  Accretion flows around black holes generally result in mass-outflows that
exhibit irregular behavior quite often. Using 2D time-dependent hydrodynamical
calculations, we show that the mass-outflow is unstable in the cases of thick
accretion flows such as the low angular momentum accretion flow and the
advection-dominated accretion flow. For the low angular momentum flow, the
inward accreting matter on the equatorial plane interacts with the outflowing
gas along the rotational axis and the centrifugally supported oblique shock is
formed at the interface of both the flows, when the viscosity parameter
$\alpha$ is as small as $\alpha \le 10^{-3}$. The hot and rarefied blobs, which
result in the eruptive mass-outflow, are generated in the inner shocked region
and grow up toward the outer boundary. The advection-dominated accretion flow
attains finally in the form of a torus disc with the inner edge of the disc at
$3R_{\rm g} \le r \le 6R_{\rm g}$ and the center at $6R_ {\rm g} \le r \le
10R_{\rm g}$, and a series of hot blobs is intermittently formed near the inner
edge of the torus and grows up along the outer surface of the torus. As a
result, the luminosity and the mass-outflow rate are modulated irregularly
where the luminosity is enhanced by 10-40% and the mass-outflow rate is
increaed by a factor of few up to ten. We interpret the unstable nature of the
outflow to be due to the Kelvin-Helmholtz instability, examining the Richardson
number for the Kelvin-Helmholtz criterion in the inner region of the flow. We
propose that the flare phenomena of Sgr A* may be induced by the unstable
mass-outflow as is found in this work.
