Ticking hour glasses: Experimental analysis of intermittent flow

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Abstract
Fluctuations in the flow of sand in an hour glass have been investigated experimentally for the case when the interaction between grains and interstitial air is important. During the active phase of the flow, a plug is formed at the narrowest constriction of the hour glass. The plug is below the free-fall arch defining the lower ‘‘edge’’ of the sand heap in the upper chamber. The low-density region between the plug and the arch defines what we call a ‘‘bubble.’’ Early during the active (flow) phase, the position of the bubble is stationary, whereas later, it rises into the bulk of the sand and disappears. The position of the arch thus oscillates with a shorter period than the one associated with the active-inactive phase oscillation. © 1996 The American Physical Society.

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