An experimental study of the flowing granular layer in a rotating tumbler

Granular flow in a rotating tumbler is of theoretical and industrial significance. However, in spite of its relative simplicity, little is known about the dynamics of the top flowing layer. Here we present an experimental study of the velocity field within the fluidized layer of monodisperse particles in a quasi-2D (two-dimensional) rotating tumbler in the rolling flow regime using particle tracking velocimetry. The granular flow is illuminated by a laser flash and recorded using a charge coupled device camera. Image processing is used to remove the experimental noise and to achieve sub-pixel accuracy in calculating the particle displacements. The ensemble-averaged streamwise and transverse velocity profiles are calculated based on the particle displacements for three angular velocities and three bead sizes. The normalized streamwise velocity profile is linear throughout the fluidized layer, but becomes logarithmic as it enters the "fixed" bed where slow particle rearrangements dominate. The rms velocities appear to be exponentially related to the depth in the layer. Nondimensionalizing the number density in the fluidized layer with a geometric factor based on the square packing results in collapse of the data over a range of bead sizes and angular velocities.