modal analysis problem

Altair Forum User

Hello, dear colleagues

I need to find the first frequency of the detail in Ansys and Altair Optistruct.

Then I have to do the topological optimization in Altair Optistruct.

natural frequencies:

 

Altair:

(I used the symmetry of the detail)                               

Mass = 1.15892E+000 (1.15892*4=4,6356)

Mode    Frequency           

     1       15.26545       

     2       39.09809  

 

Ansys:(without symmetry)

   total mass= 4,6356

  SET   TIME/FREQ  

     1       14.665                         

     2       15.974         

 

The question is: what has caused such a big difference in the first frequencies(5%)? And how is it possible to fix this?

I tried to use the complete plate (without symmetry) in Altair. The first  frequency coincided with the frequency of the symmetric model.

I think that the case is in the direction of the element's coordinate system in the orthotropic materials.

 

Altair:

   total mass= 1010,28

   Mode    Frequency      

     1         3.804491E-02   

     2         5.532950E-02   

 

Ansys:

   total mass= 1010,28

   SET   TIME/FREQ    

     1         0.38064E-01                   

     2         0.49662E-01          

 

https://drive.google.com/folderview?id=0B9edDZwpicjCfkg1SUdfeHZSc3pBaXlXbmF0V0tjazRrTFB3bUVidkliN2ZFVGhsbkxYeW8&usp=sharing

Rahul Rajan_21763

Hi,

Symmetry boundary condition  is not recommended for vibrational analysis.There is a chance that it may change mode shade. I have seen good results for Static analysis using symmetry.I too would recommend you to go for full model.

It is not necessary that you would get same results for Ansys and Optistruct.Though you would have used same meshed model for Modal analysis but it cannot ensure that you would get same results because of difference in solver code.

 

Optistruct is stringent about element quality.It uses Lanczos solver to calculate natural frequency.The Lanczos method has the advantage that the eigenvalues and associated mode shapes are calculated exactly.

For topology optimization you can refer optistruct tutorial to maximize natural frequency.

 

Regards

Rahul R