A Novel Averaging Technique for Discrete Entropy-Stable Dissipation Operators for Ideal MHD

TL;DR

This paper introduces a new averaging method for constructing entropy stable dissipation operators in ideal MHD, improving robustness and addressing limitations of existing flux formulations in astrophysical simulations.

Contribution

A novel averaging technique for entropy Jacobians enhances the robustness of entropy stable schemes in ideal MHD.

Findings

Existing entropy conserving fluxes can fail under certain initial conditions.

Specialized dissipation matrix construction is crucial for stability.

The new averaging method improves robustness in numerical simulations.

Abstract

Entropy stable schemes can be constructed with a specific choice of the numerical flux function. First, an entropy conserving flux is constructed. Secondly, an entropy stable dissipation term is added to this flux to guarantee dissipation of the discrete entropy. Present works in the field of entropy stable numerical schemes are concerned with thorough derivations of entropy conservative fluxes for ideal MHD. However, as we show in this work, if the dissipation operator is not constructed in a very specific way, it cannot lead to a generally stable numerical scheme. The two main findings presented in this paper are that the entropy conserving flux of Ismail & Roe can easily break down for certain initial conditions commonly found in astrophysical simulations, and that special care must be taken in the derivation of a discrete dissipation matrix for an entropy stable numerical scheme to be robust. We present a convenient novel averaging procedure to evaluate the entropy Jacobians of the ideal MHD and the compressible Euler equations that yields a discretization with favorable robustness properties.