[petsc-dev] controlling vector values while doing matrix free operations

[Barry Smith]

On Dec 14, 2011, at 9:22 PM, Jed Brown wrote:

> On Wed, Dec 14, 2011 at 17:55, Barry Smith <bsmith at mcs.anl.gov> wrote:
>  What Matt is trying to tell you is that changing values at this level to prevent function evaluations outside of the function domain inside Newton is a total hack that rarely works well and should be avoided. You really don't want to do this.
> 
> It sounds like for the problems Bobby is asking about, the continuum solutions never produce values at a "constraint". In gas dynamics, the continuum equations are PDEs, not variational inequalities, but the pressure and temperature are still never negative. If a bound was available, MatMFFD could potentially choose the sign of the differencing to stay within the feasible domain. This does seem quite limited, however, because the positive and negative versions could cause violations in different parts of the domain.

   If you are truly near 0 in temperature or pressure and differencing can push you over the limit then I submit it is perfectly reasonable to solve them as VI.

   Though actually our VI solvers are currently not really matrix-free so this is a bit academic.

   Barry