  We investigate different accretion disk models and viscosity prescriptions in
order to provide a basic explanation for the exotic temporal behavior in GRS
1915+105. Based on the fact that the overall cycle times are very much longer
than the rise/fall time scales in GRS 1915, we rule out the geometry of ADAF or
a hot quasi-spherical region plus a cold outer disk for this source. We thus
concentrate on geometrically thin Shakura-Sunyaev type disks (Shakura & Sunyaev
1973; hereafter SS73). We have devised a modified viscosity law that has a
quasi-stable upper branch. Via numerical simulations, we show that the model
does account for several gross observational features of GRS 1915+105. On the
other hand, the rise/fall time scales are not short enough, and no rapid
oscillations on time scales $\simlt$ 10 s emerge naturally from the model.
  We then consider and numerically test a more elaborate model that includes
the cold disk, a corona, and plasma ejections from the inner disk region and
show that this model allows us to reproduce several additional observed
features of GRS 1915+105. We conclude that the most likely structure of the
accretion flow in this source is that of a cold disk with a modified viscosity
prescription, plus a corona that accounts for much of the X-ray emission, and
unsteady plasma ejections that occur when the luminosity of the source is high.
