modopt

Mapdl.modopt(method='', nmode='', freqb='', freqe='', cpxmod='', nrmkey='', modtype='', blocksize='', freqmod='', **kwargs)

Specifies modal analysis options.

APDL Command: MODOPT

Parameters
method

Mode-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

nmode

The 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.

freqb

The beginning, or lower end, of the frequency range of interest.

freqe

The 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.

cpxmod

Complex 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 Method = UNSYM. Not supported for Method = QRDAMP.

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 Method = QRDAMP. This is the default for this method.

nrmkey

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).

modtype

Type 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.

blocksize

The 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).

freqmod

The 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.

Notes

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 DAMP and QRDAMP options cannot be followed by a subsequent spectrum analysis. The UNSYM method supports spectrum analysis when eigensolutions are real.

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. However, PCG Lanczos, SUBSP, UNSYM, DAMP, and QRDAMP 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.