Frequency analysis for cylindrical shell

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bernd
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Re: Frequency analysis for cylindrical shell

Post by bernd »

mac_the_bike wrote: Wed Jul 20, 2022 10:13 am
For me Calculix does not return any negative value for a cube ... See attached picture. May be the first one, which is even omitted by CalculiX itself.
Yes but did you ask for negative values? Calculix will always return what you request, provided it is possible, e.g. it can't return modes less than 10Hz if there aren't any. It doesn't return what you don't request, e.g. if you don't request negative eigenvalues then it doesn't return them.
Also, MAYBE is not really good enough is it? You need unequivocal evidence that it is.
It is interesting to see that the first mode that is output, 0.291E-02, has a label of "Mode 2", but the first mode in the 'PARTICIPATION FACTORS' is 1.
good point ... If you set height and low limit to 0.00 in FreeCAD solver no limit is written to CalculiX which should give all modes requested in count pref ...

inp file ...

Code: Select all

***********************************************************
** At least one step is needed to run an CalculiX analysis of FreeCAD
*STEP
*FREQUENCY
15


***********************************************************
seams there are negative ones ...

Code: Select all

     E I G E N V A L U E   O U T P U T

 MODE NO    EIGENVALUE                       FREQUENCY   
                                     REAL PART            IMAGINARY PART
                           (RAD/TIME)      (CYCLES/TIME     (RAD/TIME)

      1  -0.1298234E-02   0.0000000E+00   0.0000000E+00   0.3603102E-01
      2  -0.1148171E-03   0.0000000E+00   0.0000000E+00   0.1071527E-01
      3   0.5007874E-03   0.2237828E-01   0.3561614E-02   0.0000000E+00
      4   0.1390500E-02   0.3728941E-01   0.5934794E-02   0.0000000E+00
      5   0.2882176E-02   0.5368590E-01   0.8544376E-02   0.0000000E+00
      6   0.3887564E-02   0.6235033E-01   0.9923364E-02   0.0000000E+00
      7   0.8379889E+12   0.9154173E+06   0.1456932E+06   0.0000000E+00
      8   0.8380244E+12   0.9154368E+06   0.1456963E+06   0.0000000E+00
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Re: Frequency analysis for cylindrical shell

Post by bernd »

BTW added the branch to my branches. Rebased and squashed commits ... I need to give the example a try ... Might be good addition to examples ...

https://github.com/berndhahnebach/FreeC ... lecylshell
mac_the_bike
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Re: Frequency analysis for cylindrical shell

Post by mac_the_bike »

Click: SolverCcxTools and the lower panel appears with things like "EigenModeHighLimit".
good point ... If you set height[high] and low limit to 0.00 in FreeCAD solver no limit is written to CalculiX which should give all modes requested in count pref ...
inp file ...
I tried that and it works, BUT it doesn't seem a logical thing to do.

When you look at the data in the lower panel you shouldn't have to realize that the combination of "high and low limits set to 0.0" has a special meaning.
The data in this panel should be self explanatory, so if you want negative eigenvalues then you specify a low limit of, say, -5.

Also, instead of requesting 15 modes, how do you get all the modes below 50Hz, say, when the high limit is set to 0.0?
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