Ansys / Fluent mesh (.msh)

The Ansys Fluent .msh mesh format: fully parenthesis-nested "Scheme-like" sections, in ASCII, binary, or a mix of both within one file.

Format name	ansys
Extensions	.msh
Read / Write	✓ / ✓
Extra dependencies	—

Reading & writing

import meshioplusplus

mesh = meshioplusplus.read("mesh.msh", file_format="ansys")
meshioplusplus.ansys.write("out.msh", mesh, binary=True)

• binary — write binary (True) or ASCII section bodies.

File structure

Every section is (<index> ...); the file is opened and always read in binary mode, since ASCII and binary sections may be mixed in one file. The index may be a bare decimal (ASCII payload) or prefixed 20/30 (binary payload — 20xx = float32/int32, 30xx = float64/int64).

• (0 ...) comment, (1 "...") header, (2 <dim>) dimensionality — bracket- skipped, not otherwise interpreted.
• Nodes — (<pfx>10 (zone-id first last type ND) ( <data> )): header integers are hexadecimal; first/last give the point-count range, ND is the spatial dimension. Body is (last-first+1) × ND values — ASCII one point per line, binary a raw float32/float64 block. A self-contained line (equal ( and ) counts, no body) is a pure declaration and is skipped.
• Cells — (<pfx>12 (zone-id first last zone-type element-type) ( <data> )): zone-type == 0 marks a dead zone, skipped with no cells produced. element-type selects a fixed node count (table below); mixed (element-type 0) zones are parsed structurally but their body is not decoded at all (Fluent's own mixed-cell-type encoding is unresolved in this reader).
• Faces — (<pfx>13 (zone-id first last type element-type) ( <data> )): body rows are n0 n1 ... cr cl (face nodes plus 2 adjacent-cell ids, the latter discarded). ASCII mixed (element-type 0) faces are parsed, each row prefixed by its own per-row type index; binary mixed faces are not supported and raise ReadError. Faces are folded into the same flat cell list as volume cells — in the resulting Mesh.cells, boundary faces and interior cells are only distinguished by cell.type.
• (39 ...) and (45 ...) (zone specifications) are always warned-and- skipped, never parsed for content.

All cells/faces across every zone are collected into one flat list, then every connectivity array has the first point-zone's first index subtracted, so files whose node numbering doesn't start at 0 or 1 still normalize correctly.

Write emits, in order:

(1 "meshio++ VERSION")
(2 DIM)
(10 (0 1 N_POINTS 0))                          -- node-count declaration
(12 (0 1 TOTAL_CELLS 0))                       -- cell-count declaration
(10|3010 (1 1 N 1 DIM)( DATA ))                -- node block
(12|2012|3012 (1 FIRST LAST 1 ANSYS_TYPE)( DATA ))   -- per cell block

Cell types

Volume/element-type codes (element-type field of a 12 section):

code	meshio++ type	nodes
0	mixed (unhandled)	—
1	triangle	3
2	tetra	4
3	quad	4
4	hexahedron	8
5	pyramid	5
6	wedge	6

Face-type codes (13 sections, read-only — not used on write): 0=mixed, 2=line(2), 3=triangle(3), 4=quad(4).

meshio++ → Ansys type codes on write: triangle:1, tetra:2, quad:3, hexahedron:4, pyramid:5, wedge:6 (no writer support for mixed/polyhedral).

Data mapping

None — point_data/cell_data/field_data are always empty; this format carries geometry and zone/boundary structure only.

Quirks & limitations

• All connectivity and zone-header integers are hexadecimal, in both ASCII bodies and headers — the defining quirk of this format among the ones meshio++ supports.
• Binary vs. ASCII is signalled purely by an optional "20"/"30" prefix glued onto the section-index digits (e.g. 2010 = binary float32 nodes, 3012 = binary int64 cells).
• Dead zones (zone-type == 0) produce no cells at all.
• mixed cell zones (element-type 0) are structurally skipped — Fluent's own encoding for heterogeneous cell zones is not decoded, and no cells result from them.
• The C++ reader defers any face section (13) carrying a data body to the Python fallback — meaning any Fluent .msh with real boundary face zones (a very common real-world case) is always parsed by Python, not C++.
• 2D/3D validity (dim in {2,3}) is only checked on write, not on read.

Notes

• No reference fixture exists under tests/meshes/ansys/; tests round-trip synthetic meshes (empty_mesh, tri_mesh, tri_mesh_2d, quad_mesh, tri_quad_mesh, tet_mesh, hex_mesh, pyramid_mesh, wedge_mesh), parametrized over both ASCII and binary.
• .msh is shared with gmsh and freefem; on auto-detection ansys is tried first. Pass file_format to disambiguate.