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 analysed 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 analysed through direct flow measurement, they are found to contain several smaller scales of fluid motion within the one larger individual fluid parcel. These flow measurements demonstrate that near-bed shear control the initiation and evolution of these CFS through merging with vortex chains that originate at the bed. These vortex chains show both coalescence in the formation of the larger structures, but 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 lifespan 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.