Numerical analysis of the effect of momentum ratio on the dynamics and sediment‐entrainment capacity of coherent flow structures at a stream confluence

Abstract

The flow and turbulence structure at stream confluences are characterized by the formation of a mixing interface (MI) and, in some cases, of streamwise‐oriented vortical (SOV) cells flanking the MI. Depending on the junction angle and planform symmetry, as well as the velocity ratio across the MI, the MI can be in the Kelvin‐Helmholtz (KH) mode or in the wake mode. In the former case, the MI contains predominantly co‐rotating large‐scale quasi two‐dimensional (2‐D) eddies whose growth is driven by the KH instability and vortex pairing. In the latter case, the MI is populated by quasi 2‐D eddies with opposing senses of rotation. This study uses eddy resolving simulations to predict details of flow structure for KH‐ and wake‐mode conditions at a confluence for which field measurements are available. Results indicate that SOV cells at this confluence, which occur in both modes, redistribute momentum and mass, enhancing the potential for entrainment of bed material beneath the cells and for extraction of fluid and suspended sediment from the MI. The simulations predict that the cores of some of the primary SOV cells are subject to large‐scale bimodal oscillations toward and away from the MI that contribute to amplification of the turbulence close to the MI and enhance the capacity of the SOV cells to entrain sediment. At this confluence, which has a concordant bed and a large angle between the incoming streams ‐ conditions that generate strong adverse lateral pressure gradients adjacent to the MI ‐ the oscillating SOV cells interact with MI eddies to generate large bed friction velocities in the zone of scour immediately downstream of the confluence. Streamwise oriented vortical cells act as a pump of momentum and mass Cores of streamwise oriented vortical cells are subject to bimodal oscillations Streamwise oriented vortical cells and mixing interface cells control entrainment