On the role of initial and boundary conditions in numerical simulations of accretion flows

We study the effects of initial and boundary conditions, taking two-dimensional hydrodynamical numerical simulations of hot accretion flow as an example. The initial conditions considered include a rotating torus, a solution expanded from the one-dimensional global solution of hot accretion flows, injected gas with various angular momentum distributions, and the gas from a large-scale numerical simulation. Special attention is paid to the radial profiles of the mass accretion rate and density. Both can be described by a power-law function, Ṁ ∝ r^s and ρ ∝ r^-p. We find that if the angular momentum is not very low, the value of s is not sensitive to the initial condition and lies within a narrow range, 0.47 {less-than or approximate} s {less-than or approximate} 0.55. However, the value of p is more sensitive to the initial condition and lies in the range 0.48 {less-than or approximate} p {less-than or approximate} 0.8. The diversity of the density profile is because different initial conditions give different radial profiles of radial velocity due to the different angular momentum of the initial conditions. When the angular momentum of the accretion flow is very low, the inflow rate is constant with radius. Taking the torus model as an example, we have also investigated the effects of inner and outer boundary conditions by considering the widely adopted 'outflow' boundary condition and the 'mass flux conservation' condition. We find that the results are not sensitive to these two boundary conditions.