POWER SPECTRA OF FLOW IN AN HOURGLASS

We have investigated the density fluctuations of granular flow in an h ourglass. When there is no counterflow of air, the how is continuous w ith no visible density fluctuations. When there is a counterflow, we o bserve both clustering and ticking depending sensitively on the grain size and distribution, in qualitative agreement with the measurements of Wu et al. [Phys. Rev. Lett. 71, 1363 (1993)]. The power spectra sho w structures appropriate for the different time scales. All power spec tra are white at low frequencies. For certain grains then is also an a pparent 1/f region as found earlier by Schick and Verveen [Nature (Lon don) 251, 599 (1974)]. However, we argue that this 1/f region is not g enuine and can be explained by a simple stochastic model in which the clusters randomly block the measuring light source.