Menu

Discussion Forum

New Discussion
Filtern

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

Eigenfrequency Analysis: Rigid Body Motion

Posted 01.02.2018, 20:02 GMT-5 Structural & Acoustics, Acoustics & Vibrations, Structural Mechanics 3 Replies

Ganesh Patil
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

I did an eigen frequency analysis of a solid cube with homogeneous Isotropic material. When I extracted 10 modes, I see first 6 modes with no real values but pure imaginary number.

Since Rigid body motion has zero frequency, having zero real number makes sense, but why do I get Imaginary number even if I dont have damping in my system. Is it FEM numerical approximation? I also get imaginary number for higher frequncy modes.

For my free-free analysis, I was expecting 3 translation and 3 rotational modes, but my 6 modes are giving me all rotational mode.

3 Replies Last Post 05.02.2018, 07:27 GMT-5
Henrik Sönnerlind COMSOL Employee

Please login with a confirmed email address before reporting spam

Posted: 7 years ago 02.02.2018, 02:49 GMT-5

Hi Ganesh,

Here is a quote from the User's guide:

In practice, the natural frequencies of the rigid body modes are not computed as exactly zero, but can appear as small numbers which can even be negative or complex. If rigid body modes are present in the model, then it is important to use a nonzero value in the Search for eigenfrequencies around text field in the settings for the Eigenfrequency study step. The value should reflect the order of magnitude of the first important nonzero eigenfrequency

Your six rigid body modes are all linear combinations of the three translations and three rotations.

Regards,

Henrik

-------------------
Henrik Sönnerlind
COMSOL
Hi Ganesh, Here is a quote from the User's guide: > In practice, the natural frequencies of the rigid body modes are not computed as exactly zero, but can appear as small numbers which can even be negative or complex. If rigid body modes are present in the model, then it is important to use a nonzero value in the **Search for eigenfrequencies around** text field in the settings for the Eigenfrequency study step. The value should reflect the order of magnitude of the first important nonzero eigenfrequency Your six rigid body modes are all linear combinations of the three translations and three rotations. Regards, Henrik
Ganesh Patil
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 7 years ago 02.02.2018, 11:54 GMT-5

Hi, Thank you very much, that does resolve lot of queries. I still have one. I am getting a modal frequency with same real and imaginary values but it is shown with different sign. PFA image (Last two frequencies)

The mode shape for both of them is same. Why is that?

Hi, Thank you very much, that does resolve lot of queries. I still have one. I am getting a modal frequency with same real and imaginary values but it is shown with different sign. PFA image (Last two frequencies) The mode shape for both of them is same. Why is that?

Attachments:

Henrik Sönnerlind COMSOL Employee

Please login with a confirmed email address before reporting spam

Posted: 7 years ago 05.02.2018, 07:27 GMT-5

Hi Ganesh,

The complex conjugate pair is probably numerical noise. Note that the imaginary part is small relative to the real part. If you have a good value for Search for eigenfrequencies around (like 2e5), and still get this behavior, I would suggest tightening the Relative tolerance in the Eigenvalue Solver node.

Regards,

Henrik

-------------------
Henrik Sönnerlind
COMSOL
Hi Ganesh, The complex conjugate pair is probably numerical noise. Note that the imaginary part is small relative to the real part. If you have a good value for **Search for eigenfrequencies around** (like 2e5), and still get this behavior, I would suggest tightening the **Relative tolerance** in the **Eigenvalue Solver** node. Regards, Henrik

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.

Suggested Content