  First, a meshless simulation method is presented for multiphase
fluid-particle flows with a two-way coupled Smoothed Particle Hydrodynamics
(SPH) for the fluid and the Discrete Element Method (DEM) for the solid phase.
The unresolved fluid model, based on the locally averaged Navier Stokes
equations, is expected to be considerably faster than fully resolved models.
Furthermore, in contrast to similar mesh-based Discrete Particle Methods
(DPMs), our purely particle-based method enjoys the flexibility that comes from
the lack of a prescribed mesh. It is suitable for problems such as free surface
flow or flow around complex, moving and/or intermeshed geometries and is
applicable to both dilute and dense particle flows.
  Second, a comprehensive validation procedure for fluid-particle simulations
is presented and applied here to the SPH-DEM method, using simulations of
single and multiple particle sedimentation in a 3D fluid column and comparison
with analytical models. Millimetre-sized particles are used along with three
different test fluids: air, water and a water-glycerol solution. The velocity
evolution for a single particle compares well (less than 1% error) with the
analytical solution as long as the fluid resolution is coarser than two times
the particle diameter. Two more complex multiple particle sedimentation
problems (sedimentation of a homogeneous porous block and an inhomogeneous
Rayleigh Taylor instability) are also reproduced well for porosities 0.6 <=
\epsilon <= 1.0, although care should be taken in the presence of high porosity
gradients.
  Overall the SPH-DEM method successfully reproduces quantitatively the
expected behaviour in the test cases, and promises to be a flexible and
accurate tool for other, realistic fluid-particle system simulations.
