Abstract
In this study, the coupled effects of sediment inertia and stratification on the pattern of secondary currents in bend-flows are evaluated using a 3D numerical model. The sediment inertia effect, as well as the stratification effect induced by the non-uniform distribution of suspended sediment, is accounted for by adopting the hydrodynamic equations without the Boussinesq approximation. The 3D model is validated by existing laboratory experimental results. Simulation results of a simplified meandering channel indicate that sediment stratification effect enhances the intensity of secondary flow via reducing eddy viscosity, while sediment inertia effect suppresses it. The integrated effects result in an increase and a reduction in the secondary flow, respectively, at lower and higher concentrations (near-bed volumetric concentrations of 0.015 and 0.1 are, respectively, considered in this study). This suggests that the dominance of the suspended sediment effect depends on the sediment concentration profile. With the increase of concentration under a specific sediment size, the secondary flow rises to reach a maximum, and then decreases. Moreover, as the sediment concentration increases, an exponentially decaying rate has been found for the secondary flow. It is concluded that in the numerical simulation of flow in meandering channels, when concentration is high, the variable-density hydrodynamic equations without the Boussinesq approximation should be considered.
Highlights
In fluvial process studies, suspension has always been known to have a fundamental role in defining the preferential bend migration, since the stratification brought by suspended load modifies the turbulence, flow structure and sediment entrainment
As there was no experimental data available to us for suspended sediment motion in meandering channels, we firstly validated the model for the cases with suspended sediment at equilibrium and non-equilibrium conditions in straight channels with comparing the model results with a set of existing experimental data (Section 3.1); applied the model for a meandering channel and validated the accuracy of the model against existing experiments (Section 3.2); and numerically simulated flow and sediment motion in a meandering channel with investigating the effects of suspended sediment on the velocity structure (Section 3.3)
This paper presents 3D numerical simulations of sediment laden flows in open channel meanders
Summary
Suspension has always been known to have a fundamental role in defining the preferential bend migration, since the stratification brought by suspended load modifies the turbulence, flow structure and sediment entrainment. Vanoni showed that the von Karman constant κ reduces at high sediment concentration This was based on the hypothesis that the suspended sediment dampens turbulence and increases velocity gradient near the wall. Sediment laden flows have been widely studied by using various numerical models, most studies concern low suspension concentration using the advection-diffusion equation. Winterverp [15,16] numerically explained the sediment stratification effect by a buoyancy destruction term in the standard k-ε turbulence model at low concentration. A large vertical density gradient which has obvious inertia effect on flow can be a result of high bulk sediment concentration. The 3D model is substantially different from traditional hydrodynamic models under Boussinesq approximation which only consider the stratification effect of suspensions as the buoyance term. First, model setup is provided; second, a few cases are selected for model verification; third, numerical experiments with and without Boussinesq approximation are presented for comparison; and discussions and suggestions with regards to the sediment inertia effect are given
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