(a) Time evolution of the normalized maximal deviation from the azimuthally averaged surface density distribution for various numerical configurations. Inadequate resolution in the R-direction can damp the viscous instability, with convergence being achieved for our fiducial model with N_R = 465. (b) Comparison of the same quantity for different azimuthal resolutions and N_R = 465. The higher resolution of N_ϕ = 1300 azimuthal cells results in square grid cells but does not influence the growth of the instability.