Upstream and downstream H ii region radius as a function of time. Panel a) shows V16, b) shows V08, c) shows V06, and d) shows V04. The blue lines show the numerical results, and the red lines show an analytic approximation obtained by adding an advection term to the usual H ii region expansion equation (see text for details). The horizontal lines at 0.93 pc and 2.84 pc in panel d) show the observed upstream and downstream extent of the H ii region RCW 120.