桥墩水流特性大涡模拟研究

Impacts of multiple bridge piers upon open channel flow based on LES

采用大涡模拟(LES)数学模型对河道中单个桥墩及3个桥墩条件下的水流运动进行数值模拟。计算结果与试验结果吻合良好。对计算得到的墩前及墩后流速变化、水位变化、剪应力分布、涡量变化及频谱进行分析,基于以上分析发现:桥墩间距较窄时(s/D=2),墩前不同位置(0>x/D>-5)处的垂向流速绝对值!Uz!大于其他工况条件下相同位置处的流速,而在墩前x/D<-5区域,各计算工况条件下的垂向流速Uz基本为零,不受桥墩阻流的影响,两侧桥墩叠加效应最明显,跌水达最大值;桥墩间距较大时(s/D>5),中心桥墩两侧的桥墩对中心桥墩尾流区的干扰减弱,叠加效应可以忽略。桥墩间距较窄时(s/D=2)墩后x=0.5D处计算得到的涡漩特征长度与桥墩直径基本相同,该处涡漩主要由单个桥墩的尾流产生,而桥墩后x=5D处的涡漩特征长度与3倍的桥墩直径一致,中心桥墩后x=5D处的涡漩由3个桥墩叠加产生。

A large-eddy numerical model was used to simulate flow in a rectangular open channel blocked with abridge pier or multiple bridge piers, focusing on effects of the bridge piers upon the flow characteristics andturbulent structures. Calculations were performed for six values of pier spacings s/D with same Froude numbers. Thecalculated results were compared well with previous laboratory results. And the variations in velocities, water levelsin front of the central bridge pier and behind it and the vorticity were deeply analyzed based on the calculatedresults. Numerical simulation results show that for the closely spaced conditions （s/D = 2） , the absolute values ofthe velocity in vertical direction ／UZ｜ in the region （0 〉 x/D 〉-5） were larger than the one in the same region forother conditions. On the contrary, the velocity in vertical direction in the region （i /D 〈 5） was almost zero andthe bridge piers hardly affected the flow field ; for the closely spaced conditions （ s/D - 2） , the values of thehydraulic drop upstream and downstream of the central bridge pier were to be the maximum due to the duplicateeffects given by the bridge piers; and for the widely spaced conditions （s/ D 〉5） , the effects caused by the bridgepiers on both sides of the central pier in the wake area downstream of the central bridge pier became weaker and theduplicate effects given by the bridge piers could be ignored. The lengths of the vortices corresponding to thedominant frequency at different locations behind the central bridge pier were studied based on the spectral analysisand calculated vorticity. The length of the vortices at x - 0. 5D was equal to a pier diameter （ D ） , so that thevortices at x -0. 5D behind the central bridge pier were mainly caused by the wake downstream of a bridge pier;and the length of the vortices at x - 5D was equal to three times of the pier diameter （ 3D ） , so the vortices at x - 5Dwere caused by the duplicate effects given by the multiple bridge piers.