Granular mixtures of particles with a difference in grain size are of relevance in many industrial applications. Unfortunately, much of the research done in the field of granular materials has focused largely on mono-sized particle distributions. The dynamics of a mixture can differ significantly from those of a same-size flow, however. For example, shearing of materials often leads to unwanted segregation of particles of different sizes. The manner in which a flow will segregate is often dependent on the flow geometry and its orientation with respect to gravity, because segregation is often gravity-induced. This research uses both experiments and discrete element computer simulations to study the segregation occurring in a cylindrical Couette flow.
The experimental apparatus consists of a pair of concentric aluminum cylinders, the inner one of which rotates while the outer cylinder remains stationary. The surfaces of the cylinders are coated with sandpaper and the region between the cylinders is filled with spherical glass beads. Mixtures of 4mm and 6mm beads are sheared at various strain rates inside the apparatus. Various inner cylinder diameters can be used in the apparatus so that the ratio of gap width to particle size is varied. The segregation of the flow is visually observed through a glass window at the front. High-speed imaging and particle tracking is used to determine velocity profiles and number density profiles of each kind of particle at various angular locations.
A three-dimensional soft-particle computer simulation has been used to model the experimental flows. Simulations allow for comparison with experimental data from observations at the glass window, while providing additional information about the three-dimensionality of the flow. Information about the flow away from the glass boundary is important because the flow characteristics can vary greatly from the boundary region to the bulk flow.
Segregation is observed in both the experiments and simulations. Quantitative comparisons are made between simulation and experimental results. Segregation in this geometry is analyzed for two different gap width to diameter ratios.