Learning Forum

[deepesh.p.gurdasani]

1) If I give input PSD data such that RMS input value is around 100 g, what should be an ideal response RMS value at any given point ? 

2) I give input PSD, then check response RMS value by inserting 'response PSD' at given nodal location / vertex . Is this the correct way to check the result ?

 

Thanks in advance

[Chandra Sekaran]

The response would depend on the natural frequencies and mode shapes of the structure to which you apply the input PSD.

For the results, you can look at 1 sigma response of displacement/stress/velocity etc. Or you can do a response PSD at any location.

[deepesh.p.gurdasani]

But just like the modes extracted should be 1.5x the last input frequency, isnt there any such rule that response RMS should be some 3x, 4x, etc. of the input RMS for the result to be correct ?

[dlooman]

The rms of an acceleration psd could be applied in a static analysis to get the static part of the response.  The actual response should be greater due to the dynamic effect.  The rms of a narrow band psd could be applied in a harmonic analysis at the central frequency as a check.

[deepesh.p.gurdasani]

The RMS of any band PSD (I am assuming one of the frequencies of PSD, probably central frequence is what you mean) if applied in harmonic analysis, what check can I get from it? 

[dlooman]

The harmonic results will match the PSD 1 sigma results.

[deepesh.p.gurdasani]

suppose I have 5 frequencies with g^2/Hz values

1Hz, 2Hz, 3Hz, 4Hz, 5Hz and its g^2/Hz values (Here By central frequency you mean 3 Hz here I assume (Please correct me if central frequency is some concept here) . Now in harmonic if I give 3 Hz as frequency and a load/force for it, wouldnt it apply sinusoidal load by default at 3 Hz? whereas in PSD data, the data might not be equivalent to a sinusoidal type force.

[dlooman]

That's not a narrow band PSD, but yes if you ran a harmonic analysis at 3 hz with the rms value and the same damping you would get a similar result to the 1 sigma PSD value.  I like this analogous solution as a check and also as a way of getting a better feel for what the PSD really is.

[deepesh.p.gurdasani]

Any specific reason why 1 sigma value of PSD would match harmonic analysis done at central frequency ?

[dlooman]

Yes, it's because a PSD analysis is essentially a harmonic analysis of an acceleration distribution.  One method of solving the PSD (not used in Ansys) is to do harmonic sweep and integrate the results.

[deepesh.p.gurdasani]

but in 'Harmonic analysis', by default the load would be applied in sinusoidal type, right? where as in PSD, it depends on the PSD data and moreover its random vibration. How can both be same ? (though it could match with sigma 1 value but I am trying to understand the logic behind it)

[dlooman]

The excitation is random with respect to phase and amplitude, but the excitations are still assumed to be sinusoidal.