Learning Forum

[ILYA Giri]

Hello everyone. 

Merry Christmass to everyone. 

I want to get natural frequency of the rotor. Rotor model made as assemly and it has contacts.

 I performed 2 analysis with different mesh size. 

First analysis has 680K elements and second has 980K elements. 

First analysis results are: 10, 10, 39 Hz.

Second analysis results are: 4, 4, 19 Hz.

39 and 19 Hz its rotation around rotor axis. 

What I do wrong? Why I am getting different results on different mesh? I read that mesh size does not influence on modal analysis results.

 

 

[bhagwantP]

Hello ILYA Giri,

You may need to perform mesh convergence study over here with different mesh sizes and check when you are getting not much distinctive frequencies.

The frequency is dependent on the stiffness. The stiffness of an element is dependent on the size of the element. (This is especially true when the model is thin and bending: Just a sidenote).

You may find bellow articles useful in this context:

Why does the mesh size affect my modal analysis results? (ansys.com)

(PDF) Effect of Mesh Size of Finite Element Analysis in Modal Analysis for Periodic Symmetric Struts Support (researchgate.net)

finite element method - Why does the mesh size affect my modal analysis results? - Engineering Stack Exchange

Correct mesh size - a quick guide! - Enterfea

How to decide mesh size of larger model in the modal analysis - Autodesk Community - Inventor Nastran

Thanks

 

[ILYA Giri]

Hello. Thank you. I will see it. 

[Dave Looman]

This is very likely due to penalty based contact between curved surfaces.  It produces an artificial moment constraint.  Try switching to MPC contact.