Please reply to all, so that other people can also read this discussion on the mailing list.

I do currently have problems understanding the inner workings of PetscFE. For example how the element matrix is assembled. As far as I could deduce from your example, g3_uu_3d is the fourth-order elasticity tensor which means that we should form the double-dot product of it with the strain for it to be the integrand of the gradient of the basis functions ( int_omega [g3:epsilon]•grad(v) d omega). So for a linear elastic problem, according to my understanding, g3 should be the elasticity tensor and g0,g1,g2 = NULL if there is no volumetric force. And the residual should be f0 = 0 ; f1= the cauchy stress tensor.

Yes, that seems correct.

I also have troubles understanding how to apply boundary conditions. There seems to be the concept of DMAddBoundary() and the PetscDSSetBdJacobian() and PetscDSSetBdResidual(). I struggle to understand what sets them apart from each other. My impression is that DMAddBoundary() is for Dirichlet and Neumann conditions and that PetscDSSetBdJacobian() and PetscDSSetBdResidual() are for BC types that do not fall in those categories?

Note that you can also use PetscDSAddBoundary now (if you use the master branch). DMAddBoundary is basically used to tell which boundary region to apply which bc. If it's a Dirichlet bc you have to provide a function to prescribe the values on the boundary, if it's a Neumann bc you just give it NULL. PetscDSSetBdJacobian() and PetscDSSetBdResidual() are for Neumann bcs.
The reason why there's this difference for Dirichlet and Neumann bcs is because for finite elements you need to assemble stuff for non-zero Neumann bcs while for Dirichlet you set some dofs to some prescribed value.

ex77 works with the options you provided me with. However the solver does not seem to converge.

What do you mean that it works but it doesn't converge? Did you use all the options I provided you? In case not, you should send the output of -snes_view.

The problem I have with the documentation online so far is that I fail to fully understand how to apply petscFE. I did for example not know that you had to set the order of the finite element space and I have the feeling that there is much more I am missing.

I don't really understand what you mean with "applying PetscFE".

I think the simplest way to learn how to use it is to start from one of the examples and if you don't understand what a function does you look it up in the documentation. If you want to see what kind of options there are for some object, for example petscspace, you add "-help|grep petscspace" to the command line.

Thanks,

Sander

On 28 June 2016 at 17:47, <rickcha@googlemail.com> wrote:

Yes, that was vague, I should have formulated that better.
What I’m trying to accomplish is writing a FEM code to calculate deformation and stress of structure. As a first step analysing a linear elastic and static problem in 2D, which I plan to extend to a elastic-plastic transient analysis for a 3D case.

I do currently have problems understanding the inner workings of PetscFE. For example how the element matrix is assembled. As far as I could deduce from your example, g3_uu_3d is the fourth-order elasticity tensor which means that we should form the double-dot product of it with the strain for it to be the integrand of the gradient of the basis functions ( int_omega [g3:epsilon]•grad(v) d omega). So for a linear elastic problem, according to my understanding, g3 should be the elasticity tensor and g0,g1,g2 = NULL if there is no volumetric force. And the residual should be f0 = 0 ; f1= the cauchy stress tensor.

I also have troubles understanding how to apply boundary conditions. There seems to be the concept of DMAddBoundary() and the PetscDSSetBdJacobian() and PetscDSSetBdResidual(). I struggle to understand what sets them apart from each other. My impression is that DMAddBoundary() is for Dirichlet and Neumann conditions and that PetscDSSetBdJacobian() and PetscDSSetBdResidual() are for BC types that do not fall in those categories?

ex77 works with the options you provided me with. However the solver does not seem to converge.

The problem I have with the documentation online so far is that I fail to fully understand how to apply petscFE. I did for example not know that you had to set the order of the finite element space and I have the feeling that there is much more I am missing.

Thank you for your precious time,

M. Hartig

On 28 Jun 2016, at 14:30, Sander Arens <Sander.Arens@ugent.be> wrote:

What are you trying to do with it? I don't think there's more documentation on how to use PetscFE than what's already in the online documentation and in the examples.
What do you mean with "grasp the concept"?

Thanks,
Sander

On 27 June 2016 at 17:41, <rickcha@googlemail.com> wrote:
That did do the trick with the examples, thank you. I am still struggling with my own simple example however. Is there any documentation available on how to use petscFE? I could not find anything besides the three examples. And I yet have to grasp the concept.

Thanks in advance,
M. Hartig

On 27 Jun 2016, at 15:49, Sander Arens <Sander.Arens@ugent.be> wrote:

I think you forgot to set the order of the finite element spaces, which default to 0. Does it work if you try for example ex77 with the following options found here?

Thanks,
Sander

On 27 June 2016 at 15:28, <rickcha@googlemail.com> wrote:
Dear Petsc developer team,

while learning the ways of the petscFE I came across a curiosity I’d like to point out to you. The Jacobians that are created from the petscFE object seem to have only 0-entries (apart from the obviously empty regions) in examples 12 and 77. That might be due to a limited understanding from my side on how exactly petscFE and the mentioned examples work, but it struck me as unusual.

Thanks in advance,

M. Hartig