Granular flow in silo with moving exit

In experiments of mono-disperse plastic beads falling out of a cylindrical silo through a circular exit at the bottom, no continuous flow can be sustained when the diameter D of the exit is about 5 times that of the beads. If the exit of the silo moves horizontally (e.g. circular motion or linear oscillation) with respect to the wall of the silo, finite flow rate can be sustained even at small D. When the exit rotates at off-center distance R with angular frequency ω, the flow rate W increases with Rω. Similarly, when the exit oscillates with amplitude A and oscillation frequency f, W increases with Af. On the other hand, if D is large, W varies non-monotonically with either the angular frequency ω or oscillation frequency f.