  Physical mechanism responsible for high viscosity in accretion disks is still
under debate. Parameterization of the viscous stress as $\alpha P$ proved to be
a successful representation of this mechanism in the outer parts of the disk,
explaining the dwarf novae and X-ray novae outbursts as due to ionization
instability. We show that this parameterization can be also adopted in the
innermost part of the disk where the adoption of the $\alpha$-viscosity law
implies the presence of the instability in the radiation pressure dominated
region. We study the time evolution of such disks. We show that the
time-dependent behavior of GRS 1915+105 can be well reproduced if
$\alpha$-viscosity disk model is calculated accurately (with proper numerical
coefficients in vertically averaged equations and with advection included), and
if the model is supplemented with (i) moderate corona dissipating 50% of energy
(ii) jet carrying luminosity-dependent fraction of energy. These necessary
modifications in the form of the presence of a corona and a jet are well
justified observationally. The model predicts outbursts at luminosity larger
than 0.16$\dot M_{Edd}$, as required, correct outburst timescales and
amplitudes, including the effect of increasing outburst timescale with mean
luminosity. This result strongly suggests that the $\alpha$-viscosity law is a
good description of the actual mechanism responsible for angular momentum
transfer also in the innermost, radiation pressure dominated part of the disk
around a black hole.
