We carry out numerical simulation of accretion flows around a black hole in presence of both viscous and cooling effects. Instead of using a constant α parameter throughout the simulation grid, we assume that α is maximum on the equatorial plane and gradually goes down in the perpendicular direction We show that when the injected sub-Keplerian flow angular momentum is high enough and/or the viscosity and also cooling is high enough, Two Component Accretion Flow (TCAF) would be formed, otherwise the sub-Keplerian flow would remain sub-Keplerian. We see that a Keplerian disk is produced on the equatorial plane. Time variations of the total, Keplerian and Sub-Keplerian matter are studied with respect to various flow parameters.

We carry out a time‐dependent numerical simulation where both the hydrodynamics and the radiative transfer are coupled together. We consider a two‐component accretion flow in which the Keplerian disc is immersed inside an accreting low angular momentum flow (halo) around a black hole. The injected soft photons from the Keplerian disc are reprocessed by the electrons in the halo. We show that in presence of an axisymmetric soft‐photon source the spherically symmetric Bondi flow loses its symmetry and becomes axisymmetric. The low angular momentum flow was observed to slow down close to the axis and formed a centrifugal barrier which added new features into the spectrum. Using the Monte Carlo method, we generated the radiated spectra as functions of the accretion rates. We find that the transitions from a hard state to a soft state is determined by the mass accretion rates of the disc and the halo. We separate out the signature of the bulk motion Comptonization and discuss its significance. We study how the net spectrum is contributed by photons suffering different number of scatterings and spending different amounts of time inside the Compton cloud. We study the directional dependence of the emitted spectrum as well.

We study the accretion processes on a black hole by a numerical simulation. We use a grid‐based finite difference code for this purpose. We scan the parameter space spanned by the specific energy and the angular momentum and compare the time‐dependent solutions with those obtained from theoretical considerations. We found several important results. (a) The time‐dependent flow behaves close to a constant height model flow in the pre‐shock region and a flow with vertical equilibrium in the post‐shock region. (c) The infall time‐scale in the post‐shock region is several times higher than the free‐fall time‐scale. (b) There are two discontinuities in the flow, one being just outside of the inner sonic point. Turbulence plays a major role in determining the locations of these discontinuities. (d) The two discontinuities oscillate with two different frequencies and behave as a coupled harmonic oscillator. A Fourier analysis of the variation of the outer shock location indicates higher power at the lower frequency and lower power at the higher frequency. The opposite is true when the analysis of the inner shock is made. These behaviours will have implications in the spectral and timing properties of black hole candidates.

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A black hole accretion is necessarily transonic. In presence of sufficiently high viscosity and cooling effects, a low-angular momentum transonic flow can become a standard Keplerian disc except close to the where hole where it must pass through the inner sonic point. However, if the viscosity is not high everywhere and cooling is not efficient everywhere, the flow cannot completely become a Keplerian disc. In this paper, we show results of rigorous numerical simulations of a transonic flow having vertically varying viscosity parameter (being highest on the equatorial plane) and optical depth dependent cooling processes to show that the flow indeed segregates into two distinct components as it approaches a black hole. The component on the equatorial plane has properties of a standard Keplerian disc, though the flow is not truncated at the innermost stable circular orbit. This component extends till the horizon as a sub-Keplerian flow. This standard disc is found to be surrounded by a hot, low angular momentum component forming a centrifugal barrier dominated oscillating shock wave, consistent with the Chakrabarti-Titarchuk two component advective flow configuration.