Study On Vortex-Induced Vibration Mechanism And Control Of Split Three-Box Girders In Long-Span Bridges

Long-span and even super-long-span bridges are an intuitive reflection of a country’s science and technology level,and also the continuous pursuit of mankind in the field of bridge engineering.As span increases,characteristics such as slenderness and low damping become more and more prominent.Bridges are more and more sensitive to wind loads so that wind-resistant design has even become a determinant for long-span bridge construction.In order to achieve sufficiently high critical flutter wind speeds,stiffening girders gradually adopt split box girders.Although split box girders have excellent flutter stability,gaps between boxes make the flow around the section complicate,vortex-induced vibration(VIV)is a common problem.For now,split box girders mainly include twin-box girders and split three-box girders.Several twin-box girder bridges were built at home and abroad in the past decade;however,never even one split three-box girder bridge has been built.As a novel form of stiffening girder,wind-induced vibration performance and aerodynamic behaviors,especially VIV mechanism and control schemes of split three-box girders,are still being explored.In this thesis,numerical simulation and wind tunnel model test are combined to study VIV mechanism and control of split three-box girders,the main contents are:(1)The history of long-span bridges and their wind-resistant research was reviewed,and the development process and status of VIV was summarized.(2)Delayed detached-eddy simulation(DDES)based on shear stress transport k-ωmodel,together with pressure and displacement measurement wind tunnel test of a large-scale sectional model that can vibrate freely have laid technical foundations for study of VIV mechanism and control.As a hybrid algorithm,DDES deals with the near wall region in the way of Reynolds-averaged Navier-stokes simulation(RANS)and other regions in the way of large eddy simulation(LES),which is an ideal method for numerical study of near wall physical flow.The above wind tunnel test technology can not only obtain VIV displacements,but also collect local fluctuating wind pressure time-histories,which provides conditions for study of VIV characteristics,time-frequency evolutionary characteristics of vortex-excited force(VEF),spanwise correlations of aerodynamic force and VIV control schemes.(3)VIV characteristics of a split three-box girder were clarified through large-scale sectional model wind tunnel tests,and VIV mechanism was revealed via DDES.Based on the VIV characteristics,physical flow around the split three-box girder section was obtained by DDES,and VIV mechanism of this type of girder section was revealed from aspects of flow structures and aerodynamic forces.(4)Based on pressure measurement wind tunnel tests on a large-scale sectional model that can vibrate freely,time-frequency evolutionary characteristics of VEF and its typical indexes during VIV of the split three-box girder were analyzed.From a micro point of view,the VIV mechanism was further refined into generation and movement of large-scale vortices on lower surface of the upstream box,downstream curved surface in the upstream gap,downstream curved surface in the downstream gap,upper surface of the downstream box,the wake and the near wake.(5)Based on the VIV mechanism,two measures for VIV aerodynamic optimization of split three-box girders were proposed,which were grids between the gaps and new scheme for maintenance tracks.DDES was used to reveal aerodynamic mechanism of the measures to improve VIV performance.Through wind tunnel tests of a large-scale sectional model,vibration suppression effect of the above measures was verified,and aerodynamic sensitive parameters of the measures were identified and refined.Combined with measures such as different heights of highway wind barriers and damping ratios,two types of VIV control schemes for split three-box girders were proposed.(6)Based on the pressure measurement wind tunnel tests above,spanwise correlation of the global aerodynamic forces and local aerodynamic forces on single boxes in the process of VIV was studied.It was revealed that the global and local aerodynamic forces were not only spanwise correlated;their correlations also possessed nonlinear characteristics related to VIV amplitudes.The spanwise correlations of aerodynamic forces were expressed as the sum of incomplete correlated part and complete correlated part,and the spanwise correlation function related to vibration state was introduced.A two-parameter spanwise correlation expression of the global and local aerodynamic forces related to VIV amplitude was established by using nonlinear least squares.At the maximum VIV amplitude,the aerodynamic force reaches its maximum correlation.The complete correlated part of the global VEF accounts for 83%,and the exponential attenuation coefficient is 0.940.The local aerodynamic force correlation analysis of single boxes can provide new ideas for VIV control from the perspective of weakening spanwise correlations of aerodynamic forces.