Comparing Flow Thresholds and Dynamics
for Oscillating and Inclined Granular Layers*
C. Puls, S. Aumaitre, and J.P. Gollub
Haverford College and University of Pennsylvania
The onset and dynamics of flow in horizontally oscillating granular
layers are studied as a function of the depth of the layer (see
the experimental devices). Measurements of the flow velocity made
from the top and side are presented in the frame of reference of the
container (see movies). The rheology of the
material is found to vary in time during the cycle in surprising ways.
If the inertial force (proportional to the container acceleration
amplitude) is slightly higher than what is required to produce flow,
then the flow velocity grows as soon as the inertial force exceeds
zero in each cycle, but flow ceases long before the inertial forces
return to zero, possibly due to the buildup of internal stresses.
At higher accelerations, the motion is fluid-like over the entire
cycle (more ...). As is also found for
avalanches of inclined layers, the thresholds for starting and stopping
of flow are slightly different. The variation with depth of the starting
acceleration for the oscillating layer matches (approximately) the
corresponding variation of the tangent of the starting angle for avalanches
in the same container (see curves). However,
the fraction of the cycle during which the layer is mobile is typically
far higher than what one would predict from static considerations
or the behavior of the inclined layer.
*Supported by NSF-DMR -0405187.