A Numerical Study of the Effects of Density Contrast on Flow, Turbulence, and Mixing at the Negro/Solimoes Confluence, Brazil

Abstract

Eddy-resolving numerical simulations are conducted to investigate the effects of density contrast between the confluent flows on hydrodynamics, coherent structures and mixing downstream of the confluence between the Negro and Solimões Rivers, Brazil, which is characterized by both a large discharge (>100,000 m³/s) and a large aspect-ratio (W/D > 100). The base case corresponds to real high-flow conditions observed at the confluence for which the density of the Negro River was smaller than that of the Solimões River and the ratio between the discharges in the Negro and Solimões rivers was QR = Q₁/Q₂ = 0.32. To be able to understand and isolate density contrast effects, numerical experiments are also conducted assuming negligible density effects between the incoming flows (i.e., with Richardson number Ri = 0) and with the densities reversed in the incoming streams. Despite the small density difference and very large discharges, a well-defined near-bed intrusion of heavier colder fluid develops in the Ri > 0 simulations and strongly affects the tilting of the mixing interface (MI) compared to the Ri = 0 case where despite the absence of streamwise-oriented vortical cells, the MI is strongly tilted toward the Negro River side of the main confluence channel. Moreover, the presence/absence of density contrast between the incoming streams affects the position, size and sense of rotation of the main helical cells of crossflow motions and the coherence of the vertical MI vortices.