  We present the newly-incorporated gray radiation hydrodynamics capabilities
of the FLASH code based on a radiation flux-limiter aware hydrodynamics
numerical implementation designed specifically for applications in
astrophysical problems. The newly incorporated numerical methods consist of
changes in the unsplit hydrodynamics solver and adjustments in the flux-limited
radiation diffusion unit. Our method can treat problems in both the strong and
weak radiation-matter coupling limits as well as transitions between the two
regimes. Appropriate extensions in the "Helmholtz" equation of state are
implemented to treat two-temperature astrophysical plasmas involving the
interaction between radiation and matter and the addition of a new opacity unit
based on the OPAL opacity database, commonly used for astrophysical fluids. A
set of radiation-hydrodynamics test problems is presented aiming to showcase
the new capabilities of FLASH and to provide direct comparison to other codes
like CASTRO. To illustrate the capacity of FLASH to simulate phenomena
occurring in stellar explosions, such as shock break-out, radiative precursors
and supernova ejecta heating due to the decays of radioactive nickel-56 and
cobalt-56, we also present 1D supernova simulations and compare the computed
lightcurves to those of the SNEC code. The latest public release of FLASH with
these enhanced capabilities is available for download and use by the broader
astrophysics community.
