Dear Matthew and Jed,
Brilliant. Thank you so much!
Your changes work like a charm Matthew (I tested your branch on the gmsh
file I sent) and thank you so much for your advice Jed. The loss of one
order of convergence for an inf-sup stable pressure discretization seems
indeed a very high price to pay for the moderate increase in efficiency
by elimination of the interior modes. You have given me food for thought
and I will probably personally not use 8-node quadrilaterals.
Nevertheless, for our code it will be important to support 8-node
quadrilaterals as it is still an element widely used in solid mechanics
simulations. LibCEED looks very interesting.
Thank you so much again.
Best wishes from Paris,
Susanne
On 11.02.2022 20:27, Matthew Knepley wrote:
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 <[email protected]> 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 <[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
$Nodes
9 21 1 46
0 1 0 1
1
0 0 0
0 2 0 1
2
1 0 0
0 3 0 1
3
1 1 0
0 4 0 1
4
0 1 0
1 1 0 3
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35
36
0.5 0 0
0.25 0 0
0.75 0 0
1 2 0 3
6
37
38
1 0.5 0
1 0.25 0
1 0.75 0
1 3 0 3
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0.5 1 0
0.75 1 0
0.25 1 0
1 4 0 3
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0 0.5 0
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0.5 0.5 0
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$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/ [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/
