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Question about Vibration Analysis Loading

Hi,
I've been going through FEMAP supplied examples, particularly the vibrations ones, to familiarize myself with software and its operation.

 

I have a question regarding the definition of loading in FE softwares. For forced freq/modal analysis, we define a function of amplitude (?) vs freq and in the exmaples, it is usually defined as a constant over the freq range.

 

And in the loading card, either a unit force or acceleration value is defined along the axes of interest and the above created function is associated to it.

 

Question is, how is FE reading or interpretting the loads?

 

Lets say, I have applied a force of 1N and my freq range is from 0-5000Hz, then per my current understanding, the amplitude of the external source remains constant over the entire freq range ? Is this accurate?

 

Also lets say that I have a motor running at 15Hz attached to a structure. If I need to perform a forced freq response, how will the loading card be defined? 

 

4 REPLIES

Re: Question about Vibration Analysis Loading

Hello!,

In frequency response analysis (SOL111) the excitation is explicitly defined in the frequency domain. Excitations can be in the form of applied forces and enforced motions (displacements, velocities, or accelerations). A dynamic load is different to a static load, the simplest dynamic loading is a simple harmonic (sinusoidal) loading, where the loading is a sine wave for which the frequency, amplitude, and phase are specified.

One important aspect in dynamic frequency response analysis is the verification of the applied load by exciting your model at 0.0 Hz and comparing the results to a static solution with the same spatial load distribution. The results at a frequency of 0.0 Hz should match the static results if direct frequency response analysis (SOL108) is used without structural damping. If modal frequency response analysis (SOL111) is used without structural damping, those results should be close to the static results.

Best regards,
Blas.

Blas Molero Hidalgo, Ingeniero Industrial, Director
IBERISA • 48004 BILBAO (SPAIN)
WEB: http://www.iberisa.com
Blog Femap-NX Nastran: http://iberisa.wordpress.com/

Re: Question about Vibration Analysis Loading

 

Thanks. 

 

I learnt yesterday that how to transform freq domain loading in to time domain.

 

Conversion of TIme Domain in to Freq DomainConversion of TIme Domain in to Freq Domain 

I will have to do a FFT to understand how a loading definition of 1 across a freq range looks like in time domain (at least for me time domain loading is more intuitive). 

 

I would appreciate if some one could provide pointers as how to define a 15Hz vibration being thrown by a motor on to a structure, in FE software. Or point me to source which goes in to some more detail about the above. 

 

Cheers, 

Re: Question about Vibration Analysis Loading

"I would appreciate if some one could provide pointers as how to define a 15Hz vibration being thrown by a motor on to a structure, in FE software. Or point me to source which goes in to some more detail about the above. "

 

Any help regarding the above? I assuming that for folks who regularly deal with vibration FE analysis, the above load case is the most elementary. Would appreciate any kind of help.

 

Thx

Re: Question about Vibration Analysis Loading

OK.

 

I may have got an understanding of the loading after speaking with someone. Apparently, the freq domain function that is created is a White Noise Function i.e. we do not have good definition of the external loading or freq wrt time and hence create a White Noise.

 

For a given structure, each & every freq in the entire specified range, is applied as the driven freq (external freq) and the system response is evaluated. Of course, the amplitude of displacement should be highest near the normal modes, which I believe holds true from the output of FE analysis.

 

If my understanding is slightly inaccurate, I would appreciate corrections.