|
Random Vibration Mass Participation Factors
|
|
Source：Internet Author：Unknow Pubdate：2010-05-22
|
|
jvian (Aerospace)
20 May 10 18:35
Hello,
I am trying to run a random vibration analysis in pro/mechanica and have ran into a question about mass participation factors. I understand the need for a high mass participation factor to determine global modes over local modes but I do not understand why an increased number of modes should be used for the random analysis when the frequency range of interest is so much lower than the frequencies obtained. My preliminary modal analysis produces only a limited number of modes with a low (<35%) mpf. Increasing the number of modes did increase the mpf (>80%) but then went way beyond (~3x) the desired frequency range of the psd.
If my frequency range of interest is 10 to 2500 hz, then why should modes of upwards of 4000 hz be included in the analysis? If I use the higher modes because they have or contribute to a higher mpf, doesn't that claim that the lower modes in the frequency range of interest do not participate globally and therefore are not of much concern? 字串9
I am trying to understand this and am open to all forms of help and advice.
Thank you
GregLocock (Automotive)
20 May 10 23:17
"If my frequency range of interest is 10 to 2500 hz, then why should modes of upwards of 4000 hz be included in the analysis? "
Because the residual participation of those higher frequency modes will affect responses in your frequency range of interest.
Might be worth running a hand analysis (shock horror) on a cantilever to look at the innaccuracy you get by ignoring higher modes.
Typically in a vehicle dynamics model we include modes up to 60 Hz, even though our models have an upper frequency limit of 20 Hz.
I don't know about the wisdom of using MPFs as such an important criterion. If you have guidelines suggesting that approach I suppose you could try running it both ways and see what the difference is.
Cheers 字串6
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
jvian (Aerospace)
21 May 10 10:13
Thank you Greg.
Forgive me if I sound a bit uneducated as neither my experience nor anything that learned in school involve this subject. So far most research I have performed involving random vibration in pro/mechanica emphasized the importance of mpf's. If there is another approach other than the mpf's could you point me in a direction to research it? As far as the mpf's could it be described as a form of harmonic of the higher frequencies that is what is participating or residual in the lower frequencies or visa versa?
I appreciate the help and thank you again for responding.
GregLocock (Automotive)
21 May 10 21:18
字串2
Either you do an experiment, trying it with and without the extra modes, or a hand calc as I suggested. Or you do some heavy maths. Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
（Click：）
|
| Previous：Altair Hyperworks Optistruct For Composites Optimization Next：Nothing |
|
[ Add TO Favorites]
[TOP] [PRINT]
[CLOCE WINDOWS] |
|
|
|
