Question: how fast does an isothermal viscous flow advance down a slope, and does the solver agree with v = rho g h^2 sin(theta) / 3 eta? A constant flux per unit width is fed onto a plane; the prediction uses only the inputs.
Press the button: slopes 3 to 30 degrees at 10^4 Pa s, and viscosities 10^2 to 10^6 Pa s at 10 degrees.
Front speed against viscosity (log-log): lines are Jeffreys' law, dots are the solver.