*FREQUENCY with previous *STEP,NLGEOM=YES

1

Hi,

I’m running a *FREQUENCY analysis after preloading the system with some centrifugal force.

*STEP,NLGEOM=YES,INC=100
*STATIC
1,1,0,0
*DLOAD
Model,CENTRIF,296526.7811172,0,0,0,0,0,1
*END STEP

*STEP,PERTURBATION
*FREQUENCY
12
*SELECT CYCLIC SYMMETRY MODES,NMIN=0,NMAX=1
*NODE FILE,GLOBAL=YES
U
*END STEP

First step is using NLGEOM=YES
What I do not understand is this message in the console when the second step starts.

STEP 2
Frequency analysis was selected
Perturbation parameter is active
Nonlinear geometric effects are taken into account
Decascading the MPC’s
…

¿What does Nonlinear geometric effects are taken into account in a *Frequency calculation mean?

If I set *STEP,PERTURBATION,NLGEOM=NO the message disappear.
If I set *STEP,PERTURBATION the message is shown but the calculation works and completes.
If I set *STEP,PERTURBATION,NLGEOM=YES Frequency stops and there is a new message:

*ERROR reading *STEP: PERTURBATION and NLGEOM are mutually exclusive.
*ERROR reading *STEP. Card image: *STEP,PERTURBATION,NLGEOM=YES
*ERROR in calinput: at least one fatal error message while reading the input deck: CalculiX stops.

¿Which would be the correct parameters to take my Centrifugal NLGEOM effects computed in the previous STEP into consideration for the Frequency analysis.?

Thanks

1 Like
2

Only the PERTURBATION parameter is necessary to account for preload in frequency analysis. NLGEOM doesn’t make sense for this kind of step, it should be ignored by the solver.

3

¿And what does “Nonlinear geometric effects are taken into account” in a *Frequency calculation mean?

4

It means that geometric nonlinearity from the previous step (static with Nlgeom) is taken into account when calculating eigenvalues.

This part of the documentation sums it up:

If the perturbation parameter is activated, the stiffness matrix is augmented by contributions resulting from the displacements and stresses at the end of the last non-perturbative static step, if any (…). Thus, the effect of the centrifugal force on the frequencies in a turbine blade can be analyzed by first performing a static calculation with these loads, and selecting the perturbation parameter on the *STEP card in the subsequent frequency step. The loading at the end of a perturbation step is reset to zero.

Actually, unlike in Abaqus, preload will be taken into account even when the preceding static step doesn’t have Nlgeom enabled. Of course, geometric nonlinearity won’t be used then.

2 Likes
5

Ok Thanks. So, it is refering to the previous STEP.

1 Like
6

I find this message confusing, too. A possible fix could be to just be more specific about what the message means, i.e. what exactly is added to the constant stiffness matrix.