fipy-sim

name	fipy-sim
description	Use when driving FiPy (NIST's pure-Python finite-volume PDE solver) via Python scripts — diffusion / transient / convection / reaction PDEs on 1D/2D/3D structured / Gmsh meshes, steady or time-stepping — through sim runtime one-shot execution.

fipy-sim

You are connected to FiPy via sim-cli.

FiPy is the canonical pure-Python finite-volume PDE solver (NIST, 2003-present). Pip-installable (pip install fipy); pure Python on top of NumPy/SciPy with optional acceleration via PySparse / PyAMG / petsc4py / Trilinos.

Scripts are plain .py:

from fipy import CellVariable, Grid1D, TransientTerm, DiffusionTerm
mesh = Grid1D(nx=50, dx=0.02)
phi = CellVariable(name='phi', mesh=mesh, value=0.0)
phi.constrain(1.0, mesh.facesLeft)
phi.constrain(0.0, mesh.facesRight)
eq = TransientTerm() == DiffusionTerm(coeff=1.0)
for _ in range(100):
    eq.solve(var=phi, dt=0.01)

Same subprocess driver mode as PyBaMM / PyMFEM / SfePy.

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base/ — always relevant

Path	What's there
base/reference/workflow.md	Mesh → variable → BCs → equation → solve / sweep loop.
base/reference/meshes.md	Grid1D / Grid2D / Gmsh2D, FaceVariable / CellVariable.
base/reference/terms.md	TransientTerm / DiffusionTerm / ConvectionTerm / Source.
base/snippets/01_poisson.py	Verified steady-state 1D Poisson E2E.
base/known_issues.md	Cell-center vs face values, sweep vs solve, sparse solver fallback.

sdk/4/ — FiPy 4.x

• sdk/4/notes.md — version-specific surface notes.

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Hard constraints

1. Boundary conditions are applied via .constrain(value, faces), not by zeroing rows. mesh.facesLeft / facesRight / facesTop / facesBottom are the standard locators for grid meshes.
2. For transient problems, use TransientTerm() on the LHS: TransientTerm() == DiffusionTerm(...). Without it, FiPy treats the equation as steady and ignores dt.
3. For nonlinear equations, use .sweep(var=, dt=) in a loop until residual converges, not .solve (which does one Newton step only).
4. Acceptance != "ran without error". Always validate against an analytical or benchmark profile (e.g. linear profile for 1D Poisson with mixed Dirichlet, error function for 1D semi-infinite diffusion).
5. Print results as JSON on the last stdout line.

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Required protocol

1. Gather inputs:
   • Category A: PDE, domain, BCs, time span (transient), acceptance.
   • Category B: mesh density, time step, solver tolerance.
2. sim check fipy.
3. Write .py per base/reference/workflow.md.
4. sim lint script.py.
5. sim run script.py --solver fipy.
6. Validate JSON.