Overview of the ISM Phases: Evolution of Large and Small Scale Structures in 3D high resolution HD and MHD simulations

We review recent 3D high resolution numerical HD and MHD studies of a multi-component and multi-phase ISM. The computational grid was chosen large enough in the disk to represent typical ISM gas patterns, and in the halo to include the full extent of the Galactic fountain. The evolution of the ISM is driven by supernovae of all types and followed for 400 Myrs, long enough to get rid of memory effects of the initial distributions. The results are substantially different from previous classical analytical models and entail: (i) a very inhomogeneous distribution of ISM gas, (ii) a high level of supernova driven turbulence, (iii) an active Galactic fountain flow, reaching out to about 5 kpc on either side of the disk, (iv) a low volume filling factor for the hot gas of about 20%, (v) a very weak correlation between magnetic field and gas density, n, for n < 100 cm^-3, and, (vi) about 50% of the gas disk mass with n < 100 cm^-3, residing in classically thermally unstable temperature regimes.