PETSc CG solver uses more iterations than other CG solver

[Berend van Wachem]

Hi,

> > I have also used the VOF/level set model in our PETSc based code, and it
> > does require a lot more iterations than a single phase model because of
> > the stencil. I guess you are solving
> >
> > d/dx 1/rho d/dx p = 0
> >
> > ? Which can result in a terrible coefficient structure.
>
> That's the equation I'm solving, yes.

Which for a bubble rising in water means that the density on the water side is 
1000 and on the air side is 1. So instead of the -1 2 -1 contribution from 
the Poisson stencil you have in single phase flow for each direction, you can 
get contributions like -1.0e-3 1.0 -1.0 which has a much worse condition to 
solve. 

But if you use exactly the same solver and preconditioner, it should result in 
approximately the same amount of iterations.

Berend.