Change SMALL_NUMBER to 10^-4 in the HLLD-series Riemann solvers #457
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The previous value 10^-8 was too small and failed in some situations.
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Prerequisite checklist
Description
The SMALL_NUMBER constant was used to detect degeneracy conditions in the HLLD-series Riemann solvers (HLLD, LHLLD, and isothermal HLLD). The previous value 10^-8 was too small and failed in some situations with strong magnetic fields. This PR changes this number to 10^-4, which is more robust.
Note that the number in HLLD and LHLLD are used for comparing variables with the pressure dimension, while in the isothermal HLLD it is for comparing variables with the velocity dimension. Therefore, a different value may be more optimal for the isothermal HLLD solver.
Testing and validation
A slow-shock with perturbation. The top panel is the original implementation and the bottom is the new one. The unphysical explosions in the original are fixed with this change.