Frequency Domain, Modal
The Frequency Domain, Modal () study and study step are used, for example, to compute the response of a linear or linearized structural mechanics model subjected to harmonic excitation for one or several frequencies.
The Frequency Domain, Modal study consists of two study steps: an Eigenfrequency study step for computing the eigenfrequencies and eigenmodes of the structure followed by a second Frequency Domain, Modal study step for computing the modal response. In the mode superposition analysis, the deformation of the structure is represented by a linear combination of the structure’s eigenmodes. This means that the frequencies to be studied are limited by the frequencies of the computed eigenmodes. A Frequency, Domain Modal study usually results in a faster computation than a direct solution using the Frequency Domain study.
The following sections describe the settings for the Frequency, Domain Modal study step. See Eigenfrequency for information about the settings for the Eigenfrequency study step.
The Frequency, Domain Modal study step node corresponds to a modal frequency sweep for systems with frequency-based loads. It gives a Modal Solver.
The Include geometric nonlinearity check box and Mesh Selection are described in Common Study Step Settings. There is also detailed information in the Physics and Variables Selection and Values of Dependent Variables sections.
Study Settings
Specify the frequencies to use for the frequency sweep. Select the unit to use from the Frequency unit list (default: Hz). Enter the frequencies in the Frequencies field using space-separated numbers or the range function.
Use the Load parameter values field to select a file with parameter values. Click the Browse button () to browse the file system. You can also click the downward arrow beside the Browse button and choose Browse From () to open the fullscreen Select File window. Click the downward arrow for the Location menu () to choose Show in Auxiliary Data () to move to the row for this file in the Auxiliary Data window, Edit Location () (if the location is a database), Copy Location (), and (if you have copied a file location) Paste Location (). After selecting a file, click the Read File () button to load the parameter values into the Frequencies field.
Use the Parameter list type list to control how to interpret the parameter values entered in the Parameter values field. Select:
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Frequency (the default setting) to use the parameter values without modification.
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Fraction to multiply the parameter values by the absolute value of the largest eigenvalue in the reduced model divided by two.
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Spread to treat the parameter values as an interval around each eigenvalue in the reduced model. That is, the absolute value of each eigenvalue is multiplied by the parameter values and the resulting parameter value vectors are concatenated into one. In the case when intervals around two or more eigenfrequencies overlap, priority is given to frequencies closest to their corresponding eigenvalue to ensure the resulting intervals are disjoint.
For the last two options above, the Frequencies field changes to a Parameter values field, and the following settings becomes available:
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From the Minimum frequency , choose Automatic (the default) or Manual. If you chose Manual, also specify a minimum value (in the chosen frequency unit) in the Value field.
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From the Maximum frequency, choose Automatic (the default) or Manual. If you chose Manual, also specify a maximum value (in the chosen frequency unit) in the Value field.
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If desired, add more frequencies in the Additional frequencies field. Use the Range button () if you want to add a range of frequencies.
You can add the minimum and maximum frequency if desired (by default there is no limit) to limit the output frequencies to solve for. You can also supply a list of additional frequencies. Those frequencies will be added to the output frequencies to solve for, which makes the spread option effectively an extension of the frequency parameter list type.
These settings are always available:
For information about the Reuse solution from previous step list, see Reuse Solution from Previous Step List.
Use a scalar-valued expression for the value in the Load factor field (default: 1). The solver uses this expression as a load factor that is multiplied with the residual.
Values of Linearization Point
Use the settings in this section to specify a linearization point.
From the Settings list, choose Physics controlled (the default) to use linearization point settings controlled by the physics interfaces. Choose User defined to specify the linearization point using the Method list. Select:
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 Initial expression to use the expressions specified on the Initial Values nodes under a specific physics interface as a linearization point.
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 Solution to use a solution as a linearization point.
Use the Study list to specify which solution to use from the available studies. Select:
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 Zero solution to use a linearization point that is identically equal to zero.
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Any other available solution to use it as a linearization point. It can be the current solution in the sequence, or a solution from another sequence, or a solution that was stored with the Solution Store node. You select a stored solution by changing Use to the name of the stored solution. Choose a solution using the Selection list (see Values of Dependent Variables under Common Study Step Settings).
With the Structural Mechanics Module, see Bracket — General Periodic Dynamic Analysis, Application Library path Structural_Mechanics_Module/Tutorials/bracket_general_periodic.