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On the evolution and form of coherent flow structures over a gravel bed: Insights from whole flow field visualization and measurement

Hardy, R. J.; Best, J. L.; Parsons, D. R.; Marjoribanks, T. I.

Authors

R. J. Hardy

J. L. Best

T. I. Marjoribanks

Abstract

©2016. The Authors. The microtopography of a gravel bed river has been shown to generate turbulent flow structures that originate from shear flow generated in the near-bed region. Although field and laboratory measurements have shown that such flows contain a range of coherent flow structures (CFS), the origin, evolution, and characteristics of the turbulent structures are poorly understood. Here we apply a combined experimental methodology using planar laser-induced fluorescence and particle imaging velocimetry (LIF-PIV) to measure simultaneously the geometric, kinematic, and dynamic characteristics of these CFS. The flow structures were analyzed by applying standard Reynolds decomposition and Lagrangian vortex detection methods to understand their evolution, propagation, and growth in the boundary layer and characterize their internal dynamical complexity. The LIF results identify large, individual, fluid packets that are initiated at the bed through shear that generate a bursting mechanism. When these large individual fluid packets are analyzed through direct flow measurement, they are found to contain several smaller scales of fluid motion within the one larger individual fluid parcel. Flow measurements demonstrate that near-bed shear controls the initiation and evolution of these CFS through merging with vortex chains that originate at the bed. The vortex chains show both the coalescence in the formation of the larger structures and also the shedding of vortices from the edges of these packets, which may influence the life span and mixing of CFS in open channels. The life span and geometric characteristics of such CFS are critical in influencing the duration and intensity of near-bed stresses that are responsible for the entrainment of sediment.

Journal Article Type Article
Publication Date Sep 13, 2016
Journal Journal of geophysical research : earth surface
Print ISSN 2169-9003
Electronic ISSN 2169-9011
Peer Reviewed Peer Reviewed
Volume 121
Issue 8
Pages 1472-1493
Institution Citation Hardy, R. J., Best, J. L., Parsons, D. R., & Marjoribanks, T. I. (2016). On the evolution and form of coherent flow structures over a gravel bed: Insights from whole flow field visualization and measurement. Journal of geophysical research. JGR. Earth surface, 121(8), 1472-1493. https://doi.org/10.1002/2015JF003753
DOI https://doi.org/10.1002/2015JF003753
Keywords River channels; Gravel bed rivers; Coherent flow structures (CFS); Turbulence
Publisher URL http://onlinelibrary.wiley.com/doi/10.1002/2015JF003753/abstract
Additional Information Copy of article first published in: Journal of geophysical research : earth surface, 2016, v.121, issue 8.

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Copyright Statement
©2016. The Authors.

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.



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