Effect of finite container size on granular jet formation.

When an object is dropped into a bed of fine, loosely packed sand, a surprisingly energetic jet shoots out of the bed. In this work we study the effect that boundaries have on the granular jet formation. We did this by (i) decreasing the depth of the sand bed and (ii) reducing the container diameter to only a few ball diameters. These confinements change the behavior of the ball inside the bed, the void collapse, and the resulting jet height and shape. We map the parameter space of impact with Froude number, ambient pressure, and container dimensions as parameters. From these results we propose an explanation for the thick-thin structure of the jet reported by several groups ([J. R. Royer, Nat. Phys. 1, 164 (2005)], [G. Caballero, Phys. Rev. Lett. 99, 018001 (2007)], and [J. O. Marston, Phys. Fluids 20, 023301 (2008)]).