<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /></head><body style='font-size: 10pt; font-family: Verdana,Geneva,sans-serif'>
<p>Dear Matthew and Jed,</p>
<p>Brilliant. Thank you so much!</p>
<p>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.</p>
<p>Thank you so much again.</p>
<p>Best wishes from Paris,<br />Susanne</p>
<p><br /></p>
<p id="reply-intro">On 11.02.2022 20:27, Matthew Knepley wrote:</p>
<blockquote type="cite" style="padding: 0 0.4em; border-left: #1010ff 2px solid; margin: 0">
<div id="replybody1">
<div dir="ltr">Jed is right about the numerics. However, this does not look hard. Here is my try at it:
<div> </div>
<div> <a href="https://gitlab.com/petsc/petsc/-/merge_requests/4838" target="_blank" rel="noopener noreferrer">https://gitlab.com/petsc/petsc/-/merge_requests/4838</a></div>
<div> </div>
<div>Please tell me if this works and I will make a test and merge.</div>
<div> </div>
<div> Thanks,</div>
<div> </div>
<div> Matt</div>
</div>
<br />
<div class="v1gmail_quote">
<div class="v1gmail_attr" dir="ltr">On Thu, Feb 10, 2022 at 6:47 PM Jed Brown <<a href="mailto:jed@jedbrown.org" rel="noreferrer">jed@jedbrown.org</a>> wrote:</div>
<blockquote class="v1gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left: 1px solid #cccccc; padding-left: 1ex;">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?<br /><br />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).<br /><br />Susanne Claus <<a href="mailto:susanne.claus@onera.fr" rel="noreferrer">susanne.claus@onera.fr</a>> writes:<br /><br />> Dear Matthew,<br />><br />> Thank you so much.<br />> I have a attached a small 8-noded quadrilateral mesh file (Version 4 <br />> ASCII) generated with gmsh 4.8.4.<br />><br />> Best wishes,<br />> Susanne<br />><br />> On 10.02.2022 16:23, Matthew Knepley wrote:<br />><br />>> On Thu, Feb 10, 2022 at 10:12 AM Susanne Claus <<a href="mailto:susanne.claus@onera.fr" rel="noreferrer">susanne.claus@onera.fr</a>> <br />>> wrote:<br />>> <br />>>> Hello,<br />>>> <br />>>> I am using DMPlex for the mesh structure of a solid mechanics finite <br />>>> element code. I mainly use gmsh as input file format. When I try to <br />>>> read in 8-noded Quadrilaterals (Element type 16 in gmsh) DMPlex tells <br />>>> me that this element type is unknown. However a 9-noded Quadrilateral <br />>>> can be read without problem. On inspecting the plexgmsh.c source code <br />>>> I can see that 8-noded quadrilaterals are deactivated:<br />>>> <br />>>> #if 0<br />>>> 146: {20, GMSH_TRI, 2, 3, 3, 9, NULL},<br />>>> 147: {16, GMSH_QUA, 2, 2, 4, 8, NULL},<br />>>> <br />>>> For our application these 8-noded quadrilateral are very important.<br />>>> <br />>>> Is there any reason why they have not been implemented/deactivated in <br />>>> the dmplex gmsh reader?<br />>> <br />>> No, we can handle them in the same way I think. Let me look at it. <br />>> Hopefully it is easy.<br />>> <br />>> Thanks,<br />>> <br />>> Matt<br />>> <br />>>> Thank you for all the great work you are doing. PETSc is amazing.<br />>>> <br />>>> Best wishes,<br />>>> Susanne Claus<br />>> <br />>> --<br />>> <br />>> What most experimenters take for granted before they begin their <br />>> experiments is infinitely more interesting than any results to which <br />>> their experiments lead.