Propagation of the shock through a low-density cavity bound by a high-density shell. The density is in units of m_p cm⁻³. The dashed line shows the initial density profile, while the blue line shows the density profile evolution. When the forward shock moves through the low-density gas, the reverse shock moves outward. After the collision with the dense shell, the reverse shock propagates deep into the ejecta. These simulations are non-radiative, which is a reasonable approximation for the ejecta and the cavity. The forward shock in the dense gas is radiative; that is, its structure is not correctly reproduced by these simulations. A jump at 20 pc in the upper panel is the reflected reverse shock that appears in a spherically symmetric 1D model.