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

通过理论分析、试验和数学模拟，阐明了复合河道主槽与滩地交界面处产生的大型相干涡的作用和成因，并通过一系列对比试验和剪切层水动力稳定性理论对复合河道中相干涡的产生机理进行了探讨。实验结果证实，在没有相邻剪切层速度差的情况下，不会产生相干涡。此外，还发现沿垂直壁面的二次上升流不能在水面上产生一致的涡流。理论上得到的两个剪切层交界处最不稳定扰动的波数与观测到的复合槽道中相干涡的波数吻合得很好。用一个简单的模型估算了界面附近的上升流强度，其中涡量梯度是由主流引起的涡管伸展来确定的。采用流动显示技术，预测值与实测值吻合较好，并对深度平均速度场进行了理论求解。从理论上推导了作用在界面上的附加剪应力，并证实在宽宽深比范围内，附加剪应力与已有的实验结果吻合较好。利用理论解，确定了一维分析所需的总体水力参数，如代表性糙率系数和能量系数。与实验结果相比，较好地再现了一次流和二次流的等高线、二次流的强度以及管状流能量的分布。计算结果也为估算湍流能量平衡中各项的大小提供了可能。

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.