Initialize the 3D pressure field from a scalar value

[taeoukkim]

Current situation

Resolves #546.

This PR is a clean replacement for the previously closed pressure-initialization PR. It has been rebuilt on top of the latest main after the 3D–1D coupling changes were merged through PR #563.

Release Notes

• Add support for initializing the entire 3D pressure field from a single scalar value specified in solver.xml.
• Preserve the existing option to initialize pressure from a full 3D result file.
• Initialize the first-step under-relaxation history for 3D–1D and 3D–0D coupling using the pressure and flow values computed from the initialized 3D state.

The scalar pressure can be specified using:

<Initial_pressure_scalar>80000.0</Initial_pressure_scalar>

If both a nodewise pressure field and a scalar pressure are provided, the nodewise pressure field takes precedence.

Documentation

Initial_pressure_scalar is specified in the general simulation parameters in solver.xml.

Selecting an appropriate initial pressure can reduce initial pressure oscillations in standalone 3D simulations and coupled 3D–1D or 3D–0D simulations.

Testing

• Successfully built on macOS using AppleClang 17.
• Tested a parallel simulation using 8 MPI processes.
• Verified that the 3D pressure field is initialized with the specified scalar value.
• Verified initialization of the coupled 3D–1D pressure history using the initialized 3D state.

Code of Conduct & Contributing Guidelines

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

[claude]

Claude Code Review

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[taeoukkim]

This PR is a clean replacement for the previously closed pressure-initialization PR.