Viscous and structural damping values in mode sets (Simcenter Nastran)

For Simcenter Nastran SOL 103 Eigenvalue solutions that you solve in release 2020.2 and later, the software computes values for equivalent viscous and structural damping and writes them to the OP2 file when you specify PARAM,MDLDAMP,YES in the Nastran input file. In Pre/Post, you can select the Output Diagonalized Modal Damping Matrices (MDLDAMP) check box in the Solution dialog box to specify this parameter.

Mode shapes are not orthogonal with respect to the viscous or structural damping matrices. Modally projected damping matrices result in non-diagonal matrices. By retaining the diagonal values only, the software converts the damping values to an equivalent modal and equivalent structural damping and writes those values to the damping matrices.

Setting up a FEM file for computing viscous and structural damping

For Simcenter Nastran to compute values for equivalent viscous and structural damping values during the solution, you must create a FEM file that includes the appropriate element types and properties.

• For equivalent modal viscous damping, the FEM file must contain elements such as CVISC and CDAMP2 elements, a PDAMP physical property table, or B2GG matrices that you define in the Nastran input file with the DMIG bulk data entry.

• For equivalent modal structural damping, in the FEM file contain elements such as CELAS2 elements, physical properties that you define with a PBUSH or PELAS physical property table, a material (MATi) that includes a Structural Damping Coefficient GE value, or K4GG matrices that you define in the Nastran input file with the DMIG bulk data entry.

Specifying scale factors for modal viscous and structural damping in a mode set

When you use an OP2 file that contains viscous and structural damping values as the data source for a mode set, you can use the Edit Modes dialog box to apply scaling factors to these values. You cannot edit the actual viscous and structural damping values.

In the Edit Modes dialog box:

• The Equivalent Modal Viscous Damping (%) column lists the viscous damping values (). You can use the Scale Factor (Modal Viscous Damping) column to apply a scale factor to selected modes. To deactivate this damping value, specify a scale factor of 0.0.

• The Equivalent Modal Structural Damping (%) column lists the structural damping values (). You can use the Scale Factor (Modal Structural Damping) column to apply a scale factor to selected modes. To deactivate this damping value, specify a scale factor of 0.0.

Viscous and structural damping values in the Edit Modes dialog box

Note:

The Edit Modes table includes these columns only when the OP2 file that you use as the data source for the mode set contains equivalent modal viscous or structural damping results.

Using parameters to control damping

For Simcenter Nastran, you can use parameters to control the damping that the software computes during a SOL 103 Eigenvalue solution.

• To have the software output modal viscous and structural damping diagonal matrices, set PARAM, MDLDAMP=YES in the input file. In the Simcenter Nastran environment, in Pre/Post, on the Bulk Data page in the Solution dialog box, select the Output Diagonalized Modal Damping Matrices (MDLDAMP) check box.

• To have the software combine modal structural damping and viscous damping and write out the critical viscous damping ratio only, set PARAM, KDAMP=1 in the input file. In the Simcenter Nastran environment, in Pre/Post, use the Solution Parameters modeling object dialog box to set PARAM, KDAMP=1 for a solution.To have the software compute modal structural damping only, set PARAM, KDAMP=-1 in the input file. In the Simcenter Nastran, in Pre/Post, use the Solution Parameters modeling object dialog box to set PARAM, KDAMP=-1 for a solution.

Where do I find it?

Application	Pre/Post
Prerequisites	An assembly FEM file as the work part and displayed partSimcenter Nastran as the specified solverA mode set representation of a component FEMA Simulation file created from the assembly FEM
Simulation Navigator	Expand the Mode Sets or FRF Sets folder→right-click the set you want to edit→Edit Modes

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Displaying alternate FEM representations

Replace a FEM file with a mode set or FRF set

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Edit a mode set or FRF set

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Viscous and structural damping values in mode sets (Simcenter Nastran), Simcenter 3D 2021.1 Series

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