A 3D discrete element model of packing, entrainment and saltation of sediment mixtures
It is believed that the behaviour of the bed-load fraction of transported sediment as a whole is influenced by the behaviour of the individual grain. At this micro-scale spatial resolution it is difficult to make field and laboratory observations. For this reason, numerical models have found a particular niche. A model has been developed to investigate the interaction between the surface of a bed of multi-sized spherical particles (from a continuous size distribution) and a logarithmic flow velocity profile. The model is based upon the cycle of erosion, transport and deposition. The first phase of operation involves depositing a layer of particles to the pre-determined size distribution. This packing is shown to behave as expected, on evidence from experiments undertaken in the investigation of powders and particulate materials. The surface is exposed to the bottom boundary of a multi-slab flow model with superimposed random fluctuations, the reaction to the sum of the drag forces acting upon the surface particles acts to retard the bottom flow layer. Thus the flow model and the bed model are linked. Particle entrainment is determined from a three dimensional moment analysis of a surface particle about its nearest contacts, subject to fluid and body forces. Once entrained the particle follows a nearly ballistic trajectory until it collides with the bed, from where it can either be deposited or rebound into successive trajectories. In this way the bed is modified. The packing of spheres onto a regular foundation layer shows three distinct regions; an interface layer, an homogeneous region, and a rough surface. Calculated properties or particle size distribution, porosity, a number of particle contacts are reasonably constant in the homogeneous region, diverging towards the foundation and the surface.