A Numerical Study of the Influence of Channel-Scale Secondary Circulation on Mixing Processes Downstream of River Junctions
A Numerical Study of the Influence of Channel-Scale Secondary Circulation on Mixing Processes Downstream of River Junctions
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DOI：
10.3390/w12112969
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发表时间：
2020-10
期刊：
影响因子：
3.4
通讯作者：
T. Lyubimova;A. Lepikhin;Y. Parshakova;V. Y. Kolchanov;C. Gualtieri;B. Roux;S. Lane
中科院分区：
文献类型：
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作者：
T. Lyubimova;A. Lepikhin;Y. Parshakova;V. Y. Kolchanov;C. Gualtieri;B. Roux;S. Lane
在文献中已经确定了驱动大型河流交汇处横向混合强度的过程的快速下游减弱，并归因于通道尺度二次环流和剪切驱动混合随着距离交汇处下游的逐渐减少。本文采用雷诺平均Navier Stokes方程的三维计算结合雷诺应力湍流模型的卡马河和Vishera河在俄罗斯乌拉尔的汇合，这些过程进行了研究。对三种不同的配置进行了模拟：具有矩形横截面的理想化平面形状（R）、具有矩形横截面的自然平面形状（P）和具有实测水深的自然平面形状（N），每种配置都有三种不同的流量比。结果表明，在理想配置（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。混合是缓慢和不完全的，在10倍以上的通道宽度下游的交界角。然而，当引入自然平面形状和水深测量（N）时，混合速率在交汇角处急剧增加，并随着下游距离的增加而保持不变。与P情况的比较表明，这是水深，驱动最快速的混合，特别是当流量比是这样的，一个单一的通道规模的涡流发展的曲率在后连接通道的帮助。当与汇合后河道具有相同曲率方向的支流流量最大时，这种影响最强。需要一套全面的实地数据来检验这一结论。如果它成立，在河流中的混合过程的理论模型将需要考虑水深测量的影响后，河流流量比，二次环流的发展，和混合率之间的相互作用。
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.