Lateral momentum flux and the spatial evolution of flow within a confluence mixing interface

Mixing interfaces at confluences have been viewed as analogous to shallow mixing layers in which lateral fluxes of momentum generally are viewed as unimportant. This investigation examines the spatial evolution of flow along a confluence mixing interface, focusing on lateral fluxes of momentum. Cross-stream profiles of depth-averaged mean velocities indicate that flow within the mixing interface has wake characteristics near the upstream junction corner, mixing-layer properties within the confluence, and attributes of a weak jet within the downstream channel. Convective acceleration of downstream velocity along the path of the mixing interface is influenced by lateral fluxes of momentum from the converging mean flow. These fluxes exceed turbulence lateral momentum fluxes by one to two orders of magnitude. Flow in the mixing interface is unstable, but lateral expansion of coherent turbulent vortices over distance is limited, possibly due to the inhibiting effects of the converging flow. Similar low-frequency oscillations in near-surface velocity fluctuations occur within the stagnation zone and along the mixing interface, suggesting that the dynamics of the stagnation-zone wake may influence low-frequency characteristics of flow in the downstream portion of the mixing interface. Patterns of turbulence kinetic energy (k) are influenced by lateral momentum fluxes, which progressively shift the zone of maximum k toward the outer bank. Overall the study shows that lateral momentum fluxes are important in the spatial evolution of flow within confluence mixing interfaces and need to be accounted for in theoretical treatments of these interfaces.