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

Abstract

[1] 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.

Flow structure and channel change in a sinuous grass‐lined stream within an agricultural drainage ditch: Implications for ditch stability and aquatic habitat

“…His data showed that values of k are highest at the apex of the bend within the zone of scour near the base of the outer bank. This finding is consistent with patterns of k in a curved flow immediately downstream of a river confluence where the highest levels of k are located along the lower face of the outer bank (Rhoads and Sukhodolov, 2008). It also conforms to measurements of turbulence kinetic energy in actively migrating meander bends (Engel and Rhoads, submitted).…”

Response of bed morphology and bed material texture to hydrological conditions at an asymmetrical stream confluence

“…10B). High values of k and acceleration of flow along the mixing interface correspond to high bed shear stresses in the center of the confluence and an increase in bed shear stress over distance, both of which promote scour (Rhoads and Sukhodolov, 2008). Rhoads (1996) showed that dual surface-convergent helical cells also develop on either side of the mixing interface at the entrance to the downstream channel (cross section A), which sweep sediment laterally away from the center of the CHZ, contributing to scour and segregation of sediment loads (Fig.…”

“…10). Scour along the path of the mixing interface is probably related to several factors, including high values of near-bed turbulence kinetic energy (k) Rhoads and Sukhodolov, 2008), convective acceleration of the flow (Rhoads and Sukhodolov, 2008), and helical motion Rhoads, 1996;Rhoads and Sukhodolov, 2001). Ultimately, patterns of erosion and deposition within the CHZ reflect spatial variation in bed shear stress, which (in turn) produces spatial variation in bedload transport capacity (Best and Rhoads, 2008).…”

Experimental investigation of the flow evolution in the tributary of a 90° open channel confluence

“…Since in asymmetrical confluences mixing and shear layer characteristics can be significantly influenced by the lateral momentum flux (Rhoads and Sukhodolov 2008), the distribution of the incoming velocities over the tributary branch is key to understand downstream flow behavior.…”