*FREQUENCY

Extract natural frequencies and modal vectors.

This option is used to perform eigenvalue extraction to calculate the natural frequencies and corresponding mode shapes of a system.

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Natural Frequency Extraction

Products Abaqus/Standard Abaqus/CAE Abaqus/AMS

Type History data

LevelStep

Abaqus/CAE Step module

Optional parameters

ACOUSTIC COUPLING

For the AMS eigensolver and Lanczos eigensolver, set ACOUSTIC COUPLING=ON to include the effect of acoustic-structural coupling during the natural frequency extraction procedure in models with acoustic and structural elements coupled using the TIE option or in models with ASI-type elements. This is the default option for the Lanczos eigensolver.

For the AMS eigensolver and Lanczos eigensolver based on the SIM architecture, set ACOUSTIC COUPLING=PROJECTION to extract the uncoupled acoustic and structural modes and project the acoustic-structural coupling operator during the natural frequency extraction procedure in models with acoustic and structural elements coupled using the TIE option. This is the default option for the AMS eigensolver.

Set ACOUSTIC COUPLING=OFF to omit the projection of the acoustic-structural coupling operator and to ignore the effect of acoustic-structural coupling during natural frequency extraction in models with acoustic and structural elements coupled using the TIE option or in models with ASI-type elements.

This parameter is not relevant for the subspace iteration eigensolver.

DAMPING PROJECTION

This parameter is relevant only for the AMS eigensolver or for the Lanczos eigensolver used in conjunction with the SIM parameter.

Set DAMPING PROJECTION=ON (default) to project the viscous and structural damping operators during the natural frequency extraction procedure. If there is no damping defined in the model, Abaqus/Standard does not perform the projection.

Set DAMPING PROJECTION=OFF to omit the projection of damping operators.

EIGENSOLVER

Set EIGENSOLVER=LANCZOS (default) to invoke the Lanczos eigensolver.

Set EIGENSOLVER=AMS to invoke the automatic multilevel substructuring (AMS) eigensolver.

Set EIGENSOLVER=SUBSPACE to invoke the subspace iteration eigensolver.

NORMALIZATION

Set NORMALIZATION=DISPLACEMENT to normalize the eigenvectors so that the largest displacement, rotation, or acoustic pressure (in coupled acoustic-structural extractions) entry in each vector is unity. Displacement normalization is the default for both the subspace iteration eigensolver and for the Lanczos eigensolver when you do not include the SIM parameter.

Set NORMALIZATION=MASS to normalize the eigenvectors with respect to the structure's mass matrix (Abaqus/Standard scales the eigenvectors so that the generalized mass for each vector is unity). Mass normalization is the default and only option available for the AMS eigensolver. Abaqus/Standard switches on mass normalization for both the Lanczos eigensolver and the subspace iteration eigensolver when you use them in conjunction with the default SIM parameter.

PROPERTY EVALUATION

Set this parameter equal to the frequency at which to evaluate frequency-dependent properties for viscoelasticity, springs, and dashpots during the eigenvalue extraction. If you omit this parameter, Abaqus/Standard evaluates the stiffness associated with frequency-dependent springs and dashpots at zero frequency and does not consider the stiffness contributions from frequency domain viscoelasticity in the FREQUENCY step.

RESIDUAL MODES

This parameter is relevant only for the Lanczos and AMS eigensolvers.

Include this parameter to indicate that Abaqus/Standard should compute residual modes.

SIM

This parameter is relevant only for the Lanczos and subspace iteration eigensolvers.

Set the value of this parameter equal to NO if the non-SIM architecture is required for the Lanczos or subspace iteration eigensolvers.

Set the value of this parameter equal to YES (default) if the SIM architecture is required.

The SIM architecture is the only option if the AMS eigensolver is activated.

Optional parameter when EIGENSOLVER=AMS

NSET

Set this parameter equal to the name of the node set or include the parameter with no value to allow Abaqus/Standard to automatically select the nodes at which to compute eigenvectors. If you omit this parameter, Abaqus/Standard computes eigenvectors at all nodes.

Data line for a natural frequency extraction when EIGENSOLVER=LANCZOS

First (and only) line

  1. Number of eigenvalues to calculate. You can leave this field blank if you provide the maximum frequency of interest and want to evaluate all the eigenvalues in the given range. You must provide the number of requested eigenmodes in a cyclic symmetry analysis or if the analysis includes more than one natural frequency extraction step.

