DEM simulation involves following the motion of every particle in the flow and modelling each collision between combinations of particles and between particles and their environment (e.g. the walls of a bin or the liner of a mill). The general DEM methodology and its variants are well established and are described in review articles by Barker (1994), Campbell (1990) and Walton (1994). The implementation used here is described in more detail in Cleary (1998a, b). Briefly, the particles are allowed to overlap and the amount of overlap ? x, and normal v _n and tangential v _t relative velocities determine the collisional forces via a contact force law. We use a linear spring-dashpot model. For more complex interaction models, see Sch fer et al. (1996) and Walton (1994).

The normal force:

(1)

is modelled by a linear spring to provide the repulsive force and a dashpot to dissipate a specified proportion of the relative kinetic energy. The maximum overlap between particles is determined by the stiffness k _n of the spring in the normal direction. Typically, average overlaps of 0.1-0.5 per cent are desirable, requiring spring constants of the order of 10 ⁴-10 ⁶ N/m in three dimensions. The normal damping coefficient C _n is chosen to obtain the required coefficient of restitution ? (defined as the ratio of the post-collisional to pre-collisional normal component of the relative velocity), and is given by Cleary (1998b).

The tangential...