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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.

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

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

Topics: Gravel bed rivers, Coherent flow structures, Wavelet analysis.
Publisher: American Geophysical Union
Year: 2009
DOI identifier: 10.1029/2007JF000970
OAI identifier: oai:dro.dur.ac.uk.OAI2:7278
Journal:

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Citations

  1. (1972). A First Course in Turbulence,
  2. (1998). A practical guide to wavelet analysis, doi
  3. (1999). Are weakly mobile-bed flows a special class of wall-bounded flows?,
  4. (1991). Digital particle image velocimetry, doi
  5. (1949). Dynamics of Alluvial Flows
  6. (1985). Experimental study of secondary currents in open channel flow, paper presented at 21st IAHR Congress,
  7. (2001). Field investigation of three dimensional flow structure at stream confluences, doi
  8. (1997). Fundamental of digital particle image velocimetry, doi
  9. (1984). Large-scale structure of turbulent flow in a rectangular flume (in Russian), Trans. State Hydrol.
  10. (1973). Measurements of structure of Reynolds stress in a turbulent boundary layer, doi
  11. (1989). Mechanics of flow over ripples and dunes, doi
  12. (1991). On the dynamics of near-wall turbulence, doi
  13. (1986). On the vortex formation in the mixing layer behind dunes,
  14. (1995). Role of near-bed turbulence structure in bed load transport and bed form mechanics, doi
  15. (1986). Simultaneous flow visualization and Reynolds stress measurement in a turbulent boundary layer, doi
  16. (1990). The wavelet transform: Some applications to fluid dynamics and turbulence,
  17. (1997). Turbulence characteristics of New Zealand gravel-bed rivers, doi
  18. (1979). Turbulent flow in a depth limited boundary layer, doi
  19. (1991). Velocity distribution and bed roughness in high-gradient streams, doi
  20. (1982). Visualisation of the mixing layer behind dunes, in Mechanics of Sediment Transport,

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