Susanne, do you want PetscFE to make the serendipity (8-node) finite element space or do you just want to read these meshes? I.e., would it be okay with you if the coordinates were placed in a Q_2 (9-node, biquadratic) finite element space?
This won't matter if you're traversing the dofs per edge manually, but there are some efficiency benefits of using the Q_2 space (especially if your code can use the tensor product, perhaps via a library like libCEED). Note that Q_2 spaces have better stability properties. For example, the Q_2 space is inf-sup stable with P_1 discontinuous pressure (gives third order L^2 and second order H^1 convergence), but serendipity (8-node) is only stable with piecewise constant pressure (gives second order L^2 and first order H^1 convergence). Susanne Claus <[email protected]> writes: > Dear Matthew, > > Thank you so much. > I have a attached a small 8-noded quadrilateral mesh file (Version 4 > ASCII) generated with gmsh 4.8.4. > > Best wishes, > Susanne > > On 10.02.2022 16:23, Matthew Knepley wrote: > >> On Thu, Feb 10, 2022 at 10:12 AM Susanne Claus <[email protected]> >> wrote: >> >>> Hello, >>> >>> I am using DMPlex for the mesh structure of a solid mechanics finite >>> element code. I mainly use gmsh as input file format. When I try to >>> read in 8-noded Quadrilaterals (Element type 16 in gmsh) DMPlex tells >>> me that this element type is unknown. However a 9-noded Quadrilateral >>> can be read without problem. On inspecting the plexgmsh.c source code >>> I can see that 8-noded quadrilaterals are deactivated: >>> >>> #if 0 >>> 146: {20, GMSH_TRI, 2, 3, 3, 9, NULL}, >>> 147: {16, GMSH_QUA, 2, 2, 4, 8, NULL}, >>> >>> For our application these 8-noded quadrilateral are very important. >>> >>> Is there any reason why they have not been implemented/deactivated in >>> the dmplex gmsh reader? >> >> No, we can handle them in the same way I think. Let me look at it. >> Hopefully it is easy. >> >> Thanks, >> >> Matt >> >>> Thank you for all the great work you are doing. PETSc is amazing. >>> >>> Best wishes, >>> Susanne Claus >> >> -- >> >> What most experimenters take for granted before they begin their >> experiments is infinitely more interesting than any results to which >> their experiments lead. >> -- Norbert Wiener >> >> https://www.cse.buffalo.edu/~knepley/ [1] > > -- > > Susanne Claus > Ingénieur Chercheur > Applied Mathematics and Scientific Computing Group > DTIS > > ONERA - The French Aerospace Lab > 6 Chemin de la Vauve aux Granges, 91120 Palaiseau > > Links: > ------ > [1] http://www.cse.buffalo.edu/~knepley/ > $MeshFormat > 4.1 0 8 > $EndMeshFormat > $PhysicalNames > 2 > 1 2 "Neumann" > 2 1 "Domain" > $EndPhysicalNames > $Entities > 4 4 1 0 > 1 0 0 0 0 > 2 1 0 0 0 > 3 1 1 0 0 > 4 0 1 0 0 > 1 -9.999999994736442e-08 -1e-07 -1e-07 1.0000001 1e-07 1e-07 0 2 1 -2 > 2 0.9999999000000001 -9.999999994736442e-08 -1e-07 1.0000001 1.0000001 1e-07 > 1 2 2 2 -3 > 3 -9.999999994736442e-08 0.9999999000000001 -1e-07 1.0000001 1.0000001 1e-07 > 0 2 3 -4 > 4 -1e-07 -9.999999994736442e-08 -1e-07 1e-07 1.0000001 1e-07 0 2 4 -1 > 1 -9.999999994736442e-08 -9.999999994736442e-08 -1e-07 1.0000001 1.0000001 > 1e-07 1 1 4 1 2 3 4 > $EndEntities > $Nodes > 9 21 1 46 > 0 1 0 1 > 1 > 0 0 0 > 0 2 0 1 > 2 > 1 0 0 > 0 3 0 1 > 3 > 1 1 0 > 0 4 0 1 > 4 > 0 1 0 > 1 1 0 3 > 5 > 35 > 36 > 0.5 0 0 > 0.25 0 0 > 0.75 0 0 > 1 2 0 3 > 6 > 37 > 38 > 1 0.5 0 > 1 0.25 0 > 1 0.75 0 > 1 3 0 3 > 7 > 39 > 40 > 0.5 1 0 > 0.75 1 0 > 0.25 1 0 > 1 4 0 3 > 8 > 41 > 42 > 0 0.5 0 > 0 0.75 0 > 0 0.25 0 > 2 1 0 5 > 9 > 43 > 44 > 45 > 46 > 0.5 0.5 0 > 0.75 0.5 0 > 0.5 0.25 0 > 0.25 0.5 0 > 0.5 0.75 0 > $EndNodes > $Elements > 2 6 197 206 > 1 2 8 2 > 197 2 6 37 > 198 6 3 38 > 2 1 16 4 > 203 2 6 9 5 37 43 44 36 > 204 1 5 9 8 35 44 45 42 > 205 4 8 9 7 41 45 46 40 > 206 3 7 9 6 39 46 43 38 > $EndElements
