  The dynamics of rigid particle suspensions in a wall-bounded laminar flow
present several non-trivial and intriguing features, including particle
ordering, lateral transport, and the appearance of stable, preferential
locations like the Segr\'e-Silberberg annulus. The formation of more than one
annulus is a particularly puzzling phenomenon that is still not fully
explained. Here, we present numerical simulation results of a dilute suspension
of particles in (periodic) pipe flow based on the lattice Boltzmann and the
discrete element methods (DEM). Our simulations provide access to the full
radial position history of the particles while traveling downstream. This
allows to accurately quantify the transient and steady states. We observe the
formation of the secondary, inner annulus and show that its position invariably
shifts toward the Segr\'e-Silberberg one if the channel is sufficiently long,
proving that it is, in fact, a transient feature for Reynolds numbers (Re) up
to 600. We quantify the variation of the channel focusing length ($L_s/2R$)
with Re. Interestingly and unlike the theoretical prediction for a point-like
particle, we observe that $L_s/2R$ increases with Re for both the single
particle and the suspension.
