×
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

Rigid body modes
3

Rigid body modes

Rigid body modes

(OP)
Dear All,

I would like to ask something regarding rigid body modes. Maybe it is a trivial thing, but I don't really get it: When I create a new model, I usually run a free-free modes check as part of a set of model checks. Then, I check that there are six rigid modes with very low eigenfrequencies. The thing is that sometimes, when I have a look at the modeshapes, they don't look rigid at all. To put a simple example, if I analyze a cylinder, I would expect to see three rotational and three translation rigid movements, but what I get are some modes in which the cylinder contracts or expands radially.

Is there any explanation for this? Am I getting anything wrong?

Thanks a lot!

Cheers,

Israel.

RE: Rigid body modes

2
"To put a simple example, if I analyze a cylinder, I would expect to see three rotational and three translation rigid movements, but what I get are some modes in which the cylinder contracts or expands radially."

These are in fact rotational DOFs, but the expansion/contraction effect that you're seeing is the result of the linear underpinnings of a modal analysis. If you had a fixed-end cylinder (like a cantilever beam) with a torsional load applied on one end, then the results of a linear static analysis will show the cylinder expanding. I should point out that this is just a visual manifestation of the linear assumption; the numeric results calculated by the simulate (strains, stresses, etc) are fine (assuming you don't violate the small displacement approximation). If you were to run a non-linear analysis, then this effect would go away.

RE: Rigid body modes

Good one. Also it may just be a visual effect, for instance the animation of the 'roll' mode of a cylinder would show the cylinder's diameter changing, as all the surface points move tangentially, not around the circumference.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Rigid body modes

(OP)
Dear shaun8567 and GregLocock,

Thank you very much for your replies! For newcomers and people with just a few motnsh of experience like me, it is very useful that the community answers to questions like this :)

I see your points but I don't understand it totally, though. Here my questions:

- "If there is deformation in the elastic parts of the structure then there must be actual energy involved in the mode shape and so the frequency would be non zero, hence not an RBM"

I totally agree, but I never manage to get exactly zero energy and zero frequency in a free-free analysis. In fact, in http://femci.gsfc.nasa.gov/validitychecks/vc2.html it is said that the eigenfrequencies should be small, but not zero. I am wondering if the solution is meant to be like that or it has something to do with error introduced by the FE discretization or by the numerical algorithms used to solve the problem.

- "These are in fact rotational DOFs, but the expansion/contraction effect that you're seeing is the result of the linear underpinnings of a modal analysis. If you had a fixed-end cylinder (like a cantilever beam) with a torsional load applied on one end, then the results of a linear static analysis will show the cylinder expanding. I should point out that this is just a visual manifestation of the linear assumption; the numeric results calculated by the simulate (strains, stresses, etc) are fine (assuming you don't violate the small displacement approximation). If you were to run a non-linear analysis, then this effect would go away."

Probably I am getting something wrong from your explanation. I understand that if you have a torsional load the cylinder will expand, but I am not able to see the connection between torsional effects and rigid body mode. If the cylinder is rotating as a rigid body, it is not supposed to be suffering a torsion, right? Wouldn't that be like stating that a cylinder moving in one translational direction as a rigid body suffers, say, compresion?

- "Good one. Also it may just be a visual effect, for instance the animation of the 'roll' mode of a cylinder would show the cylinder's diameter changing, as all the surface points move tangentially, not around the circumference."

What's the difference between moving tangentially and around the circumference?


Again, thanks a lot.

Kind regards,

Israel

RE: Rigid body modes

(OP)
Dear GregLocock, shaun8567 and all,

What do you think about the last things I pointed out? Unfortunately, they are not clear to me yet :(

I don't know what the meaning of "draw it up" is(I'm not a native English speaker). Do you mean that I take a paper and draw the cylinder? XD In any case, I can see three "directions" in a cylinder: tangential, radial and length direction...

Thanks again,

Cheers!

Israel

RE: Rigid body modes

(OP)
Ok... but what about the other two questions??

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