Possibility of perfect fluid flow from granular jet impact

Axisymmetric collision of a cylindrical water jet with a circular target generates a thin conical sheet, also known as a water bell [1]. Intriguingly, recent experiments on granular jet impact in the regime of dense inertial flow reveal similar behavior: the angles by which the collimated sheets of particles are ejected from the target [2] agree closely with the angles measured in the water-bell experiments. This quantitative correspondence suggests that the collective granular motion during impact can be modeled as an incompressible, continuum fluid. Since viscous effects are weak in water-jet impact and the granular jet is comprised of non-cohesive particles (hence possessing zero surface tension), the simplest scenario is that the continuum motion corresponds to the flow of a perfect fluid. We assess this possibility by comparing exact solutions of 2D Euler-jet impact with 2D discrete-particle simulations of granular impact. We also construct approximate solutions for axisymmetric Euler-jet impact and compare these with granular-impact experiments.\newline [1] Cheng et al. Phys. Rev. Lett. 99, 2007.\newline [2] Clanet, C. J. Fluid Mech. 430, 2001.