Re: [Libmesh-users] Get XYZ location of global DOFs

[David X. <dx...@my...>]

On 7/26/06, Roy Stogner <roy...@ic...> wrote:
>
> On Wed, 26 Jul 2006, David Xu wrote:
>
> > On 7/26/06, Roy Stogner <roy...@ic...> wrote:
> >
> >> Loop over all the elements.  On each geometric element, reinit your FE
> >> object with a grid quadrature rule whose order at least equals your
> >> polynomial degree.  Get the XYZ coordinates and the solution value at
> >> each quadrature point, and output them.


I kinda get it, but still vague about how to implement it. Once I load the
eigenfunction value points from file to libMesh using
NumericVector<>::set(), how does libMesh provide me with the eigenfunction
point at requested quadrature point? I think I'm missing something here. I
know how to get xyz at requested quadrature point from the examples.


>
> > I do this after I load the eigenvector solutions back to libMesh,
> correct?
>
> Yes.
>
> > If I reinit FE object with a grid quadrature rule, how can Imake sure
> the
> > XYZ coordinates at each quadrature point are the ones correspondent to
> the
> > loaded eigenvectors?
>
> You don't.  You're not plotting eigenvector coefficients, you're
> plotting eigenfunction points.  Depending on the finite element space
> you use, your coefficients may correspond to function points, but they
> may also correspond to mixed second derivatives, projection
> magnitudes onto orthogonal function spaces, or the phase of the moon!
> So forget about trying to plot the coefficients themselves, and just
> worry about how to plot the function they represent.
>
> > Can I just ouput the xyz at each quadrature point at the element
> matrices
> > (Ke, Me) assembly step? Output them to a file and concatenate with the
> > eigenvector solutions? Will that work?
>
> No.
>
> >> That will output vertex and edge points multiple times, but the plot
> >> should look the same.  If your elements are so large that the plot
> >> still looks faceted, use a quadrature rule with more points.
> >
> > Same question here, if  I use a quadrature rule different from the one
> used
> > to assemble Ke, Me, will the xyz coordinates at the each quadrature
> point be
> > correctlycorrespondent to the eigenvector solutions with the size of
> global
> > DOFs?
>
> No, they won't.  That's why you don't just get the xyz coordinates at
> each quadrature point, you get the solution value at each quadrature
> point too.
> ---
> Roy
>