Coherent Eddies in a Compound Channel and Their Effects on Flow Resistance

通过理论分析、实验和数学模拟，阐明了复合河道中主河道与漫滩交界面产生的大型相干涡的作用和成因。通过一系列对比实验和剪切层的水动力稳定性理论，研究了复合通道中相干涡流的产生机理。实验结果证实，如果相邻剪切层之间没有速度差，就不会产生相干涡流。此外，还发现沿垂直壁面的二次上升流不能在水面上产生连贯的涡流。理论上得到的两剪切层交界面最不稳定扰动波数与复合通道内相干涡流波数的观测值吻合较好。用一个简单的模型估计了界面附近存在的上升流的强度，该模型的涡度梯度是由主流引起的涡管拉伸决定的。通过流动可视化技术，预测值与实测值吻合较好。对深度平均速度场进行了理论求解。在较宽的沟槽宽深比范围内，理论推导了施加在沟槽界面上的附加剪应力，并与已有的实验结果相吻合。利用理论解，确定了一维分析所需的总体水力参数，如具有代表性的粗糙度系数和能量系数。利用代数应力模型建立了湍流模型。与实测结果相比，一次流和二次流的等等值线、二次流的强度和湍流能量的分布都得到了很好的再现。计算结果也使湍流能量平衡各项的大小估计成为可能。

The role and origin of large coherent eddies generated at the interface between the main channel and the flood plain in compound channels are clarified in this study through theoretical analysis, experiments, and mathematical simulations.The generation mechanism of coherent eddies in compound channels was investigated by a series of comparative experiments and a hydrodynamic stability theory for shear layers. Experimental results confirmed that coherent eddies were not produced without velocity difference between neighboring shear layers. Furthermore, it was found out that secondary upwelling along a vertical wall was unable to produce coherent eddies on the water surface. The wave number of the most unstable disturbance at the interface of two shear layers obtained theoretically agrees well with observed wave number of coherent eddies in compound channels.The intensity of upwelling which exists near the interface was estimated by a simple model where the gradient of vortity was determined by a stretching of vortex tube due to the main flow. Predicted values were in good agreement with measured values by a flow visualization technique.Depth-averaged velocity field was theoretically solved. Additional shear stress which exerted on the interface was derived theoretically and confirmed to be in good agreement with existing experimental results for a wide range of width to depth ratio of the channel. Utilizing the theoretical solution, bulk hydraulic parameters, for instance, representative roughness coefficient and energy coefficient which are required for a one-dimensional analysis were determined.A turbulence model was developed using the algebraic stress model. Isovels of the primary flow abd secondary flow, the intensity of secondary flow, and distribution of tubulence energy were reproduced pretty well compared with measured results. Computed results also made the estimation possible for the magnitude of each term in turbulence energy balance.