Learning Forum

[Matthew Kirby]

I am trying to complete Random Vibration analysis of an assembly. The model contains a mixture of fixed and frictionless supports, as well as a number of displacement conditions. I have applied an input PSD (G acceleration) and probed the response at a predetermined location, giving the chart below.

What I'm unable to work out is why the response PSD is lower than the input, where usually these plots mirror each other up until close to the first eigenfrequency. I have probed the PSD response at the location of excitation to check whether the input PSD was applied correctly, which appeared correct. 

What could be the cause of this apparent offset? 

I run these analyses fairly regularly but I haven't seen this issue before so I'm hoping someone can enlighten me.

[Ashish Khemka]

Hi Matthew,

 

Do you have damping defined in the system? Did you get correct match for same setup earlier?

 

Regards,

Ashish Khemka

[Matthew Kirby]

Hi Ashish,

Thanks for your reply.

Damping ratio is defined as 0.025 across the whole frequency spectrum. And no, all previous attempts at this analysis have run into the same problem with the offset between PSD input and reponse.
We regularly run analyses with this damping ratio but have not seen this issue before. 

Regards, 

Matt

[Ashish Khemka]

Hi Matthew,

 

Please wait for other forum members to comment.

 

Regards,

Ashish Khemka