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# On the decay of dispersive motions in the outer region of rough-wall boundary layers

Meyers, Johan, Ganapathisubramani, Bharathram and Cal, Raul Bayoan (2019) On the decay of dispersive motions in the outer region of rough-wall boundary layers. Journal of Fluid Mechanics, 862, 1-13, [R5].

Record type: Article

## Abstract

In rough-wall boundary layers, wall-parallel non-homogeneous mean-flow solutions exist that lead to so-called dispersive velocity components and dispersive stresses. They play a significant role in the mean-flow momentum balance near the wall, but typically disappear in the outer layer. A theoretical framework is presented to study the decay of dispersive motions in the outer layer. To this end, the problem is formulated in Fourier space,and a set of governing ordinary differential equations per mode in wavenumber space is derived by linearizing the Reynolds-averaged Navier–Stokes equations around a constant background velocity. With further simplifications, analytically tractable solutions are found consisting of linear combinations of exp(−kz) and exp(−Kz), with z the wall distance, k the magnitude of the horizontal wavevector k, and where K(k, Re) is a function of k and the Reynolds number Re. Moreover, for k → ∞ or k1 → 0 (with k1 the stream-wise wavenumber), K → k is found, in which case solutions consist of a linear combination of exp(−kz) and z exp(−kz), and are Reynolds number independent.These analytical relations are compared in the limit of k1 = 0 to the rough boundary layer experiments by Vanderwel and Ganapathisubramani (J. Fluid Mech. 774, R2, 2015) and are in reasonable agreement for k/δ 6 0.5, with δ the boundary-layer thickness and k = 2π/k.

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On the decay of dispersive motions in the outer region of rough - Accepted Manuscript

Accepted/In Press date: 11 December 2018
e-pub ahead of print date: 25 January 2019
Published date: 10 March 2019

## Identifiers

Local EPrints ID: 426776
URI: http://eprints.soton.ac.uk/id/eprint/426776
ISSN: 0022-1120
PURE UUID: e4560967-6bff-4e0f-b291-f2121cd2bab5
ORCID for Bharathram Ganapathisubramani: orcid.org/0000-0001-9817-0486

## Catalogue record

Date deposited: 12 Dec 2018 17:30

## Contributors

Author: Johan Meyers
Author: Raul Bayoan Cal