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//
// Copyright (C) 2000 - 2016 by the deal.II authors
//
// This file is part of the deal.II library.
//
// The deal.II library is free software; you can use it, redistribute
// it, and/or modify it under the terms of the GNU Lesser General
// Public License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
// The full text of the license can be found in the file LICENSE at
// the top level of the deal.II distribution.
//
// ---------------------------------------------------------------------
#ifndef dealii__grid_refinement_h
#define dealii__grid_refinement_h
#include <deal.II/base/config.h>
#include <deal.II/base/exceptions.h>
#include <deal.II/grid/tria.h>
#include <vector>
#include <limits>
DEAL_II_NAMESPACE_OPEN
// forward declarations
template <int dim, int spacedim> class Triangulation;
template <class T> class Vector;
/**
* Collection of functions controlling refinement and coarsening of
* Triangulation objects.
*
* The functions in this namespace form two categories. There are the
* auxiliary functions refine() and coarsen(). More important for users are
* the other functions, which implement refinement strategies, as being found
* in the literature on adaptive finite element methods. For mathematical
* discussion of these methods, consider works by Dörfler, Morin,
* Nochetto, Rannacher, Stevenson and many more.
*
* @ingroup grid
* @author Wolfgang Bangerth, Thomas Richter, Guido Kanschat 1998, 2000, 2009
*/
namespace GridRefinement
{
/**
* Return a pair of double values of which the first is adjusted refinement
* fraction of cells and the second is adjusted coarsening fraction of
* cells.
*
*
* @param[in] current_n_cells The current cell number.
*
* @param[in] max_n_cells The maximal number of cells. If current cell
* number @p current_n_cells is already exceeded maximal cell number @p
* max_n_cells, refinement fraction of cells will be set to zero and
* coarsening fraction of cells will be adjusted to reduce cell number to @
* max_n_cells. If cell number is going to be exceeded only upon refinement,
* then refinement and coarsening fractions are going to be adjusted with a
* same ratio in an attempt to reach the maximum number of cells. Be aware
* though that through proliferation of refinement due to
* Triangulation::MeshSmoothing, this number is only an indicator. The
* default value of this argument is to impose no limit on the number of
* cells.
*
* @param[in] top_fraction_of_cells The requested fraction of cells to be
* refined.
*
* @param[in] bottom_fraction_of_cells The requested fraction of cells to be
* coarsened.
*
* @note Usually you do not need to call this function explicitly. Pass @p
* max_n_cells to function refine_and_coarsen_fixed_number() or function
* refine_and_coarsen_fixed_fraction() and they will call this function if
* necessary.
*/
template <int dim>
std::pair<double, double>
adjust_refine_and_coarsen_number_fraction (const unsigned int current_n_cells,
const unsigned int max_n_cells,
const double top_fraction_of_cells,
const double bottom_fraction_of_cells);
/**
* This function provides a refinement strategy with predictable growth of
* the mesh.
*
* The function takes a vector of refinement @p criteria and two values
* between zero and one denoting the fractions of cells to be refined and
* coarsened. It flags cells for further processing by
* Triangulation::execute_coarsening_and_refinement() according to the
* following greedy algorithm:
*
* <ol>
*
* <li> Sort the cells according to descending values of @p criteria.
*
* <li> Set the refinement threshold to be the criterion belonging to the
* cell at position @p top_fraction_of_cells times
* Triangulation::n_active_cells().
*
* <li> Set the coarsening threshold accordingly using the cell @p
* bottom_fraction_of_cells times Triangulation::n_active_cells() from the
* end of the sorted list.
*
* <li> Use these two thresholds in calls to refine() and coarsen(),
* respectively.
*
* </ol>
*
* As an example, with no coarsening, setting @p top_fraction_of_cells to
* 1/3 will result in approximately doubling the number of cells in two
* dimensions. The same effect in three dimensions is achieved by refining
* 1/7th of the cells. These values are good initial guesses, but should be
* adjusted depending on the singularity of approximated function.
*
* The sorting of criteria is not done actually, since we only need the
* threshold values in order to call refine() and coarsen(). The order of
* cells with higher and of those with lower criteria is irrelevant. Getting
* this value is accomplished by the @p nth_element function of the
* <tt>C++</tt> standard library, which takes only linear time in the number
* of elements, rather than <tt>N log N</tt> for sorting all values.
*
* @note This function only sets the coarsening and refinement flags. The
* mesh is not changed until you call
* Triangulation::execute_coarsening_and_refinement().
*
* @param[in,out] triangulation The triangulation whose cells this function
* is supposed to mark for coarsening and refinement.
