From 7925b4f928f44b6633df12765be06518608df492 Mon Sep 17 00:00:00 2001 From: Wolfgang Bangerth Date: Sun, 22 Jun 2014 02:33:09 +0000 Subject: [PATCH] Add to the documentation in a couple of places. git-svn-id: https://svn.dealii.org/trunk@33070 0785d39b-7218-0410-832d-ea1e28bc413d --- .../deal.II/distributed/solution_transfer.h | 9 +++ .../deal.II/numerics/solution_transfer.h | 60 +++++++++++++++++-- 2 files changed, 64 insertions(+), 5 deletions(-) diff --git a/deal.II/include/deal.II/distributed/solution_transfer.h b/deal.II/include/deal.II/distributed/solution_transfer.h index c82adf5dce..81ae1dde8b 100644 --- a/deal.II/include/deal.II/distributed/solution_transfer.h +++ b/deal.II/include/deal.II/distributed/solution_transfer.h @@ -97,6 +97,15 @@ namespace parallel parallel::distributed::SolutionTransfer sol_trans(dof_handler); sol_trans.deserialize (distributed_vector); @endcode + * + * + *

Interaction with hanging nodes

+ * + * In essence, this class implements the same steps as does + * dealii::SolutionTransfer (though the implementation is entirely + * separate). Consequently, the same issue with hanging nodes and + * coarsening can happen with this class as happens with + * dealii::SolutionTransfer. See there for an extended discussion. * * @ingroup distributed * @author Timo Heister, 2009-2011 diff --git a/deal.II/include/deal.II/numerics/solution_transfer.h b/deal.II/include/deal.II/numerics/solution_transfer.h index 3c2dd0dbb0..77606024bd 100644 --- a/deal.II/include/deal.II/numerics/solution_transfer.h +++ b/deal.II/include/deal.II/numerics/solution_transfer.h @@ -1,7 +1,7 @@ // --------------------------------------------------------------------- // $Id$ // -// Copyright (C) 1999 - 2013 by the deal.II authors +// Copyright (C) 1999 - 2014 by the deal.II authors // // This file is part of the deal.II library. // @@ -34,10 +34,12 @@ DEAL_II_NAMESPACE_OPEN /** * This class implements the transfer of a discrete FE function * (e.g. a solution vector) from one mesh to another that is obtained - * by the first by a single refinement and/or coarsening step. During + * from the first by a single refinement and/or coarsening step. During * interpolation the vector is reinitialized to the new size and * filled with the interpolated values. This class is used in the - * step-15, step-31, and step-33 tutorial programs. + * step-15, step-31, and step-33 tutorial programs. A version of this + * class that works on parallel triangulations is available as + * parallel::distributed::SolutionTransfer. * *

Usage

* @@ -199,9 +201,51 @@ DEAL_II_NAMESPACE_OPEN * * * + *

Interaction with hanging nodes

+ * + * This class does its best to represent on the new mesh the finite element + * function that existed on the old mesh, but this may lead to situations + * where the function on the new mesh is no longer conforming at hanging + * nodes. To this end, consider a situation of a twice refined mesh that + * started with a single square cell (i.e., we now have 16 cells). Consider + * also that we coarsen 4 of the cells back to the first refinement level. In + * this case, we end up with a mesh that will look as follows if we were to + * use a $Q_1$ element: + * + * @image html hanging_nodes.png "" + * + * The process of interpolating from the old to the new mesh would imply that + * the values of the finite element function will not change on all of the + * cells that remained as they are (i.e., the fine cells) but that on the + * coarse cell at the top right, the four values at the vertices are obtained + * by interpolating down from its former children. If the original function + * was not linear, this implies that the marked hanging nodes will retain + * their old values which, in general, will not lead to a continuous function + * along the corresponding edges. In other words, the solution vector obtained + * after SolutionTransfer::interpolate() does not satisfy hanging node + * constraints: it corresponds to the pointwise interpolation, but not to the + * interpolation onto the new finite element space that contains + * constraints from hanging nodes. + * + * Whether this is a problem you need to worry about or not depends on your + * application. The situation is easily corrected, of course, by applying + * ConstraintMatrix::distribute() to your solution vector after transfer, + * using a constraint matrix object computed on the new DoFHandler object (you + * probably need to create this object anyway if you have hanging nodes). This + * is also what is done, for example, in step-15. + * + * @note This situation can only happen if you do coarsening. If all cells + * remain as they are or are refined, then SolutionTransfer::interpolate() + * computes a new vector of nodel values, but the function represented is of + * course exactly the same because the old finite element space is a subspace + * of the new one. Thus, if the old function was conforming (i.e., satisfied + * hanging node constraints), then so does the new one, and it is not + * necessary to call ConstraintMatrix::distribute(). + * + * *

Implementation in the context of hp finite elements

* - * In the case of hp::DoFHandlers, it is not defined which of the finite elements + * In the case of hp::DoFHandlers, nothing defines which of the finite elements * that are part of the hp::FECollection associated with the DoF handler, should * be considered on cells that are not active (i.e., that have children). This * is because degrees of freedom are only allocated for active cells and, in fact, @@ -225,9 +269,15 @@ DEAL_II_NAMESPACE_OPEN * Q3, in this example, if the user has set the active_fe_index for a different * space post-refinement and before calling hp::DoFHandler::distribute_dofs()). * + * @note In the context of hp refinement, if cells are coarsened or the + * polynomial degree is lowered on some cells, then the old finite element + * space is not a subspace of the new space and you may run into the same + * situation as discussed above with hanging nodes. You may want to consider + * calling ConstraintMatrix::distribute() on the vector obtained by + * transfering the solution. * * @ingroup numerics - * @author Ralf Hartmann, 1999, Oliver Kayser-Herold and Wolfgang Bangerth, 2006, 2014 + * @author Ralf Hartmann, 1999, Oliver Kayser-Herold and Wolfgang Bangerth, 2006, Wolfgang Bangerth 2014 */ template, class DH=DoFHandler > class SolutionTransfer -- 2.39.5