小宽深比条件下弯道水流特性的试验研究

在弯道水流中，几何参数宽深比对水流的3维运动特性具有重要影响。由于小宽深比（< 5）河道广泛存在于山区及人工河道中，采用声速多普勒流速仪（ADV）对固定宽度不同宽深比（B/H 分别为1.5，2.0，3.0）的U型弯道内水流流速进行了精细测量，并以水面横比降、横向环流、流速重分布、紊动强度与剪切力为研究内容，讨论与分析了该情况下的弯道水流结构。试验结果表明，在小宽深比条件下增加宽深比时：1）水流动力轴线开始向凹岸偏移的位置提前，因而小流速主流线沿着凸岸流动，直到出弯道才开始向凹岸偏移，大流速则更早地向凹岸偏移；2）横向流速在凹岸开始波动的位置依次靠近上游，3种工况中波动位置分别为120°断面、90°断面及60°断面；3）凹岸表层附近出现次生环流的位置依次提前，其位置分别为120°断面、60°断面及30°断面，并且次生环流强度及范围不断增大；4）床面剪切力不断增大。需要指出的是：3种工况的剪切力在弯道入口断面到弯顶断面之间较大，最大剪切力位置均出现在弯道的前半段，可以推断在动床中弯道泥沙在此段的冲刷比较严重；过弯顶断面后，凸岸床面剪切力大于凹岸。试验结果可以有效地反映不同小宽深比参数对弯道水流结构的影响，并为该条件下的弯道泥沙冲淤研究提供参考。

Experimental Study on Flow Characteristics of Curved Channel in Small Width-depth Ratio Conditions

The geometric parameter of width-depth ratio plays an important role in three-dimensional characteristics of water flow in curved channels.Considering that bends with small width-depth ratio (< 5) widely exist in mountain rivers and artificial channels,the Acoustic Doppler Velocimeter (ADV) was employed to measure distributions of velocity in a U-shape bend with three different width-depth ratios of 1.5,2.0 and 3.0,respectively.Based on the measurements,the transverse gradient of water surface,transverse flow circulation,redistribution of velocity,turbulence intensity,and bed shear stress were investigated to analyze the structure of water flow.The experimental results demonstrated that as the increase of width-depth ratio:1) flow dynamic axis moved to concave bank earlier,which resulted in main streamline with low inlet velocity flows along the convex bank until the export of bend,while that with large inlet velocity moved to concave bank earlier;2) the locations of fluctuation of transverse velocity moved forward,which corresponded to 120° section,90° section and 60° section,respectively,in these three working conditions;3) the locations of secondary circulation on the surface of concave appeared in advance which were 120° section,60° section and 30° section,respectively,and both the strength and the range of secondary circulation increased;4) the bed shear stress increased.It should be noted that the shear stress was larger between inlet section and top section of the bend,and the location of the strongest shear stress appeared in the first half of the bend,which could infer that sediment erosion in this position was strong.The shear stress corresponding to the convex bank was larger than that of the concave bank after the top of the bend.The results could effectively describe the structure of water flow in small width-depth ratio conditions,and provide references for the sediment scouring and silting in the bend.