  The non-linear regime of Rayleigh-Taylor instability (RTI) in a radiation
supported atmosphere, consisting of two uniform fluids with different
densities, is studied numerically. We perform simulations using our recently
developed numerical algorithm for multi-dimensional radiation hydrodynamics
based on a variable Eddington tensor as implemented in Athena, focusing on the
regime where scattering opacity greatly exceeds absorption opacity. We find
that the radiation field can reduce the growth and mixing rate of RTI, but this
reduction is only significant when radiation pressure significantly exceeds gas
pressure. Small scale structures are also suppressed in this case. In the
non-linear regime, dense fingers sink faster than rarefied bubbles can rise,
leading to asymmetric structures about the interface. By comparing the
calculations that use a variable Eddington tensor (VET) versus the Eddington
approximation, we demonstrate that anisotropy in the radiation field can affect
the non-linear development of RTI significantly. We also examine the disruption
of a shell of cold gas being accelerated by strong radiation pressure,
motivated by models of radiation driven outflows in ultraluminous infrared
galaxies. We find that when the growth rate of RTI is smaller than acceleration
time scale, the amount of gas that would be pushed away by the radiation field
is reduced due to RTI.
