Rotating Accretion Flows: From Infinity to the Black Hole
  Accretion onto a supermassive black hole of a rotating inflow is a
particularly difficult problem to study because of the wide range of length
scales involved. There have been broadly utilized analytic and numerical
treatments of the global properties of accretion flows, but detailed numerical
simulations are required to address certain critical aspects. We use the ZEUS
code to run hydrodynamical simulations of rotating, axisymmetric accretion
flows with Bremsstrahlung cooling, considering solutions for which the
centrifugal balance radius significantly exceeds the Schwarzschild radius, with
and without viscous angular momentum transport. Infalling gas is followed from
well beyond the Bondi radius down to the vicinity of the black hole. We produce
a continuum of solutions with respect to the single parameter
Mdot_Bondi/Mdot_Edd, and there is a sharp transition between two general
classes of solutions at an Eddington ratio of Mdot_Bondi/Mdot_Edd ~ few x
10^(-2). Our high inflow solutions are very similar to the standard Shakura &
Sunyaev (1973) results. But our low inflow results are to zeroth order the
stationary Papaloizou and Pringle (1984) solution, which has no accretion. To
next order in the small, assumed viscosity they show circulation, with disk and
conical wind outflows almost balancing inflow. These solutions are
characterized by hot, vertically extended disks, and net accretion proceeds at
an extremely low rate, only of order alpha times the inflow rate. Our
simulations have converged with respect to spatial resolution and temporal
duration, and they do not depend strongly on our choice of boundary conditions.
