Precise measurements of granular flows in rotating tumblers using digital imaging and particle tracking velocimetry

The fundamental physics of large-scale flow of granular particles is not well understood, thus the current knowledge is based largely on heuristics. The goal of this research is to investigate several aspects of the flow of granular materials in rotating tumblers to better understand the fundamental physics of these flows. In granular flow systems, the movement of particles is constrained to a thin flowing layer at the free surface. Experiments are conducted to determine how properties of this free surface flow change at Particle, Flow, and System Levels of observation. Digital images are analyzed to measure particle positions and velocities are measured via Particle Tacking Velocimetry (PTV). Results indicate that increasing the nanoscale surface roughness of individual particles causes the dynamic angle of repose of the free surface to increase. In terms of the velocity of the flow, experiments verify the linear relationship between the local flowing layer length and the free surface streamwise velocity. The velocity results also confirm the negligible spanwise velocity in three-dimensional tumblers regardless of their shape (cylindrical, conical, or spherical). However, due to the conservation of mass, three-dimensional flow effects occur near end walls in cylindrical rotating tumblers, most evident as axial flow in regions upstream and downstream of the midlength of the flowing layer. This boundary flow near the end walls increases in magnitude with increasing radius of the tumbler. Decreasing the axial length to diameter ratio of the tumbler to less than one causes the boundary flow regions at the two end walls to merge generating streamwise velocity in quasi-two-dimensional tumblers that is twice as fast as the free stream flow that occurs in three-dimensional tumblers. Finally, the transient response of granular flow is determined from time-varying forcing conditions. Step changes in rotation rate can cause the flow characteristics to overshoot the steady state flow conditions. Time-periodic rotation rates result in time-varying streamwise velocity and flowing layer depth that mimic the input waveform of the rotation protocol, though with a slight phase lag.