Modal analysis & Modal response for structural eigenfrequency calculation


#1

Which feature do you need?

  • Getting back the modal analysis capabilities Simscale offered until the end of 2017 through the Calculix solver enhanced by some minor additional features (Eigenstresses) included in the details below.
  • Modal Analysis to be followed by an optional 2nd step to calculate the Modal Response of a linear structure: Both calculation steps are usually much faster vs. the dynamic analysis type if you need the structural vibrations for many frequencies. (Details below only adress the first step of the Modal Analysis)

What is the problem that you want to address with this feature?
Typically structures tend to fail because of excitation in their eigenfrequencies. Knowledge of Eigenfrequencies is key to decide which frequencies need special attention which may be subsequently adressed through the still available Dynamic analysis.

What is the expected outcome?
Structural Eigenfrequencies:

  • Most likely structures fail because of high force levels in their eigenfrequencies. Eigenfrequencies are key to understand dynamic failure of structures!
  • To compare different meshes for the same structures regarding their capabilities for other dynamic analysis types (still offered by Simscale) we typically compare the eigenfrequencies of the different meshes. For the frequency ranges in which multiple meshes show same eigenfrequencies you can run subsequent dynamic analysis with the coarser mesh.
  • Quick sense check if calculated eigenfrequencies match the known eigenfrequencies of simple structures (see examples for cantilever beams and simply supported plates on http://www.computational-acoustics.de/html/rechnerische_modalanalyse_-fem-.html .

Eigenforms:

  • Visual check to differentiate real physically existing eigenforms vs. artifical eigenforms (an expert in acoustics can easily differentiate between both types)
  • Quick check where the vibrating structure may have highest vibration amplitudes due to external excitation in the respective eigenfrequency.

Eigenstresses (not part of the earlier Simscale offering): Eigenstresses are normalized (Max displayed as “1”) stresses showing a normalized stress distribution through excitation of external forces in their eigenfrequencies. Consequently these stresses will not tell you if the structure fails, but it shows where the initial crack may start causing the subsequent failure.

Can you share application examples, ideally with project links on SimScale?
Earlier analysis examples which made use of this feature:
Jet Engine Eigenfrequency analysis
Eigenfrequency analysis of a railway wheel set
Some additional background from a training lecture (see course on “Grundlagen der Maschinenakustik” by Technische Akademie Essslingen) on Computational Acoustics:
Modal analysis vs. other dynamic analysis types
Simscale Blog on Modal Analysis


Modal analysis (by Calculix) - Not longer supported?
Acoustic analysis
SimScale without Eigenfrequency? This cannot be!
Get the mode frequencies from a Harmonic study
'Modal Analysis of Vibratory feeder' simulation project by ymatrix
Harmonic analysis
#2

That is wonderful, descriptive feedback @dgiljohann! Thanks for that and also for the German link you posted - the page looks super interesting!

Cheers and all the best!

Jousef


#3

Dear SimScale team,

after some first tests of the static and CFD calculation I wanted to try the Eigenfrequency calculation.
Now I see this kind of simulation is no longer supported because it wasn´t used so often…
Harmonic analysis besides static analysis is such a basic requirement that you should really reconsider this decision.
I watched a Youtube video on this calculix frequency simulation. Nothing was wrong with this, so why don´t you just reactivate it :frowning:


#4

Hi @mah1967!

I have moved your request to an existing one, just give the missing feature a vote :slight_smile: Anyway, thanks a lot for the feedback!!

Edit: Christoph (@cjquijano) told you how to get the existing feature.

Cheers!

Jousef


#5

Hello @mah1967,

When you go to choose the analysis type and you get the list of analysis that can be performed, their should be a “More” option at the bottom of the list. You will find the frequency analysis have been added back here.

Thanks,
Christopher


#6

I don’t see a “more” button!?


#7

Hi @Glanzner!

If you are interested in bringing this module back please feel free to give it a vote! :slight_smile:

Cheers and thanks!

Jousef