Self-organized critical density waves of granular particles flowing through a pipe

Density waves of granular material which flows through a pipe have been investigated. Granules (mainly sand) were drained from a hopper through a vertical glass pipe. The flow looked intermittent over the length of the pipe except for the uppermost part where the granules streamed smoothly and homogeneously from the hopper aperture. Transmission light intensity across the pipe was measured as time-series signals. The FFT power spectra were found to change from white-noise-like form to a stable and robust power-law form of 1/fα with α 1.5 as the measuring position was varied from just below the hopper to the pipe end. This indicates the occurrence of self-organized critical density waves in the granular flow through a pipe. It is pointed out that the back-flow of air is responsible for the occurrence of the density waves.