  We present numerical simulations of the hydrodynamical interactions that
produce circumstellar shells. These simulations include several scenarios, such
as wind-wind interaction and wind-ISM collisions. In our calculations we have
taken into account the presence of dust in the stellar wind. Our results show
that, while small dust grains tend to be strongly coupled to the gas, large
dust grains are only weakly coupled. As a result, the distribution of the large
dust grains is not representative of the gas distribution. Combining these
results with observations may give us a new way of validating hydrodynamical
models of the circumstellar medium.
