multigrid and multiple boundary id's

[bou...@gmail.com]

Hello everyone!

I'm trying to use multigrid preconditioner in conjunction with adaptive mesh refinement for a laplace problem as described in step-16. However, the boundary domain comprises of several boundary id's, at each of which a different non-homogeneous dirichlet condition holds. I don't know how to implement these conditions in the code, as in the step-16 tutorial only the homogeneous case of only one boundary id is presented. I suppose the only difference will be in the "interpolate_boundary_values" function in the "setup_system" function. But I don't know how to make a "dirichlet_boundary" object that will describe the boundary that I need.

Thanks in advance!

George

[Fan Bai]

Hi George,

I think in the function "interpolate_boundary_values", there is a parameter "component_mask" in it can identify how to interpolate Dirichlet BVs, you can set a pointer before this function (in the part of assembly). 

Best,

Fan

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[bou...@gmail.com]

As I read in the documentation, the first thing that I need to do is create a map of boundary ID's and functions. Therefore, each (constant in my case) function will be attached to a boundary id. So, the question now becomes, how do I create such a map that can be used as argument in the "interpolate_boundary_values" function?

And the next question will be, do I need to change anything else in the code of step-16 in order to implement the multiple boundary id's - nonhomogeneous dirichlet bcs?

Thanks again,

George

[bou...@gmail.com]

Guys please for any help! I must solve it by tomorrow!

[Martin Steigemann]

Hi George,

do you mea something like this:

    constraints.clear ();
    constraints.reinit (locally_relevant_dofs);
    DoFTools::make_hanging_node_constraints (dof_handler, constraints);

    typename FunctionMap<dim>::type  dirichlet_boundary;

    ZeroFunction<dim>  homogeneous_dirichlet_bc (dim);

    dirichlet_boundary[0] = &homogeneous_dirichlet_bc;
    dirichlet_boundary[1] = &homogeneous_dirichlet_bc;
    dirichlet_boundary[2] = &homogeneous_dirichlet_bc;

    VectorTools::interpolate_boundary_values (dof_handler,
                                              dirichlet_boundary,
                                              constraints);
    constraints.close ();

Best,

Martin

[Jean-Paul Pelteret]

Hi George,

If you look at step-18, you will see an example of the creation of non-trivial BC's for multiple boundaries. The map to which you refer is initially empty and is filled by the VectorTools::interpolate_boundary_values calls. You then use the MatrixTools::apply_boundary_values function to apply the constraints to your system.

Regards,

J-P

[bou...@gmail.com]

Guys thanks for your help!

Finally I did what Martin suggested and it worked.

Thanks again!

G.