Comparison of the radial α(r) obtained for different isothermal simulations used in this paper. Values are averaged from 1000 to 1800 yr. All simulations except “full” are for one eighth of a complete disc. The specified resolution refers to the number of radial grid cells in the simulations, where “res1024” denotes the resolution of our fiducial model and “res512” has half the resolution in all spatial directions. In addition to the inviscid fiducial model we ran also simulation with non-zero viscosity, and the labels refer to the dimensionless value of the constant kinematic viscosity coefficient ν.