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

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

doi:10.1002/2015jf003753

Abstract

AbstractThe 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.

Highlights

One key feature that determines the characteristics of river flow is its interaction with a heterogeneous sedimentary bed, which creates shear instabilities in the boundary layer that are often invoked as the mechanism in the formation of macroturbulent flow structures [e.g. Kostaschuk and Church, 1993; Bennett and Best 1995; Venditti and Bennett, 2000; Best and Kostaschuk, 2002; Jessup et al, 2013]
The geometric characteristics identified in the laser induced fluorescence (LIF) image T1 show that the Rhodamine is entrained into the shear flow generated by the topographic protrusion
This study has examined the nature of turbulent flow over a gravel bed at four different Reynolds numbers by applying a combined measurement and flow visualisation technique

Summary

Introduction

One key feature that determines the characteristics of river flow is its interaction with a heterogeneous sedimentary bed, which creates shear instabilities in the boundary layer that are often invoked as the mechanism in the formation of macroturbulent (i.e. flow-depth scale) flow structures [e.g. Kostaschuk and Church, 1993; Bennett and Best 1995; Venditti and Bennett, 2000; Best and Kostaschuk, 2002; Jessup et al, 2013] These shear instabilities are generated by flow separation induced by boundary layer flow over multi-scale bed topography [Wiberg and Smith, 1991; Dinehart, 1992; Robert et al, 1992; Buffin-Bélanger and Roy, 1998; Lacey and Roy, 2006; Hardy et al, 2007, 2009], and through the wakes of individual topographic protrusions and jetting of higher-velocity flow between such bedforms. These turbulent structures are known as eddies [Townsend, 1976] but are more commonly referred to as coherent flow structures (CFS) [Cantwell, 1981; Adrian, 2007, 2013; Venditti et al, 2013], and it is their make-up that control the structure of turbulent flows

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