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 \simeq 1 ) are stable and have a strong equatorial inflow and bipolar outflows .
The low viscosity flows ( \alpha \la 0.1 ) are convectively unstable and this induces quasi-periodic variability .