Numerical Study On Aerodynamic Characteristics And Flow Field Mechanism Of Twin Flat Box Girders With Different Width-height Ratios

With the development of society,the volumes of traffic increase day by day,twin flat box girders with upper and lower separation are widely used in bridge construction.However,due to the aerodynamic interferences between twin flat box girders,the aerodynamic characteristics of two box girders must be different from that of single box girder,which bring new challenges to the wind resistance of twin flat box girders,however,domestic and foreign scholars rarely studied the twin flat box girders,therefore,it is particularly important to study the aerodynamic characteristics and flow field mechanism of the twin flat box girders.In this paper,four kinds of single flat box girder and twin flat box girders with different width-height ratios are studied by numerical calculation method.The main research contents of this paper are as follows:(1)By numerical calculations,the variation rules of the three-point force coefficients and wind pressure coefficients of the twin flat box girders with spacing ratios and wind attack angles are obtained,the width-height ratios of the twin flat box girders are 5.0,6.0,8.0,10.0,wind attack angles are-6 to 6 degrees,spacing ratios D/H are 0.2,1.0,2.0,5.0,10.0.It is found that the drag coefficients of the upstream box girder decrease first and then increase with the increasing of spacing ratios and wind attack angles;the lift coefficients increase first and then decrease with the increasing of spacing ratios and wind attack angles;the torque coefficients increase gradually with the increasing of spacing ratios and wind attack angles.The drag coefficients of the downstream box girder increase firstly and then tend to be gentle with the increasing of the spacing ratios,the variation of wind attack angles and width-height ratios are consistent with that of upstream box girder;the lift and torque coefficients are consistent with the upstream box girder.The wind pressure coefficients at most positions of upstream box girder become smaller and smaller with the increasing of the spacing ratios;the wind pressure coefficients at most locations increase with the increasing of width-height ratios;with the increasing of the wind attack angles,the wind pressure coefficients on upper roof become smaller and smaller.The wind pressure coefficients at the straight web on windward side of downstream box girder gradually increase with the increasing of spacing ratios;the wind pressure coefficients of each side of box girder increase with the increasing of width-height ratios;with the increasing of wind attack angles,the wind pressure coefficients on the leading edge of bottom plate under box girder become larger and larger.(2)The flow field mechanisms of twin flat box girders with different width-height ratios are explained by analyzing the average flow charts of the twin flat box girders with different spacing ratios and wind attack angles.It is found that the vortexes formed by the upper roof of the upstream box girder decrease with the increasing of the spacing ratios;the vortexes formed by the upper roof and the trailing edge of the lower floor gradually decrease with the increasing of the width-height ratios;with the increasing of the wind attack angles,the vortexes formed by the upper roof become larger and larger,while the vortexes formed by the leading edge of the lower bottom plate become smaller and smaller.The upper roof of the downstream box girder appears vortexes with the increasing of the spacing ratios,while the vortexes formed by the leading edge of the lower bottom plate gradually disappear;the vortexes formed by the leading edge of the upper roof and lower floor gradually decrease with the increasing of the widthheight ratios;with the increasing of the wind attack angles,the vortexes formed by the upper roof become larger and larger,while the vortexes generated by the leading edge of the lower bottom plate become smaller and smaller.