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Flow and Clog in a Silo with Oscillating Exit
Speaker Prof. Kiwing To, Institute of Physics, Academia Sinica
Date 9 January 2018 (Tuesday)
Time 11:00 to 12:30
Venue Lecture Theatre H (Lifts 27-28), HKUST

When grains flow out of a silo, flow rate W increases with exit size D. If D is too small, an arch may form and the flow may be blocked at the exit. To recover from clogging, the arch has to be destroyed. Here we construct a two-dimensional silo with movable exit and study the effects of exit oscillation (with amplitude A and frequency f) on flow rate, clogging, and unclogging of grains through the exit. We find that, if exit oscillates, W remains finite even when D (measured in unit of grain diameter) is only slightly larger than one. Surprisingly, while W increases with oscillation strength Γ ≡ 4π2 Af 2 as expected at small D, W decreases with Γ when D ≥ 5 due to induced random motion of the grains at the exit. When D is small and oscillation speed v ≡ 2πAf is slow, temporary clogging events cause the grains to flow intermittently. In this regime, W depends only on v—a feature consistent to a simple arch breaking mechanism, and the phase boundary of intermittent flow in the D−v plane is consistent to either a power law: D ∝ v−7 or an exponential form: D ∝ e−D/0.55. Furthermore, the flow time statistic is Poissonian whereas the recovery time statistic follows a power-law distribution.