  Observations of turbulent velocity dispersions in the HI component of
galactic disks show a characteristic floor in galaxies with low star formation
rates and within individual galaxies the dispersion profiles decline with
radius. We carry out several high resolution adaptive mesh simulations of
gaseous disks embedded within dark matter haloes to explore the roles of
cooling, star-formation, feedback, shearing motions and baryon fraction in
driving turbulent motions. In all simulations the disk slowly cools until
gravitational and thermal instabilities give rise to a multi-phase medium in
which a large population of dense self-gravitating cold clouds are embedded
within a warm gaseous phase that forms through shock heating. The diffuse gas
is highly turbulent and is an outcome of large scale driving of global
non-axisymmetric modes as well as cloud-cloud tidal interactions and merging.
At low star-formation rates these processes alone can explain the observed HI
velocity dispersion profiles and the characteristic value of ~10 km/s observed
within a wide range of disk galaxies. Supernovae feedback creates a significant
hot gaseous phase and is an important driver of turbulence in galaxies with a
star-formation rate per unit area >10^-3 M_sun/yr/kpc^2.
