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Non k-type behaviour of roughness when1 in-plane wavelength approaches the2 boundary layer thickness3

Non k-type behaviour of roughness when1 in-plane wavelength approaches the2 boundary layer thickness3
Non k-type behaviour of roughness when1 in-plane wavelength approaches the2 boundary layer thickness3
A surface roughness from a recently cleaned and painted ship hull was scanned, scaled and replicated for laboratory testing to systematically investigatethe influence of the ratio of in-plane roughness wavelength,λ, with respect to the boundary layer thickness δ. The experiments were performed by geometrically scaling the surface which maintains a constant effective slopeESxand solidityΛ, while the ratio ofλ~δis varied. Here we scale the scanned roughness topography by a factor of 2.5 and, and measure the18mean velocity profiles in the turbulent boundary layers developing over these surfaces at a range of freestream velocities and streamwise measurement locations. The results show that the 2.5×scaled roughness, which hasλ~δ≪1 behaves in the expectedk-21type manner, with a roughness function∆U+that is proportional to the viscous scaled roughness height. The 15×surface, however, which hasλ~δ≈1, exhibits very different nonk-type behaviour. This larger surface does not approach the fully rough asymptote and also exhibits a drag penalty that is comparable to the 2.5×case despite the six-fold increase in the roughness height. Measurements on a spanwise/wall-normal plane reveal that the 15×surface has introduced a large scale spanwise variation in mean streamwise velocity (dispersive stresses) that extend far beyond the logarithmic region. Together this evidence suggests that a demarcation betweenk-type and nonk-type behaviour can occur in situations where the in-plane roughness wavelength approachesthe boundary layer thickness. This finding has important implications to how we scale smallscale roughness from high Relarge-scale applications for testing in lowResmall-scale laboratory facilities or simulations.
0022-1120
Nugroho, Bagus
312aeeae-cbdd-4f94-ae46-e5591a7777db
Monty, J.P.
b73588eb-5749-46f9-8efb-b2e52924c27e
Utama, I.K.A.P.
0449bcc4-a91f-46d8-b337-5885d64d01fe
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052
Hutchins, Nicholas
10ef0e3e-266c-4569-90ae-598ed9fb5edd
Nugroho, Bagus
312aeeae-cbdd-4f94-ae46-e5591a7777db
Monty, J.P.
b73588eb-5749-46f9-8efb-b2e52924c27e
Utama, I.K.A.P.
0449bcc4-a91f-46d8-b337-5885d64d01fe
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052
Hutchins, Nicholas
10ef0e3e-266c-4569-90ae-598ed9fb5edd

Nugroho, Bagus, Monty, J.P., Utama, I.K.A.P., Ganapathisubramani, Bharathram and Hutchins, Nicholas (2020) Non k-type behaviour of roughness when1 in-plane wavelength approaches the2 boundary layer thickness3. Journal of Fluid Mechanics. (doi:10.1017/jfm.2020.875). (In Press)

Record type: Article

Abstract

A surface roughness from a recently cleaned and painted ship hull was scanned, scaled and replicated for laboratory testing to systematically investigatethe influence of the ratio of in-plane roughness wavelength,λ, with respect to the boundary layer thickness δ. The experiments were performed by geometrically scaling the surface which maintains a constant effective slopeESxand solidityΛ, while the ratio ofλ~δis varied. Here we scale the scanned roughness topography by a factor of 2.5 and, and measure the18mean velocity profiles in the turbulent boundary layers developing over these surfaces at a range of freestream velocities and streamwise measurement locations. The results show that the 2.5×scaled roughness, which hasλ~δ≪1 behaves in the expectedk-21type manner, with a roughness function∆U+that is proportional to the viscous scaled roughness height. The 15×surface, however, which hasλ~δ≈1, exhibits very different nonk-type behaviour. This larger surface does not approach the fully rough asymptote and also exhibits a drag penalty that is comparable to the 2.5×case despite the six-fold increase in the roughness height. Measurements on a spanwise/wall-normal plane reveal that the 15×surface has introduced a large scale spanwise variation in mean streamwise velocity (dispersive stresses) that extend far beyond the logarithmic region. Together this evidence suggests that a demarcation betweenk-type and nonk-type behaviour can occur in situations where the in-plane roughness wavelength approachesthe boundary layer thickness. This finding has important implications to how we scale smallscale roughness from high Relarge-scale applications for testing in lowResmall-scale laboratory facilities or simulations.

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nugroho_et_al_2020
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Accepted/In Press date: 23 September 2020

Identifiers

Local EPrints ID: 444123
URI: http://eprints.soton.ac.uk/id/eprint/444123
ISSN: 0022-1120
PURE UUID: 8dc63d09-9f67-40f3-a1c7-9bd0f2181f53
ORCID for Bharathram Ganapathisubramani: ORCID iD orcid.org/0000-0001-9817-0486

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Date deposited: 28 Sep 2020 16:30
Last modified: 22 Nov 2021 08:15

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Contributors

Author: Bagus Nugroho
Author: J.P. Monty
Author: I.K.A.P. Utama
Author: Nicholas Hutchins

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