Study On The Influence Of Characteristic Size Of Inverted L-shaped Deflector On Vortex-induced Vibration Performance Of Twin-box Girder

In the process of long-span bridges designing,the twin-box girder with good flutter stability has obvious advantages,but its significant vortex-induced vibration(VIV)problems need to be further studied.In this paper,the VIV characteristics and flow field characteristics of the twin-box girder section are analyzed by wind tunnel test and CFD numerical simulation,the differences of numerical simulation methods are compared,and the influence law of characteristic size of inverted L-shaped deflector on VIV and its vibration suppression mechanism are summarized.First of all,through the segmental model wind tunnel test,it is found that there is an obvious VIV phenomenon in the original section of the twin-box girder,in which the torsional VIV has two wind speed locking intervals.In order to analyze the influence of the characteristic size of the inverted L-shaped deflector,21 test conditions are designed.It is found that the change in the horizontal plate size has an obvious effect on the vertical bending VIV.With the horizontal plate size increase,the vibration suppression effect increases at first and then decreases.And then increases the nonlinear trend,there are two turning points.On the other hand,the torsional VIV is more sensitive to the change of the vertical plate size,showing an obvious linear law,and the increase of the vertical plate size also reduces the number of wind speed locking intervals of the torsional VIV.At the same time,the vibration suppression effect of the inverted L-shaped deflector is also limited by the range of its own aspect ratio parameters.when the aspect ratio is too large,it tends to evolve into a horizontal deflector.Secondly,in order to analyze the influence of the turbulence model on the numerical simulation of VIV of twin-box girder section and ensure the accuracy of numerical simulation,the calculation results of standard k-ε model,RNG k-ε model,Realizable k-ε model,standard k-ω model and SST k-ω model commonly used in engineering practice are compared and analyzed.It is found that the calculation results of the standard k-ω model are contrary to the experimental phenomenon,and the simulation is seriously distorted.The calculated values of other models have some errors compared with the experimental values,but they can better restore the experimental phenomenon,in which the calculation cost of standard k-ε model and RNG k-ε model is the highest,and the calculation cost of Realizable k-εmodel and SST k-ω model is about 1/3 of the former.Taking this into account,the SST k-ωmodel is the relatively optimal turbulence model.Finally,the flow field around the section of the twin-box girder is analyzed by CFD numerical simulation.It is found that under the VIV wind speed,the vortex motion frequency of the flow field is very close to the structural fundamental frequency.In the case of vertical bending VIV,the distribution of co-frequency vortices on the upstream and downstream box girders is uniform,and the vortex energy distribution is balanced,while in torsion VIV,the cofrequency vortices on the upstream and downstream box girders show a quasi-antisymmetric distribution,and the surface vortex energy of the downstream box girders is much higher than that of the upstream.The distribution of vortex and vortex energy at different wind speeds leads to two VIV patterns.The existence of the inverted L-shaped deflector and the change of its characteristic size not only greatly reduce the vortex area in the flow field,but also increase the number of vortices,reconstruct the vortex structure distribution of the flow field,destroys the original co-frequency vortex and reduces the vortex energy by several orders of magnitude,and effectively suppresses the VIV.