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Double-averaging analysis and local flow characterization of near-bed turbulence in gravel-bed channel flows

Published online by Cambridge University Press:  10 January 2009

EMMANUEL MIGNOT ,
E. BARTHELEMY  and
D. HURTHER
EMMANUEL MIGNOT*
Affiliation:
Laboratoire des Ecoulements Géophysiques et Industriels (LEGI), BP 53, 38041 Grenoble Cedex 9, France
E. BARTHELEMY
Affiliation:
Laboratoire des Ecoulements Géophysiques et Industriels (LEGI), BP 53, 38041 Grenoble Cedex 9, France
D. HURTHER
Affiliation:
Laboratoire des Ecoulements Géophysiques et Industriels (LEGI), BP 53, 38041 Grenoble Cedex 9, France
*
†Email address for correspondence: emmanuel.mignot@hmg.inpg.fr
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Abstract

This investigation focuses on the characteristics of near-bed turbulence in fully rough gravel-bed open-channel flows. The analysis combines results obtained with the double-averaging methodology and local flow characterization, using velocity measurements provided by a high-resolution three-axis Acoustic Doppler Velocity Profiler (ADVP). As a result of the flow heterogeneity induced by the bed topography, the flow is not locally uniform in the near-bed region, and a double-averaging methodology is applied over a length scale much greater than the gravel size. In smooth- and rough-bed flow conditions, without macro-roughness bed elements, maximum turbulent kinetic energy (TKE) production occurs very close to z = 0, while in our case with fully rough flows with macro-roughness elements, maximum turbulence activity is found to occur at gravel crest levels zc (zc/h = 0.1). Turbulent diffusion also reaches a maximum at this elevation. The characteristics of the spatially averaged TKE budget are in good agreement with those obtained in flows over canopies. The hydrodynamic double-averaged properties have strong similarities with mixing layers and reattached mixing layers in flows over backward facing steps. Local time-averaged velocity profiles can be split into three typical classes, namely log, S-shaped and accelerated. It appears that the S-shaped class profiles, located in the wakes of the macro-roughness elements, exhibit an inflectional profile typical of mixing layers. They are of major importance in the double-averaged TKE budget, as they provide a local high contribution to the double-averaged TKE flux, TKE production and dissipation compared to the log class profiles. Consequently, double-averaged TKE production is roughly 75% greater than the dissipation rate at the point of maximal TKE production. Moreover the macro-roughness bed elements imply mixing-layer-type hydrodynamics that play a dominant role in the overall structure of mean near-bed turbulence of gravel-bed channel flows.

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Papers
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Journal of Fluid Mechanics , Volume 618 , 10 January 2009 , pp. 279 - 303
Copyright
Copyright © Cambridge University Press 2008

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