Collision process between an incident bead and a three-dimensional granular packing.

We report on experimental studies of the collision process between an incident bead and a three-dimensional granular packing (made of particles identical to the impacting one). The understanding of such a process and the resulting ejection of particles is, in particular, crucial to describe eolian sand transport. We present here an extensive experimental analysis of the collision and ejection process. The analysis is two dimensional in the sense that we determined only the vertical component V{z} of the ejection velocity of the splashed particles and the horizontal component V{x} lying in the incident plane. We extracted in particular the distribution of the ejection velocities for a wide range of impact angles theta{i} and incident velocity V{i} . We show that the mean quadratic horizontal velocity of the splashed particles is almost insensitive to changes in the impact angle and velocity, while the mean quadratic vertical velocity slightly increases with increasing impact velocity (as V{i}{1/2}). Moreover, the mean number of splashed particles per collision is found to be dependent on both the impact angle and velocity, and to scale with the impact speed as V{i}{3/2}. A consequence of these outcomes is that the sum of the kinetic energy of the splashed particles is directly proportional to the kinetic energy of the incident particle. Finally, we provide the bivariate probability distribution function P(V{x},V{z}) of the ejection velocities and show that it can be approximated by the product of a log-normal distribution and a circular normal one.