question about the neumann boundary

[Yunfei Huang]

Hi Everyone, 

Here, I have attempted to add the Neumann boundary conditions (forces) to the 2D elasticity problem. Now I can see that the top two layers of grids are under force. The force on the top layer is smaller than that on the second layer.  please see my attachment figure. 

My question is: should there be one layer or not in the boundary layer?
If it is only one layer, why did I get these results?

Here is my code: 

for (; cell!=endc; ++cell){
cell_matrix = 0;
cell_rhs = 0;

int traction_surface_id = 7;
for (unsigned int face=0; face<GeometryInfo<dim>::faces_per_cell; ++face)
if (cell->face(face)->at_boundary() &&
cell->face(face)->boundary_id() == traction_surface_id)
{
fe_face_values.reinit(cell, face);
boundary_force_neumann.vector_value_list(fe_face_values.get_quadrature_points(),
boundary_force_values);

for (unsigned int q_point=0; q_point<n_face_q_points; ++q_point)
{
for (unsigned int i = 0; i < dofs_per_cell; ++i)
{
const unsigned int component_i = fe.system_to_component_index(i).first;
// if (component_i==1)
cell_rhs(i) += fe_values.shape_value(i,q_point) *
boundary_force_values[q_point][component_i]*
// neumann_value[0][component_i]*
fe_values.JxW(q_point);
}
}
}

Best regards,

Yunfei 

[Luca Heltai]

Careful: you are still using

> fe_values.shape_value

but you should be using

> fe_face_values.shape_value

and

> fe_face_values.JxW(q_point);

You are now adding just *volumetric* forces on *some* quadrature points *inside* the cell, not on the boundary.

Best,
L.

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[Yunfei Huang]

Dear Mr. Heltai, 

Thanks a lot. it works now.

Best regards,

Yunfei 

Luca Heltai <luca....@gmail.com> 于2025年8月18日周一 16:56写道：

To view this discussion visit https://groups.google.com/d/msgid/dealii/555B48CF-9E7C-4F97-BDB7-435B55CB5290%40gmail.com.