Radiation hydrodynamics (RHD) is
essentially "normal" hydrodynamics plus the
effects of radiative transfer. When modeling a
physical entity (e.g., a star or cloud), internal electromagnetic radiation (EMR) may
or may not significantly affect the result, presenting the question
of whether to include the complication of radiative transfer.
The momentum of photons (radiation pressure) is one factor.
Much hydrodynamic code developed for astrophysics
does also include such effects of EMR.
Note that RHD is also sometimes used for
relativistic hydrodynamics,
i.e., hydrodynamics including the effects of relativity.