Viscosity Prescriptions in Accretion Disks with Shock Waves
  We study the evolution of viscous, isothermal, rotating, thin, axisymmetric accretion disks around a compact object using Smoothed Particle Hydrodynamics. We emphasize the effects of different choices of viscosity prescriptions on the evolution of angular momentum as well as other physical quantities of the disk. We show that a flow with the Shakura-Sunyaev viscosity prescription may produce only {\it shear} shock, where the angular momentum changes significantly across the shock. We present and study the effects of other prescriptions of viscosity which render the angular momentum continuous across the shock waves. In general, it is observed that for flows with a small viscosity, the shocks are weaker, form farther away and are wider as compared to the shocks in inviscid flows. If the viscosity is high, shocks do not form at all. The flow remains subsonic and Keplerian throughout the disk and becomes supersonic only very close to the horizon. 
Sandip K. Chakrabarti and Diego Molteni