  A module for the ZEUS-2D code is described which may be used to solve the
equations of radiation hydrodynamics to order unity in v/c, in the flux-limited
diffusion (FLD) approximation. In this approximation, the tensor Eddington
factor f which closes the radiation moment equations is chosen to be an
empirical function of radiation energy density. This is easier to implement and
faster than full-transport techniques, in which f is computed by solving the
transfer equation. However, FLD is less accurate when the flux has a component
perpendicular to the gradient in radiation energy density, and in optically
thin regions when the radiation field depends strongly on angle. The material
component of the fluid is here assumed to be in local thermodynamic
equilibrium. The energy equations are operator-split, with transport terms,
radiation diffusion term, and other source terms evolved separately. Transport
terms are applied using the same consistent transport algorithm as in ZEUS-2D.
The radiation diffusion term is updated using an alternating-direction implicit
method with convergence checking. Remaining source terms are advanced together
implicitly using numerical root-finding. However when absorption opacity is
zero, accuracy is improved by treating compression and expansion source terms
using time-centered differencing. Results are discussed for test problems
including radiation-damped linear waves, radiation fronts propagating in
optically-thin media, subcritical and supercritical radiating shocks, and an
optically-thick shock in which radiation dominates downstream pressure.
