LUSGS viscous spectral contribution

After the inviscid flux Jacobian increment, the viscous spectral radius damps the neighbor delta:

\[r \leftarrow r - \lambda_{\mathrm{vis}}\,\Delta q\]

Model 1 stores a per-volume radius \(\lambda_{\mathrm{vis}}=\lambda_{\mathrm{face}}/V\). Model 2 uses the face radius directly.

Face density for the viscous radius models is the arithmetic mean of the left/right states.

References

• Yoon–Jameson LUSGS; viscous spectral radius practice.

• See lusgs_flux_increment.rst and verification_layers.rst.

Pure APIs: LusgsViscousSpectrumContribution, LusgsFaceArithmeticMean, LusgsViscousRadiusPerVolume. Wiring: UNsLusgs::CalcViscousTerm.

Intermediate verification

Closed-form checks of \(\Delta r = -\lambda_{\mathrm{vis}}\Delta q\) live in lusgs_viscous_spectrum_test with theory table tools/verification/data/lusgs_viscous_spectrum_contrib_theory.csv (reference \(\lambda_{\mathrm{vis}}=2\)).