Hey David, Yes, this won't work since FEValues need a cell to reinit to get mapping > data. I looded at the class DataPostprocessor as you mentioned, but don't > quite understand the parameters passed to compute_derived_quantities_vector > method. I don't understand what exactly is the parameters "uh", "duh" ..., > I know they are the computed finite element solutions, but I don't how they > are composed, so I don't know how to use them. > Copying from the documentation for DataPostprocessor<dim>::compute_derived_quantities_scalar:
"[...]uh is a reference to a vector of data values at all points, duh the same for gradients, dduh for second derivatives and normals is a reference to the normal vectors. Note, that the last four references will only contain valid data, if the respective flags are returned by get_needed_update_flags, otherwise those vectors will be in an unspecified state.[...]" > > As I have stated in the first post, I want to compute the stress using > dierivatives of the solution, namely > > > <https://lh3.googleusercontent.com/-HucFvD1_fXU/V0J7dsthOEI/AAAAAAAAACo/lQkFJ2oHzaw7FaOxMqAOwNLr-7CgKw34gCLcB/s1600/constitutive.png> > > so how can I use it to compute u_k,k, u_i,j and u_j,i., does these > quantities the same with the parameter "duh" ? Could you (or anyone else) > please give some explanation? > Have a look at the postprocessing in compute_derived_quantities_vector in step-32 [1]. There you see how individual components of the solution vector are accessed to output the respective quantities as well as how to compute derived quantities. In particular, the strain rate is computed. Modifying this to include the divergence should then be straight forward. The generated output is a std::vector. Hence, you need to decide in which component of this vector you want to store which component of your stress. Best, Daniel [1] https://www.dealii.org/8.4.0/doxygen/deal.II/step_32.html#BoussinesqFlowProblemoutput_result -- 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.
