Issue regarding modal analysis, getting modes at extreme low frequency

Ananthan
Ananthan Altair Community Member
edited July 8 in Community Q&A

Hi,

I had modelled my rotor blade with composite properties. While attempting to do a modal analysis using HyperWorks (Optistruct) as per the tutorial from Altair on YouTube, my modes were shown at extremely low values. I tried all possible variations but still, the modes are not appearing at higher values as expected for a regular rotor blade. The mode shapes generated are also not reflect anything on the overall rotor body movement. 

Base model with All DOF constrained at the root

image

Result of modal analysis

image

For example the mode shapes of first 6 modes and mode 8  and mode 9 are like this with 0 magnitude (Real Eigen modes)

image

Mode 7 and 10 are like this but slightly different magnitude

image

Any help is highly appreciated.

Thank you for your time and expertise. 

Regards

Ananthan

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Answers

  • Rajashri_Saha
    Rajashri_Saha
    Altair Employee
    edited July 4

    Hi Ananthan,

    From your description it looks like rigid body mode exists in your model. As you have mentioned it is a constrained modal analysis, you should not see the modes which are near to zero. I will suggest you to check for connection and any modelling issue.

    If you can share the model I can have a look.

    Thanks

    Rajashri

  • Ananthan
    Ananthan Altair Community Member
    edited July 4
    Rajashri Saha_20525 said:

    Hi Ananthan,

    From your description it looks like rigid body mode exists in your model. As you have mentioned it is a constrained modal analysis, you should not see the modes which are near to zero. I will suggest you to check for connection and any modelling issue.

    If you can share the model I can have a look.

    Thanks

    Rajashri

    Hi,

    I checked the connectivity, but the model still shows similar behavior.

    As you can see, the free edges are on either side of the model, but the same is still detected as 44 free edges in total. But when I checked the preview equivalence, it said  '0' node was found. Also, I checked element connectivity, and the response was '0' elements found with connectivity issues. 

    image

    Do you know how I can share the model privately with you? 

    Regards

    Ananthan

  • Rajashri_Saha
    Rajashri_Saha
    Altair Employee
    edited July 4
    Ananthan said:

    Hi,

    I checked the connectivity, but the model still shows similar behavior.

    As you can see, the free edges are on either side of the model, but the same is still detected as 44 free edges in total. But when I checked the preview equivalence, it said  '0' node was found. Also, I checked element connectivity, and the response was '0' elements found with connectivity issues. 

    image

    Do you know how I can share the model privately with you? 

    Regards

    Ananthan

    You can send me in below link.

    https://ftin.india.altair.com/filedrop/~v8IiTt

    Thanks

    Rajashri

  • Ananthan
    Ananthan Altair Community Member
    edited July 4
    Rajashri Saha_20525 said:

    You can send me in below link.

    https://ftin.india.altair.com/filedrop/~v8IiTt

    Thanks

    Rajashri

    I had shared the file. 

    Kindly check

    Regards

    Ananthan

  • Rajashri_Saha
    Rajashri_Saha
    Altair Employee
    edited July 4
    Ananthan said:

    I had shared the file. 

    Kindly check

    Regards

    Ananthan

    I defined TIE contact between the foam and outer layer. EIGRL card is modified with lower bound as 0.1. see the warning you were getting before. It is advised not give 0 as lower bound.

     
     *** WARNING # 731
     The zero lower bound specified in METHOD card of a frequency 
     subcase (for vibrational eigenvalue analysis) is ignored.
          subcase id = 1
           METHOD id = 1

    I will send you the corrected file.

    Thanks

    Rajashri

  • Ananthan
    Ananthan Altair Community Member
    edited July 4
    Rajashri Saha_20525 said:

    I defined TIE contact between the foam and outer layer. EIGRL card is modified with lower bound as 0.1. see the warning you were getting before. It is advised not give 0 as lower bound.

     
     *** WARNING # 731
     The zero lower bound specified in METHOD card of a frequency 
     subcase (for vibrational eigenvalue analysis) is ignored.
          subcase id = 1
           METHOD id = 1

    I will send you the corrected file.

    Thanks

    Rajashri

    Hi,

    Thank you for the modifications and for including the Tie contact.

    However, the revised modal still has the eigenmodes at very low-frequency values, as shown. 

    image

    Regards

    Ananthan

  • Ananthan
    Ananthan Altair Community Member
    edited July 8

    Hi,

    I could solve the problem. The issues were basically concerning the unit of density. I had entered it earlier in g/mm3. When it is changed to Tonne/mm3, the modes get at a higher frequency.

    Followed the unit consistency as mentioned in the post below.

    https://community.altair.com/community/en/debugging-guide-for-linear-analysis-modelling-checks?id=kb_article&sys_id=8016e4fb977fdd10e3b0361e6253afb4

    Thanks for the response.

    Regards

    Ananthan

  • Rajashri_Saha
    Rajashri_Saha
    Altair Employee
    edited July 8
    Ananthan said:

    Hi,

    I could solve the problem. The issues were basically concerning the unit of density. I had entered it earlier in g/mm3. When it is changed to Tonne/mm3, the modes get at a higher frequency.

    Followed the unit consistency as mentioned in the post below.

    https://community.altair.com/community/en/debugging-guide-for-linear-analysis-modelling-checks?id=kb_article&sys_id=8016e4fb977fdd10e3b0361e6253afb4

    Thanks for the response.

    Regards

    Ananthan

    Ok, Good. Need to be careful with units always.

    You can make the query as solved now.

    Thanks

    Rajashri

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