The hydrodynamics of shock-cloud interactions in three dimensions

Following Stone & Norman, we present quantitative numerical studies of the interaction between a planar shock and small interstellar clouds with different shapes and orientations in three dimensions. We assume that the dynamical timescale of the interaction is much smaller than that of other physical processes such as radiative cooling, thermal conduction, and gravitational contraction. As a result, we neglect these effects. The problem is therefore specified by only three parameters: the Mach number of the shock M, the ratio of the density of the cloud to that of the ambient gas χ, and the adiabatic index γ. In this paper, we study strong shocks with M = 10 and overdense clouds with χ = 10, and we assume γ= 5/3. We consider the evolution of clouds using three different initial geometries: (1) a spherical cloud, (2) a prolate cloud with the long axis aligned perpendicular to the shock normal, and (3) a prolate cloud with the long axis inclined at 45° to the shock normal. We find that, as in previous two-dimensional simulations, in each case three-dimensional clouds are strongly fragmented in a few dynamical timescales, and the cloud material is strongly mixed with the ambient gas by complex turbulent motions. However, the geometry of the clouds does affect both the acceleration of the cloud and the mixing rate of the cloud with the ambient gas. Most importantly, different initial geometries can change the morphology of the clouds in late stages of their evolution substantially; this makes observational identification of shocked clouds in young supernova remnants based on morphology alone more difficult. In each case, very complex filamentary structures are observed in our simulations.