TY - JOUR
T1 - On the evolution and form of coherent flow structures over a gravel bed
T2 - Insights from whole flow field visualization and measurement
AU - Hardy, R. J.
AU - Best, J. L.
AU - Parsons, D. R.
AU - Marjoribanks, T. I.
N1 - Publisher Copyright:
©2016. The Authors.
PY - 2016/8/1
Y1 - 2016/8/1
N2 - 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.
AB - 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.
KW - coherent flow structures
KW - gravel bed rivers
KW - turbulence
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U2 - 10.1002/2015JF003753
DO - 10.1002/2015JF003753
M3 - Article
AN - SCOPUS:84983428271
SN - 2169-9003
VL - 121
SP - 1472
EP - 1493
JO - Journal of Geophysical Research: Earth Surface
JF - Journal of Geophysical Research: Earth Surface
IS - 8
ER -