Jie, > I need to constrain all the support points on a Q2 element instead of only vertices. The constrained values (inhomogeneity) are obtained from (conceptually) a function, say f(\bold{x}).

The code below suggests that you want to constraint all degrees of freedom. Is this really what you intend? If so, it might be better to just interpolate the values using VectorTools::interpolate <https://www.dealii.org/8.5.0/doxygen/deal.II/namespaceVectorTools.html#a05db6c8cebf924b417dd92f525efe3db>. > I already know how to query the coordinates of the support points, I also know how to query their global dof numbers, but I do not know how the dof numbers are ordered: for example, what is the global dof number of the ith component of velocity at support point v? > Especially my finite element consists of a 2nd order vector field (velocity) and a 1st order scalar field (pressure). Here is the code snippet: [...] Don't rely on any particular order and use the whole finite element instead of just the first base element in your code! To find out if a degree of freedom corresponds to the velocity FiniteElement you can then call system_to_component_index <https://www.dealii.org/8.5.0/doxygen/deal.II/classFiniteElement.html#a27220a135402b96c7e6eecbb04acda56> . The problem with the approach above is that there (probably?) is no DoFHandler that is responsible for the velocity degrees of freedoms alone. Hence, a cell iterator (based on a DoFHandler) can only give you the global degrees of freedoms for all the components at the same time. By using system_to_component_index you can then decide which to use. > Another related question is that, my computational results are written in vtu format as Q1 elements, which makes the resolution very coarse. Is there a way to write the results as 2nd order elements (ParaView supports it)? > I suppose the pressure component then needs to be interpolated as it is 1st order? No, as far as I know we don't support writing the results as 2nd order elements. The approach we take instead is to split a cell into several (most of the time degree^dim) using DataOut::build_patches(degree) to obtain a linear interpolation of the correct order. Best, Daniel -- 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.