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Scale interactions in velocity and pressure within a turbulent boundary layer developing over a staggered-cube array

Published online by Cambridge University Press:  21 January 2021

M.A. Ferreira Open the ORCID record for M.A. Ferreira [Opens in a new window]  and
B. Ganapathisubramani Open the ORCID record for B. Ganapathisubramani [Opens in a new window]
M.A. Ferreira*
Affiliation:
Faculty of Engineering and Physical Sciences, University of Southampton, Boldrewood Campus, SouthamptonSO16 7QF, UK
B. Ganapathisubramani
Affiliation:
Faculty of Engineering and Physical Sciences, University of Southampton, Boldrewood Campus, SouthamptonSO16 7QF, UK
*
†Email address for correspondence: m.aguiar-ferreira@soton.ac.uk
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Abstract

We experimentally investigate the surface drag characteristics of a staggered-distributed cube array and its interaction with the turbulent structure of the overlying flow. Instantaneous maps of the pressure field, inferred from in-plane velocity data are used to estimate the forces acting on a target roughness element. Coupled statistics of the force in combination with conditional flow analysis and extended proper orthogonal decomposition (POD) of the pressure field, based on the velocity POD modes, elucidate the relevant mechanisms responsible for surface drag generation. The results show that turbulent motions, at different scales, leave an imprint on the pressure field. Specifically, positive and negative fluctuations are generally associated with flow regions experiencing a local deceleration and acceleration, respectively. Although large-scale motions were found to be the single greatest contributor to the fluctuating pressure field, their direct influence on the surface drag fluctuations appears to be mitigated by the relative size of the considerably smaller roughness obstacles. We hypothesise that a pressure wave induced by the passage of alternating high- and low-momentum regions evenly affects the flow field over a broad region, coupling the forces on the windward and leeward sides of the cube, which, in turn, partially cancel each other out. Uncorrelated, intermediate and small-scale pressure events are thus more important to the overall drag fluctuations. While the direct influence of the large-scale structures on the surface drag may be smaller than expected, the results suggest that they are still significant for the role they play in modulating the small-scale pressure events in the canopy region.

JFM classification

Turbulent Flows: Turbulent boundary layers Boundary Layers: Boundary layer structure
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 910 , 10 March 2021 , A48
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Copyright
© The Author(s), 2021. Published by Cambridge University Press

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References

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