Numerical simulations on mixing of passive scalars in river confluences

Pouchoulin, Sébastien; Mignot, Emmanuel; Rivière, Nicolas; Coz, Jérôme Le

doi:10.1051/e3sconf/20184005019

Cited by 4 publications

(4 citation statements)

References 10 publications

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“…As the helical cell from the CS becomes dominant, mixing occurs as flow from the KR is advected near the bed into flow from the lateral tributary ( Figure 3). The importance of curvature-induced helical motion for mixing at asymmetrical confluences has been noted in other field and numerical modeling studies (Chen et al, 2017;Jung et al, 2019), and modeling work has suggested that the strength of helical flow can directly control mixing length for both slow-and fast-mixing cases (Pouchoulin et al, 2018).…”

Section: Water Resources Researchmentioning

confidence: 83%

Advective Lateral Transport of Streamwise Momentum Governs Mixing at Small River Confluences

Lewis

Rhoads

Sukhodolov

et al. 2020

Water Resources Research

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Confluences are important sites for mixing within river networks. Past work has shown that mixing within confluences is highly variable; in some cases flows mix rapidly and in other cases flows remain unmixed far downstream of the confluence. The fluvial processes that govern mixing within confluences remain poorly understood. This study relates patterns and amounts of mixing to three-dimensional flow structure at three small confluences. It focuses on lateral fluxes of streamwise momentum, which theoretical considerations suggest should influence lateral mixing. Patterns and amounts of mixing differ at the three sites. Considerable mixing occurs at an asymmetrical confluence with strong helical motion within flow from the lateral tributary, which produces substantial differences in advective lateral transport of streamwise momentum over depth. Minor mixing occurs at a comparatively symmetrical confluence where incoming flows have relatively equal momentum fluxes; however, helical motion within one of the flows locally increases mixing. At a symmetrical confluence where one incoming flow has much greater momentum flux than the other, mixing occurs largely through progressive lateral shifting of the mixing interface toward the minor tributary because of the strong lateral flux of streamwise momentum by the dominant tributary. At all three confluences, lateral turbulent transport of streamwise momentum is an order of magnitude less than advective lateral transport of streamwise momentum. The study indicates that generalization of mixing at confluences remains challenging but that advective lateral fluxes of streamwise momentum related to secondary currents (helical motion) or primary flow (cross currents) greatly enhance mixing at confluences.

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Section: Water Resources Researchmentioning

confidence: 83%

Advective Lateral Transport of Streamwise Momentum Governs Mixing at Small River Confluences

Lewis

Rhoads

Sukhodolov

et al. 2020

Water Resources Research

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“…In order to evaluate the mixing efficiency of pollutant, two indices were imposed for the assessment of mixing rate along the channel. The normalized root mean square of concentration Cm within the profile is an effective parameter to quantify the uniformity of the pollutant's spatial distribution, which have been widely used in similar studies [33,34]. The definition of Cm can be expressed by: ( )…”

Section: Mixing Efficiencymentioning

confidence: 99%

Numerical Study of Mixing Process by Point Source Pollution with Different Release Positions in a Sinuous Open Channel

Zhu

Wang

et al. 2022

Water

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The process of pollutant mixing is significantly influenced by secondary flow and turbulence in meandering rivers. To investigate the influence of different point source release positions on the pollutant mixing process in sinuous open channel flows, a 3D large-eddy simulation (LES) model based on OpenFOAM was established to simulate the process of passive scalar transport in a sinuous channel with a rectangular cross-section. After verification by a flume experiment, two sets of cases in which the point sources were arranged at identical intervals in spanwise and streamwise directions were configured to evaluate the mixing efficiency. The effect of flow structure, secondary motion, and the turbulent viscosity on the scalar transport and mixing was discussed. The distribution of scalar as well as the scalar flux was analyzed in detail, and the fluctuation characteristics were also described. The results demonstrate that due to the existence of secondary flow in the sinuous channel, different transverse and streamwise release positions of the point source have significant influence on mixing efficiency and spatial distribution of the pollutant. The point source placed near the center of the cross-section in transverse or near the apex of the bend in streamwise result in higher mixing efficiency. Mixing efficiency calculated by different indices can be different, which requires comprehensive assessment.

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“…To quantify the non-uniformity of mixing downstream of the confluence, the dimensionless parameter E, used e.g. by Dalmon et al (2015) and Pouchoulin et al (2018), is adopted in the present work:…”

Section: Mixing Of the Passive Scalarmentioning

confidence: 99%

“…Both the flow and the mixing of a passive scalar will be investigated by means of Large Eddy Simulation (LES). Moreover, the spatial focus of this paper will not be mainly devoted to the CHZ, as in Tang et al (2018), but will also extend somewhat further downstream, though not as far as needed for studying the length for complete mixing, as in Pouchoulin et al (2018). (= 0.4m) is larger than the width W (= 0.3m) of the main upstream channel and the tributary channel, as shown schematically in Figure 1.…”

Section: Introductionmentioning

confidence: 99%

Numerical study of the flow and passive scalar transport in an open-channel confluence with a flat and a degraded fixed bed

Jin¹,

Ramos²,

Schindfessel³

et al. 2020

River Flow 2020

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Confluences are important junctions in fluvial and artificial networks of open-channels, as they regulate the mixing phenomena of substances transported by the merging flows, as well as the scour and deposition phenomena. This paper aims at contributing to the study of how the bed morphology, such as the presence of a scour hole and a depositional bar, influences the flow and mixing phenomena at a T-shaped open-channel confluence. By means of Large Eddy Simulations (LES) of flow and passive scalar transport, a comparative numerical study is carried out of two fixed bed cases, i.e. a flat one and a degraded one, which were investigated earlier experimentally in the lab and numerically with a RANS based solver by Tang et al. (2018). The spatial focus of the present paper is not exclusively devoted to the Confluence Hydrodynamics Zone, but also extends somewhat further downstream (i.e. up to 10 post-confluence channel widths W d downstream of the upstream confluence corner). For the investigated flow condition with a dominant incoming flow from the upstream main channel, the recirculation zone in the degraded bed case is found to be significantly shorter and less wide or even absent near the bed, yielding differences in the contraction of the main channel flow and in the secondary flow. Nevertheless, the non-uniformity of passive scalar mixing was found to be very similar up to 3.5W d . Further downstream, the flat bed case shows a slightly higher mixing rate, which does not seem to be related to differences in secondary flow patterns, but rather to differences in turbulence levels.

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Numerical simulations on mixing of passive scalars in river confluences

Cited by 4 publications

References 10 publications

Advective Lateral Transport of Streamwise Momentum Governs Mixing at Small River Confluences

Advective Lateral Transport of Streamwise Momentum Governs Mixing at Small River Confluences

Numerical Study of Mixing Process by Point Source Pollution with Different Release Positions in a Sinuous Open Channel

Numerical study of the flow and passive scalar transport in an open-channel confluence with a flat and a degraded fixed bed

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