Release 11.0 Documentation for ANSYS
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MODOPT, Method, NMODE, FREQB, FREQE, Cpxmod/PRMODE, Nrmkey
Specifies modal analysis options.
MP ME ST PR PRN DS DSS <> <> EH <> PP <>
Mode extraction method to be used for the modal analysis.
LANB | — | Block Lanczos |
LANPCG | — | PCG Lanczos |
SUBSP | — | Subspace iteration |
REDUC | — | Householder (reduced) |
UNSYM | — | Unsymmetric matrix. This option cannot be followed by a subsequent spectrum analysis. |
DAMP | — | Damped system. This option cannot be followed by a subsequent spectrum analysis. |
QRDAMP | — | Damped system using QR algorithm. This option cannot be followed by a subsequent spectrum analysis. |
VT | — | Solve with the Variational Technology method of ANSYS DesignXplorer. |
The number of modes to extract. The value can depend on the value supplied for Method. Defaults to the number of master DOFs when Method = REDUC. For the other methods, NMODE has no default and must be specified. When Method = SUBSP, only half of the total number of DOFs can be extracted (that is, if the model contains 100 DOFs, only 50 modes are extracted even if NMODE is set to 100). However, if Method = LANB or LANPCG, 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 = REDUC, NMODE should be less than half of the number of master DOFs. When Method = SUBSP, NMODE should be less than half of the total number of DOFs. When Method = LANPCG, NMODE should be less than 100 to be computationally efficient.
The 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 SUBSP, UNSYM, or DAMP are zero or blank, the default value is -1.0. Eigenvalue extraction is most accurate near the shift point; multiple shift points are used in the LANB, SUBSP, and QRDAMP methods. For SUBSP method, and for LANB, LANPCG, 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 method may lead to inefficient solution times because the LANPCG method will find all eigenvalues between zero and FREQB before finding the requested number of modes between FREQB and FREQE.
The ending, or upper end, of the frequency range of interest. The default is 1e8 for Method = LANB, LANPCG, SUBSP, and QRDAMP. The default for the other methods is to calculate all modes, regardless of their maximum frequency.
CPXMOD
ON | — | Calculate complex eigenmodes. Valid only when Method = QRDAMP. |
OFF | — | Do not calculate complex eigenmodes. |
PRMODE
The number of reduced modes to print. Valid only when Method = REDUC.
Mode 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 (Nrmkey = OFF). |
Specifies modal analysis (ANTYPE,MODAL) options. Additional options used only for subspace iteration eigenvalue extraction are specified by SUBOPT. 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. Method = LANPCG is available in both shared-memory ANSYS and Distributed ANSYS.
The Block Lanczos method is strongly recommended for high-frequency magnetic eigenvalue problems. The initial frequency guess is not critical. The ratio of FREQE to FREQB can be up to 1 x 106. The PCG Lanczos method is not supported for high-frequency magnetic eigenvalue problems.
If you specify complex eigenmode calculation (CPXMOD = ON), mode shape unit normalization to unity (Nrmkey = ON) becomes the default behavior .
This command is also valid in PREP7.