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Turbulent flow over large roughness elements: effect of frontal and plan solidity on turbulence statistics and structure

Turbulent flow over large roughness elements: effect of frontal and plan solidity on turbulence statistics and structure
Turbulent flow over large roughness elements: effect of frontal and plan solidity on turbulence statistics and structure
Wind-tunnel experiments were carried out on fully-rough boundary layers with large roughness (δ/h≈10, where h is the height of the roughness elements and δ is the boundary-layer thickness). Twelve different surface conditions were created by using LEGO™ bricks of uniform height. Six cases are tested for a fixed plan solidity (λP) with variations in frontal density (λF), while the other six cases have varying λP for fixed λF. Particle image velocimetry and floating-element drag-balance measurements were performed. The current results complement those contained in Placidi and Ganapathisubramani (J Fluid Mech 782:541–566, 2015), extending the previous analysis to the turbulence statistics and spatial structure. Results indicate that mean velocity profiles in defect form agree with Townsend’s similarity hypothesis with varying λF, however, the agreement is worse for cases with varying λP. The streamwise and wall-normal turbulent stresses, as well as the Reynolds shear stresses, show a lack of similarity across most examined cases. This suggests that the critical height of the roughness for which outer-layer similarity holds depends not only on the height of the roughness, but also on the local wall morphology. A new criterion based on shelter solidity, defined as the sheltered plan area per unit wall-parallel area, which is similar to the ‘effective shelter area’ in Raupach and Shaw (Boundary-Layer Meteorol 22:79–90, 1982), is found to capture the departure of the turbulence statistics from outer-layer similarity. Despite this lack of similarity reported in the turbulence statistics, proper orthogonal decomposition analysis, as well as two-point spatial correlations, show that some form of universal flow structure is present, as all cases exhibit virtually identical proper orthogonal decomposition mode shapes and correlation fields. Finally, reduced models based on proper orthogonal decomposition reveal that the small scales of the turbulence play a significant role in assessing outer-layer similarity.
0006-8314
Placidi, M.
7c8407cd-0652-4e34-add4-b23cc73ede66
Ganapathisubramani, B.
5e69099f-2f39-4fdd-8a85-3ac906827052
Placidi, M.
7c8407cd-0652-4e34-add4-b23cc73ede66
Ganapathisubramani, B.
5e69099f-2f39-4fdd-8a85-3ac906827052

Placidi, M. and Ganapathisubramani, B. (2017) Turbulent flow over large roughness elements: effect of frontal and plan solidity on turbulence statistics and structure. Boundary-Layer Meteorology. (doi:10.1007/s10546-017-0317-3).

Record type: Article

Abstract

Wind-tunnel experiments were carried out on fully-rough boundary layers with large roughness (δ/h≈10, where h is the height of the roughness elements and δ is the boundary-layer thickness). Twelve different surface conditions were created by using LEGO™ bricks of uniform height. Six cases are tested for a fixed plan solidity (λP) with variations in frontal density (λF), while the other six cases have varying λP for fixed λF. Particle image velocimetry and floating-element drag-balance measurements were performed. The current results complement those contained in Placidi and Ganapathisubramani (J Fluid Mech 782:541–566, 2015), extending the previous analysis to the turbulence statistics and spatial structure. Results indicate that mean velocity profiles in defect form agree with Townsend’s similarity hypothesis with varying λF, however, the agreement is worse for cases with varying λP. The streamwise and wall-normal turbulent stresses, as well as the Reynolds shear stresses, show a lack of similarity across most examined cases. This suggests that the critical height of the roughness for which outer-layer similarity holds depends not only on the height of the roughness, but also on the local wall morphology. A new criterion based on shelter solidity, defined as the sheltered plan area per unit wall-parallel area, which is similar to the ‘effective shelter area’ in Raupach and Shaw (Boundary-Layer Meteorol 22:79–90, 1982), is found to capture the departure of the turbulence statistics from outer-layer similarity. Despite this lack of similarity reported in the turbulence statistics, proper orthogonal decomposition analysis, as well as two-point spatial correlations, show that some form of universal flow structure is present, as all cases exhibit virtually identical proper orthogonal decomposition mode shapes and correlation fields. Finally, reduced models based on proper orthogonal decomposition reveal that the small scales of the turbulence play a significant role in assessing outer-layer similarity.

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Accepted/In Press date: 19 October 2017
e-pub ahead of print date: 4 November 2017

Identifiers

Local EPrints ID: 414500
URI: http://eprints.soton.ac.uk/id/eprint/414500
ISSN: 0006-8314
PURE UUID: 699fe482-3725-4da6-9bf1-ee087e64d461
ORCID for B. Ganapathisubramani: ORCID iD orcid.org/0000-0001-9817-0486

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Date deposited: 03 Oct 2017 16:31
Last modified: 07 Oct 2020 01:58

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