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Coherent flow structures in a depth-limited flow over a gravel surface : the role of near-bed turbulance and influence of Reynolds number

Hardy, R.J.; Best, J.L.; Lane, S.N.; Carbonneau, P.E.

Coherent flow structures in a depth-limited flow over a gravel surface : the role of near-bed turbulance and influence of Reynolds number Thumbnail


Authors

J.L. Best

S.N. Lane



Abstract

In gravel bed rivers, the microtopography of the bed exerts a significant effect on the generation of turbulent flow structures. Although field and laboratory measurements have indicated that flows over gravel beds contain coherent macroturbulent flow structures, the origin of these phenomena, and their relationship to the ensemble of individual roughness elements forming the bed, is not quantitatively well understood. Here we report upon a flume experiment in which flow over a gravel surface is quantified through the application of digital particle imaging velocimetry, which allows study of the downstream and vertical components of velocity over the entire flow field. The results indicate that as the Reynolds number increases (1) the visual distinctiveness of the coherent flow structures becomes more defined, (2) the upstream slope of the structures increases, and (3) the turbulence intensity of the structures increases. Analysis of the mean velocity components, the turbulence intensity, and the flow structure using quadrant analysis demonstrates that these large-scale turbulent structures originate from flow interactions with the bed topography. Detection of the dominant temporal length scales through wavelet analysis enables calculation of mean separation zone lengths associated with the gravel roughness through standard scaling laws. The calculated separation zone lengths demonstrate that wake flapping is a dominant mechanism in the production of large-scale coherent flow structures in gravel bed rivers. Thus, we show that coherent flow structures over gravels owe their origin to bed-generated turbulence and that large-scale outer layer structures are the result of flow-topography interactions in the near-bed region associated with wake flapping.

Citation

Hardy, R., Best, J., Lane, S., & Carbonneau, P. (2009). Coherent flow structures in a depth-limited flow over a gravel surface : the role of near-bed turbulance and influence of Reynolds number. Journal of Geophysical Research, 114, Article F01003. https://doi.org/10.1029/2007jf000970

Journal Article Type Article
Publication Date Jan 1, 2009
Deposit Date Aug 23, 2010
Publicly Available Date Mar 29, 2024
Journal Journal of geophysical research. Earth surface.
Print ISSN 0148-0227
Publisher Wiley
Peer Reviewed Peer Reviewed
Volume 114
Article Number F01003
DOI https://doi.org/10.1029/2007jf000970
Keywords Gravel bed rivers, Coherent flow structures, Wavelet analysis.

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Copyright Statement
Hardy, R. J. and Best, J. L. and Lane, S. N. and Carbonneau, P. E. (2009) 'Coherent flow structures in a depth-limited flow over a gravel surface : the role of near-bed turbulance and influence of Reynolds number.', Journal of geophysical research : earth surface., 114, F01003, 10.1029/2007JF000970. To view the published open abstract, go to http://dx.doi.org and enter the DOI.





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