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Influence of planform geometry and momentum ratio on thermal mixing at a stream confluence with a concordant bed

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Abstract

The effects of planform geometry and momentum flux ratio on thermal mixing at a stream confluence with concordant bed morphology are investigated based on numerical simulations that can capture the dynamics of large-scale turbulence. In two simulations, the bathymetry and asymmetrical planform geometry are obtained from field experiments and the momentum flux ratio is set at values of one and four. These two conditions provide the basis for studying differences in thermal mixing processes at this confluence when the wake mode and the Kelvin–Helmholtz mode dominate the development of coherent structures within the mixing interface (MI). The effects of channel curvature and angle between the two incoming streams on thermal mixing processes are investigated based on simulations conducted with modified planform geometries. Two additional simulations are conducted for the case where the upstream channels are parallel but not aligned with the downstream channel and for the zero-curvature case where the upstream channels are parallel and aligned with the downstream channel. The simulations highlight the influence of large-scale coherent structures within the MI and of streamwise-oriented vortical (SOV) cells on thermal mixing processes within the confluence hydrodynamics zone. Simulation results demonstrate the critical role played by the SOV cells in promoting large-scale thermal mixing for cases when such cells form in the immediate vicinity of the MI and in modifying the shape of the thermal MI within cross sections of the downstream channel—predictions consistent with empirical measurements of thermal mixing at the confluence. The set of numerical simulations reveal that the degree of thermal mixing occurring within the confluence hydrodynamic zone varies dramatically with planform geometry and incoming flow conditions. In some cases thermal mixing at the downstream end of the confluence hydrodynamic zone is limited to the MI and its immediate vicinity, whereas in others substantial thermal mixing has occurred over most of the cross-sectional area of the flow. Overall, the simulations highlight the flow conditions and the controls of these conditions that influence mixing within the immediate vicinity of a confluence.

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Acknowledgments

The authors would like to thank the Transportation Research and Analysis Computing Center (TRACC) at the Argonne National Laboratory and the National High-Performance Computing Center in Taiwan (NHPC) for providing substantial computing time. Part of this work was supported by the National Science Foundation under Grant GSS1359836. Any opinions, findings or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the National Science Foundation    

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Authors and Affiliations

  1. Department of Civil and Environmental Engineering and IIHR-Hydroscience and Engineering, The University of Iowa, Iowa City, IA, 52242, USA

    George Constantinescu

  2. IIHR-Hydroscience and Engineering, The University of Iowa, Iowa City, IA, 52242, USA

    Shinjiro Miyawaki

  3. Department of Geography and Geographic Information Science, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA

    Bruce Rhoads

  4. Department of Ecohydrology, Institute of Freshwater Ecology and Inland Fisheries, Müggelseedamm 310, 12587, Berlin, Germany

    Alexander Sukhodolov

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  1. George Constantinescu
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  2. Shinjiro Miyawaki
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Correspondence to George Constantinescu.

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Constantinescu, G., Miyawaki, S., Rhoads, B. et al. Influence of planform geometry and momentum ratio on thermal mixing at a stream confluence with a concordant bed. Environ Fluid Mech 16, 845–873 (2016). https://doi.org/10.1007/s10652-016-9457-0

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