At present, a significant weakening of the intensity of transverse mixing at the confluence of large rivers, which is observed in a number of cases, is widely discussed. Since the observed features of the confluence of large watercourses are not only of research interest but also of significant economic importance associated with the characteristics of water management at these water bodies, a large number of works are devoted to their study. Water resources management requires measures for the organization of water use which can be rational only under the understanding of processes occurring in water basins. To explain the phenomenon of suppression of the transverse mixing, which is interesting and important from the point of view of ecology, a wide range of hypotheses is proposed, up to the negation of turbulence in rivers. One of the possible mechanisms for explaining the suppression of transversal mixing can be the presence of transverse circulation manifesting itself as Prandtl’s secondary flows of the second kind. The characteristic velocity of these circulation flows is very small and difficult to measure directly by instruments; however, in our opinion, they can significantly complicate the transverse mixing at the confluence. The proposed hypothesis is tested in computational experiments in the framework of the three-dimensional formulation for dimensions of a real water object at the mouth of the Vishera River where it meets the Kama. Calculations demonstrate that, at sufficiently large flow rates, the two waters practically do not mix in the horizontal direction throughout the depth over long distances from the confluence. It has been found that a two-vortex flow is formed downstream the confluence, which just attenuates the mixing; the fluid motion in the vortices is such that, near the free surface, the fluid moves from the banks to the middle of the riverbed.

中文翻译：

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水动力规律对汇流河水混合的影响

目前，人们广泛讨论了大河汇流处横向混合强度的显着减弱，这种情况在许多情况下观察到。由于观察到的大型水道汇流特征不仅具有研究意义，而且与这些水体的水管理特征相关的具有重要经济意义，因此有大量工作致力于它们的研究。水资源管理需要组织用水的措施，这些措施只有在了解水流域发生的过程的情况下才可能是合理的。为了解释从生态学的角度有趣且重要的横向混合抑制现象，提出了广泛的假设，直到否定河流湍流。解释横向混合抑制的可能机制之一可能是横向环流的存在，表现为第二类普朗特二次流。这些循环流的特征速度很小，很难用仪器直接测量；然而，在我们看来，它们会使汇合处的横向混合显着复杂化。提出的假设在与 Kama 汇合的 Vishera 河口的真实水体尺寸的三维公式框架内的计算实验中得到检验。计算表明，在足够大的流速下，两种水实际上不会在距离汇合处很远的整个深度水平方向上混合。已经发现在汇合处下游形成双涡流，这恰好减弱了混合；涡流中的流体运动是这样的，在自由表面附近，流体从河岸移动到河床中部。