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# Siemens NX Nastran - Random analysis (PSD curve)

## Siemens NX Nastran - Random analysis (PSD curve)

(OP)
Hi all,

I would like to ask something about random analysis. I´m solving a random analysis problem in NX Nastran. It's going about PSD units. I went through these posts:
https://community.plm.automation.siemens.com/t5/CA...

and many more...

But just to clarify. In my results I still get quite small RMS stress and I don't believe it... If I want the results in G, I will apply the PSD loading curve in G^2/Hz. Then similiarly, to get the results in Pa, PSD in (m/s^2)^2/Hz; to get the results in MPa, PSD in (mm/s^2)^2/Hz. Am I right? So If I have for instance PSD value 0.1 G^2/Hz and I want the results in MPa, I will simply convert it to (mm/s^2)^2/Hz => I will get 0.1*(9810*9810)=9623610 (mm/s^2)^2/Hz.

Also I want to ask you one more thing. The pre calculation for the random analysis is the harmonic analysis with the unit loading. Does it mean that If I want it in: G units => applying the unit loading of 1 G

Is it OK or Am I having something wrong?

Filip

### RE: Siemens NX Nastran - Random analysis (PSD curve)

as long as you work in consistent unit there is no issue
Are you using NX sol111 or the Response Simulation module?
If you use mm and mass = tonnes (Mg), a psd in g^2/Hz then you stress should be in MPa^2/Hz, disp in mm^2/Hz, accel in (mm/s^2)/Hz etc

### RE: Siemens NX Nastran - Random analysis (PSD curve)

(OP)
I use SOL111. I can post here the input *dat file after weekend... First time I have used: tonnes,mm, unit loading SPCD = 9810 mm/s^2, PSD in G^2/Hz => but I got very small RMS stress (something like 0.0001... MPa). Then I've tried: tonnes,mm, unit loading SPCD = 1 mm/s^2, PSD in (mm/s^2)^2/Hz => higher RMS stress values (0.01... MPa). But still I think there is an issue with units...

Filip

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