Mass participation factor and it's role in modal anlysis

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Main Question or Discussion Point

When we are doing modal analysis, how many mode should we extract? For complex piping system with thicker wall and less diameter the no of modes to be consider is a difficult task. How can I define upto how many modes should I evaluate for the modal results and check it with external excitation.

I am surprise there is no clear cut answer to this till now, as far as I know and I got. There are very few documents where they have written the mass participation factor, and it's direction, external excitation direction and mode shape determine the no of modes we have to consider. As far as my understanding, I also think like that only and do the same.

But there is a little bit confusion with my friends, who told me only first few modes (for piping four modes) is need to examine for resonance, no need to go for higher modes even though mass participation factor is high enough!!!

And can anyone tell me for complex 3d geometry, is that the nodes of vibration always lie on the geometry itself??

And is it possible that the amplitude at higher modal frequency can be higher than that of lower modal frequency??

And for continuous systems how many fundamental frequency are there??


I am seriously looking for it's answer. Please help me.
Thanks in advance...Lot's of love.....
 

Answers and Replies

  • #2
Mech_Engineer
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As a general rule, you probably want to look at as many modes as it takes to fully explore the frequency excitation range you're expecting. For piping, look at things like pumps and motors, find what frequency they operate at (probably no higher than a couple hundred Hz) and go from there.

Typically for structural excitations I look at 6 modes minimum (obvious) and don't normally look at more than 10 modes or a couple of hundred Hz (say 500 Hz).
 
  • #3
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Thank you for your response.

For me, my compressor frequency is very low(in the range of 47 to 48) and we always avoid the range from [0-compressor frequency+0.1*compressor frequency]. I don't have any doubt in that. But we are also look for higher frequencies (upto six modes) if that frequency is matching with the harmonics(like 1x, 2x,3x,4x...) of the compressor frequency.


Till now we are not considering the mass participation factor for targeting the modes. But if the first six modes only contribute 60% mass participation, and above 12 th mode also if for 13 mode the mass participation factor is higher should we check with compressor harmonics upto that or not!!!
 

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