Nastran OP2 results (.op2)

The binary result file MSC Nastran and Simcenter Nastran (NX Nastran) write with PARAM,POST,-1 (or -2), and that Femap, Patran and most post-processors read. MSC runs that write HDF5 are read by nastran_h5; OP2 is the route for Simcenter/NX Nastran and for MSC decks without MDLPRM,HDF5. meshio++ reads it as a sequence, .op2 → .vtu / VTKHDF / any meshio++ format, with every subcase, mode, time or frequency as a step.

Format name	nastran_op2
Extensions	.op2
Read / Write	✓ / — (read-only)
Extra dependencies	none

Reading

import meshioplusplus

mesh = meshioplusplus.read("job.op2")                 # the first step
mode3 = meshioplusplus.read("job.op2", time_step=2)   # the third one

mesh.point_data["DISPLACEMENT"]        # (n, 3) translations
mesh.point_data["DISPLACEMENT_ROT"]    # (n, 3) rotations
mode3.point_data["EIGENVECTOR"]        # a mode shape
mesh.cell_data["STRESS:VON_MISES"]     # solids' centre von Mises, NaN elsewhere
mesh.cell_data["STRESS:VON_MISES1"]    # shells' fibre-1 von Mises
mesh.cell_data["STRESS:X@corner"]      # (cells, nodes): solid corner values
mesh.cell_data["GRID_FORCE:F1"]        # (cells, nodes): grid point forces
mesh.field_data["nastran:subcase"], mesh.field_data["meshio:time"]

meshioplusplus.nastran_op2.time_values("job.op2")

meshioplusplus convert job.op2 'mode_{step}.vtu'

Both engines read the whole format: the C++ core, and a Python reference reader the core falls back to. .op2 is read-only.

Which files

An OP2 is a Fortran unformatted file of named tables. meshio++ reads MSC and NX/Simcenter files, 32-bit (4-byte words) and 64-bit (8-byte words, still with 4-byte record markers), in either byte order. The table names of 64-bit NX files, written four characters per word, are read as the 32-bit ones. A formatted (text) OP2 is refused. A file is recognised by its .op2 extension, or by content: the PARAM,POST,-1 header (a date and the tape code record) is sniffed in any word size and byte order.

The model

The mesh is built as the nastran_h5 reader builds it, from the same card table (detail/nastran_model.hpp):

• Points are the GEOM1 GRIDs, in file order, in the basic coordinate system (since v16.10.0): a GRID with CP != 0 is moved from its local system, which the GEOM1 CORD1R/C/S and CORD2R/C/S records define, and point_data["nastran:cp"] (["nastran:cd"]) records the system ids. The rules are those of nastran_h5. NX's 32-bit GRID records with double-precision coordinates are read as such. A mesh taken from a sibling deck keeps the deck reader's raw coordinates.
• Cells. The GEOM2 element records with a cell type: CQUAD4, CQUADR, CSHEAR, CQUAD8, CQUAD, CTRIA3, CTRIAR, CTRIA6, CTETRA, CPYRAM, CPENTA, CHEXA (linear or quadratic by their mid-side nodes), CROD, CTUBE, CONROD, CBAR, CBEAM, CBUSH, CVISC, PLOTEL as lines and CONM2 as vertices, in card-name order; NX 2019 and later's CQUAD4/CTRIA3 records (keys 15401 and 15301) are read as those cards. A card whose record length fits more than one layout (MSC and NX CQUAD4s differ by a word) takes the first whose entries validate: ids increasing, corner nodes that are GRIDs. Records with no cell type (CELAS*, CDAMP*, CMASS*, CGAP...) are named in one warning.
• Cell data. nastran:eid and nastran:pid (-1 for CONM2, CONROD, PLOTEL).
• Regions. One cell region per property id, named <PTYPE>_<pid> after its EPT record (PSHELL, PSOLID, PCOMP, PCOMPG, PBAR, PBARL, PBEAM, PBEAML, PROD, PSHEAR, PTUBE, PBUSH...), PID_<pid> when none defines it. NX writes each PCOMP a second time as a PSHELL with material ids from 100000000: that twin is ignored, and a PCOMPG wins over a PCOMP of the same id, as pyNastran reads them.
• Without geometry. A file with no GEOM1 GRIDs (written without the model, or with PARAM,POST,-2 defaults) takes its mesh from the input deck beside it: <stem>.bdf, .dat, .nas or .blk, read by the bulk Nastran reader, with nastran:eid added. Without one, the ReadError lists the paths it looked for. The bulk reader does not follow INCLUDE cards.

Results and steps

A results table alternates a 146-word header (approach, table and element-type codes, subcase, mode or time, width) with the data it describes. Every distinct (subcase, analysis code, word 5 of the header: load set, mode number, time or frequency) of the tables below is a step, in the order the file first names it: time_step picks one, read_metadata(...).time_values lists them and read_sequence walks them.

