×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Contact US

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!

*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

Modal Analysis - modal matrices export

Modal Analysis - modal matrices export

Modal Analysis - modal matrices export

(OP)
Hello.

I have question regarding the NX Nastran modal analysis. I am working on project which takes Nastran results from modal analysis for further post processing/computation. However, the values which I am interested in are modal mass matrix(Mhh), modal matrix(Phi) and matrix of eigenvalues(omega). Modal mass matrix should be NxN (N-number of DOF) diagonal matrix of ones so thats no problem. The modal matrix Phi is NxN and its columns consists of N eigenvectors. However, I a bit confused since the real eigenvectors listed in output are of AxB dimensions, where A-number of nodes in FEM model and B-number of directions(3 translations,3 rotations) therefore by using these eigenvectors the modal matrix won't be NxN. Can anyone explain how can I build the modal matrix from these eigenvalues so the dimensions would agree? Or, is there any option to directly export these matrices from nastran?

I've also attached a figure which describes the derivation of problem and variables I am interested in. Is there an option to directly obtain some of these (sub)matrices from Nastran?
Thanks.

Mat

RE: Modal Analysis - modal matrices export

The snip you provided does not tell you how to get the eigenvectors. Typically when you run modal analysis you would not request all possible modes of the model. If the model has N degrees of freedom (DOF), and stiffness and mass are defined at all these DOF (rarely the case), and there are no boundary conditions and no rigid elements or MPCs, then technically you could obtain N eigenvectors and your Mhh, omega and Phi matrices would all be NxN. However, this transformation to modal coordinates is not just for fun; the main reason for doing it is to restrict the computation to the first M important modes, or perhaps the M important modes in a frequency range. Which are the important modes rather depends on what you are trying to achieve, and this is a subject of a different discussion.

So let's assume you have decided that the first M modes ( where M<<N ) are important. If you chose mass normalisation of the eigenvectors, then by definition, as you pointed out the Mhh (modal mass) matrix is an identity matrix and will have dimensions MxM. Likewise, the omega matrix will be a diagonal matrix MxM with the eigenvalues along the diagonal. The eigenvectors in Phi, one for each of the M modes, will have N rows (one row for each DOF) and M columms (one for each mode).

If you have an eigenvector matrix AxB, where B=N and A=number of GRID points, the most likely explanation is that either you requested the number of modes equal to the number of GRID points or you requested a very large number of modes, which the eigensolver has restricted to the number of DOF with mass, and this happens to correspond to the number of GRID points.

You already have these outputs from Natran. The EIGENVALUE SUMMARY TABLE shows you Mhh and omega in the columns GENERALIZED MASS and GENERALIZED STIFFNESS respectively. The eigenvectors may be obtained by requesting DISP=ALL (VECTOR=ALL will do the same thing). These outputs are available in formatted tables (known as OFP tables), but you can also obtain any of these quantities (and others you choose to compute) at the matrix level using the DMAP language, which is again a subject of a different discussion.

DG

RE: Modal Analysis - modal matrices export

(OP)
Thank you for reply.I've already requested DISP=ALL. You're right, the generalized mass,stiffness,eigenvalues are printed in the eigenvalue summary table, thats no problem. But lets consider simple 2DOF spring-mass system. The system has obviously 2 DOF, so Mhh is 2x2, Phi(modal matrix) should be also 2x2. However, when I look at the printed eigevectors,those are in this case in shape 5x6 where 5 is number of grid points(nodes) and 6 is number of directions. Since Phi is matrix of eigenvectors, than resulting shape would be 5x12 and not 2x2. Ofc, if I ignore all the zeros in those eigenvectors I get the desired shape and result but in general 3D problems,there will be no zeros. So my main confusion is how to obtain the Phi matrix from printed eigenvectors since those eigenvectors are in shape AxB, A-no. of grid points,B-no. of directions(6). I've attached the results file for this particular case.

Mat

RE: Modal Analysis - modal matrices export

That's not a 2 DOF problem, it is a 30 DOF problem where you have computed the first 2 modes.
Mhh and Khh (omega) are 2X2 because you have 2 modes
Phi is 30 rows (DOFs) by 2 columns (modes).

Remember that in Nastran, all GRID points have 6 DOF whether you use them or not. If GRID points 5,8 and 9 are not doing anything, then remove them from the model. Then you will have a 12 DOF problem where perhaps you are using only the first DOF of each GRID point (the other 5 DOF are restrained either by direct action through an SPC or by the auto-restraint mechanism of AUTOSPC). The eigenvector will always show you all 6 DOF per GRID point, just zero values at the DOF you are not using.

DG

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! Already a Member? Login


Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close