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Spatial modulations of kinetic energy in the roughness sublayer

Published online by Cambridge University Press:  06 July 2018

Jérémy Basley Open the ORCID record for Jérémy Basley [Opens in a new window] ,
Laurent Perret  and
Romain Mathis Open the ORCID record for Romain Mathis [Opens in a new window]
Jérémy Basley*
Affiliation:
LHEEA, UMR 6598 CNRS Centrale Nantes, 44300 Nantes, France Department of Aeronautics, Imperial College London, Kensington, London SW7 2AZ, UK
Laurent Perret
Affiliation:
LHEEA, UMR 6598 CNRS Centrale Nantes, 44300 Nantes, France
Romain Mathis
Affiliation:
Institut de Mécanique des Fluides de Toulouse (IMFT), Université de Toulouse, CNRS, 31400 Toulouse, France
*
†Email address for correspondence: [email protected]
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Abstract

High-Reynolds-number experiments are conducted in the roughness sublayer of a turbulent boundary layer developing over a cubical canopy. Stereoscopic particle image velocimetry is performed in a wall-parallel plane to evidence a high degree of spatial modulation of the small-scale turbulence around the footprint of large-scale motions, despite the suppression of the inner layer by the high roughness elements. Both Fourier and wavelets analyses show that the near-wall cycle observed in smooth-wall-bounded flows is severely disrupted by the canopy, whose wake in the roughness sublayer generates a new range of scales, closer to that of the outer-layer large-scale motions. This restricts significantly scale separation, hence a diagnostic method is developed to divide carefully and rationally the fluctuating velocity fields into large- and small-scale components. Our analysis across all turbulent kinetic energy terms sheds light on the spatial imprint of the modulation mechanism, revealing a very different signature on each velocity component. The roughness sublayer shows a preferential arrangement of the modulated scales similar to what is observed in the outer layer of smooth-wall-bounded flows – small-scale turbulence is enhanced near the front of high momentum regions and damped at the front of low momentum regions. More importantly, accessing spanwise correlations reveals that modulation intensifies the most along the flanks of the large-scale motions.

JFM classification

Geophysical and Geological Flows: Atmospheric flows Boundary Layers: Boundary layer structure Turbulent Flows: Turbulent Flows
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 850 , 10 September 2018 , pp. 584 - 610
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Copyright
© 2018 Cambridge University Press 

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