We have found experimentally that the flow of sand in an hour glass is oscillatory in a certain range of parameters. The oscillatory motion results from a coupling between the flow of sand and convection of air through the sand matrix. The period of the oscillation is remarkably insensitive to the grain size, while the mass of the sand contained in one period strongly depends on it.

• Received 13 April 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.1363