Abstract
We use interface-resolved direct numerical simulations to study the dynamics of a single sediment particle in a turbulent open channel flow over a fixed porous bed. The relative strength of the gravitational acceleration, quantified by the Galileo number, is varied so as to reproduce the different modes of sediment transport – resuspension, saltation and rolling. The results show that the sediment dynamics at lower Galileo numbers (i.e. resuspension and saltation) are mainly governed by the mean flow. Here, the regime of motion can be predicted by the ratio between the gravity and the shear-induced boundary force. In these cases, the sediment particle rapidly takes off when exposed to the flow, and proceeds with an oscillatory motion. Increasing the Galileo number, the frequency of these oscillations increases and their amplitude decreases, until the transport mode switches from resuspension to saltation. In this case, the sediment travels by short successive collisions with the bed. Further increasing the Galileo number, the particle rolls without detaching from the bed. Differently from the previous modes, the motion is triggered by extreme turbulent events, and the particle response depends on the specific initial conditions, at fixed Reynolds number. The results reveal that close to the onset of sediment motion, only turbulent sweeps can effectively trigger the particle motion by increasing the stagnation pressure upstream. We show that for the parameters in this study, a criterion based on the streamwise flow-induced force can successfully predict the incipient movement.
Highlights
IntroductionParticle entrainment plays a key role in the earth surface dynamics (erosion, transport and deposition of sediments in the atmosphere and rivers), biological flows
Particle entrainment plays a key role in the earth surface dynamics, biological flowsScience, University of Iceland, Hjardarhagi 2-6, 107 Reykjavik, Iceland. 893 A24-2A
We aim to investigate the dynamics of the sediment particle for each case by performing fully resolved numerical simulations, which allow us to explore the flow physics with the three-dimensional and time-resolved insight that is still difficult to achieve in a real experiment
Summary
Particle entrainment plays a key role in the earth surface dynamics (erosion, transport and deposition of sediments in the atmosphere and rivers), biological flows. A fundamental understanding of these transport mechanisms is relevant to physicists and engineers, with a key role often played by the identification of the threshold conditions for the onset of sediment motion, known as incipient motion. Despite several attempts to address this problem, it has remained elusive owing to the large number of parameters determining the particle dynamics. The complexity rises from the fact that the particle entrainment is highly sensitive to the bed surface morphology, particle size and shape, exposure and packing conditions, and, not least, to the intermittent behaviour of turbulence in near-bed flows (Dey & Ali 2018)
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