Numerical simulations of shock-cloud interactions

This Thesis presents numerical simulations of shocks interacting with
regions containing multiple individual clouds. Firstly, the
hydrodynamic interaction is presented. It is the first study to
include 100s of clouds in a clumpy region which `mass-load' the flow.
The 'mass-loading' reduces the Mach number of the shock and leads to
the formation of a dense shell. In cases in which the `mass-loading'
is sufficient the flow slows enough that the shock degenerates into a
wave. The shock does not decelerate below a minimum velocity
determined by properties of the region.

Despite the turbulence generated behind the shock, the initial mass
loss from the clouds is weaker. Nevertheless, the shell is found to
regulate the cloud lifetimes such that all clouds are destroyed in
similar time. The one exception occurs when a
few high density clouds are distributed among lower density ones.

Secondly, 2D adiabatic magnetohydrodynamic simulations of a shock
interacting with groups of two or three cylindrical clouds are
presented.
We find (i) some clouds are stretched
along their field lines, whereas others are confined by their field
lines; (ii) upstream clouds may accelerate past downstream clouds;
(iii) clouds may also change their relative positions transverse to
the direction of shock propagation as they `slingshot' past each
other; (iv) downstream clouds may be offered some protection from the
oncoming flow as a result of being in the lee of an upstream cloud;
(v) the cycle of cloud compression and re-expansion is generally
weaker when there are nearby neighbouring clouds.

This small-scale study helps to interpret the behaviour of systems
with 100s of clouds. Infinitely wide regions can be interpreted via
interactions between individual clouds, but in regions of finite width
shocks driven from the sides of the region have different field-flow
orientations - individual clouds can experience evolving field
morphology.