[petsc-users] Gmsh 8-noded quadrilateral

[Matthew Knepley]

Jed is right about the numerics. However, this does not look hard. Here is
my try at it:

  https://gitlab.com/petsc/petsc/-/merge_requests/4838

Please tell me if this works and I will make a test and merge.

  Thanks,

    Matt

On Thu, Feb 10, 2022 at 6:47 PM Jed Brown <jed at jedbrown.org> wrote:

> 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 <susanne.claus at onera.fr> 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 <susanne.claus at onera.fr>
>
> >> 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
>


-- 
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/ <http://www.cse.buffalo.edu/~knepley/>
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