Quasi-Spherical, Time-Dependent Viscous Accretion Flow: One-Dimensional
  Results
  We investigated the instability of advective accretion flow as a consequence
of angular momentum transfer in one-dimensional, quasi-spherical transonic
accretion flow around a non-rotating black hole. The code is designed to
include the effects of viscosity; the hydrodynamics component preserves angular
momentum strictly with Lagrangian and remap method in absence of viscosity,
while the viscosity component updates viscous angular momentum transfer through
the implicit method. We performed two tests to demonstrate the suitability of
the code for accretion study. First, we simulated the inviscid, low angular
momentum, transonic accretion flow with shocks around a black hole, and then
the subsonic, self-similar ADAF solution around a Newtonian object. Both
simulations fitted the corresponding analytical curves extremely well. We then
simulated a rotating, viscous, transonic fluid with shocks. We showed that for
low viscosity parameter, stable shocks at larger distance are possible. For
higher viscosity parameter, more efficient angular momentum transfer in the
post-shock disk makes the shock structure oscillatory. Moreover, as the shock
drifts to larger distances, a secondary inner shock develops. We showed that
the inner shock is the direct consequence of expansion of the outer shock, as
well as creation of regions with $\partial l / \partial r < 0$ due to more
efficient angular momentum transfer near the inner sonic point. We showed that
all disk parameters, including emissivity, oscillate with the same period as
that of the shock oscillation. Our simulation may have implication for low
frequency QPOs, e.g., GRO J1655-40 and XTE J1550-564.
