Setting tolerances for multigrid

[Marcus Heck]

Hi,

On 2007-06-15 10:06, Knut Erik Teigen wrote:
> Thanks, Matthew. It seems like the tolerances weren't my problem,
> though. Even with a tolerance of 1e-10, the computation diverges after
> only a few time steps when using multigrid. With ordinary linear
> solvers, I can use a tolerance of 1e-4 and still get a satisfactory
> solution.

How many grid levels do you have? How coarse is the coarsest grid?
It can be possible, that a coarsgrid solution will "distroy" somehow the
solution for the finer grids an leads to divergence. I observed this for
geometric multigrid some time ago

Marcus Heck


> Could it be that the Galerkin approximation for the coarser levels is
> too inaccurate for my problem?