A comparison of shock-cloud and wind-cloud interactions: The longer
  survival of clouds in winds
  The interaction of a hot, high-velocity wind with a cold, dense molecular
cloud has often been assumed to resemble the evolution of a cloud embedded in a
post-shock flow. However, no direct comparative study of these two processes
currently exists in the literature. We present 2D adiabatic hydrodynamical
simulations of the interaction of a Mach 10 shock with a cloud of density
contrast $\chi = 10$ and compare our results with those of a commensurate
wind-cloud simulation. We then investigate the effect of varying the wind
velocity, effectively altering the wind Mach number $M_{wind}$, on the cloud's
evolution. We find that there are significant differences between the two
processes: 1) the transmitted shock is much flatter in the shock-cloud
interaction; 2) a low-pressure region in the wind-cloud case deflects the flow
around the edge of the cloud in a different manner to the shock-cloud case; 3)
there is far more axial compression of the cloud in the case of the shock. As
$M_{wind}$ increases, the normalised rate of mixing is reduced. Clouds in winds
with higher $M_{wind}$ also do not experience a transmitted shock through the
cloud's rear and are more compressed axially. In contrast with shock-cloud
simulations, the cloud mixing time normalised by the cloud-crushing time-scale
$t_{cc}$ increases for increasing $M_{wind}$ until it plateaus (at $t_{mix}
\simeq 25 \, t_{cc}$) at high $M_{wind}$, thus demonstrating the expected Mach
scaling. In addition, clouds in high Mach number winds are able to survive for
long durations and are capable of being moved considerable distances.
