  We have combined time dependent hydrodynamics with a two-fluid model for dust
driven AGB winds. Our calculations include self-consistent gas chemistry, grain
formation and growth, and a new implementation of the viscous momentum transfer
between grains and gas. This allows us to perform calculations in which no
assumptions about the completeness of momentum coupling are made. We derive new
expressions to treat time dependent and non-equilibrium drift in a hydro code.
Using a stationary state calculation for IRC +10216 as initial model, the time
dependent integration leads to a quasi-periodic mass loss in the case where
dust drift is taken into account. The time scale of the variation is of the
order of a few hundred years, which corresponds to the time scale needed to
explain the shell structure of the envelope of IRC +10216 and other AGB and
post-AGB stars, which has been a puzzle since its discovery. No such
periodicity is observed in comparison models without drift between dust and
gas.
