  Massive stars shape their surrounding medium through the force of their
stellar winds, which collide with the circumstellar medium. Because the
characteristics of these stellar winds vary over the course of the evolution of
the star, the circumstellar matter becomes a reflection of the stellar
evolution and can be used to determine the characteristics of the progenitor
star. In particular, whenever a fast wind phase follows a slow wind phase, the
fast wind sweeps up its predecessor in a shell, which is observed as a
circumstellar nebula. We make 2-D and 3-D numerical simulations of fast stellar
winds sweeping up their slow predecessors to investigate whether numerical
models of these shells have to be 3-D, or whether 2-D models are sufficient to
reproduce the shells correctly. We focus on those situations where a fast
Wolf-Rayet (WR) star wind sweeps up the slower wind emitted by its predecessor,
being either a red supergiant or a luminous blue variable. As the fast WR wind
expands, it creates a dense shell of swept up material that expands outward,
driven by the high pressure of the shocked WR wind. These shells are subject to
a fair variety of hydrodynamic-radiative instabilities. If the WR wind is
expanding into the wind of a luminous blue variable phase, the instabilities
will tend to form a fairly small-scale, regular filamentary lattice with thin
filaments connecting knotty features. If the WR wind is sweeping up a red
supergiant wind, the instabilities will form larger interconnected structures
with less regularity. Our results show that 3-D models, when translated to
observed morphologies, give realistic results that can be compared directly to
observations. The 3-D structure of the nebula will help to distinguish
different progenitor scenarios.
