Dissipative motion in galaxies with non-axisymmetric potentials
  Due to the clumpy nature of the self gravitating gas composing the interstellar medium, it is not clear whether galactic gas dynamics can be discussed in terms of standard hydrodynamics. Nevertheless, it is clear that certain generic properties related to orbital structure in a given potential and the effect of dissipation can be used to qualitatively understand gas motion in galaxies. The effect of dissipation is examined in triaxial galaxy potentials with and without rotating time dependent components. In the former case, dissipative trajectories settle around closed loop orbits when these exist. When they do not, e.g., inside a constant density core, then the only attractor is the centre and this leads to mass inflow. This provides a self regulating mechanism for accession of material towards the centre --- since the formation of a central masses destroys the central density core and eventually stops the accession. In the case when a rotating bar is present, there are usually several types of attractors, including those on which long lived chaotic motion can occur (strange attractors). Motion on these is erratic with large radial and vertical oscillations. 
Amr El-Zant