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 $\Phi$. The wave structures include compact clogs (lengths are independent of $\Phi$) and bubbles of low compactness (lengths increase with $\Phi$). 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.