(a) Maximum shell density as a function of time for 1D simulations of D-type expansion of an H ii region, run with differing spatial resolutions from 32 cells to 8192 cells, as indicated in the legend. (b) As above, but with 2D results overplotted in red, for nested grids of 32² cells with 1, 2, and 3 levels, as indicated in the legend.