CAE sample .npz

A writer and reader for the per-case .npz layout NVIDIA PhysicsNeMo's CAE datapipes read — one file per case, carrying the superset of the keys DoMINODataPipe and TransolverDataPipe consume, since each selects its own subset through keys_to_read and ignores the rest. The layout is transcribed from the Kratos PhysicsNeMoApplication's own exporter, so a meshio++ case and a Kratos case are interchangeable in one dataset directory.

Format name	cae
Extensions	.npz
Read / Write	✓ / ✓
Extra dependencies	—

This is a tabular sample export, not a mesh format in the usual sense. It reads back, but a round trip is lossy by construction: the surface half is a triangulation of the input's skin, and the volume half is node coordinates plus nodal fields with no volume connectivity at all.

Reading & writing

import meshioplusplus

meshioplusplus.write("case_0.npz", mesh)                 # one case
mesh = meshioplusplus.read("case_0.npz")                 # the surface half
volume = meshioplusplus.cae.read("case_0.npz", part="volume")

meshioplusplus.cae.write(
    "case_0.npz",
    mesh,
    surface_fields=["p", "wallShearStress"],
    volume_fields=["U"],
    global_params={"stream_velocity": 30.0, "air_density": 1.226},
    global_params_reference={"stream_velocity": 25.0},
    global_params_order=["stream_velocity", "air_density"],
    time=0.5,
    step=3,
)

A whole dataset at once, from a glob, a path list or a multi-step file:

meshioplusplus.cae.export_cases("run_*.vtu", "dataset/")

which is also the data export-cae CLI verb and the export_cae MCP tool. One mesh is alive at a time however large the run.

File structure

An uncompressed numpy.savez archive. A mesh with 3-D cells is the volume: its nodes become volume_mesh_centers, its numeric point_data becomes volume_fields, and its skin — extracted and triangulated here — becomes the surface half, with the volume's own cell_data gathered onto each triangle through its parent cell. A surface mesh is written as the surface alone, which is what DoMINO's surface-only model type reads.

Key	Shape	Dtype	Notes
stl_coordinates	(P, 3)	float32	surface vertices
stl_faces	(3T,)	int32	flattened — DoMINO feeds it straight to signed_distance_field
stl_centers	(T, 3)	float32	vertex mean, not the area centroid
stl_areas	(T,)	float32	0.5 * ‖(v1-v0) × (v2-v0)‖
surface_normals	(T, 3)	float32	unit length; winding is the input's
surface_mesh_centers / surface_areas			the aliases Transolver reads
surface_fields	(T, ΣW)	float32	omitted when empty
volume_mesh_centers	(N, 3)	float32	the volume's nodes
volume_fields	(N, ΣW)	float32	nodal only; omitted when empty
<param>	(1,)	float32	one per global parameter
global_params_values / _reference	(k, 1)	float32	stacked, in global_params_order
TIME / STEP	(1,)	float32 / int64	only when given

Point fields are averaged from a triangle's three vertices; cell fields are replicated from the parent cell. Widths flatten, so a 3-vector contributes three columns, and the blocks are concatenated in the requested order.

meshio++ adds four sidecar keys (meshioplusplus:surface_field_names, ..._widths, meshioplusplus:node_field_names, ..._widths) so a read can split the concatenated blocks back into named arrays. The datapipes ignore unknown keys, so they cost the consumer nothing; a file written elsewhere, without them, comes back with its block whole rather than guessed at.

Cell types

Triangles only, on the surface side. Quads and n-gons are triangulated and higher-order cells linearized first; a mesh whose highest cell dimension is 1 or 0 is refused by name. The volume half has no cells at all — it reads back as a vertex point cloud.

Data mapping

surface_fields becomes per-triangle cell_data, volume_fields per-node point_data, and the global parameters plus TIME/STEP become field_data. surface_normals, surface_areas and stl_centers come back as cell_data too.

Quirks & limitations

Three rules belong to the reader on the other side, and each produces silently wrong results rather than an error when broken:

• Every array must have ndim >= 1, because NpzFileReader does in_data[key][:]. Hence TIME/STEP and every scalar parameter as shape-(1,) arrays.
• No key but volume_mesh_centers and volume_fields may contain the substring volume. That reader takes the volume row count from next(key for key in in_data.keys() if "volume" in key) — the first such key in file order, over every key in the file, not just the requested ones. meshio++'s own sidecars are therefore called node_field_*, and a global parameter whose name contains volume is refused by name.
• The two volume keys must share their row count, since the slice computed from one is applied to the other.

Provenance rides a bytes array written as the archive's first member, so the ordinary head scanner finds it with no .npz-specific code — each row is NUL-padded so the block does not run into the zip's next local-file header.