Comparing Modal analysis FEA vs Experimental / High modal density problem
Comparing Modal analysis FEA vs Experimental / High modal density problem
(OP)
I have a bladed disk with 21 blades. By FEA i have obtained the First family of in plane bending modes. See the attached file(FEA_results).
http://files.engineering.com/getfile.aspx?folder=2...
After i did an experimental modal analysis using an impact hammer.
I am attaching the measured FRF for condition (impact on the blade 1 and response on the blade 1).
http://files.engineering.com/getfile.aspx?folder=4...
http://files.engineering.com/getfile.aspx?folder=9...
here i am getting a single peak may be due to high modal density Or does this response includes all nodal diameter modes of first family?How to extract from it all nodal diameter modes.
Can somebody guide me how to segregate the nodal diameters and identify the mode shape from measured FRF for such a particular case.
Thanks.
http://files.engineering.com/getfile.aspx?folder=2...
After i did an experimental modal analysis using an impact hammer.
I am attaching the measured FRF for condition (impact on the blade 1 and response on the blade 1).
http://files.engineering.com/getfile.aspx?folder=4...
http://files.engineering.com/getfile.aspx?folder=9...
here i am getting a single peak may be due to high modal density Or does this response includes all nodal diameter modes of first family?How to extract from it all nodal diameter modes.
Can somebody guide me how to segregate the nodal diameters and identify the mode shape from measured FRF for such a particular case.
Thanks.





RE: Comparing Modal analysis FEA vs Experimental / High modal density problem
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Comparing Modal analysis FEA vs Experimental / High modal density problem
By first plot i wanted to show that by FEA modal analysis i get a set of natural frequencies whose values are very close to each other (Case of high modal density).
i did measurement using two sensors which were attached to blade extremity , one on each extreme corner and the impact was made on all the blades one by one from 1 to 21.
This time I have attached the amplitude of FRF (acceleration / Force) graph with zoom out for drive point measurement ( sensor on blade and impact on same blade) :
http://files.engineering.com/getfile.aspx?folder=1....
If something specific is required please tell me.
However what i require is :
Thanks again
RE: Comparing Modal analysis FEA vs Experimental / High modal density problem
Could you show me the coherence, phase, and magnitude plot for (a) the driving point and (b) another point, across a frequency range of 500-700 Hz, if for some bizarre reason you don't want to show the whole thing. Also post a nyquist plot of the frequency range 600-700 hz for the same points.
As to your specific (good) questions
1) yes that is very common, the reality is that a system with many identical structures in it will have clusters of modes where say 10 blades move one way and 11 the other, but exactly which blade does what is determined by non linearities. I'd have to see the above plots before suggesting a strategy.
2)yes that is a typical occurrence. Again the physical system is 'selecting' which mode gets excited by non linearities. The trick there is to use non contacting forcing and response measurement, typically, ie shake the thing as a whole and measure the response with eg doppler lasers or cameras, depending on amplitude. If you can't do that then do your survey with all 21 blades instrumented at once and don't move the excitation point.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Comparing Modal analysis FEA vs Experimental / High modal density problem
Mistuning - one of many references is:
Castanier, M. P., Ceccio S. L., Epureanu, B, I., Pierre, C, (2007) “Next-Generation Modeling, Analysis, and Testing of the Vibration of Mistuned Bladed Disks,” AFOSR Grant FA9550-04-1-0099, The University of Michigan, Ann Arbor, MI, USA.
You could apply clay to all blades except one and get a quick rap test; then repeat to obtain variation of all 21 blades.