Effects of Turbulent Viscosity on A Rotating Gas Ring Around A Black Hole: Results in Numerical Simulation

TL;DR

This study uses numerical simulations to explore how turbulent viscosity affects the evolution and structure of gas rings around black holes, revealing conditions for shock formation and transitions between Keplerian and sub-Keplerian flows.

Contribution

It demonstrates the impact of turbulent viscosity on accretion flow structures and shock formation around black holes using detailed hydrodynamic simulations.

Findings

Small viscosity leads to shock-forming sub-Keplerian flows.

High viscosity results in a smooth, Keplerian disc without shocks.

Multiple Mach number solutions connect different flow regimes.

Abstract

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 hydrodynamic simulation. We show the way in which angular momentum transport is included in simulations of non-self-gravitating accretion of matter towards a black hole. We use the Shakura-Sunyaev {\alpha} 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 on to a black hole and hence forms accretion disc in the presence of turbulent viscosity. We show that the centrifugal 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 Kepleiran 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 connects 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.