Particle based modelling and simulation of natural sand dynamics in the wave bottom boundary layer

Abstract

Sand transport and morphological change occur in the wave bottom boundary layer due to sand particle interactions with an oscillatory flow and granular interactions between particles. Although these interactions depend strongly on the characteristics of the particle population, i.e. size and shape, little is known about how natural sand particles behave under oscillatory conditions and how variations in particle size influence transport behaviour. To enable this to be studied numerically, an Euler-Lagrange point-particle model is developed which can capture the individual and collective dynamics of subaqueous natural sand grains. Special treatments for particle collision, friction and hydrodynamic interactions are included to take into account the wide size and shape variations in natural sands. The model is used to simulate sand particle dynamics in two asymmetric oscillatory flow conditions corresponding to the vortex ripple experiments of Van der Werf et al. (J. Geophys. Res., vol. 112, 2007, F02012) and the sheet-flow experiments of O'Donoghue & Wright (Coast. Engng, vol. 50, 2004, pp. 117-138). A comparison of the phase resolved velocity and concentration fields shows overall excellent agreement between simulation and experiments. The particle based datasets are used to investigate the spatio-temporal dynamics of the particle-size distribution and the influence of three-dimensional vortical features on particle entrainment and suspension processes. For the first time, it is demonstrated that even for the relatively well-sorted medium-size sands considered here, the characteristics of the local grain size population exhibit significant space-time variation. Both conditions demonstrate a distinct coarse-over-fine armouring at the bed surface during low-velocity phases, which restricts the vertical mobility of finer fractions in the bed, and also results in strong pickup events involving disproportionately coarse fractions. The near-bed layer composition is seen to be very dynamic in the sheet-flow condition, while it remains coarse through most of the cycle in the vortex ripple condition. Particles in suspension spend more time sampling the upward directed parts of these flows, especially the smaller fractions, which delays particle settling and enhances the vertical size sorting of grains in suspension. For the submillimetre grain sizes considered, most † Email address for correspondence: J.Finn@liv.ac.uk

The Importance of Splat Events to the Spatiotemporal Structure of Near‐Bed Fluid Velocity and Bed Load Motion Over Bed Forms: Laboratory Experiments Downstream of a Backward Facing Step

“…Rather, only a force coupling between particles and flow is needed. Finn et al () used a LES‐DEM (large eddy simulation and distinct element method) to generate ripples under oscillatory waves. Kidanemariam and Uhlmann (, ) used a fully resolved turbulence and DEM model to generate small ripples.…”

Study of sedimentation of non-cohesive particles via CFD–DEM simulations

“…As shown in Tab. 2, when in Case 1 is 0.4, max is 0.58, which is in the range of the experimental packing density of sand particle 0.58~0.6 [44]. Therefore, the friction coefficient of Case 1 is determined as 0.4.…”

Sand transport processes in the surf and swash zones

“…This opposes process based sand transport models, that describe the physics of sediment transport processes in much more detail, are used primarily as a research tool, and operate at an intra wave time scale (e.g. Hsu et al, 2004;Revil Baudard and Chauchat, 2013;Kranenburg et al, 2014;Finn et al, 2016). Practical sand transport modeling approaches for bedload and suspended load are notably different, as detailed below.…”

“…Kranenburg et al, 2014;Finn et al, 2016) can offer another approach in understanding the effects of external (breaking generated) turbulence on bedload transport rates. Such local process based models, or process based morphodynamic models that operate at an intra wave time scale, may also be used to assess the effects of cross shore sediment influx on local bedload transport rates, i.e.…”

“…This may even lead to the formation of erosion resistant bed surface layers of coarse grains ('armouring'), which can significantly reduce sediment pick up and transport rates (Nielsen, 1992;Wiberg et al, 1994). Finn et al (2016) suggest, based on detailed simulations with a particle based numerical model, that such armour layers for sheet flow conditions may already develop after one wave cycle. Grain size observations in field (Wang et al, 1998) and laboratory (Koomans, 2000) conditions revealed a relatively large fraction of coarse sand on breaker bar crests, while bar troughs are composed of relatively fine sediment.…”