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 non-effective radiation cooling (ADAFs) can be both convectively stable or unstable depending on the value of the viscosity parameter \alpha. The high viscosity flows (\alpha~1) are stable and have a strong equatorial inflow and bipolar outflows. The low viscosity flows (\alpha<0.1) are convectively unstable and this induces quasi-periodic variability.