Counter-Rotating Accretion Discs
  Counter-rotating discs can arise from the accretion of a counter-rotating gas
cloud onto the surface of an existing co-rotating disc or from the
counter-rotating gas moving radially inward to the outer edge of an existing
disc. At the interface, the two components mix to produce gas or plasma with
zero net angular momentum which tends to free-fall towards the disc center. We
discuss high-resolution axisymmetric hydrodynamic simulations of a viscous
counter-rotating disc for cases where the two components are vertically
separated and radially separated. The viscosity is described by an isotropic
$\alpha-$viscosity including all terms in the viscous stress tensor. For the
vertically separated components a shear layer forms between them. The middle of
this layer free-falls to the disk center. The accretion rates are increased by
factors $\sim 10^2-10^4$ over that of a conventional disc rotating in one
direction with the same viscosity. The vertical width of the shear layer and
the accretion rate are strongly dependent on the viscosity and the mass
fraction of the counter-rotating gas. In the case of radially separated
components where the inner disc co-rotates and the outer disc rotates in the
opposite direction, a gap between the two components opens and closes
quasi-periodically. The accretion rates are $\gtrsim 25$ times larger than
those for a disc rotating in one direction with the same viscosity.
