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Development of turbulent boundary layers past a step change in wall roughness

Development of turbulent boundary layers past a step change in wall roughness
Development of turbulent boundary layers past a step change in wall roughness
In this study, the development of a boundary layer past a change in surface roughness (from rough to smooth, R -> S) is examined. Measurements of the flow were made by hot wires, whereas the friction velocity was estimated by Preston tube measurements. By means of a diagnostic plot of the turbulence intensity, it is shown that above the internal layer the flow exhibits characteristics of a rough, wall-bounded flow, whereas near the wall the turbulence intensity is similar to that of an isolated smooth wall. Similarly, viscous scaling of the mean streamwise velocity shows an excessive wake region downstream of the R -> S wall surface change that diminishes with the fetch from the surface change. Above the internal layer a second peak in the streamwise Reynolds stress was associated with the upstream rough-wall flow. Examination of the turbulent spectra revealed the presence of large-scale motions within this region that gradually diminish in strength with increasing distance from the change in surface roughness. The magnitude of the near-wall peak failed to collapse to that of a comparable smooth-wall boundary layer under viscous scaling, however, the wall-normal location of the peak appears to be at y+?15 at all downstream distances. A new mixed scaling is proposed for the near-wall peak based on the corrected wake deficit and the friction velocity. This shows the importance of outer region to the growth of near-wall peak in this non-equilibrium boundary layer.
0022-1120
494-523
Hanson, R.E.
32c4b840-4052-4c62-b87c-d3721c2896d4
Ganapathisubramani, B.
5e69099f-2f39-4fdd-8a85-3ac906827052
Hanson, R.E.
32c4b840-4052-4c62-b87c-d3721c2896d4
Ganapathisubramani, B.
5e69099f-2f39-4fdd-8a85-3ac906827052

Hanson, R.E. and Ganapathisubramani, B. (2016) Development of turbulent boundary layers past a step change in wall roughness. Journal of Fluid Mechanics, 795, 494-523. (doi:10.1017/jfm.2016.213).

Record type: Article

Abstract

In this study, the development of a boundary layer past a change in surface roughness (from rough to smooth, R -> S) is examined. Measurements of the flow were made by hot wires, whereas the friction velocity was estimated by Preston tube measurements. By means of a diagnostic plot of the turbulence intensity, it is shown that above the internal layer the flow exhibits characteristics of a rough, wall-bounded flow, whereas near the wall the turbulence intensity is similar to that of an isolated smooth wall. Similarly, viscous scaling of the mean streamwise velocity shows an excessive wake region downstream of the R -> S wall surface change that diminishes with the fetch from the surface change. Above the internal layer a second peak in the streamwise Reynolds stress was associated with the upstream rough-wall flow. Examination of the turbulent spectra revealed the presence of large-scale motions within this region that gradually diminish in strength with increasing distance from the change in surface roughness. The magnitude of the near-wall peak failed to collapse to that of a comparable smooth-wall boundary layer under viscous scaling, however, the wall-normal location of the peak appears to be at y+?15 at all downstream distances. A new mixed scaling is proposed for the near-wall peak based on the corrected wake deficit and the friction velocity. This shows the importance of outer region to the growth of near-wall peak in this non-equilibrium boundary layer.

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Accepted/In Press date: 4 March 2016
e-pub ahead of print date: 19 April 2016
Published date: May 2016
Organisations: Aerodynamics & Flight Mechanics Group

Identifiers

Local EPrints ID: 391420
URI: http://eprints.soton.ac.uk/id/eprint/391420
ISSN: 0022-1120
PURE UUID: d1d1ea11-610e-438a-8d07-40098205a12e
ORCID for B. Ganapathisubramani: ORCID iD orcid.org/0000-0001-9817-0486

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Date deposited: 12 Apr 2016 11:47
Last modified: 15 Mar 2024 03:37

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Author: R.E. Hanson

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