  2. Minimum frequency of interest, in cycles/time. If you leave this field blank, Abaqus/Standard does not set a minimum.

  3. Maximum frequency of interest, in cycles/time. If you leave this field blank, Abaqus/Standard does not set a maximum. This value is required if you left the first field blank.

  4. Shift point, in squared cycles per time (positive or negative). Abaqus/Standard extracts the eigenvalues closest to this point.

  5. Block size. If you omit this entry, Abaqus/Standard creates a default value, which is usually appropriate.

  6. Maximum number of block Lanczos steps within each Lanczos run. If you omit this entry, Abaqus/Standard creates a default value, which is usually appropriate.

  7. Acoustic range factor. This factor applies only to structural-acoustic problems and is used to set the maximum frequency for the acoustic stage of the uncoupled eigenproblem as a multiple of the nominal maximum frequency of interest. Abaqus/Standard supports this factor only when using the SIM architecture and you provide the maximum frequency of interest. The acoustic range factor must be greater than 0. The default value is 1.0.

Subsequent lines when default residual modes are not sufficient or you request residual modes

  1. Node number or node set label.

  2. First degree of freedom for which residual modes are requested.

  3. Last degree of freedom for which residual modes are requested. You can leave this field blank if you request residual modes for only one degree of freedom.

Repeat this line as often as necessary to request residual modes.

Data lines for a natural frequency extraction when EIGENSOLVER=AMS

First line

  1. Number of eigenvalues to calculate. If you leave this field blank, Abaqus evaluates all the eigenvalues from the minimum frequency of interest up to the maximum frequency of interest.

  2. Minimum frequency of interest, in cycles/time. If you leave this field blank, Abaqus does not set a minimum.

  3. Maximum frequency of interest, in cycles/time.

  4. A ⁢ M ⁢ S cutoff 1 multiplied by the maximum frequency of interest is used to truncate local substructure eigenmodes in the reduction phase.

  5. A ⁢ M ⁢ S cutoff 2 multiplied by the maximum frequency of interest is used to truncate local branch substructure eigenmodes for defining a starting subspace in the reduced eigensolution phase.

  6. A ⁢ M ⁢ S cutoff 3 multiplied by the maximum frequency of interest is used to truncate local leaf substructure eigenmodes for defining a starting subspace in the reduced eigensolution phase.

  7. Acoustic range factor. This factor applies only to structural-acoustic problems. Use it to set the maximum frequency for the acoustic stage of the uncoupled eigenproblem as a multiple of the nominal maximum frequency of interest. The acoustic range factor must be greater than 0. The default value is 1.0.

Subsequent lines if default residual modes are not sufficient or you request residual modes

  1. Node number or node set label.

  2. First degree of freedom for which residual modes are requested.

  3. Last degree of freedom for which residual modes are requested. You can leave this field blank if you request residual modes for only one degree of freedom.

Repeat this line as often as necessary to request residual modes.

Data line for a natural frequency extraction when EIGENSOLVER=SUBSPACE

First (and only) line

  1. Number of eigenvalues to be calculated.

  2. Maximum frequency of interest, in cycles/time. This user-specified maximum frequency is increased automatically by 12.5% to help capture closely spaced modes. Abaqus/Standard also reports all eigenvalues that converge in the same iteration as those in the specified range, even if their frequencies are more than 12.5% above the maximum frequency specified by the user. If you leave this field blank, Abaqus/Standard does not set a maximum.

  3. Abaqus/Standard extracts frequencies until either of the above limits is reached.

  4. Shift point, in squared cycles per time (positive or negative). Abaqus/Standard extracts the eigenvalues closest to this point.

  5. Number of vectors used in the iteration. If you omit this entry, Abaqus/Standard creates a default value, which is usually appropriate. The default number of vectors used is the minimum of (n+ 8, 2n), where n is the number of eigenvalues requested (the first data item on this data line). In general, the convergence is more rapid with more vectors, but the memory requirement is also larger. Therefore, if you know that a particular type of eigenproblem converges slowly, you might reduce the analysis cost by providing more vectors.

  6. Maximum number of iterations. The default is 30.