Simulation study of air-induced segregation of equal-sized bronze and glass particles.

We have investigated the effect of air on the density segregation of fine, equal-sized bronze and glass particles under vertical sinusoidal vibrations by numerical simulation, using a hybrid granular dynamics-computational fluid dynamics model. We find both the bronze-on-top and the sandwich configurations as observed in previous experiments. The simulations allow us to give a detailed explanation of the phenomena. The key observation is the precise timing of the decompaction of the bed with the oscillating gas flow, which will cause the bronze to move to the top, due to the difference in acceleration from the air drag. For higher frequencies, the different inertia in collisions will cause the bronze clusters on top to sink through the bed, leading to a sandwich formation.