  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.
