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feat(boundaries): Robin BC (a·u + b·∂u/∂n = c)

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#62 1 comentario 0 reacciones 0 asignados Ver en GitHub

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Evaluación

Dificultad
5/5
Tiempo estimado
Más de una semana
Aptitud para principiantes
45/100
Tipo de issue
Nueva funcionalidad
Claridad
Bastante claro
Estado de actividad
Tranquilo
Stack tecnológico
julia
Área
tooling

Línea de trabajo

Start in src/boundaries.jl:82 and read the existing AbstractBC implementations, especially _bc_sign, _offset_ghost!, _require_face_bc, fold_bc!, and operator_diagonal. Decide whether Robin coefficients support per-face Fields or only scalars, then trace how that choice interacts with boundary_rhs. Done means Robin is recognized through the boundary machinery and its diagonal correction is available to the multigrid path.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

enhancement

What's missing

AbstractBC is exactly Periodic, Dirichlet, Neumann, Interface. _require_face_bc (src/boundaries.jl:82) errors loudly for anything else, so a Robin condition is a genuine extension rather than a config option — it needs _bc_sign, _offset_ghost! and a _require_face_bc method, plus the matching fold_bc! row and the operator_diagonal correction.

Why (downstream)

Robin is the native boundary condition for trans-wall exchange in the microvascular model, in two places:

  1. Perivascular wall coupling. Flux across the capillary wall is g·(C_blood − C_tissue) — a conductance times a concentration difference. That is a Robin condition on the tissue side, currently hand-assembled as a separate coupling operator with its own implicit/explicit split.
  2. Proton leak regularization. The transported proton field H is no-flux on every boundary, which makes its Laplacian pure-Neumann and therefore singular. A small Robin leak gH is what regularizes it. Right now that regularization is a bespoke term rather than a boundary condition, so it is invisible to operator_diagonal and to multigrid.

Both are the same mathematical object, and both would be one BC type if MFO had it.

Sketch

Robin(a, b, c)      # a·u + b·∂u/∂n = c ; Dirichlet is b=0, Neumann is a=0

Worth deciding up front whether a, b, c may be per-face Fields or must be scalars. The downstream conductance g is genuinely per-face (it varies with local vessel geometry), so scalar-only would leave the main use case unserved — but per-face coefficients interact with the affine boundary_rhs lift, so it's a real design call, not a detail.

Payoff beyond the downstream package

operator_diagonal gaining a Robin case is what lets MultigridPreconditioner precondition wall-coupled systems. That matters: the downstream steady solve currently reaches for ILU(0) on an assembled sparse W precisely because unpreconditioned matrix-free GMRES spent ~750 iterations per linear solve. The system is diffusion-dominated — ideal MG territory — so a Robin-aware diagonal is the step that makes MFO's native preconditioner applicable and lets the sparse-matrix detour go away.

🤖 Beep — Claude wrote this one; Kyle is not yet talking to himself in his own issue tracker.

Lenguaje dominante
Julia
Estrellas
3
Forks
1
Merge medio
1 d 28 min
PR fusionados (30 d)
6

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Primeros pasos

  1. Lee el issue completo y luego la guía de contribución del proyecto.
  2. Comenta en el issue que vas a ocuparte — evita que dos personas hagan lo mismo.
  3. Haz un fork del repositorio y trabaja en una rama.
  4. Abre un pull request que haga referencia al número del issue.

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