Grain sedimentation with SPH-DEM and its validation

Our mesoscale simulation method [M. Robinson, S. Luding, and M. Ramaioli, submitted (2013)] for multiphase fluid-particle flows couples Smoothed Particle Hydrodynamics (SPH) and the Discrete Element Method (DEM) and enjoys the flexibility of meshless methods, such as being capable to handling free surface flows or flow around complex and/or moving geometries. We use this method to simulate three different sedimentation test cases and compare the results to existing analytical solutions. The grain velocity in Single Particle Sedimentation compares well (< 2% error) with the analytical solution as long as the fluid resolution is coarser than two times the particle diameter. The multiple particle sedimentation problem and Rayleigh Taylor Instability (RTI) also perform well against the theory, but it was found that the method is susceptible to fluid velocity fluctuations in the presence of high porosity gradients. These fluctuations can be damped by the addition of a dissipation term, which has no effect on the terminal velocity but can lead to slower growth rates for the RTI.