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

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.

A numerical investigation into the importance of bed permeability on determining flow structures over river dunes

“…It is also worth considering how these returning horseshoe‐shaped vortices could affect the boundary layer flow. Recent experimental work looking at coherent flow structures over gravel beds [ Hardy et al ., ] identified large individual packets of fluid, that contain several smaller scales of fluid motion, were initiated at the bed through shear that generate a bursting mechanism. No consideration in these experiments was taken of flow passing through the bed and the possible influence these structures have on the overall nature of the boundary layer flow.…”

Observations of Coherent Flow Structures Over Subaqueous High‐ and Low‐ Angle Dunes

“…Several vortex cores are also depicted in the near‐bed region and denote boundary layer turbulence. Such preferential emphasis of smaller‐scale flow structures has also been observed in PIV measurements by Hardy et al () who explored vorticity patterns over a gravel bed to examine CFS. Due to this apparent tendency to emphasize small‐scale flow structures, which are either secondary products of the large‐scale CFS (Müller & Gyr, ) or unrelated phenomena close to the bed, swirling strength does not appear to be a suitable candidate to identify the large‐scale CFS that are the focus of this study.…”

Morphodynamic styles: characterising the behaviour of gravel-bed rivers using a novel, quantitative index

“…Inspired by Strahler (1952), coupling this quantitative revolution with a shift toward process-based understanding, morphodynamic research has expanded in both its breadth and depth (e.g. Rust, 1972;Fenton and Abbott, 1977;Desloges and Church, 1989;Abrahams et al, 1995;Wheaton et al, 2013;Hardy et al, 2016). However, there has been a concomitant fragmentation of research themes and an increased focus on isolated processes from one another as dataset sizes increase.…”