Charru, François and Franklin, Erick M. Subaqueous Barchan dunes in turbulent shear flow. Part 2: Fluid flow. (2012) Journal of Fluid Mechanics, 692. 131154. ISSN 14697645

(Document in English)
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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 speedup over the dune are compared, the usual ‘sameelevation’ and the more relevant ‘Lagrangian’, showing that the latter is smaller by a factor of two. The twolayer 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 speedup, 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 mixinglength 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 speedup 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:  hal00920714 
Audience (journal):  International peerreviewed journal 
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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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