ANSYS prestressed modal analyses with linear perturbation

You can perform prestressed modal analyses with the linear perturbation solution method to calculate the frequencies and mode shapes of a deformed structure or of a structure that includes nonlinear (sliding) contact. After you perform a large deflection nonlinear statics analysis, you can restart the analysis with a modal solution that uses the linear perturbation solution method to calculate the frequencies and mode shapes of the deformed structure.

For details on how to perform this type of analysis in Pre/Post, see Set up a large deflection, prestressed modal cyclic symmetry analysis.

In Pre/Post, you can:

• Restart a Nonlinear Statics solution as a Modal solution and specify options to control the restart.

• Specify linear perturbation options.

• Modify the behavior of individual contact pairs,

Restarting the analysis

You can restart an ANSYS Modal solution from a Nonlinear Statics solution. For example, in a large deflection, prestressed cyclic modal analysis of turbo machinery, you may want to set up two solutions:

• An initial Nonlinear Statics solution in which you specify options to control the restart.

• A restart Modal solution where you can add additional loads or constraints, specify perturbation options, and modify the properties (KEYOPTS) of contact elements.

You can use the Restart Controls (RESCONTROL) modeling object to define options for restarting the analysis, including specifying the frequency at which the software writes the restart results.

The options in this dialog box correspond to the options for the ANSYS RESCONTROL command.

Specifying the linear perturbation method options

In ANSYS, you can use the linear perturbation method to capture the results of a nonlinear static analysis and use them as the initial conditions in modal or buckling eigenvalue analyses. You can use the Linear Perturbation Options (PERTURB) modeling object to define the perturbation options, such as the analysis type, material behavior, contact status of all contact elements, and the load values to retain.

The options in this dialog box correspond to the options for the ANSYS PERTURB command.

For more information, see Specifying linear perturbation options.

Modifying behavior of individual contact pairs

If you want to modify the contact status for individual pairs of contact elements, rather than modifying the status for all contact elements, you can use the Contact Element KEYOPTS Modification (CNKMOD) modeling object. You can also change the units of normal contact stiffness.

The options in this dialog box correspond to the options for the ANSYS CNKMOD command.

For more information, see Modifying the behavior of contact pairs.

Import limitations

You can only import the nonlinear statics solution part of an ANSYS linear perturbation analysis into Pre/Post. The following issues occur if you try to import the modal analysis restart portion of the analysis:

• The resulting solution in Pre/Post does not include any nodes and elements.

• During the import process, Pre/Post uses the ANSYS database file rather than /PREP7 input file.

• Pre/Post imports the modal restart solution as a linear statics solution because it includes the ANTYPE,STATIC command. The software does not process commands from the modal restart solution, such as PERTURB, CNMOD, MODOPT, and MXPAND, during the import process because these properties apply to modal solutions only.

Where do I find it?

Restart solution options

Application	Pre/Post
Prerequisite	A Simulation file as the work part and displayed partANSYS as the specified solver
Simulation Navigator	Right-click the Simulation file and choose New Solution
Location in dialog box	Restart page (Modal solutions)Restart Controls page (Nonlinear Statics solutions)

Restart Controls (RESCONTROL), Linear Perturbation Options (PERTURB), and Contact Element KEYOPTS Modification (CNKMOD) modeling object

Application	Pre/Post
Prerequisite	A Simulation file with ANSYS as the work part and displayed partANSYS as the specified solverModal or Nonlinear Statics as the specified analysis type
Command Finder	Modeling Objects

How do I

Set up a large deflection, prestressed modal cyclic symmetry analysis

Learn more

Controlling how ANSYS writes restart files

Modifying the behavior of contact pairs

Specifying linear perturbation options

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ANSYS prestressed modal analyses with linear perturbation, Simcenter 3D 2021.1 Series

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