Influences Of Grid Plates On Vortex-Induced Vibration Performance Of Separated Wide Twin Box Girders

The separated twin-box girder has good flutter stability,but the vortex-induced vibration of the bridge section is easily caused by the complex flow characteristics at the central slot.It is an effective aerodynamic measure to suppress the vortex-induced vibration of the separated double box girder by setting the vortex partition plate at the central slot.However,the research on the effect of the opening form and the effective ventilation rate on the vibration suppression effect after considering the Reynolds number effect is still relatively small in the field of bridge wind engineering.It is necessary to carry out systematic and in-depth parametric research.In this paper,the influence of different length-width ratio,hole size,hole spacing and air permeability on the wind resistance coefficient is studied by means of wind tunnel test and numerical simulation,and the local boundary layer effect is discussed.The vortex-induced vibration(VIV)characteristics of the separated twin-box girders and the influence of the air permeability and the arrangement of the vortex separators on the VIV performance were studied through the simultaneous pressure and vibration measurement tests of a 1:70 small-scale sectional model.The differences of the VIV suppression effects with different apertures and arrangements were analyzed under the same air permeability of the vortex separators.Finally,the 1:30large-scale segment model vibration test was carried out and compared with the results of the small-scale segment model test.The Reynolds number effect of the aerodynamic measures of the vortex plate and the influencing factors of its application in the real bridge were discussed.The main research contents are as follows :(1)In this paper,the wind tunnel test of different parameters of the vortex plate is carried out.Combined with numerical simulation,the effects of length-width ratio,air permeability,aperture,hole spacing,orifice thickness and opening form on the wind resistance coefficient of the vortex plate are compared.The difference of the wind resistance coefficient of the vortex plate under different parameters reflects the change of the flow separation characteristics when the airflow passes through different pores.This paper discusses the influence of pore size and pore spacing on the local boundary layer thickness and effective porosity of the vortex plate under the same porosity.It is found that under the same porosity,the pore shape has little effect on the wind resistance coefficient of the porous plate,while the pore size and pore spacing have great influence on the wind resistance coefficient of the porous plate.(2)In this paper,the vibration response time history under different wind speeds is obtained by the vibration test of the initial section and the fully closed section(the ventilation rate of the vortex plate is 0 %)of the 1 : 70 small-scale segment model.It is found that the vortex vibration performance of the initial section without the vortex plate is poor,but the vortex vibration can be effectively suppressed by using the fully closed central slot of the vortex plate.In addition,two key parameters of ventilation rate and opening form are selected to conduct synchronous vibration and pressure measurement tests on the vortex vibration performance of the section.It is found that the ventilation rate and arrangement of the vortex plate will affect the vortex vibration performance of the main beam.The lower the ventilation rate of the vortex plate,the better the suppression effect on the vertical bending vortex vibration.The change of the opening form of the vortex partition plate has obvious differences in the vertical bending and torsional vortex vibration of the separated double box girder.Under the same air permeability,the difference of the vertical bending vortex vibration of the large aperture porous plate,the small aperture porous plate and the strip grid is mainly reflected in the amplitude of the vortex vibration interval.The amplitude of the vertical bending vortex of the strip grid is the largest,and the amplitude of the large aperture porous plate is the smallest.The difference of torsional vortex vibration is mainly reflected in the suppression effect on vortex vibration.The large aperture porous plate has the best suppression effect on torsional vortex vibration,and the small aperture porous plate and the strip grille have a promoting effect on torsional vortex vibration when the ventilation rate is greater than 9.36 %.(3)Considering the maximum amplitude condition of the section of the research object in this paper,the comparative vibration test of the initial section of the 1 : 30large-scale section model,different ventilation rates and different vortex plate arrangements was carried out at the angle of attack of-3 °,and compared with the results of the 1 : 70small-scale section model.It is found that the results of the large and small scale model tests of the initial section are in good agreement with the locking range and amplitude of the vertical bending and torsional vortex vibration.In addition,the vertical vortex vibration response amplitude of the vortex plate with the same ventilation rate in the large-scale section model test is slightly larger than that of the small-scale model.Combined with the previous research results of the local boundary layer effect of the vortex plate,it is speculated that the reason is the Reynolds number effect of the vortex plate,that is,the effective ventilation rate of the small-scale model to the large-scale model will be greater even at the same ventilation rate.Therefore,when the vortex baffle is used as the aerodynamic control measure of vortex-induced vibration,in addition to considering the key parameters such as the air permeability and the opening form of the vortex baffle,the Reynolds number effect between the wind tunnel scale model and the real bridge structure should also be paid attention to.