Hostname: page-component-78c5997874-v9fdk Total loading time: 0 Render date: 2024-11-18T12:20:58.340Z Has data issue: false hasContentIssue false
  • English
  • Français

Horizontal mixing of quasi-uniform straight compound channel flows

Published online by Cambridge University Press:  15 January 2010

ALESSANDRO STOCCHINO  and
MAURIZIO BROCCHINI
ALESSANDRO STOCCHINO*
Affiliation:
Dipartimento di Ingegneria delle Costruzioni, dell'Ambiente e del Territorio, University of Genova, Via Montallegro 1, 16145 Genova, Italy
MAURIZIO BROCCHINI
Affiliation:
Department of I.S.A.C., Polytechnic University of Marche, Via Brecce Bianche 12, 60131 Ancona, Italy
*
†Email address for correspondence: jorma@dicat.unige.it
Get access Rights & Permissions [Opens in a new window]

Abstract

The generation and evolution of large-scale vortices with vertical axis (macro-vortices) in a straight compound channel under quasi-uniform flow conditions is investigated. We discuss possible similarities and clear differences with free shear layer flows induced by the meeting of shallow streams of different speeds. An experimental investigation based on particle image velocimetry (PIV) measurements of free-surface velocities forms the basis for an analysis of both the specific features of macro-vortices and of the related mean flow characteristics. Dynamical properties strongly depend on the ratio rh between the main channel flow depth (h*mc) and the floodplain depth (h*fp), and three flow classes can be identified. ‘Shallow flows’ (rh > 3) are dominated by strong shearing and large macro-vortices populating the transition region between the main channel and the floodplains. The mean streamwise velocity induced in ‘intermediate flows’ (2 ≤ rh ≤ 3) is characterized by a dip in the transition region, while it closely resembles that occurring in a rectangular channel in the case of ‘deep flows’ (rh < 2). For both the latter cases the shear in the transition region decreases and the macro-vortices are also generated in the wall boundary layer of the floodplains. The typical size of the quasi-two-dimensional macro-vortices, generated at the transition region, is found to be independent of the streamwise coordinate. This and the non-monotonic behaviour of the mean streamwise velocity suggest that in straight compound channels the topographic forcing is so dominant that conceptual models interpreting these flows as free shear layers may largely fail to describe the physics of compound channels flows.

JFM classification

Mixing: Mixing
Type
Papers
Information
Journal of Fluid Mechanics , Volume 643 , 25 January 2010 , pp. 425 - 435
Copyright
Copyright © Cambridge University Press 2010

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

REFERENCES

Adrian, R. J., Christensen, K. T. & Liu, Z. C. 2000 Analysis and interpretation of istantaneous turbulent velocity fields. Exp. Fluids 29, 275–290.CrossRefGoogle Scholar
Bousmar, D., Proust, N., Riviereand, S., Paquier, A., Morel, R. & Zech, Y. 2005 Upstream discharge distribution in compound-channel flumes. J. Hydraul. Engng ASCE 131 (5), 1408–1413.CrossRefGoogle Scholar
Chu, V. H., Wu, J. H. & Khayat, R. E. 1991 Stability of transverse shear flow in shallow open channels. J. Hydr. Engng 117 (10), 1370–1388.CrossRefGoogle Scholar
Jirka, G. H. 2001 Large scale flow structures and mixing processes in shallow flows. J. Hydraul. Res. 39, 567–573.CrossRefGoogle Scholar
Mobbs, F. R. 1968 Spreading and contraction at flows. J. Fluid Mech. 33, 227–240.CrossRefGoogle Scholar
Negretti, M. E., Vignoli, G., Tubino, M. & Brocchini, M. 2006 On shallow-water wakes: an analytical study. J. Fluid Mech. 567, 457–475.CrossRefGoogle Scholar
Nezu, I., Onitsuka, K. & Iketani, K. 1999 Coherent horizontal vortices in compound open-channel flows. In Hydraulic Modeling (ed. Singh, V. P., Seo, I. W. & Sonu, J. H.), pp. 17–32. Water Resources Publication.Google Scholar
Nikora, V., Nokes, R., Veale, W., Davidson, M. & Jirka, G. H. 2007 Large-scale turbulent structure of uniform shallow free-surface flows. Environ. Fluid Mech. 7, 159–172.CrossRefGoogle Scholar
Piattella, A., Brocchini, M. & Mancinelli, A. 2006 Topographically-controlled, breaking wave-induced macrovortices. Part 3. The mixing features. J. Fluid Mech. 559, 81–106.CrossRefGoogle Scholar
van Prooijen, B. C., Battjes, J. A. & Uijttewaal, W. S. J. 2005 Momentum exchange in straight uniform compound channel flow. J. Hydraul. Engng 131 (3), 175–183.CrossRefGoogle Scholar
van Prooijen, B. C. & Uijttewaal, W. S. J. 2002 A linear approach for the evolution of coherent structures in shallow mixing layers. Phys. Fluids 14 (12), 4105–4114.CrossRefGoogle Scholar
Sellin, R. H. J. 1964 A laboratory investigation into the interaction between the flow in the channel of a river and that over its flood plain. La Houille Blanche 7, 793–802.CrossRefGoogle Scholar
Shiono, K. & Knight, D. W. 1991 Turbulent open-channel flows with variable depth across the channel. J. Fluid Mech. 222, 617–646.CrossRefGoogle Scholar
Socolofksy, S. A. & Jirka, G. H. 2004 Large-scale flow structures and stability in shallow flows. J. Environ. Engng Sci. 3, 451–462.CrossRefGoogle Scholar
Soldini, L., Piattella, A., Brocchini, M., Mancinelli, A. & Bernetti, R. 2004 Macrovortices-induced horizontal mixing in compound channels. Ocean Dyn. 54, 333–339.CrossRefGoogle Scholar
Stephenson, D. & Kolovopoulos, P. 1990 Effects of momentum transfer in compound channels. J. Hydraul. Engng ASCE 116, 1512–1522.CrossRefGoogle Scholar
Tabeling, P. 2002 Two-dimensional turbulence: a physicist approach. Phys. Rep. 362, 1–62.CrossRefGoogle Scholar