| With the rapid development of economy,people’s demand for transportation is increasing day by day,and the demand for highway bridges and railway bridges is increasing.Considering the scarcity and uniqueness of sea crossing channel,in order to meet the traffic demand and flutter performance of railway and highway,bridge engineers put forward a new section form-separated three box girder.Compared with the separated double box girder,its flow field distribution is more complex,the vortex shedding and impact are more frequent,and it is easier to induce vortex-induced vibration（VIV）.The numerical simulation method can study the VIV characteristics of separated three box girder at a low cost,and provide a reference for solving the VIV problem of separated three box girder.In this paper,the VIV of separated three box girder is studied in the following aspects:（1）The wind tunnel test of small-scale segment model is carried out for the separated three-box girder without auxiliary facilities,and the VIV performance wind tunnel test of small-scale segment model with different angles of attack is carried out.The results show that the separated three-box girder without auxiliary facilities has obvious vertical VIV at-3°,0°and+3°angles of attack.（2）Based on two-dimensional RANS and DDES methods,6)-SST Turbulence model,by embedding Newmark method into UDF to realize fluid structure coupling operation,the VIV of separated three-box girder is numerically simulated,and the calculation results of VIV response under different simulation methods are compared.The results show that the RANS method can well simulate the stable VIV response under the calculation conditions that the minimum grid size order is 10-5m and the time step order is 10-3s,and the phenomena of fluid structure coupling and frequency locking are obvious.However,the VIV response of DDES method can not be stabilized at one amplitude.The frequency spectrum analysis results show that the frequency component of the lift force is complex,and there is no obvious fluid structure coupling and frequency locking phenomenon.（3）The changes of VIV amplitude,vibration frequency and lift frequency of separated three-box girder under different wind speeds are analyzed and compared with wind tunnel test.The results show that the wind speed range of VIV is basically consistent with the test,but the overall VIV amplitude is obviously smaller than the wind tunnel test results.（4）Based on the calculation and analysis of pressure coefficient distribution,vorticity evolution,mean square deviation of pressure coefficient on the surface of main beam in one cycle,the mechanism of vertical VIV of separated three box girder and the influence of grid on it are studied.The results show that the main reason for the VIV of the separated three box girder is the increase of the mean square deviation of the surface pressure coefficients of the windward box and the intermediate box.The grid at the slot of the separated three box girder can effectively inhibit the formation of vortices on the upper surface of the main beam,reduce the mean square deviation of the pressure coefficient in this area,and inhibit its vertical VIV.（5）Based on two-dimensional RANS and fluid structure coupling algorithm,the vertical VIV of separated three-box girder with different spacing ratio is numerically simulated,and the variation law of vertical VIV response of separated three-box girder with spacing ratio is analyzed.The results show that with the increase of the spacing ratio,the extreme value of the vertical amplitude of the main beam and the corresponding converted wind speed decrease,the VIV frequency basically remains near the main beam natural frequency of 2.026Hz,and thenumber shows an upward trend.（6）For the separated three box girder with different box spacing,the evolution of vorticity around the box,the distribution of vorticity and the distribution characteristics of surface pressure on the main beam are studied when its vertical extreme amplitude,and the relationship between the characteristics of vorticity,the distribution and variation characteristics of surface wind pressure and the ratio of box spacing is discussed.The results show that the increase of the spacing ratio leads to the weakening of the vortex shedding at the tail of the intermediate box and the local area of the leeward box,and the decrease of the mean value and mean square deviation of the pressure coefficient,resulting in the decrease of the vertical amplitude.（7）According to the surface pressure integration of each box of the three box girder,the aerodynamic lift and its time history of each box are obtained.The influence of the box spacing ratio on the phase of the aerodynamic lift of each box and the correlation between the magnitude of the aerodynamic lift of each box and the vertical amplitude are studied and discussed.The results show that when the spacing ratio is 1.6 and 1.8,the aerodynamic lift phase of each box is basically the same,while the aerodynamic lift of each box has different degrees of phase difference under other spacing ratios.The mean value of aerodynamic lift coefficient of windward box and the mean absolute value and mean square deviation of aerodynamic lift coefficient of leeward box are consistent with the variation trend of amplitude.Therefore,it is inferred that there is a certain correlation between the variation of aerodynamic lift of windward box and leeward box and vertical amplitude. |
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