Intermittent dry granular flow in a vertical pipe
  The intermittent compact flow of glass beads in a vertical glass pipe of small diameter is studied experimentally by combining particle fraction, pressure, and air and grain flow rates measurements with a spatio-temporal analysis of the flow. At the onset of the flow, a decompaction front is observed to propagate from the bottom to the top of the tube at a velocity much larger than that of the grains. The blockage front also propagates upwards and at a still higher velocity. The decompaction induces a decreasing pressure wave strongly amplified as it propagates upwards towards the top of the tube. Pressure variations of 3000 Pa or more are detected in this region while particle fraction variations are of the order of 0.02. Grain velocities during the flow period also increase strongly at the top of the tube while the corresponding fraction of total time decreases. A 1D numerical model based on a simple relation between the effective acceleration of the grains and the particle fraction variations reproduces the amplification effect and provides predictions for its dependence on the permeability of the packing. 
Yann Bertho, Frederique Giorgiutti-Dauphine and Jean-Pierre Hulin