Hi All, Just a quick question on convergence rates/error analysis.
I have a solver which does primarily what they do in step-20. The model I am trying to solve takes the pressure solution from the equations in step-20 and uses its gradient on the right hand side of a new equation. I use fe_values_p[pressure].get_function_gradients (solution_darcy, grad_p_values); on my test problem, where i just do v=grad p (v is a vector), to solve after the pressure is found from step-20 so that with the mass matrix as the system matrix, on the rhs i have: local_rhs(i) += (grad_pf_values[q_point][component_i] * fe_values_new.shape_value(i,q_point)*fe_values_new.JxW(q_point); I have noticed that convergence rates are very low, at p=1 order, convergence rate is between 2 and 3, when it should be 4. Further, i noticed that increasing the degree of the test problem also did not change the error at all. I am using fe (FE_RaviartThomas <https://www.dealii.org/8.4.0/doxygen/deal.II/classFE__RaviartThomas.html><dim>(degree1), 1, FE_DGQ <https://www.dealii.org/8.4.0/doxygen/deal.II/classFE__DGQ.html><dim>(degree1), 1) as normal as in step-20 for the u and pressure variable, and for my test problem, for the solution v, i am using FE_Q<dim>(degree2) So my main question is, how much error is get_function_gradients actually accounting for? the error does definitely decrease when degree1 is increased (without having to change degree2), but increasing the degree in the test problem alone does not do anything at all. I am guessing it is because I am using fe_values_p on the old space and not the new when i do get_function_gradients, but I thought that refining the mesh should still give me the correct convergence. Could someone explain the error that the pressure gradient calculation is contributing? (note that when i try to solve v=p using get_function_value), i get the correct convergence rate. Many thanks -- The deal.II project is located at http://www.dealii.org/ For mailing list/forum options, see https://groups.google.com/d/forum/dealii?hl=en --- You received this message because you are subscribed to the Google Groups "deal.II User Group" group. To unsubscribe from this group and stop receiving emails from it, send an email to dealii+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
