Flux-Limited Diffusion In Hydrodynamics

The radiation transport equation, correct to order upsilon/c, is used to derive a flux limiter valid in high-velocity hydrodynamic flows. In the limit upsilon = 0 the widely used result of Levermore and Pomraning (LP) is recovered. In the case of homologous flow (upsilon is-proportional-to R) the LP result remains valid, but in the presence of shock waves and other nonhomologous flows differences may be large. We calculate the Eddington factors, which are significantly altered, and discuss the importance of having them possess the correct form in the presence of a shock wave, e.g., on the neutrino transport during the formation of a supernova.