Large Eddy Simulation of Near-Bed Flow and Turbulence over Roughness Elements in the Shallow Open-Channel
Turbulent flows in rough open-channels have complex structures near the channel-bed. The near-bed flow can cause bed erosion, channel instability, and damages to fish habitats. This paper aims to improve our understanding of the structures. Transverse square bars placed at the channel-bed form two-d...
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| Format: | Article |
| Language: | English |
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MDPI AG
2020-09-01
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| Series: | Water |
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| Online Access: | https://www.mdpi.com/2073-4441/12/10/2701 |
| _version_ | 1797552494157496320 |
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| author | Zeng Zhang S. Samuel Li |
| author_facet | Zeng Zhang S. Samuel Li |
| author_sort | Zeng Zhang |
| collection | DOAJ |
| description | Turbulent flows in rough open-channels have complex structures near the channel-bed. The near-bed flow can cause bed erosion, channel instability, and damages to fish habitats. This paper aims to improve our understanding of the structures. Transverse square bars placed at the channel-bed form two-dimensional roughness elements. Turbulent flows over the bars are predicted using large eddy simulation (LES). The predicted flow quantities compare well with experimental data. The LES model predicts mean-flow velocity profiles that resemble those in the classic turbulent boundary layer over a flat plate and profiles that change patterns in the vicinity of roughness elements, depending on the pitch-to-roughness height ratio <inline-formula><math display="inline"><semantics><mrow><mi>λ</mi><mo>/</mo><mi>k</mi></mrow></semantics></math></inline-formula>. The relative turbulence intensity and normalized Reynolds shear stress reach maxima of 15% and 1.2%, respectively, at <inline-formula><math display="inline"><semantics><mrow><mi>λ</mi><mo>/</mo><mi>k</mi></mrow></semantics></math></inline-formula> = 8, compared to 9% and 0.2% at <inline-formula><math display="inline"><semantics><mrow><mi>λ</mi><mo>/</mo><mi>k</mi></mrow></semantics></math></inline-formula> = 2. The predicted bottom boundary layers constitute a large portion of the total depth, indicating roughness effect on the flow throughout the water column. Fluid exchange between the roughness cavity and outer region occurs due to turbulence fluctuations. The fluctuations increase in intensity with increasing <inline-formula><math display="inline"><semantics><mrow><mi>λ</mi><mo>/</mo><mi>k</mi></mrow></semantics></math></inline-formula> ratio. This ratio dictates the number of eddies in the cavity as well as their locations and shapes. It also controls turbulence stress distributions. LES can be used to explore strategies for erosion control, channel restoration, and habitat protection. |
| first_indexed | 2024-03-10T16:00:55Z |
| format | Article |
| id | doaj.art-8dba5dead676488391b55c81238a8d5a |
| institution | Directory Open Access Journal |
| issn | 2073-4441 |
| language | English |
| last_indexed | 2024-03-10T16:00:55Z |
| publishDate | 2020-09-01 |
| publisher | MDPI AG |
| record_format | Article |
| series | Water |
| spelling | doaj.art-8dba5dead676488391b55c81238a8d5a2023-11-20T15:17:07ZengMDPI AGWater2073-44412020-09-011210270110.3390/w12102701Large Eddy Simulation of Near-Bed Flow and Turbulence over Roughness Elements in the Shallow Open-ChannelZeng Zhang0S. Samuel Li1Department of Building, Civil and Environmental Engineering, Concordia University, 1455 de Maisonneuve Boulevard West, Montreal, QC H3G 1M8, CanadaDepartment of Building, Civil and Environmental Engineering, Concordia University, 1455 de Maisonneuve Boulevard West, Montreal, QC H3G 1M8, CanadaTurbulent flows in rough open-channels have complex structures near the channel-bed. The near-bed flow can cause bed erosion, channel instability, and damages to fish habitats. This paper aims to improve our understanding of the structures. Transverse square bars placed at the channel-bed form two-dimensional roughness elements. Turbulent flows over the bars are predicted using large eddy simulation (LES). The predicted flow quantities compare well with experimental data. The LES model predicts mean-flow velocity profiles that resemble those in the classic turbulent boundary layer over a flat plate and profiles that change patterns in the vicinity of roughness elements, depending on the pitch-to-roughness height ratio <inline-formula><math display="inline"><semantics><mrow><mi>λ</mi><mo>/</mo><mi>k</mi></mrow></semantics></math></inline-formula>. The relative turbulence intensity and normalized Reynolds shear stress reach maxima of 15% and 1.2%, respectively, at <inline-formula><math display="inline"><semantics><mrow><mi>λ</mi><mo>/</mo><mi>k</mi></mrow></semantics></math></inline-formula> = 8, compared to 9% and 0.2% at <inline-formula><math display="inline"><semantics><mrow><mi>λ</mi><mo>/</mo><mi>k</mi></mrow></semantics></math></inline-formula> = 2. The predicted bottom boundary layers constitute a large portion of the total depth, indicating roughness effect on the flow throughout the water column. Fluid exchange between the roughness cavity and outer region occurs due to turbulence fluctuations. The fluctuations increase in intensity with increasing <inline-formula><math display="inline"><semantics><mrow><mi>λ</mi><mo>/</mo><mi>k</mi></mrow></semantics></math></inline-formula> ratio. This ratio dictates the number of eddies in the cavity as well as their locations and shapes. It also controls turbulence stress distributions. LES can be used to explore strategies for erosion control, channel restoration, and habitat protection.https://www.mdpi.com/2073-4441/12/10/2701wall turbulencesurface roughnesslarge eddy simulationnear-bed flowturbulence shear stress<i>k</i>-type roughness |
| spellingShingle | Zeng Zhang S. Samuel Li Large Eddy Simulation of Near-Bed Flow and Turbulence over Roughness Elements in the Shallow Open-Channel Water wall turbulence surface roughness large eddy simulation near-bed flow turbulence shear stress <i>k</i>-type roughness |
| title | Large Eddy Simulation of Near-Bed Flow and Turbulence over Roughness Elements in the Shallow Open-Channel |
| title_full | Large Eddy Simulation of Near-Bed Flow and Turbulence over Roughness Elements in the Shallow Open-Channel |
| title_fullStr | Large Eddy Simulation of Near-Bed Flow and Turbulence over Roughness Elements in the Shallow Open-Channel |
| title_full_unstemmed | Large Eddy Simulation of Near-Bed Flow and Turbulence over Roughness Elements in the Shallow Open-Channel |
| title_short | Large Eddy Simulation of Near-Bed Flow and Turbulence over Roughness Elements in the Shallow Open-Channel |
| title_sort | large eddy simulation of near bed flow and turbulence over roughness elements in the shallow open channel |
| topic | wall turbulence surface roughness large eddy simulation near-bed flow turbulence shear stress <i>k</i>-type roughness |
| url | https://www.mdpi.com/2073-4441/12/10/2701 |
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