Pebble sizes D_p (top panel), the corresponding Stokes numbers τ (bottom panel), and their dependence on the radial distance r in individualsimulations. The value of D_p changes in such a way as to keep the pebble flux Ṁ_p initially constant. On the other hand, the value of τ is inversely proportional to the coupling between the gas and pebbles. Both quantities are increased in the damping zones (grey strips), but this does not affect our simulation in any way.