Stability issues of entropy-stable and/or split-form high-order schemes

TL;DR

This paper investigates the local energy stability of high-order entropy-stable schemes, revealing that many such methods, including split-form SBP and flux-based schemes, can be inherently unstable and cause error growth in simulations.

Contribution

It identifies the inherent local energy instability of entropy-conserving fluxes in high-order schemes and demonstrates their adverse effects on nonlinear conservation law simulations.

Findings

Entropy-conserving fluxes can be dissipative or anti-dissipative.

Numerical error growth observed in Burgers and Euler equations.

Common split-forms are also locally energy unstable.

Abstract

The focus of the present research is on the analysis of local energy stability of high-order (including split-form) summation-by-parts methods, with e.g. two-point entropy-conserving fluxes, approximating non-linear conservation laws. Our main finding is that local energy stability, i.e., the numerical growth rate does not exceed the growth rate of the continuous problem, is not guaranteed even when the scheme is non-linearly stable and that this may have adverse implications for simulation results. We show that entropy-conserving two-point fluxes are inherently locally energy unstable, as they can be dissipative or anti-dissipative. Unfortunately, these fluxes are at the core of many commonly used high-order entropy-stable extensions, including split-form summation-by-parts discontinuous Galerkin spectral element methods (or spectral collocation methods). For the non-linear Burgers equation, we further demonstrate numerically that such schemes cause exponential growth of errors during the simulation. Furthermore, we encounter a similar abnormal behaviour for the compressible Euler equations, for a smooth exact solution of a density wave. Finally, for the same case, we demonstrate numerically that other commonly known split-forms, such as the Kennedy and Gruber splitting, are also locally energy unstable.