Stress Result in a cylinder Shell

[jake77]

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v 2.10

Do you really use ccx 2.10 !!! This is years old. You should use 2.15 or at least 2.14

For all the rest i will give some input later on ...

that was the one I found before installing FreeCAD and finding out it has one bundled. I'll check on this.

[jake77]

runs here too apart from the pressure problem (which is a FreeCAD limitation ATM for curved shell geometry and CalculiX DLOAD) ... has quite funny mode shapes ...


Screenshot_20190117_202128.png

This is probably some higher mode (unless the way supports are defined interferes...) but yes, this is what was expected. I'm actually quite happy to see it. Could you pass the relevant parts (i.e. everything except for the load, face, node definitions) of the input?

[bernd]

take your file and change in solver from static to frequency and run the analysis. The pic was 10th mode shape ...

[jake77]

take your file and change in solver from static to frequency and run the analysis. The pic was 10th mode shape ...

That much is clear now. Linear buckling and modal results should look similar.
My problem is not so much choosing the analysis type, but getting a correct mesh. What settings did you use when generating the mesh for the modal analysis you provided a pic from? That one looks perfect. Do you use the bundled gmsh.exe?

[jake77]

runs here too apart from the pressure problem (which is a FreeCAD limitation ATM for curved shell geometry and CalculiX DLOAD) ... has quite funny mode shapes ...

Could you elaborate what exactly is the limitiation? I changed the way the external pressure is applied. It is on the cylinder now, not on the outer faces, meaning, pressure load is applied only on singly curved surface. This should be OK I hope.

[bernd]

open your file and just remesh, no change ... I use gmsh 3.0.6 on Debian Buster. Remeshing takes some time ( only on of12 core runs on 100%, but thats another story ... ) one or two minutes.

- open your doppelstock file (BTW: what does doppelstock means in the regard of this cylinder? I only know doppelstockzug und -bus ?)
- rmesh
- set analysis
- run ccx
-

Just tried with Windows version 0.18 Py3, Qt5 and bundled Gmsh. I can confirm problems ... (Unexpected error when creating mesh: <class 'RuntimeError'>) Mhh never had this on Linux ...

@harry, you gone work with Windows too sometime. Can you confirm, and have you had this error before?

bernd

[bernd]

runs here too apart from the pressure problem (which is a FreeCAD limitation ATM for curved shell geometry and CalculiX DLOAD) ... has quite funny mode shapes ...

Could you elaborate what exactly is the limitiation? I changed the way the external pressure is applied. It is on the cylinder now, not on the outer faces, meaning, pressure load is applied only on singly curved surface. This should be OK I hope.

It has to do with the work flow. In FreeCAD we gone define the constraints on the geometry but the analysis is done with the mesh. This pressure does not work on curved shell geometry ATM. I need to have a closer look what exact the problem was. See https://github.com/FreeCAD/FreeCAD/blob ... ls.py#L712


@harry we gone had this topic already. You have been asking this when you came to the forum. There should be a topic around which exactly describes the problem. I am quite in a hurry and do not have the time to get into this. Last work day before ski holidays and there is a lot of snow in the european alps ATM

[jake77]

Guys,

I just managed to create a mesh (in 20 mins... ) that then did not work due to memory allocation error (20.8M nodes). I have an 8 years old laptop with 8 GB RAM but a mobile processor - currently I'm just buring time trying to solve this problem. I'm glad you have results though

For me FreeCAD is not production-ready yet (something I suspected but still gave a try), but I forked the code and will look into the whole on toy problems first.

Anyways, thank you very much for the support, it's been a great experience to get prompt help and I certainly hope to work on the software in the future.

Cheers
jake

[jake77]

open your file and just remesh, no change ... I use gmsh 3.0.6 on Debian Buster. Remeshing takes some time ( only on of12 core runs on 100%, but thats another story ... ) one or two minutes.

- open your doppelstock file (BTW: what does doppelstock means in the regard of this cylinder? I only know doppelstockzug und -bus ?)
- rmesh
- set analysis
- run ccx
-

Just tried with Windows version 0.18 Py3, Qt5 and bundled Gmsh. I can confirm problems ... (Unexpected error when creating mesh: <class 'RuntimeError'>) Mhh never had this on Linux ...

@harry, you gone work with Windows too sometime. Can you confirm, and have you had this error before?

bernd

I guess switching to Linux would be a reasonable step at this point. Quite a step to take to get FEM results

The inverstigated structure is the pressurized part of a pressure vessel that is to be checked for stability according to DIN EN 1993-1-6 for external pressure (full vacuum in the vessel). The vessel itself has two such compartments, that are placed above each other. From the outside it looks like a giant pipe (height ~22 m, diameter 3.6 m) with the helical halpfpipes (one on the upper, one on the lower part) around it. The speciality is that the cylinder is thin (9,5 mm) so the helix acts as a stiffener. However, the helix pitch is quite large so the formulae of the norm may not be applied (although they yield correct results). Hence the fight with the FE model.

[HarryvL]

@harry, you gone work with Windows too sometime. Can you confirm, and have you had this error before?

bernd

Bernd, the only Windows-specific problem I ever encountered was with building FreeCAD. Apart from that the windows version works the same as the Linux version.

PS: actually Windows works better for me because I don’t get Netgen as a meshing option in Linux.