CONVERGENCE ANALYSIS OF GRAD'S HERMITE EXPANSION FOR LINEAR KINETIC EQUATIONS

In [Commun. Pure. Appl. Math., 2 (1949), pp. 331-407], Grad proposed a Hermite series expansion for approximating solutions to kinetic equations that have an unbounded velocity space. However, for initial boundary value problems, poorly imposed boundary conditions lead to instabilities in Grad's Hermite expansion, which could result in nonconverging solutions. For linear kinetic equations, a method for posing stable boundary conditions was recently proposed for (formally) arbitrary order Hermite approximations. In the present work, we study L-2-convergence of these stable Hermite approximations and prove explicit convergence rates under suitable regularity assumptions on the exact solution. We confirm the presented convergence rates through numerical experiments involving the linearized BGK equation of rarefied gas dynamics.