Impact of Junction Angle on Density‐Driven Secondary Flow and Mixing Rates at River Confluences: A Numerical Investigation

Abstract

It is now well understood that a small difference in the density of two rivers can cause density‐driven streamwise vortices (SOVs) to form and their mixing interface to tilt downstream of their confluence. However, much less knowledge exists pertaining to how these processes are affected by other key confluence parameters such as the discharge ratio and junction angle. This study uses eddy‐resolving simulations of an experimental confluence flume to parametrically investigate how increasing junction angle under a range of and discharge ratios affect flow structure. The findings demonstrate that increasing junction angle increases the width of the mixing interface near the apex as well as mixing rates. However, the contribution of junction angle is shown to be incidental compared to that of . Increasing has an overwhelming effect on the lateral expansion of the mixing interface, the magnitude of streamwise circulation, and ultimately, mixing rates compared to the junction angle. Increasing also increases the tilting of the mixing interface, leading to increased lateral stretching of the vertically oriented Kelvin‐Helmholtz vortices, resulting in a rapid loss in their coherence downstream. The simulations indicate the SOVs decay in the streamwise direction due to interaction with the free surface and the bed. Our use of smooth downstream corners, similar to those present at natural river confluences, suggests that the role of the junction angle has likely been overestimated in previous laboratory and modeling studies, which focused on asymmetrical junctions featuring a sharp downstream corner and a wide separation zone.