Microscopic dissipation in a cohesionless granular jet impact.

Sufficiently fine granular systems appear to exhibit continuum properties, though the precise continuum limit obtained can be vastly different depending on the particular system. In the present paper the continuum limit of an unconfined, dense granular flow is investigated. To do this a two-dimensional dense cohesionless granular jet impinging upon a target is used as a test system. This is simulated via a time-step-driven hard-sphere method and apply a mean-field theoretical approach to connect the macroscopic flow with the microscopic material parameters of the grains. It is observed that the flow separates into a cone with an interior cone angle determined by the conservation of momentum and the dissipation of energy. From the cone angle a dimensionless quantity A-B that characterizes the flow is extracted. This quantity is found to depend both on whether or not a dead zone, i.e., a stationary region near the target, is present and on the value of the coefficient of dynamic friction. A theory is presented for the scaling of A-B with the coefficient of friction that suggests that dissipation is primarily a perturbative effect in this flow rather than the source of qualitatively different behavior.