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Subaqueous Barchan dunes in turbulent shear flow. Part 2: Fluid flow.

Charru, François and Franklin, Erick M. Subaqueous Barchan dunes in turbulent shear flow. Part 2: Fluid flow. (2012) Journal of Fluid Mechanics, 692. 131-154. ISSN 1469-7645

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Official URL: http://dx.doi.org/10.1017/jfm.2011.528

Abstract

We report an experimental study of the turbulent flow above a barchan dune in a channel, from particle image velocimetry measurements, for Reynolds numbers ranging from 9000, just below the threshold for particle motion, up to 24 000, where the dune moves. Two calculations of the speed-up over the dune are compared, the usual ‘same-elevation’ and the more relevant ‘Lagrangian’, showing that the latter is smaller by a factor of two. The two-layer structure of the flow disturbance – an essentially inviscid outer layer and a turbulent inner layer of thickness δi – is assessed. In the outer layer, streamline curvature is shown to be responsible for half of the Lagrangian speed-up, from the comparison of the velocity measurements with two Bernoulli calculations. In the inner layer, detailed measurements of the velocity and stresses are provided, down to γ+ ≈ 1, and the momentum budget is discussed. The Reynolds shear stress decreases monotonically towards the dune surface, according to the standard mixing-length closure, whereas the total shear stress increases strongly in the viscous sublayer. Along the dune surface, the shear stress increases up to the crest where it reaches twice its unperturbed value. A good estimate of the surface stress is provided by a parabolic fit of the inner velocity profile matching the outer flow at γd ≈ δi. Doubling the Reynolds number, the surface shear stress and the speed-up decrease by ∼30 %. The implications of these results on the dune motion, presented in Part 1 of this study (Franklin & Charru, J. Fluid Mech., vol. 675, 2011, pp. 199–222), are finally discussed.

Item Type:Article
Additional Information:Thanks to Cambridge University Press editor. The original PDF of the article can be found at Journal of Fluid Mechanics website : http://journals.cambridge.org/action/displayJournal?jid=FLM
HAL Id:hal-00920714
Audience (journal):International peer-reviewed journal
Uncontrolled Keywords:
Institution:French research institutions > Centre National de la Recherche Scientifique - CNRS (FRANCE)
Université de Toulouse > Institut National Polytechnique de Toulouse - INPT (FRANCE)
Université de Toulouse > Université Toulouse III - Paul Sabatier - UPS (FRANCE)
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Deposited By: Francois CHARRU
Deposited On:19 Dec 2013 08:48

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