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We investigated the instability of advective accretion flow as a consequenceof angular momentum transfer in one-dimensional, quasi-spherical transonicaccretion flow around a non-rotating black hole. The code is designed toinclude the effects of viscosity; the hydrodynamics component preserves angularmomentum strictly with Lagrangian and remap method in absence of viscosity,while the viscosity component updates viscous angular momentum transfer throughthe implicit method. We performed two tests to demonstrate the suitability ofthe code for accretion study. First, we simulated the inviscid, low angularmomentum, transonic accretion flow with shocks around a black hole, and thenthe subsonic, self-similar ADAF solution around a Newtonian object. Bothsimulations fitted the corresponding analytical curves extremely well. We thensimulated a rotating, viscous, transonic fluid with shocks. We showed that forlow viscosity parameter, stable shocks at larger distance are possible. Forhigher viscosity parameter, more efficient angular momentum transfer in thepost-shock disk makes the shock structure oscillatory. Moreover, as the shockdrifts to larger distances, a secondary inner shock develops. We showed thatthe inner shock is the direct consequence of expansion of the outer shock, aswell as creation of regions with $\partial l / \partial r < 0$ due to moreefficient angular momentum transfer near the inner sonic point. We showed thatall disk parameters, including emissivity, oscillate with the same period asthat of the shock oscillation. Our simulation may have implication for lowfrequency QPOs, e.g., GRO J1655-40 and XTE J1550-564.

Quasi-Spherical, Time-Dependent Viscous Accretion Flow: One-Dimensional Results