Hi Roy, Yes, I meant introduction_ex3 - sorry for that. I will move forward with your advices. Thanks for the answer.
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Renato On Tue, Aug 8, 2017 at 11:10 AM, Roy Stogner <royst...@ices.utexas.edu> wrote: > > 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 > ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, Slashdot.org! http://sdm.link/slashdot _______________________________________________ Libmesh-users mailing list Libmesh-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/libmesh-users
