[deal.II] finite difference method to calculate the gradient of density in cells by reference file of cook_membrane.cc

[Lance Zhang]

Hello ,

I would like to use finite difference method to calculate the gradient of 
density in cells.

But I got issue when the program was processed at the part of 
compute_gradient();

Here is the code below  about density and gradient.


//>>>>>>>>>>>>>>>Density>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
template <int dim,typename NumberType>

void Solid<dim,NumberType>::compute_density()
{
  typename DoFHandler<dim>::active_cell_iterator cell = 
dof_handler_ref.begin_active(),
                                                 endc = 
dof_handler_ref.end();

  for (; cell != endc; ++cell)
  {
    // Compute the grid density for each cell,
    density[cell->active_cell_index()] = 1.0 / cell->measure();
  }
}



//>>>>>>>>>>>>>>>>>>>Gradient>>>>>>>>>>>>>>>>>>>>>>>>
template <int dim,typename NumberType>


void Solid<dim,NumberType>::compute_gradient()
{
  typename DoFHandler<dim>::active_cell_iterator cell = 
dof_handler_ref.begin_active(),
                                                 endc = 
dof_handler_ref.end();

  for (; cell != endc; ++cell)
  {
    for (unsigned int face = 0; face < GeometryInfo<dim>::faces_per_cell; 
++face)
    {
      if (cell->face(face)->at_boundary())
      {
        // Compute the gradient at each boundary face by differencing the 
neighboring cell densities
        const typename DoFHandler<dim>::cell_iterator neighbor = 
cell->neighbor(face);
        gradient[cell->face(face)->index()] = 
density[neighbor->active_cell_index()] - density[cell->active_cell_index()];
      }
    }
  }
}
//<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

The error comes out at the part of compute_gradient().
----------------------------------------------------------------------------------------------------------------
Error information:
Thread 1 "cook_membrane" received signal SIGSEGV, Segmentation fault.
0x00007ffff4bfcdaa in dealii::CellAccessor<3, 3>::active_cell_index() const 
() from /lib/x86_64-linux-gnu/libdeal.ii.so.9.3.2
----------------------------------------------------------------------------------------------------------------
I tried to reduce the numbers of grids and dofs ,but I sill got the same 
issue.

in the part of void Solid<dim,NumberType>::run(),
I add  the code below  

 template <int dim,typename NumberType>
  void Solid<dim,NumberType>::run(){
 .//other codes
 .//other codes
 .//other codes
    density.reinit(dof_handler_ref.n_dofs());
    gradient.reinit(dof_handler_ref.n_dofs());
    //<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
    print_vertical_tip_displacement();

    std::cout<<"Density claculation"<<std::endl;
    compute_density();//sucessfully
    std::cout<<"Gradient calculation"<<std::endl;
    compute_gradient();//error comes out
}

Could anyone provide any hint or suggestions?

Thanks in advance!
Best regards
Lance

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