MODOPT,
Method, NMODE,
FREQB, FREQE,
Cpxmod, Nrmkey,
ModType, BlockSize,
--, --, --,
FREQMOD
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 (or
eigenvalue range if FREQMOD is specified) 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 (or eigenvalue
range if FREQMOD is specified) 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 accurate the
solution will be and the more eigenvalues it could produce; but the solution time will also be
longer. 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. The accuracy of the
SNODE solution is controlled by both FREQE and the
RangeFact value on the SNOPTION command. Refer
to SNOPTION for more information on using the SNODE eigensolver options to
control solution efficiency and accuracy.
CpxmodComplex eigenmode key. (Valid
only when Method = QRDAMP or Method = UNSYM).
AUTO | — | Determine automatically if the eigensolutions
are real or complex and output them accordingly. This is the default
for |
ON or CPLX | — | Calculate and output complex eigenmode shapes. |
OFF or REAL | — | Do not calculate complex eigenmode
shapes. This is required if a mode-superposition analysis is intended
after the modal analysis for |
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.
--Unused field.
FREQMODThe specified frequency when the solved eigenvalues are no longer frequencies (for example, the model has the Floquet periodic boundary condition). In a modal analysis, the Floquet periodic boundary condition (body load FPBC) is only valid for the acoustic elements FLUID30, FLUID220, and FLUID221.
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. Additional options used only for the Block Lanczos (LANB) eigensolver are specified by the LANBOPTION command. Additional options used only for the QRDAMP eigensolver are specified by the QRDOPT 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 DAMP and QRDAMP options cannot be followed by a subsequent spectrum analysis. The UNSYM method supports spectrum analysis when eigensolutions are real.
In a modal analysis, the Floquet periodic boundary condition (body load FPBC) is only valid for the acoustic elements FLUID30, FLUID220, and FLUID221.
This command is also valid in PREP7.
Distributed ANSYS Restriction The VT extraction method is not supported in Distributed ANSYS. All other extraction methods are supported. Block Lanczos, PCG Lanczos, SUBSP, UNSYM, DAMP, and QRDAMP are distributed eigensolvers that will run a fully distributed solution. However, the Supernode eigensolver is not a distributed eigensolver; therefore, you will not see the full performance improvements with this method that you would with a fully distributed solution.
| Command Option Method | Available Products |
| LANB | DesSpc | Pro | Premium | Enterprise | Ent PP | Ent Solver | – |
| LANPCG | DesSpc | Pro | Premium | Enterprise | Ent PP | Ent Solver | – |
| SNODE | – | Pro | Premium | Enterprise | Ent PP | Ent Solver | – |
| SUBSP | DesSpc | Pro | Premium | Enterprise | Ent PP | Ent Solver | – |
| UNSYM | – | – | Premium | Enterprise | Ent PP | Ent Solver | – |
| DAMP | – | – | Premium | Enterprise | Ent PP | Ent Solver | – |
| QRDAMP | – | – | Premium | Enterprise | Ent PP | Ent Solver | – |