Numerical simulations of shocks encountering clumpy regions

We present numerical simulations of the adiabatic interaction of a shock with a clumpy region containing many individual clouds. Our work incorporates a sub-grid turbulence model which for the first time makes this investigation feasible. We vary the Mach number of the shock, the density contrast of the clouds, and the ratio of total cloud mass to inter-cloud mass within the clumpy region. Cloud material becomes incorporated into the flow. This "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 interaction evolves through up to four stages: initially the shock decelerates; then its speed is nearly constant; next the shock accelerates as it leaves the clumpy region; finally it moves at a constant speed close to its initial speed. Turbulence is generated in the post-shock flow as the shock sweeps through the clumpy region. Clouds exposed to turbulence can be destroyed more rapidly than a similar cloud in an "isolated" environment. The lifetime of a downstream cloud decreases with increasing cloud-to-intercloud mass ratio. We briefly discuss the significance of these results for starburst superwinds and galaxy evolution.Comment: 17 pages, 19 figures, accepted for publication in MNRA

Numerical simulations of a shock interacting with multiple magnetized clouds

We present 2D adiabatic magnetohydrodynamic simulations of a shock interacting with groups of two or three cylindrical clouds. We study how the presence of a nearby cloud influences the dynamics of this interaction, and explore the resulting differences and similarities in the evolution of each cloud. The understanding gained from this small-scale study will help to interpret the behaviour of systems with many 10s or 100s of clouds.We observe a wide variety of behaviour in the interactions studied, which is dependent on the initial positions of the clouds and the orientation and strength of the magnetic field. 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 (though magnetic tension can prevent this); (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; (vi) the plasma β in cloud material can vary rapidly as clouds collide with one another, but low values of β are always transitory. This work is relevant to studies of multiphase regions, where fast, low-density gas interacts with dense clouds, such as in circumstellar bubbles, supernova remnants, superbubbles and galactic winds