Viscous spreading ring test from Speith & Kley (2003) on a logarithmic grid N_r × N_ϕ= 512 × 256. The ring is initialized with Σ(x, τ = 0.016) (black line) and then evolved to τ = 439.82. The vertical dashed cyan line marks the position of the initial ring at time τ = 0. The blue line is the azimuthal averaged surface density, which matches the analytical formula (red line) well. The viscous instability can be seen as waves in a density slice along the x axis (magenta line).