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Reynolds number dependence of the near-wall flow over irregular rough surfaces

Reynolds number dependence of the near-wall flow over irregular rough surfaces
Reynolds number dependence of the near-wall flow over irregular rough surfaces

The Reynolds-number dependence of turbulent channel flow over two irregular rough surfaces, based on scans of a graphite and a grit-blasted surface, is studied by direct numerical simulation. The aim is to characterise the changes in the flow in the immediate vicinity of and within the rough surfaces, an area of the flow where it is difficult to obtain experimental measurements. The average roughness heights and spatial correlation of the roughness features of the two surfaces are similar, but the two surfaces have a significant difference in the skewness of their height distributions, with the graphite sample being positively skewed (peak-dominated) and the grit-blasted surface being negatively skewed (valley-dominated). For both cases, numerical simulations were conducted at seven different Reynolds numbers, ranging from Re=90 to Re=720. The positively skewed surface gives rise to higher friction factors than the negatively skewed surface in all cases. For the highest Reynolds numbers, the flow has values of the roughness function ΔUwell in excess of 7 for both surfaces and the bulk flow profile has attained a constant shape across the full height of the channel except for the immediate vicinity of the roughness, which would indicate fully rough flow. However, the mean flow profile within and directly above the rough surface still shows considerable Reynolds-number dependence and the ratio of form to viscous drag continues to increase, which indicates that at least for some types of rough surfaces the flow retains aspects of the transitionally rough regime to values of ΔUor kwell in excess of the values conventionally assumed for the transitionally to fully rough threshold. This is also reflected in the changes that the near-wall flow undergoes as the Reynolds number increases: the viscous sublayer, within which the surface roughness is initially buried, breaks down and regions of reverse flow intensify. At the highest Reynolds numbers, a layer of near-wall flow is observed to follow the contours of the local surface. The distribution of thickness of this ‘blanketing’ layer has a mixed scaling, showing that viscous effects are still significant in the near-wall flow.
0022-1120
196-224
Busse, Angela
0430b320-341b-4c73-9cb5-f35632d562a4
Thakkar, Manan
0c465d92-64b4-4c9e-b34f-4095c89a240f
Sandham, Neil
0024d8cd-c788-4811-a470-57934fbdcf97
Busse, Angela
0430b320-341b-4c73-9cb5-f35632d562a4
Thakkar, Manan
0c465d92-64b4-4c9e-b34f-4095c89a240f
Sandham, Neil
0024d8cd-c788-4811-a470-57934fbdcf97

Busse, Angela, Thakkar, Manan and Sandham, Neil (2017) Reynolds number dependence of the near-wall flow over irregular rough surfaces. Journal of Fluid Mechanics, 810, 196-224. (doi:10.1017/jfm.2016.680).

Record type: Article

Abstract


The Reynolds-number dependence of turbulent channel flow over two irregular rough surfaces, based on scans of a graphite and a grit-blasted surface, is studied by direct numerical simulation. The aim is to characterise the changes in the flow in the immediate vicinity of and within the rough surfaces, an area of the flow where it is difficult to obtain experimental measurements. The average roughness heights and spatial correlation of the roughness features of the two surfaces are similar, but the two surfaces have a significant difference in the skewness of their height distributions, with the graphite sample being positively skewed (peak-dominated) and the grit-blasted surface being negatively skewed (valley-dominated). For both cases, numerical simulations were conducted at seven different Reynolds numbers, ranging from Re=90 to Re=720. The positively skewed surface gives rise to higher friction factors than the negatively skewed surface in all cases. For the highest Reynolds numbers, the flow has values of the roughness function ΔUwell in excess of 7 for both surfaces and the bulk flow profile has attained a constant shape across the full height of the channel except for the immediate vicinity of the roughness, which would indicate fully rough flow. However, the mean flow profile within and directly above the rough surface still shows considerable Reynolds-number dependence and the ratio of form to viscous drag continues to increase, which indicates that at least for some types of rough surfaces the flow retains aspects of the transitionally rough regime to values of ΔUor kwell in excess of the values conventionally assumed for the transitionally to fully rough threshold. This is also reflected in the changes that the near-wall flow undergoes as the Reynolds number increases: the viscous sublayer, within which the surface roughness is initially buried, breaks down and regions of reverse flow intensify. At the highest Reynolds numbers, a layer of near-wall flow is observed to follow the contours of the local surface. The distribution of thickness of this ‘blanketing’ layer has a mixed scaling, showing that viscous effects are still significant in the near-wall flow.

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Accepted/In Press date: 5 August 2016
e-pub ahead of print date: 24 November 2016
Published date: 10 January 2017
Organisations: Aerodynamics & Flight Mechanics Group

Identifiers

Local EPrints ID: 401104
URI: http://eprints.soton.ac.uk/id/eprint/401104
ISSN: 0022-1120
PURE UUID: 428ba277-bf12-43fb-8adf-1a55aa234673
ORCID for Neil Sandham: ORCID iD orcid.org/0000-0002-5107-0944

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Date deposited: 10 Oct 2016 09:29
Last modified: 09 Jan 2022 07:03

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Contributors

Author: Angela Busse
Author: Manan Thakkar
Author: Neil Sandham ORCID iD

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