  We report on density waves in granular material, investigated both
experimentally and numerically. When granular material falls through a long
narrow pipe one observes recurrent clogging. The kinetic energy of the falling
particles increases up to a characteristic threshold corresponding to the onset
of recurrent clogging and density waves of no definite wavelength. The
distances between regions of high density depend strongly on the initial
conditions. They vary irregularly without any characteristic time and length
scale. The particle-flow was investigated using 2D Molecular Dynamics
simulations. Experimental investigations lead to equivalent results.
