On shallow mixing interfaces and their relevance for understanding mixing at river confluences
Although confluence flows are ubiquitous, occurring in geophysical systems, such as river networks, mechanical systems, such as duct or pipe networks, and biological systems, such as arterial or venous networks, understanding of the fluid dynamics of confluences is limited.
This research proposes a generalized framework characterizing the fluid dynamics of open-channel confluences with a concordant bed, including an-in depth investigation of the physics of shallow mixing layers developing over a no-slip surface. Using eddy-resolving numerical simulations in idealized geometries, we systematically examine the main parameters (confluence angle, velocity ratio, density ratio, curvature of the downstream channel) that control confluence fluid dynamics and the influence of this dynamics on mixing between the two incoming streams. In particular, we investigate the role played by the quasi-2D large-scale eddies generated within the mixing interface and by the streamwise-oriented vortical (SOV) cells forming in the vicinity of the mixing interface in mixing.
We show that the main reason why SOV cells have a large capacity to enhance mixing and to entrain sediment from the bed in the case of a loose-bed boundary is because their cores are subject to large-scale bimodal oscillations toward and away of the mixing interface.
Finally we discuss some examples where shallow mixing interfaces form at river confluences with natural bathymetry and in particular the role played by density differences between the incoming streams on hydrodynamics and mixing of concordant bed confluences.
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