Problem with 3D Fem sparse matrix ( plz help me)
hi,
i have calculated the local stiffness matrix and form global stiffness matrix. but when i am solving the equation it is saying the matrix (stiffness) is badly scaled and near to singular matrix. i think there is one problem in assembling the local matrix. anybody can give the idea what will be the probabal solution. i have done the 2d and its working fine. i have some doubt on assembling. what i have done is that i have use the same algorithm of 2d( linear basis is triangle) assemble but different degrees of freedom and ofcourse node number will be differents for 3D ( basia is linear tetrahedron).
plz help me i am on my deadline
»
- sameer2008's blog
- Login or register to post comments
- 432 reads
Re: Problem with 3D Fem sparse matrix ( plz help me)
What I would do in your case is to try with a simple example done by hand. Thus, you know what numbers the stiffness matrix should have. There are some reasons to get a singular matrix. One of them might be that the global stiffness matrix is not variationally correct meaning that the essential boundary conditions are not properly included in your stiffness matrix. After you form the global stiffness matrix you need to modify it along with the force vector to include the essential boundary conditions. Otherwise the matrix will be singular. If this is not your problem then a wrong assembly procedure might be the cause.
Best wishes in your project,
Alejandro A. Ortiz
thank you very much. i
thank you very much. i will do the things that u told.
but if i dont include the boundary condition and simpliy find the stiffness matrix for each local tetrahedron and to get the global stiffness matrix, i just add the local matrices in there corresponding positions with respect to the global matrix, even after that it will show any problem?
for assembling the local matrices i just did these things
kk=zeros(total_nod,total_nod); global stiffnes matrix
kk(t(iel,:),t(iel,:))=kk(t(iel,:),t(iel,:))+k; where k is local stiffness matrix for t(iel,:) element
i am using matlab software
once again thank you ( if u have any suggestion plz suggest )
You should include the BC's
Hi Sameer,
With out the BC's, the stiffness matrices are simply singular.
This isn't a numerical issue. Even when you solve a DE analytically your
solution is not uniquely defined until you supply appropriate boundary
conditions. So include the BC before you go ahead.
I am Temesgen and I approve this message!
thank you very much Mr.
thank you very much Mr. Temesgen