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Turbulent spots in oscillatory boundary layers

Published online by Cambridge University Press:  19 September 2011

Marco Mazzuoli ,
Giovanna Vittori  and
Paolo Blondeaux
Marco Mazzuoli
Affiliation:
Department of Civil, Environmental and Architectural Engineering, University of Genoa, 16145, Genoa, Italy
Giovanna Vittori*
Affiliation:
Department of Civil, Environmental and Architectural Engineering, University of Genoa, 16145, Genoa, Italy
Paolo Blondeaux
Affiliation:
Department of Civil, Environmental and Architectural Engineering, University of Genoa, 16145, Genoa, Italy
*
†Email address for correspondence: vittori@dicat.unige.it
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Abstract

Detailed knowledge of the dynamics of vortex structures in an oscillatory boundary layer is essential for the correct modelling of transport processes in many engineering problems and, in particular, of the pick-up and transport of sediments at the bottom of sea waves. In the present contribution, the formation of turbulent spots in an oscillatory boundary layer is investigated by means of direct numerical simulations. Two of the laboratory experiments of Carstensen, Sumer and Fredsøe are reproduced and, after a comparison of the numerical results with laboratory measurements, a detailed and quantitative characterization of the turbulent spots is also given on the basis of further simulations. The speeds of the head () and tail () of the spots are found to scale with the instantaneous free stream velocity and to be similar to those observed in steady boundary layers. The ratios and seem to increase with the Reynolds number () while the streamwise expansion rate of the spots appears to be independent of .

JFM classification

Instability: Transition to turbulence
Type
Papers
Information
Journal of Fluid Mechanics , Volume 685 , 25 October 2011 , pp. 365 - 376
Copyright
Copyright © Cambridge University Press 2011

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