Discussion Groups

Simulation Mechanical and Multiphysics

Reply
Contributor
cody.macneil
Posts: 18
Registered: ‎01-11-2012

Thin Wall Vessel with Stiffening Rings - Natural Frequency Analysis

203 Views, 3 Replies
12-05-2013 02:07 PM

Hello,

 

I am trying to model a thin walled vessel (L/t ratio > 375  D/t > 225) with stiffening rings. I obviously can't model this using bricks since in order to get three elements across the thickness I would require millions of elements. I am also having difficulty modeling this using plate elements for the vessel and brick elements for the stiffeners due to meshing errors. Any suggestions on how to model this and check it against some known problems to ensure the results make sense?

 

Thanks,

 

CM

Please use plain text.
Distinguished Contributor
ilyas
Posts: 139
Registered: ‎06-13-2012

Re: Thin Wall Vessel with Stiffening Rings - Natural Frequency Analysis

12-05-2013 07:13 PM in reply to: cody.macneil

Hi CM,

 

Why not model both vessel and stiffener as plate elements? Another possibility is perhaps to analyse as 2D axisymmetric.

 

Regards

Ilyas

 

Please use plain text.
Contributor
cody.macneil
Posts: 18
Registered: ‎01-11-2012

Re: Thin Wall Vessel with Stiffening Rings - Natural Frequency Analysis

12-06-2013 12:37 PM in reply to: ilyas

Hi ilyas,

 

I tried modelling the stiffening rings as plates as well and found that the two parts do not seem to restraining each other. The Vessel moves through the rings and vice versa in the different modes. See attached pic for what I'm talking about.

 

Regarding the 2D modelling... I can not seem to run a modal analysis of even a simple axisymmetric model. Even if I am able to get this to work, since I am looking for higher modes of vibration, I feel like that won't help since the axisymmetric condition will ensure the modes of vibration do not vary with angle.

 

Any additional input would be great.

 

Thanks,

 

CM

 

Plate-Plate Vessel.png

Please use plain text.
Distinguished Contributor
ilyas
Posts: 139
Registered: ‎06-13-2012

Re: Thin Wall Vessel with Stiffening Rings - Natural Frequency Analysis

12-07-2013 04:15 AM in reply to: cody.macneil

From what I can see, your stiffeners are actually connected to the blue cylinder (which I assume is the main body of your vessel). However, it seems that you have another surface underneath that vessel that is deforming. Do you intend to have that surface there? If you do, just keep in mind that by default, the displaced view for linear analyses are exaggerated, which means that the inner surface might not actually penetrate your outer surface.

 

If you are as surprised as I am that there is a second surface there, then perhaps you are using the wrong mesh type. Use the midplane mesh to mesh CAD solid models into plate elements. Use plate/shell mesh only when meshing CAD surface models as plate elements.

 

You are right  that you shouldn't use symmetry when doing a model analysis. One of the conditions to use symmetry other that geometry and load is that the expected result must be symmetrical as well. The lower frequency mode shapes are rarely symmetrical. That's what I think.

 

Regards

Ilyas

Please use plain text.