Effects of bed discordance on flow, turbulence, and mixing at an idealized asymmetrical confluence between two shallow streams

When two streams of equal densities but of unequal velocities and different flow directions come into contact, large-scale coherent structures are generated downstream of the confluence apex. They include vertical vortices inside the region of high mean shear between the fluids originating in the two tributaries [i.e., the mixing interface (MI)] and streamwise-oriented-vortical (SOV) cells inside and in the vicinity of the MI. The flow physics becomes even more complex when the bed elevations of the tributary channels are not the same as that of the downstream channel. Eddy resolving simulations are used to investigate how the gradual increase in the level of bed discordance between the minor tributary channel and the main downstream channel affect flow, turbulence structure, sediment entrainment mechanisms and mixing at an asymmetrical confluence with a confluence angle of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:msup> <a:mn>70</a:mn> <a:mo>∘</a:mo> </a:msup> </a:math> . To better isolate the effects of bed discordance, simulations are performed with idealized conditions in which the bottom surfaces of the open channels are horizontal except near the region where the minor tributary connects with the main channel. The other main flow parameter that is varied is the discharge ratio between the two streams, QR. Results show that for constant QR, the coherence of the vertical MI vortices increases with increasing bed discordance, and the wake mode becomes gradually dominant even though the velocity and momentum ratios are not close to unity. Increasing the bed discordance reduces the coherence and number of SOV cells forming on the minor tributary side of the MI and promotes the formation of a near-bed intrusion of mixed fluid into the minor tributary side of the main channel near the confluence apex. This is the main mechanism that is responsible for the increase in the rates of mixing between the two streams with increasing bed discordance for constant QR. While for small bed discordance levels, sediment entrainment inside the confluence hydrodynamic zone is driven primarily by the SOV cells, for sufficiently large bed discordance levels and QR, sediment entrainment on the minor tributary side is mainly driven by the plunging core of high free surface velocities as it enters the main channel.