  Classical Wolf Rayet (WR) stars are direct supernova progenitors undergoing
vigorous mass-loss. Understanding the dense and fast outflows of such WR stars
is thus crucial for understanding advanced stages of stellar evolution, the
dynamical feedback of massive stars on their environments, and characterizing
the distribution of black hole masses. In this paper, we develop first
time-dependent, multi-dimensional, radiation-hydrodynamical models of the
extended optically thick atmospheres and wind outflows of hydrogen-free
classical WR stars. A flux limiting radiation hydrodynamics approach is used on
a finite volume mesh to model WR outflows. The opacities are described using a
combination of tabulated Rosseland mean opacities and the enhanced line
opacities expected within a supersonic flow. For high-luminosity models, a
radiation-driven, dense, supersonic wind is launched from deep sub-surface
regions associated with peaks in the Rosseland mean opacity. For a model with
lower luminosity, on the other hand, the Rosseland mean opacity is not
sufficient to sustain a net-radial outflow in the sub-surface regions. Rather,
what develops in this case is a "standard" line-driven wind launched from the
optically thin regions above an extended and highly turbulent atmosphere. We
thus find here a natural transition from optically thick outflows of classical
WR stars to optically thin winds of hot, compact sub-dwarfs; in our simulations
this transition occurs approximately at a luminosity that is about 40% of the
Eddington luminosity. Because of the changing character of the wind-launching
mechanism, this transition is also accompanied by a large drop (on the
low-luminosity end) in average mass-loss rate.
