Rotating-disk-type flow over loose boundaries |
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Authors: | Peter J Thomas Farzam Zoueshtiagh |
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Affiliation: | (1) Fluid Dynamics Research Centre, School of Engineering, University of Warwick, Coventry, CV4 7AL, UK;(2) Laboratoire de Mécanique de Lille, Bd Paul Langevin, Cité Scientifique, 59655 Villeneuve d’Ascq, France |
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Abstract: | Rotating-disk-type flow of a liquid over a loose boundary, such as a layer of sand, is investigated. For this flow the formation
of a new large-scale spiral pattern has been discovered. The new pattern is reminiscent of the Type-I spiral-vortex structures
which characterize the laminar–turbulent transition region of boundary layers over rigid rotating disks. Flow visualizations
reveal that the new pattern and the Type-I spiral vortices co-exist in the loose-boundary flow. The research investigating
the origin of the new large-scale pattern is reviewed. Then photographs from flow visualizations are analysed to obtain estimates
for the critical Reynolds number for which Type-I spiral vortices first appear for the loose-boundary flow and for the critical
Reynolds numbers for the laminar–turbulent transition of the boundary layer. The results suggest that Type-I vortices appear
at much lower Reynolds numbers over loose boundaries in comparison with flow over rigid rotating disks and that transition
also appears to be advanced to much lower Reynolds numbers. The discussion of the results suggests that advanced transition
arises from disturbances introduced into the flow after the loose boundary has been mobilized and not from disturbances associated
with the roughness that the surfaces of the granular layer represents to the flow while grains are at rest. |
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Keywords: | Loose-boundary flow Ripple formation Rotating-disk flow |
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