Thermochemically consistent backflow treatment for FGM

[joshkellyjak]

Proposed Changes

Added thermochemically consistent backflow treatment for FGM calculations to stabilise early convergence in the presence of reversed-velocity at the outlet. In the flow solver, reversed-velocity ghost states at outlet faces are corrected by reflecting the normal velocity component. In the flamelet scalar solver, a new BC_Outlet override blends the enthalpy ghost state between the interior value and a thermodynamically consistent inlet enthalpy, computed via Newton iteration on the FGM LUT, while clamping the progress variable to it's unburnt value and preventing burned scalars from re-entering the domain and pushing FGM lookups outside manifold bounds. The backflow prevention deactivates automatically after a user-specified number of outer iterations, allowing physically real backflow to develop once the solution has stabilised.

Related Work

N/A

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.