A rapid downstream weakening of the processes that drive the intensity of transverse mixing at the confluence of large rivers has been identified in the literature and attributed to the progressive reduction in channel scale secondary circulation and shear-driven mixing with distance downstream from the junction. These processes are investigated in this paper using a three-dimensional computation of the Reynolds averaged Navier Stokes equations combined with a Reynolds stress turbulence model for the confluence of the Kama and Vishera rivers in the Russian Urals. Simulations were carried out for three different configurations: an idealized planform with a rectangular cross-section (R), the natural planform with a rectangular cross-section (P), and the natural planform with the measured bathymetry (N), each one for three different discharge ratios. Results show that in the idealized configuration (R), the initial vortices that form due to channel-scale pressure gradients decline rapidly with distance downstream. Mixing is slow and incomplete at more than 10 multiples of channel width downstream from the junction corner. However, when the natural planform and bathymetry are introduced (N), rates of mixing increase dramatically at the junction corner and are maintained with distance downstream. Comparison with the P case suggests that it is the bathymetry that drives the most rapid mixing and notably when the discharge ratio is such that a single channel-scale vortex develops aided by curvature in the post junction channel. This effect is strongest when the discharge of the tributary that has the same direction of curvature as the post junction channel is greatest. A comprehensive set of field data are required to test this conclusion. If it holds, theoretical models of mixing processes in rivers will need to take into account the effects of bathymetry upon the interaction between river discharge ratio, secondary circulation development, and mixing rates.

中文翻译：

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河道规模二次循环对河道交汇处下游混合过程影响的数值研究

在文献中已经确定了驱动大河流汇合处横向混合强度的过程的下游快速减弱，这归因于河道规模的二级循环和剪切驱动的混合随着距离汇合处下游的距离逐渐减少。本文使用雷诺平均 Navier Stokes 方程的三维计算结合雷诺应力湍流模型研究了这些过程，用于俄罗斯乌拉尔的 Kama 河和 Vishera 河的汇合处。对三种不同的配置进行了模拟：具有矩形横截面 (R) 的理想化平面、具有矩形横截面 (P) 的自然平面和具有测量水深 (N) 的自然平面，每个用于三种不同的放电比。结果表明，在理想化配置 (R) 中，由于通道尺度压力梯度而形成的初始涡流随着下游距离的增加而迅速下降。在汇合角下游通道宽度的 10 倍以上时，混合缓慢且不完整。然而，当引入自然平面和水深测量 (N) 时，交汇处拐角处的混合速率急剧增加，并随着下游的距离保持不变。与 P 案例的比较表明，是水深测量驱动最快速的混合，特别是当排放比使得单个通道尺度涡流在后连接通道中的曲率的帮助下发展。当与后连接通道具有相同曲率方向的支流的流量最大时，这种效果最强。需要一组全面的现场数据来检验这一结论。如果成立，河流混合过程的理论模型将需要考虑水深测量对河流流量比、二次循环发展和混合速率之间相互作用的影响。