×
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 response FEA

frequency response FEA

frequency response FEA

(OP)
Hi

I have carried out a FE frequency sweep analysis of an assembly using modal superimposition. The assembly is shaken using the same acceleration amplitude across a frequency range along one direction only.

When plotting the response (acceleration) Vs frequency for several nodes there are frequency intervals where this becomes NEGATIVE.

Does a negative acceleration make sense in this case?? If so, what is the underlying physical meaning?

Thanks

Gio1
 

RE: frequency response FEA

Depends what you have plotted, but if you have plotted the real part or the imaginary part of the transfer function, yes it makes sense.

If you have plotted the magnitude in dB then it also makes sense, but means something completely different.

Perhaps you could save us some guesswork and post one of these puzzling plots.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

RE: frequency response FEA

(OP)

All I have done is choosing the nodal response result "acceleration" and picked a node. What I get is a graph with frequency (Hz) on the X axis and acceleration (mm/s2 - consistently with my units) on the Y axis. I can clearly locate the resonances, but the line goes into negative Y a few times. Will post it as soon as I can.

Cheers

Gio1

RE: frequency response FEA

If you are plotting Magnitude on the y axis and frequency on the x- axis you should only get positive values. The phase information will tell you if the value is negative or positive.

 

RE: frequency response FEA

True, IF he is plotting linear magnitude. That's why I'd rather look at a plot than play guessing games.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

RE: frequency response FEA

True. Lets see what the plot say.

RE: frequency response FEA

(OP)
I tried, tried again and then again, but that plot never came back as it was yesterday.
I think it was some bug that was affecting the session of the postprocessor I had open.
Today (after a reboot) all plots had POSITIVE acceleration magnitude - as I would expect.

I was slightly puzzled by Greg's last post. What do you mean by "linear magnitude"?

Thanks everybody!

Gio1  

RE: frequency response FEA

Plotting abs(v), is linear magnitude, where v is a complex vector.

It would be an unusual, but possible, choice.

abs(v)^2 is more common.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

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