Thank you so much, Prof. Bangerth.
On Friday, September 15, 2017 at 11:19:01 AM UTC-5, Wolfgang Bangerth wrote:
>
> On 09/15/2017 08:32 AM, Yiyang Zhang wrote:
> > For example, consider the sine-Gordon equation, which is in Step-25.
> > With the help of the DataPostprocessor class, we can output a plot for
> > the energy density.
> > But then, how could we calculate the total energy?
> >
> > The best scenario would be computing the energy density and total energy
> > at the same time...
>
> Yiyang -- it's not easily possible to do it both at the same time
> (because DataPostprocessor only creates output, but not do any kind of
> integration -- so you are missing some information). But it's easy to do
> by hand. For example, for the Laplace equation, you would do something
> like this (pseudo-code):
>
> double compute_energy ()
> {
> FEValues fe_values (...);
> std::vector<Tensor<1,dim> > solution_gradients (n_q_points);
>
> double energy = 0;
>
> for (cell=...)
> {
> fe_values.reinit(cell);
> fe_values.get_function_gradients (solution,
> solution_gradients);
>
> for (q=0...n_q_points)
> energy += solution_gradients[q] * solution_gradients[q] *
> fe_values.JxW(q);
> }
>
> return energy;
> }
>
> Best
> W.
>
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