Size segregation, convection, and arching effect.

We present a numerical study based on the contact dynamic procedure for the size segregation in a two-dimensional vibrating container. In agreement with previous studies, we find that the rising of the larger particle is accompanied by convection rolls. However, at low enough acceleration, rolls do not penetrate the entire cell and a local vault mechanism is observed, in which the larger particle moves up by steps. This mechanism, described in experiments by Duran and co-workers [Phys. Rev. E. 50, 5138 (1994)], occurs in a very narrow region of parameter space. Once the larger intruder reaches the exponential tail of convecting roll, its vertical motion becomes continuous and the corresponding rising speed is dramatically increased.