Add external tissue support Robin BC to CMM deformable wall

Problem

Currently there is no external tissue support BC available for a CMM wall simulation. This was a feature available in svSolver as a Robin BC and is/was useful for pulmonary deformable wall CMM simulations [1,2].

The current solver treats Type = CMM and Type = Robin as separate BC paths. As a result, a user cannot specify a CMM deformable wall with Robin-type external tissue support directly on the same wall BC.

Replacing the wall BC with Type = Robin is not correct, because that removes the dedicated CMM wall handling rather than augmenting it.

Solution

Allow Robin support to be specified inline on a Type = CMM boundary condition, so the wall remains a CMM deformable wall while also receiving the external support contribution.

Support the following fields on Type = CMM:

• Stiffness
• Damping
• Apply_along_normal_direction
• Spatial_values_file_path for spatially varying Stiffness and Damping

This would allow users to represent effective perivascular or external tissue support in a way that is common in the literature, while preserving the existing CMM wall formulation and preload/prestress workflow.

Additional context

[1] Lan et al., J Am Heart Assoc, 2022 — Virtual transcatheter interventions for peripheral pulmonary artery stenosis (DOI: 10.1161/JAHA.121.023532)
[2] Dong et al., Biomech Model Mechanobiol, 2021 — Computational simulation-derived hemodynamic and biomechanical properties of the pulmonary arterial tree early in ventricular septal defects (DOI: 10.1007/s10237-021-01519-4)

Code of Conduct

• I agree to follow this project's Code of Conduct and Contributing Guidelines