Time-dependent simplified PN approximation to the equations of radiative transfer

The steady-state simplified PN approximation to the radiative transport equation has been successfully applied to many problems involving radiation. This paper presents the derivation of time-dependent simplified PN (SPN) equations (up to N=3) via two different approaches. First, we use an asymptotic analysis, similar to the asymptotic derivation of the steady-state SPN equations. Second, we use an approach similar to the original derivation of the steady-state SPN equations and we show that both approaches lead to similar results. Special focus is put on the well-posedness of the equations and the question whether it can be guaranteed that the solution satisfies the correct physical bounds. Several numerical test cases are shown, including an analytical benchmark due to Su and Olson B. Su, G.L. Olson, An analytical benchmark for non-equilibrium radiative transfer in an isotropically scattering medium, Ann. Nucl. Energy 24 (1997) 1035-1055.].