Learning Forum

[alperen]

Hello,

I have been trying to solve modal analysis for a model which I am later going to investigate the response under forced vibration. Since the moda analysis I am conducting is free-free, I was expecting to get th first six modes 0 or close to 0. Yet the results for the 4th,5th and6th modes frequencies (can be seen in the attachment) are figures such as 10-20 Hz. The modes seem like rigid body modes but I can not be sure if the solution is correct.

After simplifying the model (which is not acceptable for the purpose, just to see the change) and uniting the bodies with boolean method, I get the results I expected; first 3 modes 0 and other 3 modes close to zero.

I am suspecting this behaviour is somewhat related to the mesh settings and element type since I am sure the geometry does not have problems such as intersecting bodies etc.

 Could anyone help me with this matter?

Thanks in advance.

[peteroznewman]

Hello,

It was a good idea to check the Modal Free-Free response prior to a dynamic analysis.

In your connections folder, are there any joints or springs to ground?

Do you have joints that are connecting faces/edges that are a large distance apart?

Try plotting Equiv Elastic Strain and look for where the strain appears in the model in modes 4,5,6.

Does mode 7 look like the expected first bending mode?

Note that ANSYS staff are not permitted to open attachments, so please use the Insert Image button.

[alperen]

Hello Peter, 

I can not calculate the equivalent stress in the modal analysis, I am guessing it is somewhat underdefined since there is a question mark. I am currently running through the connections as you you have suggested.

-Is there anyway I can calcualate the stress as you sad in the modal analysis?

-Do you have any other suggestions?

(There are no connections to the ground)

Thanks