SELF-ORGANIZED CRITICAL DENSITY WAVES OF GRANULAR MATERIAL FLOWING THROUGH A PIPE

Fourier analysis has been applied to density waves of granular material (sand) which flows through a pipe. We let sand flow from a hopper through a vertical glass pipe due to gravity. The flow appeared intermittent, reminiscent of a chain of traffic jams on a crowded highway, over the length of the pipe except for the uppermost part where the sand flowed smoothly and homogeneously from the hopper. The FFT power spectra were found to change from a rather structureless form to a stable power-law form of 1/f(alpha) with alpha similar or equal to 1.5 as we varied the measuring position from the hopper down to the pipe end. This result shows the occurrence of self-organized critical density waves in granular material flow through a pipe.