The interaction of stellar ejecta and the circumstellar medium depends on the density structure of the expanding ejecta, the density structure of the circumstellar medium, and the physics of strong shock waves. Possibilities to consider include:

• Ejecta
  angle-averaged radial profile; inhomogeneities such as clouds and bubbles; macroscopic mixing of the heavy elements
• Circumstellar Medium
  radial profile: constant density, relic wind, circumstellar shells; inhomogeneities: clumpy winds, large clouds, global asymmetries
• Shock Physics
  radiative cooling of shock-heated gas; electron heating; particle acceleration; magnetic fields

We start with the simplest case of spherically-symmetric ejecta and CSM and an ideal, adiabatic gas. In this case a convenient model of young SNRs is provided by the self-similar driven wave solution, which assumes that the density profile of the ejecta is described by a powerlaw in radius with index n, and the csm has a powerlaw in radius with index s. The two examples below are for a constant density csm (s=0) and a relic progenitor wind (s=2).