<br />>> -- Norbert Wiener<br />>> <br />>> <a href="https://www.cse.buffalo.edu/~knepley/" target="_blank" rel="noopener noreferrer">https://www.cse.buffalo.edu/~knepley/</a> [1]<br />><br />> -- <br />><br />> Susanne Claus<br />> Ingénieur Chercheur<br />> Applied Mathematics and Scientific Computing Group<br />> DTIS<br />><br />> ONERA - The French Aerospace Lab<br />> 6 Chemin de la Vauve aux Granges, 91120 Palaiseau<br />><br />> Links:<br />> ------<br />> [1] <a href="http://www.cse.buffalo.edu/~knepley/" target="_blank" rel="noopener noreferrer">http://www.cse.buffalo.edu/~knepley/</a><br />> $MeshFormat<br />> 4.1 0 8<br />> $EndMeshFormat<br />> $PhysicalNames<br />> 2<br />> 1 2 "Neumann"<br />> 2 1 "Domain"<br />> $EndPhysicalNames<br />> $Entities<br />> 4 4 1 0<br />> 1 0 0 0 0 <br />> 2 1 0 0 0 <br />> 3 1 1 0 0 <br />> 4 0 1 0 0 <br />> 1 -9.999999994736442e-08 -1e-07 -1e-07 1.0000001 1e-07 1e-07 0 2 1 -2 <br />> 2 0.9999999000000001 -9.999999994736442e-08 -1e-07 1.0000001 1.0000001 1e-07 1 2 2 2 -3 <br />> 3 -9.999999994736442e-08 0.9999999000000001 -1e-07 1.0000001 1.0000001 1e-07 0 2 3 -4 <br />> 4 -1e-07 -9.999999994736442e-08 -1e-07 1e-07 1.0000001 1e-07 0 2 4 -1 <br />> 1 -9.999999994736442e-08 -9.999999994736442e-08 -1e-07 1.0000001 1.0000001 1e-07 1 1 4 1 2 3 4 <br />> $EndEntities<br />> $Nodes<br />> 9 21 1 46<br />> 0 1 0 1<br />> 1<br />> 0 0 0<br />> 0 2 0 1<br />> 2<br />> 1 0 0<br />> 0 3 0 1<br />> 3<br />> 1 1 0<br />> 0 4 0 1<br />> 4<br />> 0 1 0<br />> 1 1 0 3<br />> 5<br />> 35<br />> 36<br />> 0.5 0 0<br />> 0.25 0 0<br />> 0.75 0 0<br />> 1 2 0 3<br />> 6<br />> 37<br />> 38<br />> 1 0.5 0<br />> 1 0.25 0<br />> 1 0.75 0<br />> 1 3 0 3<br />> 7<br />> 39<br />> 40<br />> 0.5 1 0<br />> 0.75 1 0<br />> 0.25 1 0<br />> 1 4 0 3<br />> 8<br />> 41<br />> 42<br />> 0 0.5 0<br />> 0 0.75 0<br />> 0 0.25 0<br />> 2 1 0 5<br />> 9<br />> 43<br />> 44<br />> 45<br />> 46<br />> 0.5 0.5 0<br />> 0.75 0.5 0<br />> 0.5 0.25 0<br />> 0.25 0.5 0<br />> 0.5 0.75 0<br />> $EndNodes<br />> $Elements<br />> 2 6 197 206<br />> 1 2 8 2<br />> 197 2 6 37 <br />> 198 6 3 38 <br />> 2 1 16 4<br />> 203 2 6 9 5 37 43 44 36 <br />> 204 1 5 9 8 35 44 45 42 <br />> 205 4 8 9 7 41 45 46 40 <br />> 206 3 7 9 6 39 46 43 38 <br />> $EndElements</blockquote>
</div>
<br clear="all" />
<div> </div>
-- <br />
<div class="v1gmail_signature" dir="ltr">
<div dir="ltr">
<div>
<div dir="ltr">
<div>
<div dir="ltr">
<div>What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.<br />-- Norbert Wiener</div>
<div> </div>
<div><a href="http://www.cse.buffalo.edu/~knepley/" target="_blank" rel="noopener noreferrer">https://www.cse.buffalo.edu/~knepley/</a></div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
<p><br /></p>
<div id="signature">-- <br />
<p><img src="cid:3bb899cc22ffbad8f707be3717357bb5@onera.fr" alt="" width="138" height="31" /> </p>
<p><span style="color: #0000ff; font-size: 8pt;">Susanne Claus </span><br /><span style="color: #0000ff; font-size: 8pt;">Ingénieur Chercheur</span><br /><span style="color: #0000ff; font-size: 8pt;">Applied Mathematics and Scientific Computing Group</span><br /><span style="color: #0000ff; font-size: 8pt;">DTIS</span><br /><br /><span style="font-size: 8pt;">ONERA - The French Aerospace Lab </span><br /><span style="font-size: 8pt;">6 Chemin de la Vauve aux Granges, 91120 Palaiseau </span></p>
</div>
</body></html>