非均匀封闭护栏钢箱梁断面绕流特性

Flow Characteristics of Steel Box Girder Cross-section with Non-uniformly Closed Railing

从绕流场的速度分布、三维旋涡结构的发展和演化及主梁表面压力分布等角度,应用大涡模拟技术探究顺桥向非均匀封闭护栏对钢箱梁断面绕流特性的影响及其抑制涡激振动的内在机制。数值模拟结果显示,由于斜腹板钢箱梁的原始断面在顺桥向方向基本一致,当流场流经上游护栏时,各护栏区域产生的旋涡结构较为一致,旋涡主要流场中旋涡结构围绕顺桥向旋转且具有明显的顺风向特征。在5°风攻角条件下,中央防撞护栏对上游桥面板处生成的旋涡影响很小,因具有更大尺寸和能量的脱落旋涡出现在模型下游,其振幅相对于3°风攻角的工况更大。当封闭顺桥向外侧护栏时,较大的负压区出现在上游桥面板背风侧,致使顺风向气流在顺桥向较为分散,从而降低了顺风向旋涡的能量;在桥面板负压作用下,边界层上部主流速度向主梁壁面下倾,加剧了旋涡的耗散,使得尾流中的旋涡尺寸和能量均有所下降,从而实现了对结构振动的抑制。结果表明:2种封闭措施在抑制涡激振动的机理上存在差异,在护栏开二闭一的措施下,旋涡在顺桥向的分量较大,且表面旋涡受主流下倾的抑制作用,从而降低了尾流第一对脱落涡的顺风向尺寸及能量;而在护栏开三闭一的措施下,上游桥面板的负压区较弱且位置后移,尾流旋涡能量低且强涡范围最小,上下表面压力差减小。

From the velocity distribution around flow field,the development and evolution of 3D vortex structures,and the pressure distribution on main girder surface,etc.,the large eddy simulation was used to investigate the influence of non-uniformly closed railings along bridge span on the flow characteristics of steel box girder cross-section,and to investigate the intrinsic mechanisms of suppressing vortex-induced vibration.The numerical simulation result shows that the aerodynamic shape of original cross-section of inclined web steel box girder has little difference in span-wise direction.When the flow field passes through the upstream railing,the vortex structure at each railing position is relatively similar.In the main flow field of vortex,the vortex structure rotates along the bridge and has obvious downwind characteristics.Under the condition of 5°wind attack angle,the influence of central railing on the vortex generated at the upstream bridge deck is very little,while the vortex size and energy detached downstream of model are larger,causing the vibration amplitude of model to be greater than that of 3°wind attack angle.The large negative pressure appears on the leeward side of upstream bridge deck if the outer railings are closed.That would induce larger downstream airflow in the span-wise direction,and decrease vortex energy in the downstream.Due to the negative pressure of bridge deck,the vortex induced from the downstream is dissipated significantly,leading to the reduced vortex-induced vibration amplitude.The result indicates that the vibration suppression mechanism of the two measures are different.Under the measure of opening-two-space and closing-one-space among the railings,the vortex has a larger component in the span direction,and the main stream down-dip consumes the surface vortex,resulting in the downwind size and energy decrease of wake first vortex.Under the measure of opening-three-space and closing-one-space among the railings,the negative pressure area of upstream bridge panel is weak and backward.The wake vortex energy is low,and the strong vortex range is minimal.The pressure difference between the upper and lower surfaces is reduced.