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.

Flow Patterns in an Open Channel Confluence with Increasingly Dominant Tributary Inflow

“…Compared with mixing layers in natural bathymetries [16,17], the mixing layer of the q = 0.25 case is relatively curved, as is typical for asymmetrical 90° confluences [11]. Due to the momentum inflow from the tributary, the mixing layer extends nearly to the opposing bank when entering the downstream channel ( Figure 6).…”

“…In the present simulations, the free surface is modeled by means of a so-called rigid lid. This approach has been applied to many types of open channel flows, see e.g., [16,17,23,24,26,27,29], although more complex surface tracking methods are also in use [9,13,25,31]. However, because of the small Froude number in the present experiments, the rigid lid is certainly a valid approach, and preferable because of reduced computational efforts.…”

“…The large eddies are retained and are resolved on a mesh in space and time, by solving a discretized version of the filtered equations. LESs have already been applied successfully to study confluence hydrodynamics (e.g., [13,16,17,25]). …”

“…In order to reduce the required computational efforts, and to simulate rough walls more easily, the near-wall boundary layer is not fully resolved (as in [13]), neither is a near-wall RANS model applied (as in [16,17]) but it is simply modeled by means of a wall function (as in [25]). The wall function that was implemented by the authors in OpenFOAM discerns three sublayers of the turbulent boundary layer: …”

“…Field-based research also acknowledged the importance of the discharge ratio (e.g., [14,15]), not only on time-averaged flow, but also on intermittent features [16,17]. According to the literature overview provided by Leite Ribeiro et al [18], the only morphological study including q < 0.1 was provided by Rhoads et al [15].…”