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In this paper, we present the time evolution of a rotationally axisymmetric gas ring around a non rotating black hole using two dimensional grid‐based hydrodynamics simulations. We show the way in which angular momentum transport is included in simulations of non‐selfgravitating accretion of matter towards a black hole. We use the Shakura‐Sunyaev  α  viscosity prescription to estimate the turbulent viscosity for all major viscous stress tensors. We investigate how a gas ring which is initially assumed to rotate with Keplerian angular velocity is accreted onto a black hole and hence forms an accretion disc in the presence of turbulent viscosity. We show that a centrifugally pressure supported sub‐Keplerian flow with shocks forms when the ring starts to disperse with inclusion of relatively small amount of viscosity. But, if the viscosity is above the critical value, the shock disappears altogether and the whole disc becomes Keplerian which is subsonic everywhere except in a region close to the horizon, where it supersonically enters to the black hole. We discovered a multiple valued Mach number solution and the corresponding density distributions that connect matter (a) from the initial Keplerian gas ring to a sub‐Keplerian disc with shocks in presence of small amount of viscosity and (b) from the sub‐Keplerian flow to a Keplerian disc in presence of huge amount of viscosity. We calculate the temporal variations of the magnitude of various time scales which ensure us about the stability of the flow. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effects of turbulent viscosity on a rotating gas ring around a black hole: Results of numerical simulations