High-resolution, two-dimensional hydrodynamical simulations with a large dynamic range are performed to study the turbulent nature of the interstellar medium (ISM) in galactic disks. The simulations are global, where the self-gravity of the ISM, realistic radiative cooling, and galactic rotation are taken into account. In the analysis undertaken here, feedback processes from the stellar energy source are omitted. We find that the velocity field of the disk in a nonlinear phase shows a steady power-law energy spectrum over 3 orders of magnitude in wavenumber. This implies that the random velocity field can be modeled as fully developed, stationary turbulence. Gravitational and thermal instabilities under the influence of galactic rotation contribute to the formation of the turbulent velocity field. The Toomre effective Q-value, in the nonlinear phase, covers a wide range, and gravitationally stable and unstable regions are distributed patchily in the disk. These results suggest that large-scale galactic rotation coupled with the self-gravity of the gas can be the ultimate energy sources that maintain the turbulence in the local ISM. Our models of turbulent rotating disks are consistent with the velocity dispersion of an extended H I disk in the dwarf galaxy NGC 2915, where there is no prominent active star formation. Numerical simulations show that the stellar bar in NGC 2915 enhances the velocity dispersion, and it also drives spiral arms, as observed in the H I disk.