November 2024 - petsc-users - mailman3.cels.anl.gov

Rigid body nullspace for Stokes operator
by Amneet Bhalla 05 Nov '24

05 Nov '24

Hi Folks, I am trying to solve the momentum equation in a projection preconditioner using GAMG or Hypre solver. The equation looks like for velocity variable *v* looks like: [image: Screenshot 2024-10-28 at 4.15.17 PM.png] Here, μ is spatially varying dynamic viscosity and λ is spatially varying bulk viscosity. I understand that I need to specify rigid body nullspace modes to the multigrid solver in order to accelerate its convergence. Looking into this routine MatNullSpaceCreateRigidBody() ( https://urldefense.us/v3/__https://petsc.org/release/manualpages/Mat/MatNul… ), I see that I need to provide the coordinates of each node. I am using staggered grid discretization. Do I need to provide coordinates of staggered grid locations? Thanks, -- --Amneet

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Advice on setting BCs in a PLEX
by Aldo Bonfiglioli 04 Nov '24

04 Nov '24

Dear users, I have been using petsc's KSP for over 20 years and I am considering using DMPLEX to replace my own data structure in a mixed FEM/FVM CFD code. To do so, I am trying to understand DMPLEX by writing a 1D code that solves u_t = u_xx using PSEUDOTS. In order to prescribe Dirichlet BCs at the endpoints of the 1D box, I use PetscSectionSetConstraintDof when building the PetscSection. > A global vector is missing both the shared dofs which are not owned by > this process, as well as /constrained/ dofs. These constraints > represent essential (Dirichlet) boundary conditions. They are dofs > that have a given fixed value, so they are present in local vectors > for assembly purposes, but absent from global vectors since they are > never solved for during algebraic solves. My global Vec has indeed two entries less than the local one. When initializing the solution or evaluating the rhs function I transfer data from the global to local representation, do the calculation, then transfer back. I am doing something wrong, though, which shows up in the attached log file. My simple code is also attached. Thank you for your advice. Aldo -- Dr. Aldo Bonfiglioli Associate professor of Fluid Machines Scuola di Ingegneria Universita' della Basilicata V.le dell'Ateneo lucano, 10 85100 Potenza ITALY tel:+39.0971.205203 fax:+39.0971.205215 web:https://urldefense.us/v3/__http://docenti.unibas.it/site/home/docente.html?m=002423__;!!G_uCfscf7eWS!cWgDpzKW9KGEkqxu775gDU488CBQ5itLl2l_HUsDKcPstbcuSm0_bZPHkMciXajTi3539fuMPe8TD1YZuirDfmSBMSXXxrQZWns$

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Example of SNES using matrix free jacobian
by Daniel Pino Munoz 04 Nov '24

04 Nov '24

Dear all, I have a linear problem that I am currently solving with a KSP matrix-free. I would like to move on to a non linear problem, so figure I could start by solving the same linear problem using SNES. So I am setting the problem as follows: SNESCreate(PETSC_COMM_WORLD, &snes); MatCreateShell(PETSC_COMM_WORLD, n_dofs, n_dofs, PETSC_DETERMINE, PETSC_DETERMINE, &ctx, &J); MatCreateShell(PETSC_COMM_WORLD, n_dofs, n_dofs, PETSC_DETERMINE, PETSC_DETERMINE, &ctx, &B); MatShellSetOperation(J, MATOP_MULT, (void (*)(void))(Multiplication)); MatCreateVecs(J, &x_sol, &b); VecDuplicate(x_sol, &r); SNESSetFromOptions(snes); SNESSetFunction(snes, r, &(computeResidual), &ctx); SNESSetUseMatrixFree(snes, PETSC_FALSE, PETSC_TRUE); SNESGetLineSearch(snes, &linesearch); SNESGetKSP(snes, &ksp); KSPSetOperators(ksp, J, J); KSPSetInitialGuessNonzero(ksp, PETSC_TRUE); I tested it with a small problem (compiled in debug) and it works. When I compiled it in Release, it crashes with a segfault. I tried running the Debug version through valgrind, but even for a small problem, it is too slow. So I was wondering if you guys could see any rocky mistake on the lines I used above? Otherwise, is there any example that uses a SNES combined with a matrix free KSP operator? Thank you, Daniel

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