topic Re: pseudo rigid modes in Simulation Mechanical Forums (Read-Only)

pseudo rigid modes

drutel — Thu, 09 Feb 2017 09:10:46 GMT

Is anyone here knows if the rigid pseudo modes are included in spectral analysis made by SimMech?

Thanks

Re: pseudo rigid modes

John_Holtz — Thu, 09 Feb 2017 14:30:08 GMT

Let me make sure that I (and the other readers) understand your question. Is this an accurate description of your process?

You perform a modal analysis. Because the model is not fully constrained, some of the first few modes have a frequency near 0 Hz, and the model "vibrates" as a rigid body. (It moves back-and-forth without flexing.)
You then perform a vibration analysis (response spectrum, frequency response, random vibration, ...) which uses the results of the modal analysis.
The question is: are those rigid body modes included in the vibration analysis.

The answer is yes: technically, the rigid body modes are included in the vibration analysis.

However, the rigid body modes should not affect the vibration results because of the following:

the modal effective mass (the mass participation factor) for the rigid body modes is close to 0, so the rigid body modes have no affect. (Check the log file to confirm that the effective mass is close to 0, or at least much smaller than the "real" modes.)
In response spectrum or random vibration, I assume that the input load is small at 0 Hz, so the rigid body modes have no affect.
In frequency response, I assume that you are vibrating the model at a frequency much higher than 0 Hz, so the rigid body modes have no affect.

Re: pseudo rigid modes

benalain13 — Thu, 09 Feb 2017 15:59:34 GMT

We were talking about the ability of the spectral analysis (and not the modal one) which sometimes includes the pseudo rigid modes into its solution prediction.

It is very used in the nuclear industry...

Re: pseudo rigid modes

marwan_azzam — Thu, 09 Feb 2017 19:33:08 GMT

Alain,

To put John's answer in different, and less, words:

A model analysis can produce near zero pseudo modes if the model is not statically stable.

Of course, it is only the modal analysis that can generate pseudo, along with the valid, modes. The other restart analyses such as response spectrum will use all the frequencies (pseudo and valid non-zero) in calculations.

The restart analyses (e.g. spectral) do not know, or care, that the model is not statically stable. Results of such analyses should not be effected by the pseudo modes.

Re: pseudo rigid modes

benalain13 — Fri, 10 Feb 2017 08:12:36 GMT

The model is statically stable

but since the first frequency is very high and the spectra not goes til that frequency.

So a method allows to use rigid pseudo modes to send excitations to the structure to be sure to recover all the response

please see this link for example : http://www.freelem.com/theorie/spectrale.htm

Regards

Re: pseudo rigid modes

John_Holtz — Fri, 10 Feb 2017 14:07:28 GMT

Hi Alain,

Thanks for the clarification. The question is different than I thought.

The answer is no: Simulation Mechanical does not have that type of capability.

The key paragraph from the Freelem website is as follows:

This method has one disadvantage: it requires the extraction of all the modes, which can prove to be long. We prefer to use the pseudo-mode technique corresponding to all frequencies higher than the cutoff frequency of the ZPA spectrum (more dynamic amplification in this frequency range).

The page goes on to describe how to calculate the response due to frequencies above the range of the spectrum (and frequencies not included in the modal analysis, if I understand it correctly.)

It would be interesting to know how much of an affect those pseudo-modes have on the result, versus the frequency. For example, a natural frequency that is X times higher than the response spectrum adds a% to the stress, a frequency Y times higher than the response spectrum adds b% to the stress, and so on. As the frequency gets higher (X to Y to Z to ...), the contribution gets lower (a% to b% to ....).