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Frequency analysis

Question asked by Devi Prasad Samal on Feb 6, 2009
Latest reply on Mar 19, 2009 by Jose Eduardo Fernandes de Camargo

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Hi all,

i am very new to cosmos proffesional. although i have done some basic , i have some doubt regarding frequency.

actually we know there is chances of collapsing in resonance frequency. if we want to know what is the resonant frequency of a part. then just use restraints and run study.

then we get resonance frequency value according which we have set in properties ( how many no of frequency to be calculated).

i want to know which is the required frequency in between all calculated value?

for me this is just a basic question. i want to understand how to calculate the resonant frequency at which the part fails.

if i am asking some wrong question please clarify my doubts.

Ameer Chilakala Feb 16, 2009 3:02 AM

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Hi Devi Prasad,

Sorry i mistakely mentioned that the natural frequency should be less than the first natural frequency, this is the wrong.

we need to design a structure such that its natural frequencies do not coincide with the frequency of the structure under the loading the condition.

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Devi Prasad Samal Feb 11, 2009 3:09 AM
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Hi,

I have got ideas why frequency analysis?
how to do?

and lots of other things.

but still one doubt... if 1st frequency is the required frequency...what about the others?
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- Ameer Chilakala @ Devi Prasad Samal on Feb 11, 2009 3:35 AM
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  Frequency analysis will be useful to predict the mode shapes of the structures. From the software we will get the natural freqencies, at that particular frequency how the structure is going to move. this is the output we are expecting from the natural frequency analysis. most of people will tell you, the first mode is the fundamental natural frequency or resonant frequency. If we are not applying any constriant to the structure means we need to neglect the first six natural frequencies i.e. rigid body modes, then we need to consider from the 7th mode.
  
  we need to design the system, under operating conditions, the natural frequency should be less than the first natural frequency under free vibrations.
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  - Devi Prasad Samal @ Ameer Chilakala on Feb 12, 2009 10:54 AM
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    u mean to say that the operating frequency of a system should be lie low to the fundamental natural frequency which we calculated from the frequency analysis.
    
    But i have a question, in a example a tuning fork is designed to operate at 400 hz.
    
    i have calculated frequency in two ways 1 is normal & other is rigid body mode.
    i got 223hz in normal mode & 443 in rigid mode.
    
    so normal mode is wrong then.
    
    then i have to go always for rigid mode.
    
    please clarify my doubts which to select at what time?
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    - Ameer Chilakala @ Devi Prasad Samal on Feb 16, 2009 3:02 AM
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      Hi Devi Prasad,
      
      Sorry i mistakely mentioned that the natural frequency should be less than the first natural frequency, this is the wrong.
      
      we need to design a structure such that its natural frequencies do not coincide with the frequency of the structure under the loading the condition.
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      - Devi Prasad Samal @ Ameer Chilakala on Feb 16, 2009 6:57 AM
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        thanks for the answer....
        
        but my question is what is the best method to find resonance frequency... rigid body mode or normal with restraints.
        
        which gives the correct result in between these?
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        paul collins @ Devi Prasad Samal on Mar 2, 2009 9:16 PM
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        Devi,
        Frequency response can be affected by contraints. Generally, I'm most concerned about the natural frequencies of a structure as it is used in real life. Thus, I model contraints in the FEM as they would be employed in the real life situation.
        Regards, Paul
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        sandeep pawar @ Devi Prasad Samal on Mar 3, 2009 12:54 PM
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        Natural frequency would change significantly depending upon the restraints. first 5-6 frequencies are usually of interest. restain the part so as to reflect the actual physical restrains.
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        Jose Eduardo Fernandes de Camargo @ sandeep pawar on Mar 19, 2009 5:54 PM
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        Whenever you run a frequency analysis, if your model is an assembly and it's completly restrained you should the first frequency of the assemby, not the individual component modes (sometimes they appear in the assembly as well...)
        
        Frequency analysis is just to know when and how the ressonant modes shall occur (if it's with 100Hz and it's a bending mode in the z direction for example). This study won't say how much stress or deformation your model has in this mode!!! (IF SOMEONE HAD TOLD YOU THIS, THINK TWICE ABOUT IT!!!! Because modal analysis doesn't consider external loads or excitations, how I've said it's just an analysis to know when and how ressonancy will occur!!!).
        
        If you wanna know how much, you should run a frequency response analysis (as harmonics, psd, random vibration as well), pick a frequency range that your assembly first natural frequency is in an them run it whit and accelarion of 1G for example!!! Then you should get the real values for your model!!!
        
        Best regards,
        
        Eduardo Camargo
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