MODOPT, Method, NMODE, FREQB, FREQE, Cpxmod, Nrmkey, ModType, BlockSize, --, --, Scalekey
Specifies modal analysis options.
MethodMode-extraction method to be used for the modal analysis.
LANB | — | Block Lanczos |
LANPCG | — | PCG Lanczos |
SNODE | — | Supernode modal solver |
SUBSP | — | Subspace algorithm |
UNSYM | — | Unsymmetric matrix |
DAMP | — | Damped system |
QRDAMP | — | Damped system using QR algorithm |
VT | — | Variational Technology |
NMODEThe number of modes to extract. The value can depend
on the value supplied for Method. NMODE has no default and must be specified. If Method = LANB, LANPCG, or SNODE, the number of modes
that can be extracted can equal the DOFs in the model after the application
of all boundary conditions.
Recommendation:
When Method = LANPCG, NMODE should be less than 100 to be computationally
efficient. |
When Method = SNODE, NMODE should be greater than 100 for 2-D plane or 3-D
shell/beam models and greater than 250 for 3-D solid elements to be
computationally efficient. |
FREQBThe beginning, or lower end, of the frequency range of interest.
For Method = LANB, SUBSP, UNSYM, DAMP, and QRDAMP, FREQB also represents the first shift point for the eigenvalue iterations.
If values for UNSYM or DAMP are zero or blank, the default value is
-1.0. For the other methods, the default is internally computed. Eigenvalue
extraction is most accurate near the shift point; multiple shift points
are used internally in the LANB, SUBSP, UNSYM, and QRDAMP methods.
For LANB, LANPCG, SUBSP, UNSYM, DAMP, and QRDAMP methods with a positive FREQB, eigenvalues are output beginning at the shift
point and increase in magnitude. For UNSYM and DAMP methods with a
negative FREQB value, eigenvalues are output
beginning at zero magnitude and increase.
Choosing higher FREQB values with
the LANPCG and SNODE methods may lead to inefficient solution times
because these methods will find all eigenvalues between zero and FREQB before finding the requested modes between FREQB and FREQE.
FREQEThe ending, or upper end, of the frequency range of
interest (in Hz). The default for Method = SNODE is described below. The default for all other methods is
to calculate all modes, regardless of their maximum frequency.
The default is 100 Hz for Method =
SNODE. To maintain solution efficiency, you should not set the FREQE value too high; for example, not higher than
5000 Hz for an industrial problem. The higher the FREQE value used for the SNODE method, the more solution time it will
take and the more eigenvalues it could produce. For example, if FREQE is set to 1e8, it will cause the underlying supernodal
structures to find all the possible eigenvalues of each group of supernodes;
hence, it will take an excessive amount of solution time.
CpxmodComplex eigenmode key. (Valid only when Method = QRDAMP).
ON | — | Calculate complex eigenmode shapes. |
OFF | — | Do not calculate complex eigenmode shapes. This is required if a mode-superposition analysis is performed after the modal analysis. This is the default. |
NrmkeyMode shape normalization key:
OFF | — | Normalize the mode shapes to the mass matrix (default). |
ON | — | Normalize the mode shapes to unity
instead of to the mass matrix. If a subsequent spectrum or mode superposition
analysis is planned, the mode shapes should be normalized to the mass
matrix ( |
ModTypeType of modes calculated by the eigensolver. Only applicable to the unsymmetric eigensolver.
Blank | — | Right eigenmodes. This value is the default. |
BOTH | — | Right and left eigenmodes. The left eigenmodes are written to Jobname.LMODE. This option must be activated if a mode superposition analysis is intended. |
BlockSizeThe block vector size to be
used with the Block Lanczos or Subspace eigensolver (used only when Method = LANB or SUBSP). BlockSize must be an integer value between 0 and 16. When BlockSize = zero
or blank, the code decides the block size internally (normally, a
value of 8 is used for LANB and a value of 6 is used for SUBSP). Typically,
higher BlockSize values are more efficient
under each of the following conditions:
When running in out-of-core mode and there is not enough physical memory to buffer all of the files written by the Block Lanczos or Subspace eigensolver (and thus, the time spent doing I/O is considerable).
Many modes are requested (>100).
Higher-order solid elements dominate the model.
The memory usage only slightly increases as BlockSize is increased. It is recommended that you
use a value divisible by 4 (4, 8, 12, or 16).
--Unused field.
--Unused field.
ScalekeyMatrices scaling key for acoustic-structural interaction:
OFF | — | Do not scale the matrices (default). |
ON | — | Scale the matrices. |
Specifies modal analysis (ANTYPE,MODAL) options. Additional options used only for the Supernode (SNODE)
eigensolver are specified by the SNOPTION command.
Additional options used only for the Subspace (SUBSP) eigensolver
are specified by the SUBOPT command. If Method = LANPCG, ANSYS automatically switches to the
PCG solver internally for this modal analysis. You can further control
the efficiency of the PCG solver with the PCGOPT and EQSLV commands.
For models that involve a non-symmetric element stiffness matrix, as in the case of a contact element with frictional contact, the QRDAMP eigensolver (MODOPT, QRDAMP) extracts modes in the modal subspace formed by the eigenmodes from the symmetrized eigenproblem. The QRDAMP eigensolver symmetrizes the element stiffness matrix on the first pass of the eigensolution, and in the second pass, eigenmodes are extracted in the modal subspace of the first eigensolution pass. For such non-symmetric eigenproblems, you should verify the eigenvalue and eigenmode results using the non-symmetric matrix eigensolver (MODOPT, UNSYM ).
The UNSYM, DAMP, and QRDAMP options cannot be followed by a subsequent spectrum analysis.
This command is also valid in PREP7.
Distributed ANSYS Restriction. All extraction methods are supported within Distributed ANSYS. However, PCG Lanczos, SUBSP, UNSYM, and DAMP are the only distributed eigensolvers that will run a fully distributed solution. The Block Lanczos and Supernode eigensolvers are not distributed eigensolvers; therefore, you will not see the full performance improvements with these methods that you would with a fully distributed solution. The QRDAMP method is supported, but does not use distributed memory parallelism within Distributed ANSYS.
| Command Option Method | Available Products |
| LANB | MP ME ST PR PRN DS <> <> PP EME MFS [1] |
| LANPCG | MP ME ST PR PRN DS <> <> PP EME MFS [1] |
| SNODE | MP ME ST PR PRN <> <> <> PP EME MFS |
| SUBSP | MP ME ST PR PRN DS <> <> PP EME MFS [1] |
| UNSYM | MP ME ST <> <> <> <> <> PP EME MFS |
| DAMP | MP ME ST <> <> <> <> <> PP EME MFS |
| QRDAMP | MP ME ST <> <> <> <> <> PP EME MFS |