*
* @param[in] criteria The refinement criterion for each mesh cell. Entries
* may not be negative.
*
* @param[in] top_fraction_of_cells The fraction of cells to be refined. If
* this number is zero, no cells will be refined. If it equals one, the
* result will be flagging for global refinement.
*
* @param[in] bottom_fraction_of_cells The fraction of cells to be
* coarsened. If this number is zero, no cells will be coarsened.
*
* @param[in] max_n_cells This argument can be used to specify a maximal
* number of cells. If this number is going to be exceeded upon refinement,
* then refinement and coarsening fractions are going to be adjusted in an
* attempt to reach the maximum number of cells. Be aware though that
* through proliferation of refinement due to Triangulation::MeshSmoothing,
* this number is only an indicator. The default value of this argument is
* to impose no limit on the number of cells.
*/
template <int dim, class VectorType, int spacedim>
void
refine_and_coarsen_fixed_number
(Triangulation<dim,spacedim> &triangulation,
const VectorType &criteria,
const double top_fraction_of_cells,
const double bottom_fraction_of_cells,
const unsigned int max_n_cells = std::numeric_limits<unsigned int>::max());
/**
* This function provides a refinement strategy controlling the reduction of
* the error estimate.
*
* Also known as the <b>bulk criterion</b>, this function computes the
* thresholds for refinement and coarsening such that the @p criteria of
* cells getting flagged for refinement make up for a certain fraction of
* the total error. We explain its operation for refinement, coarsening
* works analogously.
*
* Let <i>c<sub>K</sub></i> be the criterion of cell <i>K</i>. Then the
* total error estimate is computed by the formula
* @f[
* E = \sum_{K\in \cal T} c_K.
* @f]
*
* If <i> 0 < a < 1</i> is @p top_fraction, then we refine the
* smallest subset $\cal M$ of the Triangulation $\cal T$ such that
* @f[
* a E \le \sum_{K\in \cal M} c_K
* @f]
*
* The algorithm is performed by the greedy algorithm described in
* refine_and_coarsen_fixed_number().
*
* @note The often used formula with squares on the left and right is
* recovered by actually storing the square of <i>c<sub>K</sub></i> in the
* vector @p criteria.
*
* From the point of view of implementation, this time we really need to
* sort the array of criteria. Just like the other strategy described
* above, this function only computes the threshold values and then passes
* over to refine() and coarsen().
*
* @param[in,out] tria The triangulation whose cells this function is
* supposed to mark for coarsening and refinement.
*
* @param[in] criteria The refinement criterion computed on each mesh cell.
* Entries may not be negative.
*
* @param[in] top_fraction The fraction of the total estimate which should
* be refined. If this number is zero, no cells will be refined. If it
* equals one, the result will be flagging for global refinement.
*
* @param[in] bottom_fraction The fraction of the estimate coarsened. If
* this number is zero, no cells will be coarsened.
*
* @param[in] max_n_cells This argument can be used to specify a maximal
* number of cells. If this number is going to be exceeded upon refinement,
* then refinement and coarsening fractions are going to be adjusted in an
* attempt to reach the maximum number of cells. Be aware though that
* through proliferation of refinement due to Triangulation::MeshSmoothing,
* this number is only an indicator. The default value of this argument is
* to impose no limit on the number of cells.
*/
template <int dim, class VectorType, int spacedim>
void
refine_and_coarsen_fixed_fraction
(Triangulation<dim,spacedim> &tria,
const VectorType &criteria,
const double top_fraction,
const double bottom_fraction,
const unsigned int max_n_cells = std::numeric_limits<unsigned int>::max());
/**
* Refine the triangulation by flagging certain cells to reach a grid that
* is optimal with respect to an objective function that tries to balance
* reducing the error and increasing the numerical cost when the mesh is
* refined. Specifically, this function makes the assumption that if you
* refine a cell $K$ with error indicator $\eta_K$ provided by the second
* argument to this function, then the error on the children (for all
* children together) will only be $2^{-\text{order}}\eta_K$ where
* <code>order</code> is the third argument of this function. This makes the
* assumption that the error is only a local property on a mesh and can be
* reduced by local refinement -- an assumption that is true for the
* interpolation operator, but not for the usual Galerkin projection,
* although it is approximately true for elliptic problems where the Greens
* function decays quickly and the error here is not too much affected by a
* too coarse mesh somewhere else.
