Density waves in dry granular media falling through a vertical pipe

We report experimental measurements of density waves in granular materials flowing down in a capillary tube. The density wave regime occurs at intermediate flow rates between a low density free fall regime and a high compactness slower flow. We observe this intermediate state when the ratio of the tube diameter to the particle size lies between 6 and 30. The propagation velocity of the waves is constant along the tube length and increases linearly with the total mass flow rate Φ. The wave structures include compact clogs (lengths are independent of Φ) and bubbles of low compactness (lengths increase with Φ). Both length distributions are invariant along the tube length. A model assuming a free fall regime in the bubbles and a compactness of 35% inside the clogs allows to account for the mass distribution in the flow.