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Effects of heterogeneous surface geometry on secondary flows in turbulent boundary layers

Effects of heterogeneous surface geometry on secondary flows in turbulent boundary layers
Effects of heterogeneous surface geometry on secondary flows in turbulent boundary layers

The effect of spanwise heterogeneous surface geometry on turbulent boundary layer secondary flows and on skin friction is investigated experimentally. The surfaces consist of smooth streamwise-aligned ridges of different shapes and widths with spanwise wavelengths comparable to the boundary layer thickness . Cross-stream stereoscopic particle image velocimetry combined with oil-film interferometry is used to investigate the flow field and assess the drag. Results show that the spanwise distribution of the skin friction varies as a consequence of the mean flow heterogeneity and is highly dependent on surface geometry. The swirling strength maps revealed remarkable changes in the secondary flow structures for different ridge shapes. For wide ridges, topological changes occur showing the appearance of tertiary vortices coexisting with the large-scale secondary structures. An imbalance in favour of these tertiary structures occurs over a certain width which take over the secondary structures, causing a swap in the locations of the low- and high-momentum pathways. Furthermore, the results indicate that the spanwise spacing alone is insufficient to characterise the surface heterogeneity. A new parameter , which is based on the ratio of the perimeter over and below the mean surface height, is shown to adequately capture the changes in skin friction and streamwise circulation of the secondary motions. Triple decomposition allowed the quantification of the dispersive stresses for all these cases, which can contribute up to of the total shear stress when strong secondary motions occur.

Key wordsturbulent boundary layers
0022-1120
1-36
Medjnoun, Takfarinas
49775b3f-b926-4bb9-b667-07a84ed46fd6
Vanderwel, Christina
fbc030f0-1822-4c3f-8e90-87f3cd8372bb
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052
Medjnoun, Takfarinas
49775b3f-b926-4bb9-b667-07a84ed46fd6
Vanderwel, Christina
fbc030f0-1822-4c3f-8e90-87f3cd8372bb
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052

Medjnoun, Takfarinas, Vanderwel, Christina and Ganapathisubramani, Bharathram (2020) Effects of heterogeneous surface geometry on secondary flows in turbulent boundary layers. Journal of Fluid Mechanics, 886, 1-36, [A31]. (doi:10.1017/jfm.2019.1014).

Record type: Article

Abstract

The effect of spanwise heterogeneous surface geometry on turbulent boundary layer secondary flows and on skin friction is investigated experimentally. The surfaces consist of smooth streamwise-aligned ridges of different shapes and widths with spanwise wavelengths comparable to the boundary layer thickness . Cross-stream stereoscopic particle image velocimetry combined with oil-film interferometry is used to investigate the flow field and assess the drag. Results show that the spanwise distribution of the skin friction varies as a consequence of the mean flow heterogeneity and is highly dependent on surface geometry. The swirling strength maps revealed remarkable changes in the secondary flow structures for different ridge shapes. For wide ridges, topological changes occur showing the appearance of tertiary vortices coexisting with the large-scale secondary structures. An imbalance in favour of these tertiary structures occurs over a certain width which take over the secondary structures, causing a swap in the locations of the low- and high-momentum pathways. Furthermore, the results indicate that the spanwise spacing alone is insufficient to characterise the surface heterogeneity. A new parameter , which is based on the ratio of the perimeter over and below the mean surface height, is shown to adequately capture the changes in skin friction and streamwise circulation of the secondary motions. Triple decomposition allowed the quantification of the dispersive stresses for all these cases, which can contribute up to of the total shear stress when strong secondary motions occur.

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Accepted/In Press date: 29 November 2019
e-pub ahead of print date: 17 January 2020
Published date: 10 March 2020
Keywords: Key wordsturbulent boundary layers

Identifiers

Local EPrints ID: 436226
URI: http://eprints.soton.ac.uk/id/eprint/436226
ISSN: 0022-1120
PURE UUID: dff0efb0-a264-4b15-86fa-8991211105e4
ORCID for Bharathram Ganapathisubramani: ORCID iD orcid.org/0000-0001-9817-0486

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Date deposited: 04 Dec 2019 17:30
Last modified: 02 Mar 2021 05:01

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Author: Takfarinas Medjnoun

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