OpenPFC / From arrays to an inspectable result

A result series remembers geometry and time

A frame number is not a simulation step, and neither one tells us the physical time. Store all three deliberately.

Includes the model, inputs, required headers and a short run guide.

This refactoring of 11_write_results uses native SnapshotSeries in place of the old example-only VTK wrapper. The field is deliberately padded, the geometry is nontrivial, and the three saved states have steps 0,4,8 at times 0,.25,.5:

\[ u(x,y,z,t)=1+x+2y+3z+t. \tag{1}\]

These are prescribed fields for studying output. No PDE solver is hidden inside this example.

Register a field, then write output opportunities

pfc::io::SnapshotSeries series(
    domain, field.box(),
    {.directory = "results", .prefix = "lesson", .comm = comm});
series.add_field("density", field, "results/density_%04d.vti",
                 pfc::io::SnapshotFormat::Vtk);
series.add_field("density_raw", field);
for (int frame = 0; frame < 3; ++frame) {
  const double t = .25 * frame;
  field.apply([&](double x, double y, double z) {
    return output_lesson::value(x, y, z, t);
  });
  series.write(4 * frame, t);
}
series.close();

The VTK slot and raw binary slot borrow the same field. The series packs only owned samples before writing, so halo storage does not leak into either file. The field and its allocation must remain valid while the series uses them. All communicator ranks participate in each write and in close.

The frame ordinal chooses the filename suffix 0000,0001,0002. Simulation steps and physical times are separate metadata. close() writes the manifest and XDMF information and reports errors on the successful path; relying only on destructor cleanup would lose that opportunity.

Inspect the files before interpreting a picture

results/lesson_manifest.json records dimensions, spacing, origin, field names, filename patterns, steps and times. VTK files include physical geometry and an appended binary scalar array. The raw binary slot also gets a generated XDMF time series, which supplies geometry and time to the otherwise headerless arrays.

In serial, open density_0000.vti in a VTK-compatible viewer. In parallel, the writer emits .vti pieces and a .pvti master with non-overlapping point extents. Some VTK/ParaView readers do not assemble those pieces into a single image; individual pieces remain readable. The generated XDMF series describes the global binary field and is another route for viewing the whole result.

Verify the content, not just the existence of a file

openpfc run model.cpp input.json --source-root .. --launcher=none
openpfc run model.cpp input.json --source-root .. --ranks=2 --launcher=mpirun

The tests parse each VTK piece’s XML and binary payload, reconstruct the whole field, check origin and spacing, and compare each saved value with Equation 1. They separately decode the binary frames and inspect the manifest and XDMF times. None of the halo sentinel values should appear in the saved fields.

Change the geometry in the helper, then check that the header and physical sample values both change. Keep the model and input with it. For actual simulation evolution, continue with Cahn–Hilliard.