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Hi Sir,

 

I performed the modal analysis using optistruct as a solver. Can you please tell me which mode is the natural frequency of the system?

How many frequency I needs to extract. If (Freq. range is 10 to 1000).

What is rigid body mode? Can I considered this modes to find out the response of the system while doing the FRA

 

 

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Hi Rajesh,

 

There is no thumb rule on how many frequencies need to be extracted, but a general practice is 3 to 4 times the freq range. Say if my freq is 200 Hz then extract 800-1000 Hz.

 

If the frequency of the mode is way less (say an order of 1 e-4) then it can be called as a rigid mode. Usually rigid modes comes in sequence, like first 6 modes are rigid modes.

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Hi,

First mode would be fundamental frequency of the system.If you are interested in frequency range then specify N1& N2 in EIGRL card don't specify ND in that case.

If you are interested in first 25 modes then use only ND as 25.

Rigid body modes you would get for free free analysis(No constraint).Some people prefer to carryout FRF with rigid mode.

Regards

Rahul R

Prithvi Raj likes this

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