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What's the equation for hollow cylinder eignfrequency calculation???

Posted Apr 5, 2010, 4:34 p.m. EDT 5 Replies

Hattie Liu
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Hi,
I'm using comsol to solve eignfrequency for hollow cylinder. The FEM result has a big difference from the computational result. So I need to compare whether FEM and computational solution are using the same equations. And I don't know where to check the equation in Comsol. I use Structure Mechanics Module--Solid,stress strain--eignfrequency analysis.

Is there anyone who knows that???

Thanks!
5 Replies Last Post Apr 8, 2010, 11:14 a.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Apr 5, 2010, 6:34 p.m. EDT
Hi

Are you in 2D or 2D axisymmetry ?

For the equations you have to look up the documentation, as for structural, the equation is not displayed in the windows

Have fun Comsoling
Ivar
Hi Are you in 2D or 2D axisymmetry ? For the equations you have to look up the documentation, as for structural, the equation is not displayed in the windows Have fun Comsoling Ivar
Jeff Lastofka
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Posted: 1 decade ago Apr 5, 2010, 11:29 p.m. EDT
It sounds like Ivar missed one of the words in your question email. You said solid stress strain module, so you're in 3D, right?

It also sounds like you might be trying to compare the COMSOL result to an analytical result from perhaps a handbook formula? Or you got an equation from somewhere for one or more natural frequencies for a hollow cylinder?

COMSOL wouldn't be using an equation for a cylinder. COMSOL would be breaking the cylinder up into thousands of little elements and then building a matrix of thousands of simple element behavior equations and solving them simultaneously to model the behavior of the meshed model of the cylinder.

You won't find anything to compare between your two methods, except for the results.

Unless I've missed something, too. I always welcome corrections and additions to my 'knowledge'...

-Jeff
It sounds like Ivar missed one of the words in your question email. You said solid stress strain module, so you're in 3D, right? It also sounds like you might be trying to compare the COMSOL result to an analytical result from perhaps a handbook formula? Or you got an equation from somewhere for one or more natural frequencies for a hollow cylinder? COMSOL wouldn't be using an equation for a cylinder. COMSOL would be breaking the cylinder up into thousands of little elements and then building a matrix of thousands of simple element behavior equations and solving them simultaneously to model the behavior of the meshed model of the cylinder. You won't find anything to compare between your two methods, except for the results. Unless I've missed something, too. I always welcome corrections and additions to my 'knowledge'... -Jeff
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Apr 6, 2010, 1:48 a.m. EDT
Hi

Certainly you are right, I did not read carefully enough ;) it was 3D

When I get differences between my FEM simulated results and the expected analytical ones, I redo my analysis on the simples possible model, and I start generally in 2D. So for a hollow cylinder I would start in 2D-axi and check the results, and then compare to 3D solid, at least until I understand the differences.

In anycase Jeff is right that you cannot compare the equations directly as in FEM you have a set of "small" finite elements.

But for similar boundary conditions, you should indeed get similar results than your analytical equations

Have fun Comsoling
Ivar
Hi Certainly you are right, I did not read carefully enough ;) it was 3D When I get differences between my FEM simulated results and the expected analytical ones, I redo my analysis on the simples possible model, and I start generally in 2D. So for a hollow cylinder I would start in 2D-axi and check the results, and then compare to 3D solid, at least until I understand the differences. In anycase Jeff is right that you cannot compare the equations directly as in FEM you have a set of "small" finite elements. But for similar boundary conditions, you should indeed get similar results than your analytical equations Have fun Comsoling Ivar
Hattie Liu
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Posted: 1 decade ago Apr 8, 2010, 11:06 a.m. EDT
Right, I did it in 3D. And I already have the analytical solution based on cantilever beam theory.
Do you mean I do the FEM just in 2D-axilsym, no need to revolve the geometry into 3D?
Right, I did it in 3D. And I already have the analytical solution based on cantilever beam theory. Do you mean I do the FEM just in 2D-axilsym, no need to revolve the geometry into 3D?
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Apr 8, 2010, 11:14 a.m. EDT
Hi

depending on you load case mostly it is enough to do the analysis in 2D or 2D axi for your case with cylindrical symmetry (I believe). This solves much quicker.

In anycase you should get similar results in 2Daxi, 3D and by analytical means, to within 5-10% at least, if not something is wrong, mostly it happens to be the boundary conditions

Have fun COMSOLing
Ivar
Hi depending on you load case mostly it is enough to do the analysis in 2D or 2D axi for your case with cylindrical symmetry (I believe). This solves much quicker. In anycase you should get similar results in 2Daxi, 3D and by analytical means, to within 5-10% at least, if not something is wrong, mostly it happens to be the boundary conditions Have fun COMSOLing Ivar

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