Are you handling the null space? Like with MatSetNullSpace()?
Can you try src/ksp/ksp/tutorials/ex32.c it solves the same problem but with a simpler DMDA. I don't see any reason why using DMStag with the cell-centered values should be different. Barry > On Oct 1, 2024, at 3:37 PM, Gautam Luhana <gkluh...@cs.ubc.ca> wrote: > > Hello, > > I am solving the stokes equations on the MAC grid using DMStag with an > approximate Schur complement based preconditioner. The preconditioner needs a > subdomain solve of the Poisson equation on both the velocity as well as the > pressure space. I am using multigrid preconditioned CG for both the subdomain > solves. While the subdomain solve on the velocity space converges in 4-5 > iterations and is independent of mesh-size, the pressure solve is not > mesh-size independent. Relevant information: > > Pressure space operator has Neumann boundary conditions. > > Coarse-grid operators are being generated using the Galerkin approach (A = > P*Af*Pt) for both of the subdomains. > > For the pressure space, the jacobi and sor smoothers were tried with several > omega and damping parameter choices. > > Symmetric smoothing (same number of pre- and post-smoothing iterations) > > Has anyone tried solving the poisson operator on the cell-centered pressure > space with DMStag? Or know what might be going wrong with the mg-cg > convergence? > > Thanks! > > -Gautam > >
