Study on stability of organized horizontal vortices and sediment transport in compound open channel flow

在两段式河道中，剪切失稳会在主河道和洪泛平原交界处产生较大的水平涡。通过改变主河道宽度进行了一系列实验室试验，并对3种水深情况进行了试验。采用激光多普勒测速仪和容型波计分别测量了流速和水面高程。研究发现，当b/H=5时，两个涡街阵列变得非常稳定，其中b为主通道宽度，H为主通道深度。在所有水深情况下，交界处的雷诺应力均在b/H=5处达到峰值。利用线性不稳定性分析成功地预测了涡旋的波长。在有河岸植被的明渠流中进行了一系列的室内试验，结果表明，与复合明渠流相比，明渠流的流场更具二维特征。同样，在复合明渠流动中，当涡街形成稳定的交错排列时，雷诺数应力达到峰值。雷诺应力峰值大于复合明渠流的峰值。采用Nadaoka和Yagi最初提出的SDS和2DH湍流模型进行数值计算，计算二维流场，支持测量结果。

It has been known that shear instability generates large horizontal vortices at the junctions of main channel and flood plains in two-stage channel. A series of laboratory tests were performed by varying the main channel width, and the tests were conducted for 3 cases of water depth. Fluid velocity and water surface elevation were measured by using laser-Doppler velocimetry and capacity-type wave gage, respectively. It was found that two arrays of vortex streets become very stable at b/H=5, in which b is main channel width and H is main channel depth. The Reynolds stress at the junction shows a peak value at b/H=5 for all cases of water depth. The wavelength of vortices was predicted successfully in terms of linear instability analysis.A series of laboratory tests were also performed in open channel flows with bank vegetation, in which the flow fields become more 2-dimensional than compound open channel flow. Similarly in compound open channel flow, when the vortex streets becomes stable staggered array, the Reynolds stress takes a peak value. The peak value of Reynolds stress is larger than that of compound open channel flow. A numerical computation employing SDS and 2DH turbulence model originally proposed by Nadaoka and Yagi was performed to calculate the 2-dimanesional flow field, supporting the measurements.