Numerical models of rotating accretion flows around black holes

Numerical, two-dimensional, time-dependent hydrodynamical models of geometrically thick accretion discs around black holes are presented. Accretion flows with inefficient radiation cooling (ADAFs) can be both convectively stable or unstable depending on the value of the viscosity parameter alpha. The high viscosity flows (alpha similar or equal to 1) are stable and have a strong equatorial inflow and bipolar outflows. The low viscosity flows (alpha less than or equal to 0.1) are convectively unstable and this induces quasi-periodic variability.