Thread Subject: Help implementing the subspace iteration- or Lanczos method in matlab

Hello all!

I am need to create an eigensolver using either the Subspace iteration method or the lanczoz method in matlab. The problem is related to dynamics in structures and the eigenfrequencies is wanted.

The structure is built with beams and bar elements, and the stiffness matrix "K" and the mass matrix "M" is made. I am having some big problems on getting started with the implementation on the eigensolvers... So help is deeply wanted!

I really appreciate any help!

Hi Tommy,

I think you can try to use this command for your analysis:
[MODE,PULSATION]=eig(K,M,freq_number,'SM');

K is stiffness matrix
M is mass matrix
freq_number is the number of natural frequencies you want.
I think eigs is for sparse matrix.
MODE is the eigenvector
PULSATION is the eigenvalue
You can recover the natural frequencies from the eigenvalue (PULSATION).

I hope this helps.

"Tommy Olsen" <tommyolse@gmail.com> wrote in message <gtva38$g30$1@fred.mathworks.com>...
> Hello all!
>
> I am need to create an eigensolver using either the Subspace iteration method or the lanczoz method in matlab. The problem is related to dynamics in structures and the eigenfrequencies is wanted.
>
> The structure is built with beams and bar elements, and the stiffness matrix "K" and the mass matrix "M" is made. I am having some big problems on getting started with the implementation on the eigensolvers... So help is deeply wanted!
>
> I really appreciate any help!

"Andrea" <alaskaice75.nospam@libero.it> wrote in message <gv0m55$elh$1@fred.mathworks.com>...
> Hi Tommy,
>
> I think you can try to use this command for your analysis:
> [MODE,PULSATION]=eig(K,M,freq_number,'SM');
>
> K is stiffness matrix
> M is mass matrix
> freq_number is the number of natural frequencies you want.
> I think eigs is for sparse matrix.
> MODE is the eigenvector
> PULSATION is the eigenvalue
> You can recover the natural frequencies from the eigenvalue (PULSATION).
>

Indeed, the resonance frequency is

f = 1/(2*pi)*sqrt(PULSATION)

Bruno

You can take a look at fe_eig in OpenFEM http://gforge.inria.fr/scm/?group_id=370