Mode-extraction method to be used for the modal analysis.

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.

For Method = LANB, SUBSP and UNSYM, you can specify NMODE = ALL to extract all eigenvalues in a given frequency range. To use this option, you must also define the frequency range (that is, a FREQE value must be set).

Recommendation:

The 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. For UNSYM and DAMP, the default = -1.0 For other methods, the default is calculated internally.

Eigenvalue extraction is most accurate near the shift point. Multiple shift points are used internally in the LANB, SUBSP, UNSYM, and QRDAMP methods. For the LANB, LANPCG, SUBSP, UNSYM, DAMP, and QRDAMP methods with a positive FREQB value, eigenvalues are output beginning at the shift point and increase in magnitude. For the 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.

The ending, or upper end, of the frequency range (or eigenvalue range if FREQMOD is specified) of interest (in Hz). Default = 100 Hz when Method = SNODE. The default for all other methods is to calculate all modes, regardless of their maximum frequency.

To maintain solution efficiency, do 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 and the more eigenvalues produced; however, the solution time also increases. For example, if FREQE is set to 1e8, it causes the underlying supernodal structures to find all possible eigenvalues of each group of supernodes, requiring excessive solution time. The accuracy of the SNODE solution is controlled by FREQE and by the RangeFact value on the SNOPTION command.

Complex eigenmode key. (Valid only when Method = QRDAMP or Method = UNSYM).

Mode shape normalization key:

Type of modes calculated by the eigensolver. Only applicable to the unsymmetric eigensolver.

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:

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.

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.