Asymptotic equilibrium diffusion analysis of time-dependent Monte Carlo methods for grey radiative transfer

The equations of nonlinear, time-dependent radiative transfer are known to yield the equilibrium diffusion equation as the leading-order solution of an asymptotic analysis when the mean-free path and mean-free time of a photon become small. We apply this same analysis to the Fleck–Cummings, Carter–Forest, and Nʹkaoua Monte Carlo approximations for grey (frequency-independent) radiative transfer. Although Monte Carlo simulation usually does not require the discretizations found in deterministic transport techniques, Monte Carlo methods for radiative transfer require a time discretization due to the nonlinearities of the problem. If an asymptotic analysis of the equations used by a particular Monte Carlo method yields an accurate time-discretized version of the equilibrium diffusion equation, the method should generate accurate solutions if a time discretization is chosen that resolves temperature changes, even if the time steps are much larger than the mean-free time of a photon. This analysis is of interest because in many radiative transfer problems, it is a practical necessity to use time steps that are large compared to a mean-free time. Our asymptotic analysis shows that: (i) the Nʹkaoua method has the equilibrium diffusion limit, (ii) the Carter–Forest method has the equilibrium diffusion limit if the material temperature change during a time step is small, and (iii) the Fleck–Cummings method does not have the equilibrium diffusion limit. We include numerical results that verify our theoretical predictions.