  The equilibrium state of a turbulent clumpy gas disk is analytically
investigated. The disk consists of distinct self-gravitating clouds.
Gravitational cloud-cloud interactions transfer energy over spatial scales and
produce a viscosity, which allows mass accretion in the gas disk. Turbulence is
assumed to be generated by instabilities involving self-gravitation and to be
maintained by the energy input from differential rotation and mass transfer.
Disk parameters, global filling factors, molecular fractions, and star
formation rates are derived. The application of our model to the Galaxy shows
good agreement with observations. They are consistent with the scenario where
turbulence generated and maintained by gravitation can account for the
viscosity in the gas disk of spiral galaxies. The role of the galaxy mass for
the morphological classification of spiral galaxies is investigated.
