FluidSim

Use FluidSim 0.9.0 as a framework for Python-defined numerical solvers, especially periodic Cartesian pseudospectral CFD. Upstream FluidSim is CeCILL-2.1; the MIT frontmatter license applies only to this skill.

This skill does not treat a completed run, a stable time step, a smooth plot, or a closed program exit as evidence of numerical convergence or physical validity.

Required workflow

1. State equations, units or nondimensionalization, geometry, boundaries, initial conditions, forcing, observables, and acceptance criteria.
2. Select a verified solver and inspect its generated default parameters.
3. Create a strict JSON plan with explicit CPU, RAM, disk, wall-time, output-file, timestep, CFL, resolution, and dealiasing bounds.
4. Run the bundled validator and resource estimator.
5. Generate and review a dry-run script. It does nothing unless executed with an explicit config-ID acknowledgement.
6. Run one tiny serial pilot. Inspect budgets, divergence/constraints, spectral tails, CFL/time-step history, and output growth.
7. Refine grid and time step independently. Check conservation/budget residuals and observable sensitivity.
8. Only then prepare a site-specific MPI job. Never submit or launch MPI automatically.
9. Preserve config, script, uv.lock, package/platform/backend versions, logs, output inventory, checksums, and restart lineage.

Stop if physical assumptions, units, boundary conditions, forcing semantics, resolution criteria, resource limits, or acceptance criteria are missing.

Version and installation

As verified on 2026-10-01:

• Latest stable PyPI release: fluidsim==0.9.0 (2025-12-04).
• Package metadata requires Python >=3.11 and lists Python 3.11–3.14.
• Pseudospectral parameter creation needs FluidFFT; bare fluidsim imported in the smoke test, but ns2d.create_default_params() failed until the fft extra was installed.
• Current companion versions tested here: fluidfft==0.4.5 and pyFFTW==0.15.1.

Prefer a project lock:

uv init --python 3.12
uv add "fluidsim[fft]==0.9.0" "fluidfft==0.4.5" "pyFFTW==0.15.1"
uv lock
uv sync --frozen

For an isolated disposable environment:

uv venv --python 3.12
uv pip install "fluidsim[fft]==0.9.0" "fluidfft==0.4.5" "pyFFTW==0.15.1"

The project lock is the reproducibility record; direct pins alone do not freeze all transitive artifacts. Do not reuse a lock across incompatible platforms or MPI ABIs.

MPI is optional and native:

uv add "mpi4py==4.1.2" "fluidfft-mpi-with-fftw==0.0.1" "fluidfft-fftwmpi==0.0.1"
uv lock

Those packages still require a compatible MPI runtime and FFTW development libraries. The optional native plugins are:

• fluidfft-fftw==0.0.1: sequential fft2d.with_fftw1d, fft2d.with_fftw2d, fft3d.with_fftw3d.
• fluidfft-mpi-with-fftw==0.0.1: MPI fft2d.mpi_with_fftw1d, fft3d.mpi_with_fftw1d.
• fluidfft-fftwmpi==0.0.1: MPI-enabled FFTW fft2d.mpi_with_fftwmpi2d, fft3d.mpi_with_fftwmpi3d.
• fluidfft-p3dfft==0.0.1: fft3d.mpi_with_p3dfft; requires P3DFFT.
• FluidFFT also declares PFFT and P3DFFT extras; audit and pin their native stacks for the target cluster.

FluidFFT documents cuFFT historically, but FluidFFT 0.4.5 declares no CUDA extra or installed GPU plugin in its package metadata, and its CUDA installation page is unfinished. Do not claim GPU acceleration or install an unrelated CUDA wheel as a FluidSim backend. Treat GPU work as source-level experimental integration requiring separate validation.

See installation for system dependencies, MPI ABI, HDF5-MPI, backend discovery, and verification.

API snapshot

Use direct, versioned imports:

from fluidsim.solvers.ns2d.solver import Simul

params = Simul.create_default_params()
params.oper.nx = params.oper.ny = 32
params.oper.Lx = params.oper.Ly = 2 * 3.141592653589793
params.oper.coef_dealiasing = 2 / 3
params.time_stepping.USE_CFL = True
params.time_stepping.cfl_coef = 0.5
params.time_stepping.deltat0 = 0.001
params.time_stepping.deltat_max = 0.01
params.time_stepping.t_end = 0.1
params.time_stepping.max_elapsed = "00:05:00"
params.init_fields.type = "noise"
params.init_fields.noise.velo_max = 0.01
params.output.HAS_TO_SAVE = False
params.output.ONLINE_PLOT_OK = False

Important 0.9 corrections:

• CFL field: params.time_stepping.cfl_coef, not CFL.
• Time-correlated forcing:

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