Dr. Bangerth,
Yes, your comment is exactly what I need, to resolve my question.
I just gave a tiny adjustment on the quadrature parameter, guess what, that
returned me exactly my expected data information.
This is wonderful, thank you so much! I will keep up my work.
Have a great weekend!
Judy
On Thursday, December 2, 2021 at 5:24:50 PM UTC-5 Wolfgang Bangerth wrote:
>
> Judy,
>
> > (1) I reset <dim = 1> thru step-8 and step-3. Was that sufficient to
> > make the program ready for 1-dimensional(1D) PDE?
> > If not sufficient, I might get more coordinate returned for each
> > quadrature point; for example, q_index = {0, 1} can be 2D coordinate for
> > each quadrature point, that caused error?
>
> I don't understand what q_index={0,1} means? q_index is just a single
> number, the number of the quadrature point.
>
>
> > (2) Regarding to "obtain quadrature point coordinate locally", my .cc
> > file for 1D PDE with pure Dirichlet BCs is attached ready to compile for
> > testing, which was made with reference to step-8 that looped quadrature
> > points inside each cell locally:
> >
> > void::assemble_system(){
> > ...
> > for (const unsigned int q_index : fe_values.quadrature_point_indices())
> > {
> > std::cout << fe.degree << std::endl; //added line to output;
> > std::cout << q_index << std::endl; //added line to output;
> > std::cout << fe_values.quadrature_point << std::endl; //added line to
> > output;
>
> I don't believe that this last line actually compiles.
> fe_values.quadrature_point is a function, it needs an argument in
> (parentheses), probably q_index.
>
>
> > The added lines returned data for each cell, like:
> > (cell 0:)
> > 2
> > 0
> > 0.00211325
> > 0.005
> >
> > 2
> > 1
> > 0.00788675
> > 0.005
>
> OK, 0.0021... and 0.0078... are the quadrature points on the first cell.
> I assume that the cell has size 0.01. The two values are then the Gauss
> points on this cell, in the global coordinate system. (On the reference
> cell in 1d, that is [0,1], they would have coordinates 0.211325 and
> 0.788675.)
>
>
> > The returned data from fe_values.quadrature_point {0.00211325,
> > 0.00788675, 0.0121132, 0.0178860, ...}, look like quadrature points in
> > physical coordinate, moving along 1D domain of (0, 0.1) with looping
> > cells. However, I did not figure out more information on them.
>
> Yes, this is correct. What more would you want to know about them?
>
>
> > When I switch to Q2 element, fe.degree became 2, but
> > fe_values.JxW(q_index) does not return expected weights of {1./6.,
> > 2./3., 1./6.}. Was that due to Jacobians of Q2 element? I must be wrong
> > in guessing and obtaining quadrature point coordinate either locally or
> > globally, as well as their quadrature weights.
>
> The weights of quadrature points are determined by the quadrature object
> you choose, not the finite element object you choose. Have you changed
> the quadrature object to QGauss(3) when you changed the finite element
> to FE_Q(2)?
>
> Best
> W.
> --
> ------------------------------------------------------------------------
> Wolfgang Bangerth email: [email protected]
> www: http://www.math.colostate.edu/~bangerth/
>
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