# Modal analysis using cylindrical coordinates
**URL:** https://community.freefem.org/t/modal-analysis-using-cylindrical-coordinates/2366
**Category:** General Discussion
**Created:** [March 30, 2023, 12:53pm UTC](https://community.freefem.org/t/modal-analysis-using-cylindrical-coordinates/2366 "2023-03-30T12:53:01Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Markov](https://community.freefem.org/u/Markov)
#### Post date: [March 30, 2023, 12:53pm UTC](https://community.freefem.org/t/modal-analysis-using-cylindrical-coordinates/2366/1 "2023-03-30T12:53:01Z")
Hello,
I’m trying to find the natural frequencies of a structure that is an axisymmetric cut of a circular disc. I wrote the FreeFem code using cylindrical coordinates, but the results are different compared to the full disc configuration. I suspect that i need to specifiy periodic boundary conditions on theta, how can i implement it in the code?
Thank you in advance
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### Author:  [@julienG](https://community.freefem.org/u/julienG)
#### Post date: [March 30, 2023, 9:51pm UTC](https://community.freefem.org/t/modal-analysis-using-cylindrical-coordinates/2366/2 "2023-03-30T21:51:14Z")
By Theta, I understand you mean the polar angle?
If so, it is not periodic, but 2 pi periodic, and thus discontinuous…
what kind of periodicity do you have in mind?
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### Author:  [@cmd](https://community.freefem.org/u/cmd)
#### Post date: [March 31, 2023, 6:58am UTC](https://community.freefem.org/t/modal-analysis-using-cylindrical-coordinates/2366/3 "2023-03-31T06:58:50Z")
If the structure is axisymmetric and your problem is linear, I would suggest you keep your problem in a 2D framework and expand along theta in a Fourier series of integer azimuthal wavenumbers.
Otherwise, you can try what @frederichecht suggests here: [Cylindrical coordinate for flow simulation](https://community.freefem.org/t/cylindrical-coordinate-for-flow-simulation/1941)
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### Author:  [@Markov](https://community.freefem.org/u/Markov)
#### Post date: [March 31, 2023, 7:47am UTC](https://community.freefem.org/t/modal-analysis-using-cylindrical-coordinates/2366/4 "2023-03-31T07:47:24Z")
Yes theta is the polar angle. I’m solving only a 1 degree cut of the full disc and i want to impose a rotational periodicity on the lateral faces to make it seem like it’s the full geometry
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### Author:  [@Markov](https://community.freefem.org/u/Markov)
#### Post date: [March 31, 2023, 7:49am UTC](https://community.freefem.org/t/modal-analysis-using-cylindrical-coordinates/2366/5 "2023-03-31T07:49:25Z")
i’m solving the problem in a 3D framework (r,theta,z). This is why i need to impose a rotational periodicity