IGR integration

[StevenXue98]

Description

This PR integrates IGR with more of the shared RHS/Riemann infrastructure while preserving the existing IGR numerical behavior. This is an intermediate integration milestone, not the final IGR architecture. It intentionally preserves the existing IGR reconstruction/Sigma/viscous-source behavior while moving RHS accumulation, RK update behavior, state layout, and LF Riemann dispatch toward shared MFC infrastructure.

Main changes:

• Store the final volume fraction for IGR so the IGR state layout matches the normal model-equations layout more closely.
• Route IGR face states through the shared s_riemann_solver path using the LF Riemann solver.
• Use shared RHS accumulation helpers for IGR:
  • s_compute_advection_source_term
  • s_compute_additional_physics_rhs
• Split the old fused s_igr_riemann_solver behavior into smaller IGR-specific routines called from the shared RHS path:
  • s_igr_reconstruct_cell_boundary_values
  • s_igr_correct_lf_fluxes
  • s_igr_sigma
• Preserve legacy IGR behavior where needed:
  • skip the normal LF MPP limiter for IGR, since it was not part of the old IGR path
  • skip normal LF viscous-source construction for IGR, since IGR now fills those source-flux slots itself
• Remove the special padded rhs_vf allocation previously needed by IGR-specific direct RHS updates.

IGR-specific reconstruction, Sigma correction, and viscous source-flux construction are still present in m_igr.fpp. Subroutine s_igr_riemann_solver is also still present in m_igr.fpp, which explains the increased line count.

Deferred follow-up work:

• replace IGR-specific face reconstruction with the standard reconstruction path
• move IGR viscous source-flux construction onto the normal viscous pipeline
• revisit compatibility with MHD and other currently prohibited physics

Type of change

• Refactor

Testing

Local testing:

./mfc.sh test --add-new-variables --only IGR
./mfc.sh test --add-new-variables --from F926448C --to F926448C
./mfc.sh test --add-new-variables --from 18A71AB2 --to 18A71AB2
./mfc.sh test --add-new-variables --from 43B5FEBD --to 43B5FEBD
./mfc.sh test --add-new-variables --from E49EF7B6 --to E49EF7B6
./mfc.sh test --only IGR

Additional local comparison/debug checks used during development:

python3 local_check_igr_full_alpha.py --default-igr
python3 local_igr_diagnostics/scripts/compare_igr_packs.py --default-igr

The IGR golden files were updated with --add-new-variables to include the newly stored final volume fraction. Existing golden entries were preserved, and the added final-alpha fields satisfy the expected volume-fraction sum checks, which is verified by the local Python scripts. The updated IGR tests match the previous conserved-variable outputs to machine-level roundoff, aside from the newly added final volume-fraction fields.

Checklist

Check these like this [x] to indicate which of the below applies.

• I added or updated tests for new behavior
• I updated documentation if user-facing behavior changed

See the developer guide for full coding standards.

GPU changes (expand if you modified src/simulation/)

• GPU results match CPU results
• Tested on NVIDIA GPU or AMD GPU
• Verified CPU only

AI code reviews

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[codecov]

Codecov Report

❌ Patch coverage is 82.46753% with 162 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.86%. Comparing base (66254db) to head (8a772de).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/simulation/m_igr.fpp	90.72%	0 Missing and 63 partials ⚠️
src/simulation/m_rhs.fpp	68.39%	24 Missing and 31 partials ⚠️
src/simulation/m_time_steppers.fpp	31.11%	15 Missing and 16 partials ⚠️
src/simulation/m_riemann_state.fpp	11.11%	6 Missing and 2 partials ⚠️
src/simulation/m_riemann_solver_lf.fpp	0.00%	0 Missing and 3 partials ⚠️
src/simulation/m_riemann_solvers.fpp	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1615      +/-   ##
==========================================
- Coverage   60.43%   57.86%   -2.58%     
==========================================
  Files          83       83              
  Lines       19868    20547     +679     
  Branches     2956     3011      +55     
==========================================
- Hits        12007    11889     -118     
- Misses       5860     6675     +815     
+ Partials     2001     1983      -18     

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[StevenXue98]

I ran some local performance comparisons against the legacy IGR path.

Representative CPU results:

18A71AB2, 50 steps, 100 runs:

• runtime: 3.73s candidate vs 2.62s legacy (~1.42x)
• peak RSS: 75.5 MB candidate vs 57.5 MB legacy (~1.31x)

18A71AB2, 500 steps, 10 runs:

• runtime: 32.09s candidate vs 21.45s legacy (~1.50x)
• peak RSS: 75.5 MB candidate vs 57.4 MB legacy (~1.31x)

18A71AB2, larger grid, ~100^3, 20 steps, 3 runs:

• runtime: 65.4s candidate vs 52.3s legacy (~1.25x)
• peak RSS: 762.6 MB candidate vs 276.3 MB legacy (~2.76x)

Raw output comparisons against legacy IGR passed to roundoff.

I also trimmed two unused IGR allocations from the refactored path:

• flux_gsrc_n
• qL_prim/qR_prim

The remaining memory overhead is mainly from the shared RHS/Riemann storage arrays required by the integrated path.