Hi Prof. Bangerth,
I would like to post that snippet of code. To be honest, I verified the
solution in a naive way, that is, I compare the two different number of
processes solutions in paraview. The comparison(including data range, plot
curve over time at a point) shows they are exactly the same.
template<int dim>
Tensor<1,dim> Postprocessor<dim>::
compute_load_force(const DoFHandler<dim> &dof_handler,
const LA::MPI::Vector &solution,
const double &lame_mu,
const double &lame_lambda,
const unsigned int °ree,
const unsigned int &boundary_id,
const MPI_Comm &mpi_comm) {
const QGauss<dim-1> face_quadrature_formula (degree + 1);
FEFaceValues<dim> fe_face_values (dof_handler.get_fe(),
face_quadrature_formula,
update_gradients | update_normal_vectors |
update_JxW_values);
const unsigned int dofs_per_cell = dof_handler.get_fe().dofs_per_cell;
const unsigned int n_face_q_points = face_quadrature_formula.size();
std::vector<unsigned int> local_dof_indices(dofs_per_cell);
std::vector<std::vector<Tensor<1, dim>>> u_grads_per_cell(n_face_q_points,
std::vector<Tensor<1, dim>>(dim));
// Declare load force per processor
Tensor<1,dim> locally_owned_load_force;
// Declare identity tensor
SymmetricTensor<2, dim> identity = unit_symmetric_tensor<dim>();
for (const auto &cell : dof_handler.active_cell_iterators())
if (cell->is_locally_owned()) {
for (unsigned int f = 0; f < GeometryInfo<dim>::faces_per_cell; ++f)
// Find the top boundary based on the id
if (cell->face(f)->at_boundary()
&& cell->face(f)->boundary_id() == boundary_id) {
fe_face_values.reinit (cell, f);
fe_face_values.get_function_gradients(solution, u_grads_per_cell);
for (unsigned int q = 0; q < n_face_q_points; ++q) {
// Compute strain and stress
Tensor<2, dim> u_grad;
for (unsigned int d = 0; d < dim; ++d)
u_grad[d] = u_grads_per_cell[q][d];
const SymmetricTensor<2, dim>
epsilon = 0.5 * (u_grad + transpose(u_grad));
const SymmetricTensor<2, dim>
sigma = 2.0 * lame_mu * epsilon
+ lame_lambda * trace(epsilon) * identity;
locally_owned_load_force += sigma * fe_face_values.normal_vector(q)
* fe_face_values.JxW(q);
}
}
}
// Collect values from all processors
Tensor<1,dim> load_force;
load_force[0] = Utilities::MPI::sum(locally_owned_load_force[0], mpi_comm);
load_force[1] = Utilities::MPI::sum(locally_owned_load_force[1], mpi_comm);
return load_force;}
Thank you!
Best,
Qing
On Wed, Mar 28, 2018 at 12:47 PM, Wolfgang Bangerth <bange...@colostate.edu>
wrote:
>
> Qing,
>
> I meet a MPI problem when I simulate a three point bending test. I try to
>> compute the load force traction on the top boundary. The results are correct
>> with the single process. But When I switched to a multi-process, the
>> results changed.
>> Let's say, if we wan to get the load force in y direction, the results
>> are like this:
>>
>> * 1 core: -0.83
>> * 2 cores: -0.83
>> * 3 cores: -0.92
>> * 4 cores: -0.415
>> * 5 cores: -0.002
>>
>> I already use `sum` collective operation after computing locally owned
>> load force value.
>> In order to double check it, I try to output each processes' local value,
>> and get the results
>> like this:
>>
>> * 1 core: locally load force y: -0.831775
>> * 2 core: locally load force y: (0)-0.415888 (1)-0.415888
>> * 4 core: locally force y: (0)0 (1)-0.408131 (2)-0.00237332 (3)-0.004908
>>
>> I do not know why I get smaller values with more cores.
>>
>> I also have checked the boundary conditions and mesh size. In fact, the
>> displacement results
>> are correct and same under different cores. So the problem should be just
>> in this load force
>> computation.
>>
>
> There is not enough information in your email to say what may be wrong. Do
> I read your description correctly to say that you have verified that the
> *solution* is the same independent of the number of processors? If that is
> so, then the problem must indeed be in the postprocessing step where you
> compute the resulting forces. But without seeing your code that does this
> computation, it's impossible to say what's wrong.
>
> Best
> W.
>
> --
> ------------------------------------------------------------------------
> Wolfgang Bangerth email: bange...@colostate.edu
> www: http://www.math.colostate.edu/~bangerth/
>
>
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