Stable Boundary Conditions and Discretization for $P_N$ Equations

Stable Boundary Conditions and Discretization for $P_N$ Equations

Year:    2022

Author:    Jonas Bünger, Neeraj Sarna, Manuel Torrilhon

Journal of Computational Mathematics, Vol. 40 (2022), Iss. 6 : pp. 977–1003

Abstract

A solution to the linear Boltzmann equation satisfies an energy bound, which reflects a natural fact: The energy of particles in a finite volume is bounded in time by the energy of particles initially occupying the volume augmented by the energy transported into the volume by particles entering the volume over time. In this paper, we present boundary conditions (BCs) for the spherical harmonic $(P_N)$ approximation, which ensure that this fundamental energy bound is satisfied by the $P_N$ approximation. Our BCs are compatible with the characteristic waves of $P_N$ equations and determine the incoming waves uniquely. Both, energy bound and compatibility, are shown on abstract formulations of $P_N$ equations and BCs to isolate the necessary structures and properties. The BCs are derived from a Marshak type formulation of BC and base on a non-classical even/odd-classification of spherical harmonic functions and a stabilization step, which is similar to the truncation of the series expansion in the $P_N$ method. We show that summation by parts (SBP) finite difference on staggered grids in space and the method of simultaneous approximation terms (SAT) allows to maintain the energy bound also on the semi-discrete level.

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Journal Article Details

Publisher Name:    Global Science Press

Language:    English

DOI:    https://doi.org/10.4208/jcm.2104-m2019-0231

Journal of Computational Mathematics, Vol. 40 (2022), Iss. 6 : pp. 977–1003

Published online:    2022-01

AMS Subject Headings:   

Copyright:    COPYRIGHT: © Global Science Press

Pages:    27

Keywords:    Boundary conditions Energy stability Spherical harmonic ($P_N$) approximation Kinetic theory Moment method Boltzmann Linear transport.

Author Details

Jonas Bünger

Neeraj Sarna

Manuel Torrilhon

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