[petsc-dev] components in PetscSection

[Matthew Knepley]

On Fri, Aug 31, 2012 at 3:52 PM, Chris Eldred <chris.eldred at gmail.com>wrote:

> Ok- so if I have a multicomponent field, I should define the number of
> dofs at a point for that field to be the total number of dofs I want
> for that field. Ex a 2 component field with 2 dofs per component
> should have 4 dofs defined, not 2. Num components just serves to give
> additional info about the field but it is not used in defining or
> manipulating the PetscSection.


Yes.

   Matt


> On Fri, Aug 31, 2012 at 2:29 PM, Matthew Knepley <knepley at gmail.com>
> wrote:
> > On Fri, Aug 31, 2012 at 1:04 PM, Chris Eldred <chris.eldred at gmail.com>
> > wrote:
> >>
> >> I was wondering how components were treated in PetscSection- my
> >> understanding is that a field with multiple components and multiple
> >> dofs would have those dofs duplicated for each component, but that
> >> does not appear to be the case. For example, let's say I have a field
> >> with 2 components and 2 dofs that covers 10 points. I expected that it
> >> would have size 40: 4 dofs per point x 10 points. Instead, it appears
> >> to have size 20: 2 dofs x 10 pts.
> >
> >
> > Yep, that is not the case. What if the components are discretized with
> > different numbers of dofs on different points? This definitely happens.
> >
> >>
> >> What then are components in PetscSection- is it just a way of telling
> >> the code that the dofs associated with a field at a point are really
> >> split across multiple components (ie for my example, I should define 4
> >> dofs at each pt instead of 2)? How does that then interact with the
> >> re-ordering done in DMComplexVecGetClosure?
> >
> >
> > The idea of components is to give you an idea what kind of field you are
> > dealing with. I know I don't go so far as to tell you its a vector, or a
> > pseudo-vector, etc. I am not sure where to draw the line yet. Within
> > GetClosure(), the dofs for reversed points are separately reversed per
> > component.
> >
> >    Matt
> >
> >>
> >>
> >> --
> >> Chris Eldred
> >> DOE Computational Science Graduate Fellow
> >> Graduate Student, Atmospheric Science, Colorado State University
> >> B.S. Applied Computational Physics, Carnegie Mellon University, 2009
> >> chris.eldred at gmail.com
> >
> >
> >
> >
> > --
> > What most experimenters take for granted before they begin their
> experiments
> > is infinitely more interesting than any results to which their
> experiments
> > lead.
> > -- Norbert Wiener
>
>
>
> --
> Chris Eldred
> DOE Computational Science Graduate Fellow
> Graduate Student, Atmospheric Science, Colorado State University
> B.S. Applied Computational Physics, Carnegie Mellon University, 2009
> chris.eldred at gmail.com
>



-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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