Menu

Discussion Forum

New Discussion
Filtrer

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

Random vibration

Posted 17 févr. 2021, 10:37 UTC−5 Acoustics & Vibrations Version 5.6 6 Replies

Likitha Thimmaraju Girijadevi
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Last time when I asked about how to plot the stress due to random vibration, Answer: To plot the stress (i.e von Mises) you use the rvib1.q2 operator on a variable that is sigma_mises^2 and then we take the square root of it. the expression will then be sqrt(rvib1.q2(3*solid.ii2s, fL, fU, nF).

Question: When i take a 3D plot and select the random vibration study I set up, I select surface under 3D plot- used expression rvib1.rms(solid.ii2s,10,500,nF) or sqrt(rvib1.q2(3*solid.ii2s, fL, fU, nF).

----I have been getting errors

Can someone suggest me of how can I plot random vibration- von mises stress?

Thank you

6 Replies Last Post 24 févr. 2021, 10:55 UTC−5
Likitha Thimmaraju Girijadevi
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 4 years ago 17 févr. 2021, 10:57 UTC−5

error message: Failed to evaluate operator. - Operator: rvib1.rms - Geometry: geom1

error message: Failed to evaluate operator. - Operator: rvib1.rms - Geometry: geom1
Henrik Sönnerlind COMSOL Employee

Please login with a confirmed email address before reporting spam

Posted: 4 years ago 18 févr. 2021, 02:46 UTC−5

The first expression, rvib1.rms(solid.ii2s,10,500,nF), is fundamentally wrong since the rms() operator is only valid for expressions that are linear functions of the degrees of freedom, such as individual stress and strain components (and since solid.iis2 is not an existing variable).

The second expression, sqrt(rvib1.q2(3*solid.ii2s, fL, fU, nF), is in principle correct, except that the name of the variable should be solid.II2s and not solid.iis2 and that the number of parentheses should match.

-------------------
Henrik Sönnerlind
COMSOL
The first expression, *rvib1.rms(solid.ii2s,10,500,nF)*, is fundamentally wrong since the *rms()* operator is only valid for expressions that are linear functions of the degrees of freedom, such as individual stress and strain components (and since *solid.iis2* is not an existing variable). The second expression, *sqrt(rvib1.q2(3\*solid.ii2s, fL, fU, nF)*, is in principle correct, except that the name of the variable should be *solid.II2s* and not *solid.iis2* and that the number of parentheses should match.
Likitha Thimmaraju Girijadevi
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 4 years ago 22 févr. 2021, 02:13 UTC−5
Updated: 4 years ago 22 févr. 2021, 02:47 UTC−5

Thank you so much. I can now plot the 3sigma expression.

Could you also suggest me on how do I plot the stress PSD response for a point on the structure for the random vibration study. I would like to plot the various response spectrum of the structure.

Thank you so much. I can now plot the 3sigma expression. Could you also suggest me on how do I plot the stress PSD response for a point on the structure for the random vibration study. I would like to plot the various response spectrum of the structure.
Likitha Thimmaraju Girijadevi
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 4 years ago 22 févr. 2021, 02:52 UTC−5

I have been trying to replicate this and would like to obtain the same plot and graph

https://www.researchgate.net/publication/338432119_Vibration_Fatigue_Analyses_of_a_Cantilever_Beam_in_Transportation_Based_on_Military_Standard/link/5e14a33d4585159aa4b9e621/download

I have been trying to replicate this and would like to obtain the same plot and graph https://www.researchgate.net/publication/338432119_Vibration_Fatigue_Analyses_of_a_Cantilever_Beam_in_Transportation_Based_on_Military_Standard/link/5e14a33d4585159aa4b9e621/download
Henrik Sönnerlind COMSOL Employee

Please login with a confirmed email address before reporting spam

Posted: 4 years ago 24 févr. 2021, 10:38 UTC−5

You can find PSD plots in all random vibration examples in Application Galleries. You create them from a Global Evaluation Sweep node.

As in many reports similar to the one you reference, the definition of 'stress' as a bit fuzzy. So it may be difficult to reproduce the result. I would guess that they are using the stress tensor component in the x-direction, because luckily in their geometry that one will be critical. If the object had been rotated in the XY-plane, things would be more difficult, since a stress component in a local coordinate system would have to be used. If the stress state is not locally uniaxial, it is even more difficult...

The whole fatigue evaluation approach used in that paper is a bit suspect in my opinion.

-------------------
Henrik Sönnerlind
COMSOL
You can find PSD plots in all random vibration examples in Application Galleries. You create them from a Global Evaluation Sweep node. As in many reports similar to the one you reference, the definition of 'stress' as a bit fuzzy. So it may be difficult to reproduce the result. I would guess that they are using the stress tensor component in the x-direction, because luckily in their geometry that one will be critical. If the object had been rotated in the XY-plane, things would be more difficult, since a stress component in a local coordinate system would have to be used. If the stress state is not locally uniaxial, it is even more difficult... The whole fatigue evaluation approach used in that paper is a bit suspect in my opinion.
Likitha Thimmaraju Girijadevi
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 4 years ago 24 févr. 2021, 10:55 UTC−5

I would like to show you my model and just verify small thing of plotting the psd response of various kinds and also about how I have extracted them in tabular columns.

Is that even possible for tomorrow?

I would like to show you my model and just verify small thing of plotting the psd response of various kinds and also about how I have extracted them in tabular columns. Is that even possible for tomorrow?

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.

Suggested Content