Effect of two different artificial-viscosity prescriptions on the flow structure. The constant-Pr flow (Pr_x, Pr_z = const.; left panel) is compared with the constant-Re flow (Pr_x, Pr_z ∝ e^{− 2z}; right panel). The vertical velocity component w is in both cases plotted on a split-logarithmic colour scale. The length of the velocity vectors (arrows) is scaled in a non-linear way to aid visualisation.