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| Titel |
Passive scalar mixing efficiency in open-channel confluences |
| VerfasserIn |
Emmanuel Mignot, Sebastien Pouchoulin, Nicolas Riviere, Gislain Lipeme-Kouyi |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250140716
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| Publikation (Nr.) |
 EGU/EGU2017-4143.pdf |
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| Zusammenfassung |
| The work deals with the efficiency of passive scalar mixing within and downstream
open-channel confluences. Mixing in confluences is imposed by the local 3D hydrodynamics
including: i) the slow recirculation zone, ii) the contraction zone with flow acceleration on its
side, iii) downstream secondary currents and iv) a mixing-layer at the interface between the
two incoming flows. The mixing efficiency thus strongly depends on the characteristics of
these large structures that exchange fluid from one flow region to another and that are,
themselves, strongly impacted by the characteristics of the confluence geometry
(angle, roughness, bed forms…) and of the inflows (momentum and density ratio,
density...). The present work aims at investigating the influence of i) the confluence
geometry and ii) inflow characteristics on the mixing efficiency at the confluence.
In the literature, this efficiency is for example related to the evolution, along the
streamwise axis of the downstream branch, of parameter E, accounting for the
variability of scalar concentration ci measured in cell i (normalized by the upstream
concentration cu of the polluted incoming flow) within a section of mean concentration cm,
as:
┌ ----------------
││ ∑n ( )2
E = ∘ 1- ci-− cm-
n i=1 cu
where n is the number of measurements in each section.
These results then permit to estimate the so-called “length for perfect mixing” Lm,
defined as the distance from the confluence to the section where the differences of local
concentrations from a uniform one become lower than a given threshold value (typically
5%).
The selected approach combines an experimental and a numerical approach. In a first
step, the experiments aim at measuring for a selected configuration, the 3D spatial
distribution of passive scalar in a simplified subcritical 90˚ angle confluence in order to
better understand the mixing processes and to validate the selected numerical calculation
methods. A novel laboratory technique able to measure the 3D concentration field within a
closed-loop flume is presented along with corresponding limits, precautions and
uncertainties. The data set obtained experimentally is then used to calibrate and validate
3D-LES and 3D-RANS numerical codes. In a second step, a series of calculations using the
calibrated model will be undertaken in order to estimate the impact of the confluence
geometry and of the inflows parameters on the passive scalar dispersion efficiency. |
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