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Surface correlations of hydrodynamic drag for transitionally rough engineering surfaces

Surface correlations of hydrodynamic drag for transitionally rough engineering surfaces
Surface correlations of hydrodynamic drag for transitionally rough engineering surfaces
Rough surfaces are usually characterised by a single equivalent sand-grain roughness height scale that typically needs to be determined from laboratory experiments. Recently this method has been complemented by a direct numerical simulation approach, whereby representative surfaces can be scanned and the roughness effects computed over a range of Reynolds number. This development raises the prospect over the coming years of having enough data for different types of rough surfaces to be able to relate surface characteristics to roughness effects, such as the roughness function that quantifies the downward displacement of the logarithmic law of the wall. In the present contribution, we use simulation data for 17 irregular surfaces at the same friction Reynolds number, for which they are in the transitionally rough regime. All surfaces are scaled to the same physical roughness height. Mean streamwise velocity profiles show a wide range of roughness function values, while the velocity defect profiles show a good collapse. Profile peaks of the turbulent kinetic energy also vary depending on the surface. We then consider which surface properties are important and how new properties can be incorporated into an empirical model, the accuracy of which can then be tested. Optimised models with several roughness parameters are systematically developed for the roughness function and profile peak turbulent kinetic energy. In determining the roughness function, besides the known parameters of solidity (or frontal area ratio) and skewness, it is shown that the streamwise correlation length and the root-mean-square roughness height are also significant. The peak turbulent kinetic energy is determined by the skewness and root-mean-square roughness height, along with the mean forward-facing surface angle and spanwise effective slope. The results suggest feasibility of relating rough-wall flow properties throughout the range from hydrodynamically smooth to fully-rough to surface parameters.
1468-5248
1-29
Thakkar, Manan
0c465d92-64b4-4c9e-b34f-4095c89a240f
Busse, Angela
0430b320-341b-4c73-9cb5-f35632d562a4
Sandham, Neil
0024d8cd-c788-4811-a470-57934fbdcf97
Thakkar, Manan
0c465d92-64b4-4c9e-b34f-4095c89a240f
Busse, Angela
0430b320-341b-4c73-9cb5-f35632d562a4
Sandham, Neil
0024d8cd-c788-4811-a470-57934fbdcf97

Thakkar, Manan, Busse, Angela and Sandham, Neil (2016) Surface correlations of hydrodynamic drag for transitionally rough engineering surfaces. Journal of Turbulence, 1-29. (In Press)

Record type: Article

Abstract

Rough surfaces are usually characterised by a single equivalent sand-grain roughness height scale that typically needs to be determined from laboratory experiments. Recently this method has been complemented by a direct numerical simulation approach, whereby representative surfaces can be scanned and the roughness effects computed over a range of Reynolds number. This development raises the prospect over the coming years of having enough data for different types of rough surfaces to be able to relate surface characteristics to roughness effects, such as the roughness function that quantifies the downward displacement of the logarithmic law of the wall. In the present contribution, we use simulation data for 17 irregular surfaces at the same friction Reynolds number, for which they are in the transitionally rough regime. All surfaces are scaled to the same physical roughness height. Mean streamwise velocity profiles show a wide range of roughness function values, while the velocity defect profiles show a good collapse. Profile peaks of the turbulent kinetic energy also vary depending on the surface. We then consider which surface properties are important and how new properties can be incorporated into an empirical model, the accuracy of which can then be tested. Optimised models with several roughness parameters are systematically developed for the roughness function and profile peak turbulent kinetic energy. In determining the roughness function, besides the known parameters of solidity (or frontal area ratio) and skewness, it is shown that the streamwise correlation length and the root-mean-square roughness height are also significant. The peak turbulent kinetic energy is determined by the skewness and root-mean-square roughness height, along with the mean forward-facing surface angle and spanwise effective slope. The results suggest feasibility of relating rough-wall flow properties throughout the range from hydrodynamically smooth to fully-rough to surface parameters.

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Accepted/In Press date: 28 October 2016
Organisations: Aerodynamics & Flight Mechanics Group

Identifiers

Local EPrints ID: 402147
URI: https://eprints.soton.ac.uk/id/eprint/402147
ISSN: 1468-5248
PURE UUID: 0054ca1d-1008-4298-8d6a-1f23582d9f01
ORCID for Neil Sandham: ORCID iD orcid.org/0000-0002-5107-0944

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Date deposited: 02 Nov 2016 13:42
Last modified: 06 Jun 2018 12:55

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