Hi Roy,

Yes, I meant introduction_ex3 - sorry for that.
I will move forward with your advices.
Thanks for the answer.


On Tue, Aug 8, 2017 at 11:10 AM, Roy Stogner <royst...@ices.utexas.edu>

> On Mon, 7 Aug 2017, Renato Poli wrote:
> I am getting started in libmesh - I've been using it for a month now.
>> I used libmesh introductory example 3 (libmesh_ex3) as a reference for my
>> problem.
> I assume you mean introduction_ex3?
> The idea is to replace the mesh by one of my one and start from there.
>> I noticed that libmesh_ex3 uses TRI6 or QUAD elements.
>> In my application I was using TRI3 and it did not work.
>> I realized that the example does not work with TRI3 either.
> I assume you're using higher than first order elements in your
> application, like we do in the example?
> Is there a reason for that? Should I use TRI6 instead?
> Yes, and yes.
> libMesh stores degree of freedom (DoF) indices on element nodes and
> element interiors, and it requires those indices to be properly
> located topologically.  Shape functions whose support is on the patch
> of elements surrounding a node need to have corresponding DoF indices
> located on that node, shape functions which live on only one element
> need to have indices located on that element or on a node internal to
> that element...
> And most critically in your case:, shape functions whose support is on
> the two triangles sharing an edge need to have indices located on that
> edge.  Since libMesh doesn't *have* edge objects, we use edge nodes to
> store such indices.  If you have a TRI3, then you don't have any edge
> nodes, and so you can't use any finite element types which have edge
> dofs.  That means you're restricted to combinations of discontinuous
> finite elements (XYZ and MONOMIAL, which live within a single element)
> and first-order finite elements (which have only vertex dofs).
> Any advice?
> In 2D, if you want to use 2nd order through 11th order basis
> functions, then use 2nd order geometric elements and you're fine.
> (roundoff error blows up at higher polynomial degrees, so we currently
> cap p refinement at 11).  Likewise for 3D on hexes.
> If you want to use 3rd order or higher bases on a mesh with tets,
> prisms, and/or pyramids, then my advice is to warn us *way* in
> advance.  We don't currently have any elements whose triangular faces
> have face nodes, so all our elements with triangular faces support a
> max of p=2.  Fixing that has been on my todo list for a decade, but
> probably won't get *off* my todo list for years to come unless there's
> a user chomping at the bit for it.
> ---
> Roy
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