*
* With this, we can define the objective function this function tries to
* optimize. Let us assume that the mesh currently has $N_0$ cells. Then, if
* we refine the $m$ cells with the largest errors, we expect to get (in $d$
* space dimensions)
* @f[
* N(m) = (N_0-m) + 2^d m = N_0 + (2^d-1)m
* @f]
* cells ($N_0-m$ are not refined, and each of the $m$ cells we refine yield
* $2^d$ child cells. On the other hand, with refining $m$ cells, and using
* the assumptions above, we expect that the error will be
* @f[
* \eta^\text{exp}(m)
* =
* \sum_{K, K\; \text{will not be refined}} \eta_K
* +
* \sum_{K, K\; \text{will be refined}} 2^{-\text{order}}\eta_K
* @f]
* where the first sum extends over $N_0-m$ cells and the second over the
* $m$ cells that will be refined. Note that $N(m)$ is an increasing
* function of $m$ whereas $\eta^\text{exp}(m)$ is a decreasing function.
*
* This function then tries to find that number $m$ of cells to refine for
* which the objective function
* @f[
* J(m) = N(m)^{\text{order}/d} \eta^\text{exp}(m)
* @f]
* is minimal.
*
* The rationale for this function is two-fold. First, compared to the
* refine_and_coarsen_fixed_fraction() and refine_and_coarsen_fixed_number()
* functions, this function has the property that if all refinement
* indicators are the same (i.e., we have achieved a mesh where the error
* per cell is equilibrated), then the entire mesh is refined. This is based
* on the observation that a mesh with equilibrated error indicators is the
* optimal mesh (i.e., has the least overall error) among all meshes with
* the same number of cells. (For proofs of this, see R. Becker, M. Braack,
* R. Rannacher: "Numerical simulation of laminar flames at low Mach number
* with adaptive finite elements", Combustion Theory and Modelling, Vol. 3,
* Nr. 3, p. 503-534 1999; and W. Bangerth, R. Rannacher: "Adaptive Finite
* Element Methods for Differential Equations", Birkhauser, 2003.)
*
* Second, the function uses the observation that ideally, the error behaves
* like $e \approx c N^{-\alpha}$ with some constant $\alpha$ that depends
* on the dimension and the finite element degree. It should - given optimal
* mesh refinement - not depend so much on the regularity of the solution,
* as it is based on the idea, that all singularities can be resolved by
* refinement. Mesh refinement is then based on the idea that we want to
* make $c=e N^\alpha$ small. This corresponds to the functional $J(m)$
* above.
*
* @note This function was originally implemented by Thomas Richter. It
* follows a strategy described in T. Richter, "Parallel Multigrid Method
* for Adaptive Finite Elements with Application to 3D Flow Problems", PhD
* thesis, University of Heidelberg, 2005. See in particular Section 4.3,
* pp. 42-43.
*/
template <int dim, class VectorType, int spacedim>
void
refine_and_coarsen_optimize (Triangulation<dim,spacedim> &tria,
const VectorType &criteria,
const unsigned int order=2);
/**
* Flag all mesh cells for which the value in @p criteria exceeds @p
* threshold for refinement, but only flag up to @p max_to_mark cells.
*
* The vector @p criteria contains a nonnegative value for each active cell,
* ordered in the canonical order of of Triangulation::active_cell_iterator.
*
* The cells are only flagged for refinement, they are not actually refined.
* To do so, you have to call
* Triangulation::execute_coarsening_and_refinement().
*
* This function does not implement a refinement strategy, it is more a
* helper function for the actual strategies.
*/
template <int dim, class VectorType, int spacedim>
void refine (Triangulation<dim,spacedim> &tria,
const VectorType &criteria,
const double threshold,
const unsigned int max_to_mark = numbers::invalid_unsigned_int);
/**
* Flag all mesh cells for which the value in @p criteria is less than @p
* threshold for coarsening.
*
* The vector @p criteria contains a nonnegative value for each active cell,
* ordered in the canonical order of of Triangulation::active_cell_iterator.
*
* The cells are only flagged for coarsening, they are not actually
* coarsened. To do so, you have to call
* Triangulation::execute_coarsening_and_refinement().
*
* This function does not implement a refinement strategy, it is more a
* helper function for the actual strategies.
*/
template <int dim, class VectorType, int spacedim>
void coarsen (Triangulation<dim,spacedim> &tria,
const VectorType &criteria,
const double threshold);
/**
* An exception thrown if the vector with cell criteria contains negative
* values
*/
DeclException0(ExcNegativeCriteria);
/**
* One of the threshold parameters causes trouble. Or the refinement and
* coarsening thresholds overlap.
*/
DeclException0 (ExcInvalidParameterValue);
}
DEAL_II_NAMESPACE_CLOSE
#endif //dealii__grid_refinement_h
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