Approximating Continuous Spectra of Hyperbolic Systems with Summation-by-Parts Finite Difference Operators

In this work we explore the fidelity of numerical approximations to continuous spectra of hyperbolic partial differential equation systems. We are particularly interested in the ability of discrete methods to accurately discover sources of physical instabilities. By focusing on the perturbed equations that arise in linearized problems, we apply high-order accurate summation-by-parts finite difference operators, with weak enforcement of boundary conditions through the simulataneous-approximation-term technique, which leads to a provably stable numerical discretization with formal order of accuracy given by $p = 2, 3, 4$ and $5$. We derive analytic solutions using Laplace transform methods, which provide important ground truth for ensuring numerical convergence at the correct theoretical rate. We find that the continuous spectrum is better captured with mesh refinement, although dissipative strict stability (where the growth rate of the discrete problem is bounded above by the continuous) is not obtained. However, we also find that sole reliance on mesh refinement can be a problematic means for determining physical growth rates as some eigenvalues emerge (and persist with mesh refinement) based on spatial order of accuracy but are non-physical. We suggest that numerical methods be used to approximate discrete spectra when numerical stability is guaranteed and convergence of the discrete spectra is evident with both mesh refinement and increasing order of accuracy.