Fixing slow nonlinear convergence for URIS valve

[hanzhao2020]

Current situation

Resolves #448

Release Notes

• Fixed the stagnated nonlinear convergence for the RIS valve in CFD simulations by removing the up term in the RIS formulation in fluid.cpp.
• Improved the slow nonlinear convergence for the RIS valve in FSI simulations by implementing the explicit geometric coupling approach in pic.cpp. In each time step of the FSI simulation, the fluid–structure equation is solved to nonlinear convergence with fixed mesh displacement from the previous time step, then the mesh equation is solved.
• Updated test cases for the URIS valve with CFD and FSI simulations.

Code of Conduct & Contributing Guidelines

• I agree to follow the Code of Conduct and Contributing Guidelines.

[codecov]

Codecov Report

❌ Patch coverage is 87.80488% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.77%. Comparing base (869da67) to head (d8e0eb1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Code/Source/solver/pic.cpp	87.09%	4 Missing ⚠️
Code/Source/solver/read_files.cpp	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #508      +/-   ##
==========================================
+ Coverage   67.70%   67.77%   +0.06%     
==========================================
  Files         168      168              
  Lines       32752    32779      +27     
  Branches     5750     5761      +11     
==========================================
+ Hits        22176    22215      +39     
+ Misses      10439    10427      -12     
  Partials      137      137              

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[ktbolt]

@hanzhao2020 Looks ok to me.

[aabrown100-git]

@hanzhao2020 Looks good thanks for this! Just a couple comments.

BTW, I have already tested these changes (using Han's latest branch which also includes some other changes) with my recent cardiac FSI simulations and I can confirm greatly improved nonlinear convergence.

[Copilot]

Pull request overview

This pull request addresses slow nonlinear convergence issues when using the URIS (unfitted resistive immersed surfaces) valve model in fluid simulations. The fix involves two main changes: (1) removing the up term from the RIS residual formulation in the fluid equation solver for CFD simulations, and (2) implementing explicit geometric coupling for FSI simulations where the fluid-structure equation converges with fixed mesh displacement before solving the mesh equation.

Changes:

• Removed the up (subscale velocity) term from the URIS/RIS resistance contribution in the fluid residual, keeping only u (velocity), which fixes stagnated convergence in CFD
• Added explicit geometric coupling functionality for FSI equations to improve convergence by decoupling the FSI and mesh equation solves within each time step
• Updated test cases to separate CFD and FSI URIS valve scenarios with appropriate field validations

Reviewed changes

Copilot reviewed 62 out of 81 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
Code/Source/solver/fluid.cpp	Removed up term from RIS valve residual contribution (lines 2242, 2244, 2246)
Code/Source/solver/pic.cpp	Implemented explicit geometric coupling logic to conditionally update FSI displacement fields and control equation sequencing
Code/Source/solver/Parameters.h	Added explicit_geometric_coupling boolean parameter to equation configuration
Code/Source/solver/Parameters.cpp	Registered the new parameter with default value false
Code/Source/solver/ComMod.h	Added expl_geom_cpl flag to equation type structure
Code/Source/solver/read_files.cpp	Read explicit geometric coupling parameter and propagate it from FSI to mesh equation
Code/Source/solver/distribute.cpp	Added broadcasting of explicit geometric coupling flag for parallel execution
tests/test_uris.py	Split into separate CFD and FSI tests with appropriate field lists
tests/cases/uris/pipe_uris_cfd/*	New CFD test case with fluid equation only
tests/cases/uris/pipe_uris_fsi/*	New FSI test case with explicit geometric coupling enabled
tests/cases/uris/pipe_uris_3d/*	Removed old FSI test with prescribed wall motion

Comments suppressed due to low confidence (1)

Code/Source/solver/fluid.cpp:2239

• The comment "Birkman term" appears to be a typo. This should be "Brinkman term" which refers to the Navier-Stokes-Brinkman equation for porous media flow, as mentioned in the linked issue. However, this comment appears to be pre-existing and not part of the changes in this PR.

  // Local residue

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[aabrown100-git]

@hanzhao2020 Looks good to me, thanks!