Turbulence structure of open channel flows over permeable and impermeable beds: a comparative study
Turbulence structure of open channel flows over permeable and impermeable beds: a comparative study
The behavior of turbulent open channel flows over permeable surfaces is not well understood. In particular, it is not clear how the surface and the subsurface flow within the permeable bed interact and influence each other. In order to clarify this issue we carried out two sets of experiments, one involving velocity measurements in open channel flows over an impermeable bed composed of a single layer of spheres, and another one where velocities were measured over and within a permeable bed made of five such layers. Comparison of surface flow velocity statistics between the two sets of experiments confirmed that bed permeability can significantly affect flow resistance. It was also confirmed that even in the hydraulically rough regime, the friction factors for the permeable bed increase with increasing Reynolds number. Such an increase in flow resistance implies a different distribution of normal form-induced stress between the permeable and impermeable bed cases. Subsurface flow measurements performed within the permeable bed revealed that there is an intense transport of turbulent kinetic energy (TKE) occurring from the surface to the subsurface flow. We provide evidence that the transport of TKE toward the lower bed levels is driven mainly by pressure fluctuations, whereas TKE transport due to turbulent velocity fluctuations is limited to a thinner layer placed in the upper part of the bed. It was also confirmed that the turbulence imposed by the surface flow gradually dissipates while penetrating within the porous medium. Dissipation occurs faster for the small scales than for the large ones, which instead are persistent, although weak, even at the lowest bed levels
125109-[12pp]
Manes, C.
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Pokrajac, D.
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McEwan, I.
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Nikora, V.
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31 December 2009
Manes, C.
7d9d5123-4d1b-4760-beff-d82fe0bd0acf
Pokrajac, D.
bf6480dd-6965-46f1-880b-8290c86f6965
McEwan, I.
c768d60c-6fe7-4b35-89a2-105cbf0a19b5
Nikora, V.
eb1445fd-f8be-429c-af56-2d24d0b9ff7d
Manes, C., Pokrajac, D., McEwan, I. and Nikora, V.
(2009)
Turbulence structure of open channel flows over permeable and impermeable beds: a comparative study.
Physics of Fluids, 21, .
(doi:10.1063/1.3276292).
Abstract
The behavior of turbulent open channel flows over permeable surfaces is not well understood. In particular, it is not clear how the surface and the subsurface flow within the permeable bed interact and influence each other. In order to clarify this issue we carried out two sets of experiments, one involving velocity measurements in open channel flows over an impermeable bed composed of a single layer of spheres, and another one where velocities were measured over and within a permeable bed made of five such layers. Comparison of surface flow velocity statistics between the two sets of experiments confirmed that bed permeability can significantly affect flow resistance. It was also confirmed that even in the hydraulically rough regime, the friction factors for the permeable bed increase with increasing Reynolds number. Such an increase in flow resistance implies a different distribution of normal form-induced stress between the permeable and impermeable bed cases. Subsurface flow measurements performed within the permeable bed revealed that there is an intense transport of turbulent kinetic energy (TKE) occurring from the surface to the subsurface flow. We provide evidence that the transport of TKE toward the lower bed levels is driven mainly by pressure fluctuations, whereas TKE transport due to turbulent velocity fluctuations is limited to a thinner layer placed in the upper part of the bed. It was also confirmed that the turbulence imposed by the surface flow gradually dissipates while penetrating within the porous medium. Dissipation occurs faster for the small scales than for the large ones, which instead are persistent, although weak, even at the lowest bed levels
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Published date: 31 December 2009
Organisations:
Energy & Climate Change Group
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Local EPrints ID: 204113
URI: http://eprints.soton.ac.uk/id/eprint/204113
ISSN: 1070-6631
PURE UUID: c5fd40d3-0cdb-44a7-a924-8ee8d43e4509
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Date deposited: 24 Nov 2011 11:55
Last modified: 14 Mar 2024 04:30
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Author:
C. Manes
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D. Pokrajac
Author:
I. McEwan
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V. Nikora
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