# Generalized multiscale finite element method for the steady state linear Boltzmann equation

## TL;DR

This paper develops and analyzes a generalized multiscale finite element method for the steady state linear Boltzmann equation, effectively handling highly oscillatory media and small Knudsen numbers with spectral convergence.

## Contribution

The paper introduces a GMsFEM approach with proven well-posedness and spectral convergence, capable of homogenizing and preserving asymptotic limits across scales.

## Key findings

- Spectral convergence of the basis functions independent of media oscillation
- Method achieves numerical homogenization and asymptotic preservation
- Proven well-posedness of the GMsFEM for the Boltzmann equation

## Abstract

The Boltzmann equation, as a model equation in statistical mechanics, is used to describe the statistical behavior of a large number of particles driven by the same physics laws. Depending on the media and the particles to be modeled, the equation has slightly different forms. In this article, we investigate a model Boltzmann equation with highly oscillatory media in the small Knudsen number regime, and study the numerical behavior of the Generalized Multi-scale Finite Element Method (GMsFEM) in the fluid regime when high oscillation in the media presents. The Generalized Multi-scale Finite Element Method (GMsFEM) is a general approach to numerically treat equations with multi-scale structures. The method is divided into the offline and online steps. In the offline step, basis functions are prepared from a snapshot space via a well-designed generalized eigenvalue problem (GEP), and these basis functions are then utilized to patch up for a solution through DG formulation in the online step to incorporate specific boundary and source information. We prove the wellposedness of the method on the Boltzmann equation, and show that the GEP formulation provides a set of optimal basis functions that achieve spectral convergence. Such convergence is independent of the oscillation in the media, or the smallness of the Knudsen number, making it one of the few methods that simultaneously achieve numerical homogenization and asymptotic preserving properties across all scales of oscillations and the Knudsen number.