Positivity-Preserving Analysis of Numerical Schemes for Ideal Magnetohydrodynamics

TL;DR

This paper provides the first rigorous positivity-preserving analysis for conservative schemes in ideal MHD, revealing a crucial connection between positivity and a discrete divergence-free condition, with practical implications for high-order numerical methods.

Contribution

It introduces a novel theoretical link between positivity preservation and a discrete divergence-free condition in ideal MHD schemes, supported by rigorous analysis and numerical validation.

Findings

PP property proven for 1D schemes with viscosity

DDF condition is necessary for 2D positivity preservation

Numerical examples confirm theoretical results

Abstract

Numerical schemes provably preserving the positivity of density and pressure are highly desirable for MHD, but the rigorous positivity-preserving (PP) analysis remains challenging. The difficulties mainly arise from the intrinsic complexity of the MHD equations as well as the indeterminate relation between the PP property and the divergence-free condition on magnetic field. We present the first rigorous PP analysis of conservative schemes with Lax-Friedrichs (LF) flux for ideal MHD. The significant innovation is the discovery of theoretical connection between PP property and a discrete divergence-free (DDF) condition. This connection is established through the generalized LF splitting properties, which are alternatives of the usually-expected LF splitting property that does not hold for ideal MHD. The generalized LF splitting properties involve a number of admissible states strongly coupled by DDF condition, making their derivation very difficult. We derive these properties via a novel equivalent form of the admissible state set and an important inequality skillfully constructed by technical estimates. Rigorous PP analysis is presented for finite volume and discontinuous Galerkin schemes with LF flux on uniform Cartesian meshes. In 1D case, PP property is proved for the first-order scheme with proper numerical viscosity, and also for arbitrarily high-order schemes under conditions accessible by a PP limiter. In 2D case, we show that the DDF condition is necessary and crucial for achieving PP property. It is observed that even slightly violating the proposed DDF condition may cause failure to preserve the positivity of pressure. We prove that the 2D LF type scheme with proper numerical viscosity preserves the positivity and DDF condition. Sufficient conditions are derived for 2D PP high-order schemes, and extension to 3D is discussed. Numerical examples confirm the theoretical findings.