  We consider the diffusion-advection problem in two simple cellular flow
models (often invoked as examples for subdiffusive tracer's motion) and
concentrate on the intermediate time range, in which the tracer's motion indeed
may show subdiffusion. We have performed extensive numerical simulations of the
systems under different initial conditions, and show that the pure
intermediate-time subdiffusion regime is only evident when the particles start
at the border between different cells, i.e. at the separatrix, and is less
pronounced or absent for other initial conditions. The motion moreover shows
quite peculiar aging properties which are also mirrored in the behavior of the
time-averaged mean squared displacement for single trajectories. This kind of
behavior is due to the complex motion of tracers trapped inside the cell, and
is absent in classical models based on continuous time random walks (CTRW) with
no dynamics in the trapped state.
