Using Fast RMS Fitted PSD functions

Fast RMS Fit PSD (Power Spectral Density) functions can be used as excitations to compute root mean square results quickly. Using Fitted PSD functions is a tradeoff of accuracy versus computation speed. Results of a properly set up model should closely match the results when using standard RMS methods, where integration at all frequencies is performed, but which can be a long process.

You use Fitted PSD functions when:

• You have a FEM with a large number of nodes and elements and want to compute RMS results quickly.

• You want to quickly perform comparative studies based on RMS results.

Requirements to use Fast RMS Fit functions as excitations to compute PSD responses are:

• The Fast RMS Method can produce incorrect results if you have active normal modes outside the frequency range of the Fitted PSD. Therefore, you should examine the mode list (in the Response Dynamics Details View) and deactivate modes that are outside of your excitation frequency range. For example, if your excitation ranges from 20 Hz to 500 Hz, deactivate all modes lower than 20 Hz and higher than 500 Hz.

• In addition, you may deactivate modes inside your frequency range that would not be excited, such as those with very small effective mass or small modal participation. However, if you deactivate dominant modes inside the Fitted PSD frequency range, the RMS results may be incorrect.

Tip:

• When running a comparative study, keep viscous and hysteretic damping constant across FEM models so you can compare results easily.

• When you create a Random Event in Simcenter 3D Response Dynamics select Use Fast RMS Evaluation Method to use a Fitted PSD function stored in an AFU.

How do I

Convert SRS/PSD/Time functions

Import test data into Response Dynamics

Create nodal and enforced motion excitations

Create distributed load excitations

Create static excitations

Create a drop impact or constant velocity impact simulation

Calculate random RMS functions from PSD input

Create rotating force excitations

Correlate two PSD excitations

Create DDAM excitations

Learn more

Excitation loads

Response Dynamics Function Toolkit

Using pulse functions for shock analysis

Velocity Impact excitations

Rotating forces and unbalanced masses

PSD correlation

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Function parameters by event type

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Using Fast RMS Fitted PSD functions, Simcenter 3D 2021.1 Series

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