# \*FREQUENCY with previous \*STEP,NLGEOM=YES **URL:** \ **Category:** Uncategorized\ **Created:** [April 2, 2023, 1:56pm UTC](https://calculix.discourse.group/t/frequency-with-previous-step-nlgeom-yes/1569 "2023-04-02T13:56:24Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Disla](https://yyz2.discourse-cdn.com/free1/user_avatar/calculix.discourse.group/disla/32/3548_2.png) [@Disla](https://calculix.discourse.group/u/Disla)\ **Post date:** [April 2, 2023, 1:56pm UTC](https://calculix.discourse.group/t/frequency-with-previous-step-nlgeom-yes/1569/1 "2023-04-02T13:56:24Z")
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 ---
**Author:** ![Calc\_em](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Calc\_em](https://calculix.discourse.group/u/Calc_em)\ **Post date:** [April 2, 2023, 3:21pm UTC](https://calculix.discourse.group/t/frequency-with-previous-step-nlgeom-yes/1569/2 "2023-04-02T15:21:38Z")
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. ---
**Author:** ![Disla](https://yyz2.discourse-cdn.com/free1/user_avatar/calculix.discourse.group/disla/32/3548_2.png) [@Disla](https://calculix.discourse.group/u/Disla)\ **Post date:** [April 2, 2023, 4:31pm UTC](https://calculix.discourse.group/t/frequency-with-previous-step-nlgeom-yes/1569/3 "2023-04-02T16:31:10Z")
¿And what does “Nonlinear geometric effects are taken into account” in a \*Frequency calculation mean? ---
**Author:** ![Calc\_em](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Calc\_em](https://calculix.discourse.group/u/Calc_em)\ **Post date:** [April 2, 2023, 4:39pm UTC](https://calculix.discourse.group/t/frequency-with-previous-step-nlgeom-yes/1569/4 "2023-04-02T16:39:13Z")
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. ---
**Author:** ![Disla](https://yyz2.discourse-cdn.com/free1/user_avatar/calculix.discourse.group/disla/32/3548_2.png) [@Disla](https://calculix.discourse.group/u/Disla)\ **Post date:** [April 2, 2023, 4:46pm UTC](https://calculix.discourse.group/t/frequency-with-previous-step-nlgeom-yes/1569/5 "2023-04-02T16:46:10Z")
> [@Calc\_em](#): > > 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. Ok Thanks. So, it is refering to the previous STEP. ---
**Author:** ![OliverKunc](https://avatars.discourse-cdn.com/v4/letter/o/aca169/32.png) [@OliverKunc](https://calculix.discourse.group/u/OliverKunc)\ **Post date:** [April 17, 2023, 4:59am UTC](https://calculix.discourse.group/t/frequency-with-previous-step-nlgeom-yes/1569/6 "2023-04-17T04:59:03Z")
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.