I also studied granular jets as compared to classical Worthington liquid-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, will help pinpoint the role of surface tension for singular liquid jets. See Movie.

  • S. T. Thoroddsen, A. Q. Shen, Granular jets, Physics of Fluids, 13 (2001), 4--6.