one million.With the increase of bridge span,the problem of bridge vibration is gradually prominent.In terms of the suspender,one of the most vital components of the suspension bridge,various wind-induced vibrations are prone to occur.Influenced by the upstream bridge tower,which has complex wake field,an extensive tower-wake-induced vibration(TWIV)of the suspender will be aroused in the region.In this thesis unfolding the study centering on the problem,the vibration features are explored through the wind tunnel test at first,and then a theoretical model is constructed to reveal the vibration mechanism,the flow control and mechanical control methods are finally developed to control vibration of the kind.In the first place,domestic large-span suspension bridges are taken as the background for the wind tunnel test of the dynamic response of the suspender’s rigid sectional model,so as to reproduce the TWIV phenomenon of the suspender at different positions of the wake field.The tower wake field is measured by the wind speed probe for the purpose of studying the turbulent flow feature and frequency feature of the wake field while the laser displacement meter is used to acquire the vibration signal of the suspender aiming at exploring the frequency feature of the suspender vibration,with results showing that the response amplitude curve of the TWIV changing with wind speed can be divided into two major parts,that is,the peak resonant response of the suspender to the wake characteristic frequency and the cumulative response of the amplitude increasing with the wind speed,which can be regarded as the effect of vortex-induced responses and aerodynamic self-exciting forces respectively.Meanwhile,the particle image velocimetry(PIV)is employed to study the relationship between suspender structure and wake field.A wind tunnel test is also carried out on the suspender flexible aeroelastic model in the tower wake field.The visual recognition technology is utilized to extract the overall displacement information of the flexible suspender.Its three-dimensional vibration characteristics are studied by modal analysis and the proper orthogonal decomposition(POD)is used to extract the suspender modal information while the tension sensor is for the variation law of the suspender tension.The test successfully reproduces the phenomenon that the suspension vibration frequency in the wake field is locked by the vortex shedding frequency of the upstream tower.On the basis of the wind tunnel test,the two-dimensional aerodynamic force model considering the unsteady influence of the tower wake and the motion differential equation of the suspender are established while the numerical solution is verified,with the experimental results consistent with the resonance range and amplitude velocity curve trend.Then,three kinds of parameters affecting the suspender motion,that is,structure parameters,wake field parameters and position parameters are analyzed one by one to study the influence law of different parameters on the suspender vibration characteristics in the wake field of the bridge tower.Results have it that the mass ratio plays a key role in the composition and proportion of the suspender vibration mode,and other parameters only regulate the suspender amplitude while having an insignificant role in changing the suspender motion mode.Meanwhile,increasing the damping ratio and reducing the turbulence in wake field can effectively reduce the vibration amplitude.Relying on the two-dimensional suspender motion equation and taking into account the influence of wind speed profile and the suspender three-dimensional effect,the nonlinear suspender structure equation considering the tension change with the suspender deformation are deduced while the three-dimensional aerodynamic force of the suspender in the wake field of bridge tower is developed,and the partial differential equation for suspender motion is established.Meanwhile,the spatiotemporal discretization method is used to solve the problem numerically to study the phenomenon and mechanism of suspender frequency lock-in in the wake field.The results turned out show that the structural nonlinear parameter of the suspender is the direct factor of the occurrence of frequency lock-in,which will expand the wind speed range of the suspender TWIV.In order to further explore the change trend of the nonlinear frequency of the suspender with the amplitude,the differences of the dynamic parameters in the two vibration modes are identified,the frequency formula related to the amplitude of the suspender is proposed,and the lock-in phenomenon of the TWIV in the two motion modes is reproduced.Meanwhile,combined with the actual engineering and considering the change of aerodynamic force caused by the wind profile,the suspender vibration of four different positions in the bridge tower wake is simulated.The results reveal that the suspender TWIV is a large vibration in a low-order mode under high wind speed.In allusion to the TWIV of the suspender,the corresponding flow control and mechanical damping control methods are developed while the passive jet ring is installed in the upstream bridge tower column to control the flow of the bridge tower’s wake.The control effect of the flow field around the column and its mechanism are analyzed.The results show that the ring changes the vortex shedding process of the upstream bridge tower.Besides,the correspondence of the lower-order mode flow field energy changes and the suppressed vortex shedding are explored by extracting the POD eigenvalues and the mode coefficients.In addition,the model force measurement and surface pressure measurement method are utilized to analyze the influence law of the ring on the aerodynamic force of the bridge tower.The results demonstrate that the aerodynamic force of the bridge tower is effectively controlled.Then,the damper is used for the mechanical damping control of the TWIV.The effect of the damper parameters on the additional damping ratio and the control effect under the optimal control parameters are analyzed.Finally,a main-cableend and girder-end damper construction scheme is proposed and the control effects by the combination of these two dampers are studied.