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Issue regarding modal analysis, getting modes at extreme low frequency

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

Result of modal analysis

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

Mode 7 and 10 are like this but slightly different magnitude

Any help is highly appreciated.

Thank you for your time and expertise.

Regards

Ananthan

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HyperMesh

v2022

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Rajashri_Saha

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

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.

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

Regards

Ananthan

Rajashri_Saha

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.

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

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

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

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.

Regards

Ananthan

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

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