Recent experiments by Baxter et al. [Phys. Rev. Lett. 62, 2825 (1989)] showed the existence of density waves in granular material flowing out of a hopper. We show, using molecular dynamics simulations, that this effect is a consequence of static friction and find that these density fluctuations follow a 1/f^α spectrum. The effect is enhanced when the opening angle of the hopper decreases.
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Risto et al. (1994) studied this question.
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