FRF sets

Frequency response functions (FRFs) are the response of a structural or vibro-acoustic model under applied loads. The response is a measure of magnitude and phase of the output as a function of frequency in comparison to the input. These responses can be acceleration, acoustic pressure (vibro-acoustic analyses only), displacement, single-point constraint (SPC) forces, and velocity. These responses are computed in function (SORT2) format. The FRFs are plotted as frequency and response.

You can use FRF results as standard result functions, and you can import the FRF results from a Simcenter Nastran (.op2) file, Simcenter Testlab (.lms) file, or universal (.unv) file, which you then use to replace a FEM component in an assembly FEM. Using an FRF set as a FEM component can reduce solution time and resources.

To compute an FRF set, use the following solutions:

• SOL 108 Direct Frequency Response Functions or SOL 111 Modal Frequency Response Functions in Simcenter Nastran or MSC Nastran structural solutions

• SOL 108 Direct Frequency Response Functions or SOL 111 Modal Frequency Response Functions in Simcenter Nastran vibro-acoustic solutions.

To compute FRFs, use the following loads:

• Unit Force — The unit force is an excitation that you can apply to FE entities on a few, or all, degrees of freedom. The response from unit force loads can be scaled to a larger or smaller value and new responses will be computed.For more information see Unit Force dialog box.

• Unit Enforced Motion — Unit enforced motion of displacement, velocity, or acceleration can be applied to FE entities with few, or all, degrees of freedom. Use this load when the known motion is specified as displacement, velocity, or acceleration to FE entities instead of directly applied loads.For more information see Unit Enforced Motion dialog box.

For linear analyses, the software scales the Unit Force and Unit Enforced Motion loads for larger or smaller loads, thus saving computation time to compute additional responses.

FEM Structural mesh	Computed FRF
FEM structural mesh converted to an FRF set

You can plot the following FRF results using scenario-based post-processing:

• Displacement

• Velocity

• Acceleration

• Acoustic Pressure (vibro-acoustic analyses only)

• SPC Forces

Where do I find it?

Creating FRF solutions

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter Nastran or MSC Nastran as the specified solverStructural (Simcenter Nastran and MSC Nastran) or Vibro-Acoustic (Simcenter Nastran) as the specified analysis type
Command Finder	Solution
Simulation Navigator	Right-click the simulation file→New Solution

Creating unit force or unit enforced motion loads

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter Nastran or MSC Nastran as the specified solverStructural (Simcenter Nastran and MSC Nastran) or Vibro-Acoustic (Simcenter Nastran) as the specified analysis typeSOL108 Direct Frequency Response Functions or SOL111 Modal Frequency Response Functions as the specified solution type
Command Finder	Unit Force Unit Enforced Motion
Simulation Navigator	Expand the Solution Step of an active solution → right-click Loads → New Load

How do I

FRF set in assembly FEM workflow

Replace a FEM file with a mode set or FRF set

Add a new FEM component from an FRF set to an assembly FEM

Editing modes or functions in mode sets and FRF sets

Edit a mode set or FRF set

Learn more

FRF set workflow

Using Modal Expansion to expand measurements to a FEM

Quick links

Command reference

Pre/Post video examples

Bulk Entry Descriptions

Simcenter 3D tutorials

Browse Simcenter 3D help by product area

Related Topics

Assembly FEM

FRF sets, Simcenter 3D 2021.1 Series

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