[petsc-users] query DMPlexCreateSection

[Matthew Knepley]

On Fri, Jan 3, 2014 at 1:37 AM, Dharmendar Reddy <dharmareddy84 at gmail.com>wrote:

> Hello,
>          I did not fully understand the flow. Need more help.
> If i look at the flow in DMPlexCreateSectionInitial
> (
> http://www.mcs.anl.gov/petsc/petsc-current/src/dm/impls/plex/plex.c.html#DMPlexCreateSection
> )
>
> 6023:   for (d = 0; d <= dim; ++d) {
> 6024:     numDofTot[d] = 0;
> 6025:     for (f = 0; f < numFields; ++f) numDofTot[d] +=
> numDof[f*(dim+1)+d];
> 6026:   }
> 6027:   PetscSectionCreate(PetscObjectComm((PetscObject)dm), section);
> 6028:   if (numFields > 0) {
> 6029:     PetscSectionSetNumFields(*section, numFields);
> 6030:     if (numComp) {
> 6031:       for (f = 0; f < numFields; ++f) {
> 6032:         PetscSectionSetFieldComponents(*section, f, numComp[f]);
> 6033:       }
> 6034:     }
> 6035:   }
> 6036:   DMPlexGetChart(dm, &pStart, &pEnd);
> 6037:   PetscSectionSetChart(*section, pStart, pEnd);
> 6038:   for (d = 0; d <= dim; ++d) {
> 6039:     DMPlexGetDepthStratum(dm, d, &pStart, &pEnd);
> 6040:     for (p = pStart; p < pEnd; ++p) {
> 6041:       for (f = 0; f < numFields; ++f) {
> 6042:         PetscSectionSetFieldDof(*section, p, f, numDof[f*(dim+1)+d]);
> 6043:       }
> 6044:       PetscSectionSetDof(*section, p, numDofTot[d]);
> 6045:     }
> 6046:   }
>
> In line 6027, number of fields for entire section is defined. Now,
> dose the flow in your earlier replies requires me to do that ? I.e,
> define numFields to 2. Then
>
> Inside the loop 6040, i need to check if the point belongs to region 1 or
> 2.
>
> If it belongs to region 1 then numFields is 1 and if it belongs to
> region 2, numFields is 2.
>
> Where do i set the number of fields in the flow you mentioned in the
> earlier email ?
>

The number of fields is always 2 and the chart is always the whole domain.
Its just that
one field only takes values on part of the chart. The other points will
have dof = 0.

   Matt


> Thanks
> Reddy
>
> On Thu, Jan 2, 2014 at 9:56 AM, Matthew Knepley <knepley at gmail.com> wrote:
> > On Thu, Jan 2, 2014 at 4:11 AM, Dharmendar Reddy <
> dharmareddy84 at gmail.com>
> > wrote:
> >>
> >> Hello,
> >>          I am trying to use DMPlexCreateSection from fortran. I was
> >> able to solve a Poisson equation earlier using DMPlex for handling
> >> mesh data.
> >>
> >> Now i need to solve a system of equations: (poisson + continuity
> >> equations)
> >>
> >> - div (grad phi) = (C + n)  --- (1)
> >>   div (J ) = 0  ---------------  (2)
> >>   J = n grad(phi)
> >> ( C is constant)
> >> Simulation domain is defined as, rectangular regions 1 and 2 shown
> below.
> >>
> >> -------------
> >> |     1        |
> >> -------------
> >> |               |
> >> |       2      |
> >> |               |
> >> --------------
> >>
> >> Now, equation 1 is defined in region 1 and 2
> >> and equation 2 is defined only for region 2.
> >>
> >> How do i setup the section  ? DMPlexCreateSection applies the given
> >> DOF layout  per cell to all cells in the mesh.
> >>
> >> I am not sure if all the function calls inside
> >> DMPlexCreateSectionIntial and DMPlexCreateSectionBCDof  are accessible
> >> from Fotran.
> >
> >
> > You don't want them anyway since they apply to the whole domain. The
> control
> > flow could be:
> >
> >   DMPlexSetChart()
> >   <loop over region 1>
> >     DMPlexSetDof() and DMPlexSetFieldDof()
> >   <loop over region 2>
> >     DMPlexSetDof() and DMPlexSetFieldDof()
> >   <loop over BC>
> >     DMPlexSetConstraintDof() and DMPlexSetFieldConstraintDof()
> >   DMPlexSetUp()
> >   <loop over BC>
> >     DMPlexSetConstraintIndices() and DMPlexSetFieldConstraintIndices()
> >
> > We could try and package some of this up if it looks generic.
> >
> >   Thanks,
> >
> >      Matt
> >
> >>
> >> Thanks
> >> Reddy
> >>
> >> --
> >> -----------------------------------------------------
> >> Dharmendar Reddy Palle
> >
> >
> >
> >
> > --
> > 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
>
>
>
> --
> -----------------------------------------------------
>



-- 
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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