Positivity enhancements to the Pn transport equations

Abstract

One of the common methods for solving the radiation transport equation is to use a polynomial expansion for the angle variable(s). While each solution technique has its disadvantages, in some instances this method can result in oscillations large enough to give a negative radiation energy density, which is nonphysical. Some authors have recast the closure of the transport equations to guarantee positivity. Unfortunately, this results in messy nonlinear equations. Other authors post process each time step’s solution to improve positivity. In this work, the Pn equations are modified to be more diffusive, less oscillatory, more positive, and more accurate. No new solution techniques are required, just the scaling of some terms with constant factors and the addition of constant filter opacities; therefore, the solutions are still linear. This new solution method is called stretched and filtered Pn. Test problems are presented in one and two dimensions. Comparisons to the Dn and TPn closures are made in one dimension. Modifications to Dn and TPn are suggested.