Units of a PSD function for Random Response Analysis

[mashst24]

Hello,

I am performing a random vibration response analysis with an input acceleration spectral density (ASD) function that has units G^2/Hz. My model uses in/s^2 as the units for acceleration, so 386.1 in/s^2 is equal to one "G". Does NX Nastran interpret the PSD function in terms of Gs, or the models base acceleration units? I know that in order to apply a 1 G acceleration load to my model I have to multiply the 1 G by 386.1 and then use that as my acceleration load magnitude. Is the same multiplication required to convert my ASD function into units of (in/s^2)^2/Hz instead of G^2/Hz? I followed the "Random Response of the Hinge Model" example in the help section, which does not perform any multiplication but my stress and displacement results seem very small.

Thanks for your help.

 

[BlasMolero]

Hello!,
Entering the PSD as G^2/Hz will cause all the output to be in G, including stress.
Most analysts prefer their stresses in psi or Pa. For output to be in psi, you need to scale the PSD function so that it is in consistent units, instead of G.

For instance: If the input is a vibration with a PSD input of 0.30 G^2/Hz from 10 to 2000 Hz, then you need to enter a value of 0.30*(386)^2. Since the input is now in (in/s2)2/Hz, all the output will be in inches, psi and in/s2.

Scaling either the acceleration load, or the acceleration load curve will produce the same results, but it is more generally accepted to scale the PSD curve as described above.

Best regards,
Blas.

~~~~~~~~~~~~~~~~~~~~~~
Blas Molero Hidalgo
Ingeniero Industrial
Director

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