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Many thanks for your concrete replies. They are very helpful.
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<div class="">Best Wishes,</div>
<div class="">Xiaofeng<br class="">
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<div class="">On Sep 27, 2022, at 20:01, Mark Adams <<a href="mailto:mfadams@lbl.gov" class="">mfadams@lbl.gov</a>> wrote:</div>
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<div dir="ltr" class="">Shells can be pretty hard.
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<div class="">Start with an easy problem with well-shaped elements and probably best to start with a well-supported structure (eg, not a long cantilever).</div>
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<div class="">You should also configure with hypre and try that. I don't know if they deal with shells, but it is a well developed solver.</div>
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<div class="">For GAMG, you need to set the coordinates (PCSetCoordinates) which will create the rigid body modes for GAMG so that it can construct the rotational RBMs.</div>
<div class="">This code has not been used for shells in a long time (petsc/src/ksp/pc/impls/gamg/agg.c:138), but it did once work (20 years ago in another code).</div>
<div class="">This code constructs the 6 rigid body modes for 3D shell problems with my ording (6 dof, x,y,z,xx,yy,zz).</div>
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<div class="">Or you can construct the RBMs yourself and set them with <a href="https://petsc.org/release/docs/manualpages/Mat/MatNullSpaceCreateRigidBody.html" class="">https://petsc.org/release/docs/manualpages/Mat/MatNullSpaceCreateRigidBody.html</a></div>
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<div class="">There is some discussion here: <a href="https://www2.eecs.berkeley.edu/Pubs/TechRpts/2000/CSD-00-1103.pdf" class="">https://www2.eecs.berkeley.edu/Pubs/TechRpts/2000/CSD-00-1103.pdf</a></div>
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<div class="">Mark</div>
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<div dir="ltr" class="gmail_attr">On Tue, Sep 27, 2022 at 6:20 AM 晓峰 何 <<a href="mailto:tlanyan@hotmail.com" class="">tlanyan@hotmail.com</a>> wrote:<br class="">
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<div style="overflow-wrap: break-word;" class="">Hello,
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<div class="">A00 comes from shell structures and discretized by FEM.</div>
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<div class="">Thanks,</div>
<div class="">Xiaofeng<br class="">
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<div class="">On Sep 27, 2022, at 17:48, Mark Adams <<a href="mailto:mfadams@lbl.gov" target="_blank" class="">mfadams@lbl.gov</a>> wrote:</div>
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<div dir="ltr" class="">what equations and discetizations are in A00?</div>
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<div dir="ltr" class="gmail_attr">On Tue, Sep 27, 2022 at 1:45 AM 晓峰 何 <<a href="mailto:tlanyan@hotmail.com" target="_blank" class="">tlanyan@hotmail.com</a>> wrote:<br class="">
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<div class="">Hi Barry,
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<div class="">A00 is formed from elliptic operator. </div>
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<div class="">I tried GAMG with A00, but it was extremely slow to solve the system with field-split preconditioner(I’m not sure I did it with the right way).</div>
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<div class="">Thanks,</div>
<div class="">Xiaofeng</div>
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<div class="">On Sep 26, 2022, at 23:11, Barry Smith <<a href="mailto:bsmith@petsc.dev" target="_blank" class="">bsmith@petsc.dev</a>> wrote:</div>
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<div class=""> What is your A00 operator? ILU is almost never a good choice for large scale problems. If it is an elliptic operator that using a PC of gamg may work well for the A00 preconditioner instead of ILU.</div>
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<div class=""> Barry</div>
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<div class=""> For moderate size problems you can use a PC type LU for AOO to help you understand the best preconditioner to use for the A11 (the Schur complement block), once you have a good preconditioner for the A11 block you would then go back and determine
a good preconditioner for the A00 block.<br class="">
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<div class="">On Sep 26, 2022, at 10:08 AM, 晓峰 何 <<a href="mailto:tlanyan@hotmail.com" target="_blank" class="">tlanyan@hotmail.com</a>> wrote:</div>
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<div class="">Hello Jed,
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<div class="">The saddle point is due to Lagrange multipliers, thus the size of A11 is much smaller than A00.</div>
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<div class="">Best Regards,</div>
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<div class="">Xiaofeng</div>
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<div class="">On Sep 26, 2022, at 21:03, Jed Brown <<a href="mailto:jed@jedbrown.org" target="_blank" class="">jed@jedbrown.org</a>> wrote:</div>
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multipliers</span></div>
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