[petsc-users] Multigrid Preconditioned CG on cell-centered Poisson with DMStag

[Gautam Luhana]

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