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Uniform momentum zones in a turbulent boundary layer subjected to freestream turbulence

Uniform momentum zones in a turbulent boundary layer subjected to freestream turbulence
Uniform momentum zones in a turbulent boundary layer subjected to freestream turbulence

The instantaneous structure of a turbulent boundary layer that has developed beneath freestream turbulence (FST) was investigated with planar particle image velocimetry. Measurements were performed in the streamwise-wall-normal plane at a position downstream of the boundary layer origin. FST was generated with an active grid placed upstream of a boundary layer plate. Three cases were investigated with FST intensity increasing from approximately zero to 12.8 %. It is shown that internal interfaces, separating areas of approximately uniform momentum, are present for all cases and show a dependence on the FST. In particular, the number and length of the uniform-momentum zones (UMZs) decrease with increasing FST. The modal velocities associated with the UMZs are distributed in approximately the same way regardless of the condition of the outer flow, suggesting a degree of resilience of the instantaneous structure to the freestream. Tracking the top edge of the upper-most UMZ revealed that this contour approaches the wall for increasing FST. Finally, it is demonstrated that the observed first-order phenomena in a turbulent boundary layer subjected to FST can be modelled by superimposing an isotropic turbulence field on a turbulent boundary layer field, which supports the hypothesis that the underlying structure of the boundary layer appears to remain largely intact if the naturally occurring fluctuations in the turbulent boundary layer exceed the energy of the FST.

boundary layer structure, homogeneous turbulence, turbulent boundary layers
0022-1120
Hearst, Jason R.
907294f5-fbed-4c83-93ac-9af35ec6fcfc
de Silva, Charitha
003c6300-d3bc-4981-8b4c-99e6214b766f
Dogan, Eda
2b7b95f2-47bc-445d-808d-7995714caf2e
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052
Hearst, Jason R.
907294f5-fbed-4c83-93ac-9af35ec6fcfc
de Silva, Charitha
003c6300-d3bc-4981-8b4c-99e6214b766f
Dogan, Eda
2b7b95f2-47bc-445d-808d-7995714caf2e
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052

Hearst, Jason R., de Silva, Charitha, Dogan, Eda and Ganapathisubramani, Bharathram (2021) Uniform momentum zones in a turbulent boundary layer subjected to freestream turbulence. Journal of Fluid Mechanics, [A109]. (doi:10.1017/jfm.2021.102).

Record type: Article

Abstract

The instantaneous structure of a turbulent boundary layer that has developed beneath freestream turbulence (FST) was investigated with planar particle image velocimetry. Measurements were performed in the streamwise-wall-normal plane at a position downstream of the boundary layer origin. FST was generated with an active grid placed upstream of a boundary layer plate. Three cases were investigated with FST intensity increasing from approximately zero to 12.8 %. It is shown that internal interfaces, separating areas of approximately uniform momentum, are present for all cases and show a dependence on the FST. In particular, the number and length of the uniform-momentum zones (UMZs) decrease with increasing FST. The modal velocities associated with the UMZs are distributed in approximately the same way regardless of the condition of the outer flow, suggesting a degree of resilience of the instantaneous structure to the freestream. Tracking the top edge of the upper-most UMZ revealed that this contour approaches the wall for increasing FST. Finally, it is demonstrated that the observed first-order phenomena in a turbulent boundary layer subjected to FST can be modelled by superimposing an isotropic turbulence field on a turbulent boundary layer field, which supports the hypothesis that the underlying structure of the boundary layer appears to remain largely intact if the naturally occurring fluctuations in the turbulent boundary layer exceed the energy of the FST.

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JFM-20-S-0936.R3_Proof_hi - Author's Original
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More information

Submitted date: 26 January 2021
e-pub ahead of print date: 29 March 2021
Published date: 29 March 2021
Additional Information: Publisher Copyright: © 2021 The Author(s),. Published by Cambridge University Press.
Keywords: boundary layer structure, homogeneous turbulence, turbulent boundary layers

Identifiers

Local EPrints ID: 446722
URI: http://eprints.soton.ac.uk/id/eprint/446722
ISSN: 0022-1120
PURE UUID: 6d385d5e-7916-4bb1-9e89-0a964b475596
ORCID for Bharathram Ganapathisubramani: ORCID iD orcid.org/0000-0001-9817-0486

Catalogue record

Date deposited: 19 Feb 2021 17:30
Last modified: 17 Mar 2024 06:18

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Contributors

Author: Jason R. Hearst
Author: Charitha de Silva
Author: Eda Dogan

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