×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

frequency peaks in modal frequency response

frequency peaks in modal frequency response

frequency peaks in modal frequency response

(OP)

Hi,

doing a frequency response, I requested the output to be calculated also at the eigenfrequency of my structure by mean of a FREQ3 card.

Looking at the results, I noticed that the eigenfrequencies reported were slightly different that those I could calculate directly from a modal analysis.

Could someone explain? Is this a matter of accuracy?
I thought that it should be the same since the NASTRAN method is based on a modal frequency response. Could the residual vectors improve the results?

Thanks in advance for your answers.

RE: frequency peaks in modal frequency response

Could this be due to damping?  Try to run a model with zero damping and see if they are they same.

Brian
www.espcomposites.com

RE: frequency peaks in modal frequency response

(OP)
Hi,

sorry for the late answer but I was out of this office for some weeks. Thanks for the answer, I will look at it. This could be an explaination.

Cheers.

RE: frequency peaks in modal frequency response

The solver first uses modal analysis to solve eigenvalues. If a damped solution is requested, then you will get the damped natural frequency, which is different than the original undamped values. Not sure if that is your problem, but something to think about.

Brian
www.espcomposites.com

RE: frequency peaks in modal frequency response

Do you have any frequency dependent springs?
If yes, then the eigenfrequency will differ. Becasue they don't have any influence in Modal Analysis.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources