[petsc-users] Nullspace issue

[Jed Brown]

On Tue, Jun 12, 2012 at 3:35 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

> G-S and Shur complement type fieldsplit PCs almost always work right so
> long as you have decent preconditioners for the original block(s). Often
> you don't need much of anything to precondition the Schur complement.
>

Thomas, note that you'll need Schur or some surrogate preconditioner to
deal with incompressibility. It'll be worth understanding fieldsplit
variants while working with Navier-Stokes and Cahn-Hilliard separately.
Once you understand how to "drive" fieldsplit on those systems separately,
you can put them together.


>
>    As I said before you can construct a preconditioner for the N.S. block
> by again using a PCFIELDSPLIT on that (recursive use of PCs).
>
>    Note that once you have defined your blocks (in the simple case if you
> have all your degrees of freedom collocated (no staggered grid) just set
> the Vec and Mat block size; otherwise you need to construct an IS that
> defines each block) then experimenting is easy and can be done at the
> command line without changing the code.
>
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