Vortex-induced Vibration Of Streamlined Box Girder Under Inflow With Large Angles Of Attack

Long-span bridges are characterized by small stiffness and low damping ratio,thus they are typical wind-sensitive structures,and wind resistance stability is one of the controlling factors for the design of long-span bridges.Streamlined box girders is the preferred beam type for most long-span bridges due to its good flutter stability.With the implementation of national strategies such as “the Belt and Road” and “the Connectivity”,the transportation networks in western mountain area have been further developed,and more and more large-span bridges will be built in complex terrains such as canyons.The canyon bridge site area is dominated by mountains and gullies,and the wind environment is extremely complicated,thence,the wind resistance stability of large-span bridges in mountain areas will face more severe challenges,among which the vortex-induced vibrations(VIVs)for the long-span streamlined box girder in complex mountainous areas have not been fully concerned and explored.In this study,the following research were carried out on the VIV of the streamlined box girder of long-span suspension bridges under mountain wind at large angles of attack.Firstly,the inflow with large angle of attack in mountain canyon bridge site area was analyzed.The ranges of wind attack angle in different bridge site areas were summarized and compared at first.Then,taking a canyon bridge site area in western mountain area as the research object,a numerical topography model with a length and width of 20 km and a height of more than 15 km centered on the bridge position was established.The computational fluid dynamics(CFD)method was utilized to explore the characteristics of the attack angle,the influence of different incoming wind directions on the wind environment of the bridge site was then analyzed,the characteristics of the mountain wind were clarified later,and reasonable values of wind parameters for long-span suspension bridges in canyons were determined,providing a reference for the further research.Then,the three-dimensional flow field distributions in the bridge site area were further analyzed,and the causes of formation for the large angle of attack of the mountain wind were discussed.Secondly,the VIV performance of streamlined box girder under mountain wind at large angle of attack was investigated.A high-precision numerical simulation process for VIV was developed based on SST k-? turbulence model,the basic equations of structural dynamics and the fourth-order Runge-Kutta iterative method were combined,and then the numerical simulation of VIV was realized by dynamic mesh technology.Then,a numerical model with high spatial resolution and simulation accuracy was established,the effects of parameters such as time step and adherent layer grid size on numerical simulation results were analyzed,and then the appropriate parameters are selected taking into account the calculation accuracy and efficiency.Subsequently,based on the existing wind tunnel test results,the reliability of the numerical simulation of vortex vibration was verified.Later,the VIV performance of streamlined box girder for long-span suspension bridges in the mountainous area at large angles of attack was further investigated.Thirdly,a mathematical model of vortex-induced force(VIF)for streamlined box girder was established.VIF was obtained and its accuracy was verified based on the VIV simulation results of streamlined box girder,the recognition accuracy of the VIF and VIV amplitude of the existing VIF model were then inspected.A self-adaptive nonlinear fitting method was proposed,and the reliability and superiority of this method were verified based on the experimental data.Then,a mathematical model of vortex force with high accuracy was established,which lays the foundation for the subsequent study of VIV mechanisms.Then,the mesoscopic mechanisms of VIV of streamlined box girder were discussed in detail.Based on the vortex distributions,the causes of the difference in VIV performance of the box girder under different wind angles of attack were discussed,and the development law of the vortex in the whole process of vortex vibration was explored.The vortex-induced pressures(VIPs)on the surface of box girder were extracted,and the VIPs were decomposed by the mathematical model to explore the distribution law of VIPs,thus,the relationship between vortex and VIPs was revealed.Herafter,the energy distribution and energy evolution law of the VIPs and aerodynamic damping components were further discussed.Finally,the influence law and its mechanism of aerodynamic shape on flow field and VIV performance of streamlined box girder were studied systematically.For the aerodynamic shape parameters such as the angle of wind fairing,the form of railing and the position of inspection vehicle track,the static and dynamic numerical simulations of the box girder after changing the above parameters were carried out.The influences of the aerodynamic shape parameters on the aerodynamic coefficients,pressure distributions and VIV performances of the streamlined box girder were investigated in detail,and the hydrodynamic mechanisms of the VIV performance of box girder changing with aerodynamic shape were further revealed.