Structure of turbulent flow at a river confluence with momentum and velocity ratios close to 1: Insight provided by an eddy‐resolving numerical simulation

Abstract

[1] River confluences are complex hydrodynamic environments where convergence of incoming flows produces complicated patterns of fluid motion, including the development of large-scale turbulence structures. Accurately simulating confluence hydrodynamics represents a considerable challenge for numerical modeling of river flows. This study uses an eddy-resolving numerical model to simulate the mean flow and large-scale turbulence structure at an asymmetrical river confluence with a concordant bed when the momentum ratio between the two incoming streams is close to 1. Results of the simulation are compared with field data on mean flow and turbulence structure. The simulation shows that the mixing interface is populated by quasi-two-dimensional eddies. Successive eddies have opposing senses of rotation. The mixing layer structure resembles that of a wake behind a bluff body (wake mode). Strong streamwise-oriented vortical (SOV) cells form on both sides of the mixing layer, a finding consistent with patterns inferred from the field data. The predicted mean flow fields show that flow curvature has an important influence on streamwise variation of circulation within the cores of the two primary SOV cells. These SOV cells, along with vortices generated by flow over a submerged block of sediment at one of the banks, strongly influence distributions of the streamwise velocity and turbulent kinetic energy downstream of the junction. Comparison of the eddy-resolving simulation results with predictions from the steady Spalart-Allmaras RANS model shows that the latter fails to predict important features of the measured distributions of streamwise velocity and turbulent kinetic energy because the RANS model underpredicts the strength of the SOV cells. Analysis of instantaneous and mean bed shear stress distributions indicates that the SOV cells enhance bed shear stresses to a greater degree than the quasi-two-dimensional eddies in the mixing interface.

Flow resistance in natural, turbulent channel flows: The need for a fluvial fluid mechanics

“…Eddy-resolving simulation techniques such as large-eddy simulation (LES) and detached eddy simulation (DES), or lattice Boltzmann methods, then become necessary. The utility of eddy-resolving methods has been demonstrated by the explosion of the number of studies in the last few years, with applications to flow around channel obstacles and their associated scour holes [Kirkil et al, 2008;Koken and Constantinescu, 2009], flow through vegetation [Kim and Stoesser, 2011], as well as sediment transport [Nabi et al, 2012[Nabi et al, , 2013aSchmeeckle, 2014] and flow over bed forms [Nabi et al, 2013b;Chang and Constantinescu, 2013;Omidyeganeh and Piomelli, 2013], and flow through channel confluences [Constantinescu et al, 2011a]. See Keylock et al [2005] for an early introduction to the use of LES in channel flow hydraulics and Keylock et al…”

A fluid-mechanics based classification scheme for surface transient storage in riverine environments: quantitatively separating surface from hyporheic transient storage

“…The minimum radius of curvature is associated with the smallest point bar that can form along a meander bend, which will decrease mean residence time. Larger main channel widths form larger point bar widths, which Miyawaki et al, 2010 andConstantinescu et al, 2011. ) increases mean residence time.…”

“…14a), whereas when the large-scale vortical structures rotate in the same direction (co-rotating), the mixing interface is in the KelvinHelmholtz mode ( Fig. 14b; Constantinescu et al, 2011). In the wake mode, the counter-rotating vortices are shed from the separation point and grow downstream (from weak Kelvin-Helmholtz instabilities) without an increase to their circulation, which limits their ability to entrain sediment.…”

“…Vortical structures in the free mixing interface are shed from each stream at the junction point and advect downstream, accelerating the downstream flow within the mixing interface (Rhoads and Kenworthy, 1995). Vortical structure pairing has been documented in the mixing interface, whereby shedded vortical structures from each stream are advected downstream in pairs from the junction point (e.g., Rhoads and Kenworthy, 1998;Rhoads and Sukhodolov, 2001;Constantinescu et al, 2011).…”

“…14; Miyawaki et al, 2009Miyawaki et al, , 2010Constantinescu et al, 2011). When the large-scale streamwise-oriented vortical structures rotate in opposite directions (counter-rotating), the mixing interface is in the wake mode (Fig.…”