Provably Positive High-Order Schemes for Ideal Magnetohydrodynamics: Analysis on General Meshes

TL;DR

This paper develops high-order discontinuous Galerkin and finite volume methods for ideal MHD that provably preserve positivity of density and pressure on general meshes, addressing divergence errors and ensuring physical reliability.

Contribution

It introduces provably positivity-preserving high-order schemes for ideal MHD on general meshes, with divergence control techniques and rigorous analysis of the PP property.

Findings

Proved PP property for 1D DG and finite volume methods with HLL flux.

Constructed PP schemes for multidimensional MHD using divergence-controlling techniques.

Numerical tests confirm the effectiveness and positivity preservation of the proposed schemes.

Abstract

This paper proposes and analyzes arbitrarily high-order discontinuous Galerkin (DG) and finite volume methods which provably preserve the positivity of density and pressure for the ideal MHD on general meshes. Unified auxiliary theories are built for rigorously analyzing the positivity-preserving (PP) property of MHD schemes with a HLL type flux on polytopal meshes in any space dimension. The main challenges overcome here include establishing relation between the PP property and discrete divergence of magnetic field on general meshes, and estimating proper wave speeds in the HLL flux to ensure the PP property. In 1D case, we prove that the standard DG and finite volume methods with the proposed HLL flux are PP, under condition accessible by a PP limiter. For multidimensional conservative MHD system, standard DG methods with a PP limiter are not PP in general, due to the effect of unavoidable divergence-error. We construct provably PP high-order DG and finite volume schemes by proper discretization of symmetrizable MHD system, with two divergence-controlling techniques: locally divergence-free elements and a penalty term. The former leads to zero divergence within each cell, while the latter controls the divergence error across cell interfaces. Our analysis reveals that a coupling of them is important for positivity preservation, as they exactly contribute the discrete divergence-terms absent in standard DG schemes but crucial for ensuring the PP property. Numerical tests confirm the PP property and the effectiveness of proposed PP schemes. Unlike conservative MHD system, the exact smooth solutions of symmetrizable MHD system are proved to retain the positivity even if the divergence-free condition is not satisfied. Our analysis and findings further the understanding, at both discrete and continuous levels, of the relation between the PP property and the divergence-free constraint.