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

[Jed Brown]

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.
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