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Turbulent boundary layer flow over regular multiscale roughness

Turbulent boundary layer flow over regular multiscale roughness
Turbulent boundary layer flow over regular multiscale roughness
In this experimental study, multiscale rough surfaces with regular (cuboid) elements are used to examine the effects of roughness scale hierarchy on turbulent boundary layers. Three iterations have been used with a first iteration of large-scale cuboids onto which subsequent smaller cuboids are uniformly added, with their size decreasing with a power-law as the number increases. The drag is directly measured through a floatingelement drag balance, while particle image velocimetry allowed the assessment of the flow field. The drag measurements revealed the smallest roughness iteration can contribute to nearly 7% of the overall drag of a full surface, while the intermediate iterations are responsible for over 12% (at the highest Reynolds number tested). It is shown that the aerodynamic roughness lengthscale between subsequent iterations varies linearly, and can be described with a geometrical parameter proportional to the frontal solidity. Mean and turbulent statistics are evaluated using the drag information, and highlighted substantial changes within the canopy region as well as in the outer flow, with modifications to the inertial sublayer (ISL) and the wake region. These changes are shown to be caused by the presence of large-scale secondary motions in the cross-plane, which itself is believed to be a consequence of the largest multiscale roughness phase (spacing between largest cuboids), shown to be of the same order of magnitude as the boundary layer thickness. Implications on the classical similarity laws are additionally discussed.
0022-1120
Medjnoun, Takfarinas
49775b3f-b926-4bb9-b667-07a84ed46fd6
Rodriguez-lopez, Eduardo
0aef9cc6-1abf-48b6-bafb-e20201b44d6a
Amaro Matoso Aguiar Ferreira, Manuel
bdfedfb9-85f6-4dbb-b031-a977cb8f145d
Griffiths, Tom
ae432712-032b-4856-951b-36aee79eb329
Meyers, Johan
0e2a737d-b393-43b2-8e55-e6829fd6e25e
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052
Medjnoun, Takfarinas
49775b3f-b926-4bb9-b667-07a84ed46fd6
Rodriguez-lopez, Eduardo
0aef9cc6-1abf-48b6-bafb-e20201b44d6a
Amaro Matoso Aguiar Ferreira, Manuel
bdfedfb9-85f6-4dbb-b031-a977cb8f145d
Griffiths, Tom
ae432712-032b-4856-951b-36aee79eb329
Meyers, Johan
0e2a737d-b393-43b2-8e55-e6829fd6e25e
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052

Medjnoun, Takfarinas, Rodriguez-lopez, Eduardo, Amaro Matoso Aguiar Ferreira, Manuel, Griffiths, Tom, Meyers, Johan and Ganapathisubramani, Bharathram (2021) Turbulent boundary layer flow over regular multiscale roughness. Journal of Fluid Mechanics, 917. (doi:10.1017/jfm.2021.228).

Record type: Article

Abstract

In this experimental study, multiscale rough surfaces with regular (cuboid) elements are used to examine the effects of roughness scale hierarchy on turbulent boundary layers. Three iterations have been used with a first iteration of large-scale cuboids onto which subsequent smaller cuboids are uniformly added, with their size decreasing with a power-law as the number increases. The drag is directly measured through a floatingelement drag balance, while particle image velocimetry allowed the assessment of the flow field. The drag measurements revealed the smallest roughness iteration can contribute to nearly 7% of the overall drag of a full surface, while the intermediate iterations are responsible for over 12% (at the highest Reynolds number tested). It is shown that the aerodynamic roughness lengthscale between subsequent iterations varies linearly, and can be described with a geometrical parameter proportional to the frontal solidity. Mean and turbulent statistics are evaluated using the drag information, and highlighted substantial changes within the canopy region as well as in the outer flow, with modifications to the inertial sublayer (ISL) and the wake region. These changes are shown to be caused by the presence of large-scale secondary motions in the cross-plane, which itself is believed to be a consequence of the largest multiscale roughness phase (spacing between largest cuboids), shown to be of the same order of magnitude as the boundary layer thickness. Implications on the classical similarity laws are additionally discussed.

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Accepted/In Press date: 24 February 2021
e-pub ahead of print date: 21 April 2021

Identifiers

Local EPrints ID: 447418
URI: http://eprints.soton.ac.uk/id/eprint/447418
ISSN: 0022-1120
PURE UUID: 0eb2022b-f715-4523-8680-a26a12063ff1
ORCID for Eduardo Rodriguez-lopez: ORCID iD orcid.org/0000-0003-3927-8811
ORCID for Manuel Amaro Matoso Aguiar Ferreira: ORCID iD orcid.org/0000-0002-2428-0284
ORCID for Bharathram Ganapathisubramani: ORCID iD orcid.org/0000-0001-9817-0486

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Date deposited: 11 Mar 2021 17:30
Last modified: 24 Aug 2021 04:01

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Contributors

Author: Takfarinas Medjnoun
Author: Eduardo Rodriguez-lopez ORCID iD
Author: Manuel Amaro Matoso Aguiar Ferreira ORCID iD
Author: Tom Griffiths
Author: Johan Meyers

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