Hi,
Using the postprocessor approach according to step-33 I have computed the
strain tensor successfully by means of the
Postprocessor<dim>::compute_derived_quantities_vector function and its
displacement gradient duh.
Unfortunately, within an arbitrary function called right after I solve the
system, exactly before postprocessing, I try the same, but obtain different
results:
const QGauss<dim> quadrature_formula(2);
const unsigned int n_q_points = quadrature_formula.size();
FEValues<dim> fe_values(fe, quadrature_formula, update_values |
update_gradients | update_quadrature_points | update_JxW_values);
LA::MPI::Vector rel_solution;
rel_solution = locally_relevant_solution;
std::vector<Vector<double> > solution_values(n_q_points, Vector<double>(dim
+ 1));
std::vector<std::vector<Tensor<1, dim> > > solution_grads(n_q_points, std::
vector<Tensor<1, dim> >(dim + 1));
typename DoFHandler<dim>::active_cell_iterator cell = dof_handler.
begin_active(), endc = dof_handler.end();
for (; cell != endc; ++cell)
{
if ( cell->is_locally_owned() )
{
fe_values.reinit(cell);
fe_values.get_function_values(rel_solution, solution_values);
fe_values.get_function_gradients(rel_solution, solution_grads);
int cell_index = cell->active_cell_index();
// std::cout << cell_index << std::endl;
Tensor<2, dim> grad_u, eps;
for (unsigned int q = 0; q < n_q_points; ++q)
{
std::cout << "\n - in energy GAUSS point " << q + 1 << " -
\n" << std::endl;
for (unsigned int i = 0; i < dim; ++i)
{
std::cout << "GRADIENT 1: " << solution_grads[q][0]
<< std::endl;
std::cout << "GRADIENT 2: " << solution_grads[q][1]
<< std::endl;
// Compute displacement gradients
grad_u[i] = solution_grads[q][i];
// Compute strains
eps = 0.5 * (grad_u + transpose(grad_u));
}
print_tensor(grad_u, "DISPLACEMENT GRADIENT");
}
}
}
The above code gives me the following output:
- in GAUSS point 1 -
GRADIENT 1: -0.000312
GRADIENT 2: -0.001801
GRADIENT 1: -0.000312
GRADIENT 2: -0.001801
DISPLACEMENT GRADIENT
0.001081 -0.001477
-0.000000 -0.008525
STRAIN TENSOR
0.001081 -0.000738
-0.000738 -0.008525
- in GAUSS point 2 -
GRADIENT 1: 0.000312
GRADIENT 2: -0.001801
GRADIENT 1: 0.000312
GRADIENT 2: -0.001801
DISPLACEMENT GRADIENT
0.001081 0.001477
-0.000000 -0.008525
STRAIN TENSOR
0.001081 0.000738
0.000738 -0.008525
- in GAUSS point 3 -
GRADIENT 1: -0.001164
GRADIENT 2: -0.006723
GRADIENT 1: -0.001164
GRADIENT 2: -0.006723
DISPLACEMENT GRADIENT
0.004034 -0.001477
-0.000000 -0.008525
STRAIN TENSOR
0.004034 -0.000738
-0.000738 -0.008525
- in GAUSS point 4 -
GRADIENT 1: 0.001164
GRADIENT 2: -0.006723
GRADIENT 1: 0.001164
GRADIENT 2: -0.006723
DISPLACEMENT GRADIENT
0.004034 0.001477
-0.000000 -0.008525
STRAIN TENSOR
0.004034 0.000738
0.000738 -0.008525
And the below are results from the postprocessor computation for a single
element (already checked them, they should be correct):
- in GAUSS point 1 -
DISPLACEMENT GRADIENT
0.000000 -0.002557
0.000000 -0.008525
STRAIN TENSOR
0.000000 -0.001279
-0.001279 -0.008525
- in GAUSS point 2 -
DISPLACEMENT GRADIENT
0.000000 0.002557
0.000000 -0.008525
STRAIN TENSOR
0.000000 0.001279
0.001279 -0.008525
- in GAUSS point 3 -
DISPLACEMENT GRADIENT
0.005115 -0.002557
-0.000000 -0.008525
STRAIN TENSOR
0.005115 -0.001279
-0.001279 -0.008525
- in GAUSS point 4 -
DISPLACEMENT GRADIENT
0.005115 0.002557
-0.000000 -0.008525
STRAIN TENSOR
0.005115 0.001279
0.001279 -0.008525
Why is the fe_values.get_function_gradients() function giving me wrong
gradients?
Kind regards,
S. A. Mohseni
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