Strategies to cure numerical shock instability in the HLLEM Riemann solver.

Summary: The HLLEM scheme is a popular contact and shear preserving approximate Riemann solver that is known to be plagued by various forms of numerical shock instability. In this paper, we clarify that the shock instability exhibited by this scheme is primarily triggered by the spurious activation of the antidiffusive terms present in the first and third Riemann flux components on the transverse interfaces adjoining the shock front due to numerical perturbations. These erroneously activated terms are shown to counteract the favorable damping mechanism provided by its inherent HLL‐type diffusive terms, causing an unphysical variation of the conserved quantity ρu both along and across the numerical shock. To prevent this, two distinct strategies are proposed termed as Selective Wave Modification and Anti Diffusion Control. The former focuses on enhancing the quantity of the favorable HLL‐type dissipation available on these critical flux components by carefully increasing the magnitudes of certain nonlinear wave speed estimates, while the latter focuses on directly controlling the magnitude of these critical antidiffusive terms. A linear perturbation analysis is performed to gauge the effectiveness of these cures and to estimate a von Neumann–type stability bounds on the CFL number associated with their use. Results from a variety of classic shock instability test cases show that the proposed strategies are able to provide excellent shock stable solutions even on grids that are highly elongated across the shock front without compromising the accuracy on inviscid contact or shear dominated viscous flows. Numerical shock instability in the HLLEM scheme is found to be triggered by the erroneous activation of certain antidiffusive terms, within the numerical shock region, which counteract the stabilizing action of the inherent HLL‐type diffusive terms. Two distinct strategies named 'Selective Wave Modification' and 'Anti Diffusion Control' are proposed to avert the instability. The schemes resulting from these, are found to be robust on a wide variety of problems involving a wide range of grid aspect ratios. [ABSTRACT FROM AUTHOR]