  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.
