Mechanisms for Axial Band Formation in a Rotating Drum of Granular Material
Newey, Michael Kent
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We study granular particles, like sand or glass beads, that are mixed in a partially filled, horizontal, rotating, cylindrical drum. When particles of different sizes are placed in the drum spatial segregation of the particles by size is observed. This segregation occurs in two phases. During the first phase, called radial segregation, the smaller particles form a radial core. In the second, called axial segregation, particles segregate into alternating bands along the axis of the drum. We perform a detailed study of the characteristics of the flow to determine the physical mechanisms driving axial segregation. We characterize the top surface of the flowing layer by tracking particles using a high-speed camera. We then extract average quantities such as velocity and diffusion. The average velocities show surprising behavior: Particles in small particle bands have a higher downhill flow velocity than particles in large particle bands. We also observe that there is a pattern of sideways velocity as a function of position down the flow. Particles flow into small particle bands in the middle of the flow but flow out of small particle bands at the bottom. We present the framework for a new model based on our experimentally observed results. We explain the axial band formation in terms of the observed surface flow patterns. Our framework connects differences in flow velocities on the surface of the drum with the radial segregation in the bulk of the drum. We compare these results to current models. We test the general model assumption that the particles always flow in the direction of steepest descent by measuring the surface height of the banded state with a laser line. We find that although there is some indication that particles flow in the direction of steepest descent, there is strong flow that is not in the direction of steepest descent, contrary to the given models. Finally, we study oscillating patterns in mixtures of three sizes of glass beads. Ternary mixtures of particles form bands within bands of the different particle sizes. For certain experimental conditions we observe traveling and oscillating patterns.