Analysis Of Vortex-induced Vibration Characteristics Of Separated Twin Box Girder Based On Proper Orthogonal Decomposition And Dynamic Mode Decomposition

With the development of long-span bridges,the separated double box girder is more and more widely used as the main girder section of the bridge due to its superior flutter performance.However,due to its unique aerodynamic shape,it is prone to vortex-induced vibration.In order to study the vortex vibration characteristics of the separated twin box girder,find out the area that has a great influence on the vortex vibration,and explore the vortex vibration mechanism of the separated twin box girder,this paper mainly does the following work and draws the corresponding conclusions:(1)The wind tunnel vibration test was carried out on the combination of five kinds of central slot width and four kinds of track position of maintenance vehicle.The results of separated double box girder under different working conditions were compared.It was found that vertical bending and torsional vortex vibration were not easy to occur when the slot width was small.When the groove width increases,the width of the vertical bending vortex vibration locking interval and the peak value of the vertical bending amplitude increase first and then decrease.The width of the torsional vortex vibration locking interval decreases,the interval moves to the high wind speed as a whole,and the torsional peak also increases first and then decreases.The influence of central slot width on the vertical bending and torsional vortex vibration performance of separated twin box girder is related to the track of maintenance vehicle.The vortex-induced vibration performance of the separated double box girder is better when the groove width is smaller and the track position of the maintenance vehicle is close to the inside of the beam.(2)The wind tunnel synchronous pressure test was carried out on the segment model of the same working condition.Based on the pressure measurement results,the intrinsic orthogonal decomposition and dynamic mode decomposition were carried out on the data.It was found that the area with the greatest influence on the torsional vortex vibration was the upper surface and the lower surface of the box girder.The oblique web area,and the POD method and the DMD method identify that the first two-order modal pressure coefficient changes at the x / B = 0.25 position on the upper surface of the downstream box girder are related to torsional vortex vibration.Under different wind speeds,the change of the energy ratio of the first two POD and DMD modes excluding the average flow field is basically consistent with the change of the vortex vibration amplitude of the model.The first two POD and DMD modes are the main contribution modes during torsional vortex vibration.(3)The numerical simulation of two typical working conditions is carried out.The analysis results show that the positive and negative pressure alternating control will appear in the straight web and the lower surface leading edge inclined web in one cycle,which has a great influence on the torsional vortex vibration.When the torsional vortex vibration occurs,the downstream box girder will appear alternating wake vortex shedding,and under the larger slot width,the wake vortex shedding of the upstream box girder will alternately hit the oblique web and straight web of the leading edge of the lower surface of the downstream box girder,providing torsional vortex excitation force for torsional vortex vibration.(4)The numerical simulation results are decomposed by POD and DMD.The POD method does not decompose the original data completely,and each order POD mode contains a variety of frequency information.The DMD method decomposes the original data more thoroughly.Each order DMD mode basically contains only a single frequency information,and the frequency is the mode of the model ’s torsional fundamental frequency and double frequency.The DMD method can more intuitively observe the time evolution process of each frequency mode in different stages of torsional vortex vibration.In the development stage of torsional VIV,the flow pattern is determined by the DMD modes of torsional fundamental frequency and multi-frequency and DMD modes similar to torsional fundamental frequency and multi-frequency,but the latter decays rapidly,while the former grows slowly.In the stable stage,only the DMD modes of torsional fundamental frequency and its multi-frequency dominate the flow pattern,which tends to a competition-induced VIV mechanism.