DYNAMIC ARTIFICIAL OPACITY FOR FLUX-LIMITED DIFFUSION IN HYDRODYNAMICS

We propose a simple way for deriving a flux limiter valid in high velo city hydrodynamic flows. Following the work of Cooperstein & Baron (19 92) (CB) we use for the derivation the radiative transport equation va lid to first order in v/c. CB used 2 parameters to represent the angul ar distribution function: the Levermore-Pomraning (LP) parameter valid for stationary matter situations, and a seperate, additional paramete r representing the hydrodynamical effects. We, instead, use the genera lized treatment developed by Janka (1990, 1991a, 1991b) for the statio nary matter case, and modify the LP parameter with a term 'dynamic art ificial opacity' which acts like a non physical opacity. This 'dynamic opacity' represents the effects of relative velocities within the sta r on the radiation angular distribution function. The 'dynamic artific ial opacity' term allows to eliminate the error in the momentum balanc e between the moving matter and the radiation associated with the flux limited diffusion schemes.