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Title: On the evolution and form of coherent flow structures over a gravel bed: insights from whole flow field visualization and measurement
Authors: Hardy, Richard J.
Best, J.L.
Parsons, Daniel R.
Marjoribanks, Timothy I.
Issue Date: 2016
Publisher: American Geophysical Union © The Authors
Citation: HARDY, R.J. ... et al, 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: Earth Surface, 121 (8), pp. 1472-1493.
Abstract: 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.
Description: This is an Open Access Article. It is published by AGU under the Creative Commons Attribution 4.0 Unported Licence (CC BY). Full details of this licence are available at http://creativecommons.org/licenses/by/4.0/
Sponsor: UK NERC. Grant Number: NE/F010060/1, NERC JREI. Grant Number: GR3/JE140, HEFCE SRIF2
Version: Published
DOI: 10.1002/2015JF003753
URI: https://dspace.lboro.ac.uk/2134/22846
Publisher Link: http://dx.doi.org/10.1002/2015JF003753
ISSN: 2169-9003
Appears in Collections:Published Articles (Civil and Building Engineering)

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