(285b) Dem Simulation on Pattern Formation of Radial Segregation of Granular in Rotating Drums

Segregation and de-mixing often occur when a granular mixture is vibrated or made to flow. Two types of particle systems, i.e., D-systems and S-systems, have received more attention in recent years, where particles have the same size and different density in D-systems, and have the same density and different size in S-systems. Both systems present theoretical challenges in understanding the physical mechanisms behind the segregation and de-mixing, of which the S-systems often exhibit interesting segregation patterns. For example, a binary-solid mixture in rotating drums generally shows two cases: one is the axial segregation when the axis of the drum is much longer than the diameter and the other is the radial segregation when the axis of the drum is much shorter. In the latter case, rotation of a mixture of particles in different sizes in a thin drum often results in radial segregation with the smaller particles concentrating in a central core and the larger particles in the periphery. However, some different flow patterns may appear under certain conditions. For instance, the pattern of radial segregation could reverse at high rotational speed, i.e., smaller particles at the periphery and larger particles in the core. When the drums are slightly more than half-filled and the rotational speeds are low that just in the avalanching regime, radial streaks, called petals patterns or sun patterns, would be found in thin drums.

This work focused attention on the dynamics of pattern formation during the radial segregation of a binary-solid system in thin rotating drums. A discrete element model (DEM) considering the normal contact force, the tangential contact force, and the rolling friction torque for granular dynamics was employed to simulate the segregation of the S-type binary granular mixtures in a horizontal drum. The simulation results successfully predicted the dynamic evolution during the pattern formation of radial granular segregation started from a uniformly mixed binary-solid system as the initial state. In particular, the dependence of different patterns on the rotary speed and the load of the drum was well predicted, which agreed well with the experimental observations in the literature. As a higher rotary speed is applied, the moon pattern that the smaller particles concentrating in a central core and the larger particles in the periphery appears. As the rotary speed is lower, the petals pattern forms in the thin drum, and the number of petals increases as the rotary speed increases. If the particles loaded in the drum are few, the petals pattern cannot form in the drum. As all these results were obtained from DEM simulations, micro-scale analysis can be made to understand the evolution and formation of the segregation patterns.