ZEUS-2D: A Radiation Magnetohydrodynamics Code for Astrophysical Flows in Two Space Dimensions. III. The Radiation Hydrodynamic Algorithms and Tests

The radiation hydrodynamical (RHD) algorithms in ZEUS-2D are described. These algorithms use a tensor variable Eddington factor, computed from a solution of the static transfer equation, to close the system of radiation moment equations. This full transport algorithm, which differs significantly from more commonly used methods based on the diffusion approximation, is applicable in both optically thin and thick media. Results computed by these algorithms on a new collection of RHD test problems are presented. For the transfer equation, very detailed tests are possible which compare the numerical solution for the specific intensity at every point on a given tangent plane to the known analytic solution. It is shown that, while the interpolation inherent in the method of short characteristics makes the solution diffusive, it is not overly so. For certain restricted geometries and physical conditions, these methods reproduce the expected results either identically or qualitatively.