Comment on two recent changes.

git-svn-id: https://svn.dealii.org/trunk@17956 0785d39b-7218-0410-832d-ea1e28bc413d

diff --git a/deal.II/examples/step-31/step-31.cc b/deal.II/examples/step-31/step-31.cc
index 7ef6eb8f98964292c5e9d373d853589f1fd449c6..3944da2da0085058b2cf775c5b4087e71e49a84f 100644 (file)
--- a/deal.II/examples/step-31/step-31.cc
+++ b/deal.II/examples/step-31/step-31.cc
@@ -705,7 +705,20 @@ BoussinesqFlowProblem<dim>::BoussinesqFlowProblem ()
                                 // domain (at the quadrature points, in
                                 // fact). How it works should be relatively
                                 // obvious to all who have gotten to this
-                                // point of the tutorial.
+                                // point of the tutorial. Note that since we
+                                // are only interested in the velocity,
+                                // rather than using
+                                // <code>stokes_fe_values.get_function_values</code>
+                                // to get the values of the entire Stokes
+                                // solution (velocities and pressures) we use
+                                // <code>stokes_fe_values[velocities].get_function_values</code>
+                                // to extract only the velocities part. This
+                                // has the additional benefit that we get it
+                                // as a Tensor<1,dim>, rather than some
+                                // components in a Vector<double>, allowing
+                                // us to process it right away using the
+                                // <code>norm()</code> function to get the
+                                // magnitude of the velocity.
                                 //
                                 // The only point worth thinking about a bit
                                 // is how to choose the quadrature points we
@@ -776,7 +789,8 @@ double BoussinesqFlowProblem<dim>::get_maximal_velocity () const
   for (; cell!=endc; ++cell)
     {
       fe_values.reinit (cell);
-      fe_values[velocities].get_function_values (stokes_solution, velocity_values);
+      fe_values[velocities].get_function_values (stokes_solution,
+                                                velocity_values);
 
       for (unsigned int q=0; q<n_q_points; ++q)
        max_velocity = std::max (max_velocity, velocity_values[q].norm());
@@ -2002,20 +2016,29 @@ void BoussinesqFlowProblem<dim>::assemble_temperature_system ()
 
   const FEValuesExtractors::Vector velocities (0);
   
-                                  // Now, let's start the loop over all
-                                  // cells in the triangulation. Again, we
-                                  // need two cell iterators that walk in
-                                  // parallel through the cells of the two
-                                  // involved DoFHandler objects for the
-                                  // Stokes and temperature part. Within
-                                  // the loop, we first set the local rhs
-                                  // to zero, and then get the values and
-                                  // derivatives of the old solution
-                                  // functions at the quadrature points,
-                                  // since they are going to be needed for
-                                  // the definition of the stabilization
-                                  // parameters and as coefficients in the
-                                  // equation, respectively.
+                                  // Now, let's start the loop over all cells
+                                  // in the triangulation. Again, we need two
+                                  // cell iterators that walk in parallel
+                                  // through the cells of the two involved
+                                  // DoFHandler objects for the Stokes and
+                                  // temperature part. Within the loop, we
+                                  // first set the local rhs to zero, and
+                                  // then get the values and derivatives of
+                                  // the old solution functions at the
+                                  // quadrature points, since they are going
+                                  // to be needed for the definition of the
+                                  // stabilization parameters and as
+                                  // coefficients in the equation,
+                                  // respectively. Note that since the
+                                  // temperature has its own DoFHandler and
+                                  // FEValues object we get the entire
+                                  // solution at the quadrature point (which
+                                  // is the scalar temperature field only
+                                  // anyway) whereas for the Stokes part we
+                                  // restrict ourselves to extracting the
+                                  // velocity part (and ignoring the pressure
+                                  // part) by using
+                                  // <code>stokes_fe_values[velocities].get_function_values</code>.
   typename DoFHandler<dim>::active_cell_iterator
     cell = temperature_dof_handler.begin_active(),
     endc = temperature_dof_handler.end();