Field data	Meaning
meshio:time	the time (transient) or frequency, the eigenvalue of a mode (real modes: λ = ω²) or the post-buckling eigenvalue, 0 for a static subcase; as written, not converted
nastran:subcase, nastran:analysis	the subcase and the approach's analysis code (1 statics, 2 modes, 5 frequency, 6 transient, 8 post-buckling...)
nastran:mode	the mode number (analysis 2, 8 and 9), else 0

Nodal results

Real SORT1 tables with 8 words per node (OUG*, BOUG*, OQG*, OQMG*, OPG*) become point data by their table code, with the nastran_h5 names: DISPLACEMENT, EIGENVECTOR, VELOCITY, ACCELERATION, SPC_FORCE, MPC_FORCE (the OQMG tables, which share the SPC forces' code) and APPLIED_LOAD, each (n, 3) with a _ROT twin; a thermal displacement table is TEMPERATURE (n,). A point with no row is NaN, and scalar-point rows are dropped. When two tables give the same point a value in the same step, the first one read keeps it. Vectors of a GRID with CD != 0 are rotated from that system to basic (since v16.10.0), except in the BOUG* tables, which Nastran already writes in basic.

Element results

Real SORT1 stress (OES*) and strain (OSTR*) tables become cell data STRESS:<M> or STRAIN:<M>, the centre value per element (a beam's first station), NaN on cells without it:

Element type (code)	Members
CROD (1), CONROD (10)	A, MSA, T, MST
CTUBE (3)	AS, MSA, TS, MST
CSHEAR (4)	TMAX, TAVG, MS
CBAR (34)	X1A...X4A, AX, MAXA, MINA, MST, X1B...X4B, MAXB, MINB, MSC
CQUAD4 (33), CTRIA3 (74), and the centre of CQUAD8 (64), CTRIAR (70), CTRIA6 (75), CQUADR (82), corner CQUAD4 (144)	per fibre k = 1, 2: FD<k>, X<k>, Y<k>, TXY<k>, ANGLE<k>, MAJOR<k>, MINOR<k>, VON_MISES<k> (or MAX_SHEAR<k>)
CTETRA (39), CHEXA (67), CPENTA (68), CPYRAM (255)	X, Y, Z, TXY, TYZ, TZX, PRINCIPAL_A, PRINCIPAL_B, PRINCIPAL_C (in the file's A, B, C order, not sorted), PRESSURE, VON_MISES (or OCT_SHEAR)
CBEAM (2)	SD, XC, XD, XE, XF, MAX, MIN, MST, MSC
CBAR stations (100)	SD, XC, XD, XE, XF, AX, MAX, MIN, MS
composite CQUAD4 (95), CQUAD8 (96), CTRIA3 (97), CTRIA6 (98), NX CQUADR (232), CTRIAR (233)	per ply: X1, Y1, T1, L1, L2, ANGLE, MAJOR, MINOR, VON_MISES (or MAX_SHEAR)

The members common to nastran_h5 (X1, TXY1, X, TZX, A, TMAX...) have its names, so an OP2 and an HDF5 file of the same run give the same arrays; the derived values the HDF5 schema does not store have their own names. Whether the last plate and solid column is von Mises or the maximum (octahedral) shear, and whether a table holds stress or strain, come from the header's stress code. Elements share results by id; CONM2 elements, which have none, are left out.

Since v16.10.0 the values beyond the centre are read too, with the arrays and nastran:layout:<name> field data of nastran_h5: <name>@corner for the corners of solids (39, 67, 68, 255) and corner-output shells (64, 70, 75, 82, 144), <name>@ply for the composite plies (column PLY - 1) and <name>@station for CBEAM's 11 stations and CBAR's station entries (stations not output stay NaN). The grid point force tables (OGPFB*) give GRID_FORCE:F1...M3 per element node and GRID_FORCE:<label>:<M> point data for the rows of no element (*TOTALS*, APP-LOAD, F-OF-SPC..., named without blanks and asterisks: GRID_FORCE:TOTALS:F1), rotated from each GRID's CD to basic. MSC writes a different header word 5 in OGPFB1 than in the other tables of the subcase, so grid point forces join the step the subcase's other tables made instead of starting their own.

Not read, and named in one warning: complex, random and SORT2 tables, every other element type (springs, NX's newer solids...), and every other table (OEF forces, ONRGY energies, contact and bolt tables...).

Verification

The test suite reads eleven files from the pyNastran test models (BSD 3-Clause, credited in tests/python/meshes/nastran_op2/README.md): MSC and NX output, 32- and 64-bit, SOL 101 statics (among them static_solid_shell_bar, a solid, shell and bar model), modes, buckling, transient heat transfer, an NX 2412 file, and one with its geometry tables removed next to its deck. On every step of every file the nodal vectors, the centre stress and strain, and the corner, ply, station and grid point force values equal pyNastran 1.4.1's reading to float precision, and the two engines agree exactly. The nodal vectors and centre values also match on 40 further pyNastran models checked outside the suite (before the corner, ply, station and grid point force values were read). No file from an MSC or NX version newer than those samples, and no big-endian file Nastran wrote, has been read.

The coordinate systems are checked on static_solid_shell_bar_cord.op2, derived from static_solid_shell_bar.op2 by pointing every GRID's CP and CD at the file's own CORD2R, CORD2C and CORD2S systems (tools/gen_nastran_cord_reference.py): points match pyNastran's to 1e-12 and displacements to the file's single precision. They are compared with upstream pyNastran's rotation matrices, because 1.4.1 composes cylindrical and spherical ones the wrong way round.