Granular jets

When a solid sphere impacts on a deep layer of granular medium, it generates an ejecta sheet and a transient axisymmetric crater. The gravity-driven radial collapse of this crater generates a pressure spike, as the cavity closes up. This pressure spike drives up a narrow granular jet along the axis of symmetry. The maximum height of the jet is found to depend on the impact velocity, gravity as well as the effective viscosity of the granular medium, through a simple product of the Reynolds and Froude numbers. The presence of such granular jets, where surface tension is absent, may help pinpoint the role of surface tension for similar liquid jets. (C) 2001 American Institute of Physics.