[petsc-dev] components in PetscSection

[Chris Eldred]

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.

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