Beltrami structures in disk-jet system: alignment of flow and   generalized vorticity
  The combination of a thin disk and a narrowly-collimated jet is a typical structure that is observed in the vicinity of a massive object such as AGN, black hole or YSO. Despite a large variety of their scales and possible diversity of involved processes, a simple and universal principle dictates the geometric similarity of the structure; we show that the singularity at the origin ($r=0$) of the Keplerian rotation ($V_\theta \propto r^{-1/2}$) is the determinant of the structure. The collimation of jet is the consequence of the alignment ---so-called Beltrami condition--- of the flow velocity and the "generalized vorticity" that appears as an axle penetrating the disk (the vorticity is generalized to combine with magnetic field as well as to subtract the friction force causing the accretion). Typical distributions of the density and flow velocity are delineated by a similarity solution of the simplified version of the model. 
Z. Yoshida and N.L. Shatashvili
astro-ph.GA physics.flu-dyn physics.plasm-ph