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// $Id: theta_timestepping.h 30036 2013-07-18 16:55:32Z maier $
//
// Copyright (C) 2010 - 2013 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 __deal2__theta_timestepping_h
#define __deal2__theta_timestepping_h
#include <deal.II/base/smartpointer.h>
#include <deal.II/algorithms/operator.h>
#include <deal.II/algorithms/timestep_control.h>
DEAL_II_NAMESPACE_OPEN
class ParameterHandler;
namespace Algorithms
{
/**
* A little structure, gathering the size of a timestep and the
* current time. Time stepping schemes can use this to provide time
* step information to the classes actually performing a single step.
*
* The definition of what is considered "current time" depends on the
* scheme. For an explicit scheme, this is the time at the beginning
* of the step. For an implicit scheme, it is usually the time at the
* end.
*/
struct TimestepData
{
/// The current time
double time;
/// The current step size times something
double step;
};
/**
* Application class performing the theta timestepping scheme.
*
* The theta scheme is an abstraction of implicit and explicit Euler
* schemes, the Crank-Nicholson scheme and linear combinations of
* those. The choice of the actual scheme is controlled by the
* parameter #theta as follows.
* <ul>
* <li> #theta=0: explicit Euler scheme
* <li> #theta=1: implicit Euler scheme
* <li> #theta=½: Crank-Nicholson scheme
* </ul>
*
* For fixed #theta, the Crank-Nicholson scheme is the only second
* order scheme. Nevertheless, further stability may be achieved by
* choosing #theta larger than ½, thereby introducing a first order
* error term. In order to avoid a loss of convergence order, the
* adaptive theta scheme can be used, where <i>#theta=½+c dt</i>.
*
* Assume that we want to solve the equation <i>u' + Au = 0</i> with a
* step size <i>k</i>. A step of the theta scheme can be written as
*
* @f[
* (M + \theta k A) u_{n+1} = (M - (1-\theta)k A) u_n.
* @f]
*
* Here, <i>M</i> is the mass matrix. We see, that the right hand side
* amounts to an explicit Euler step with modified step size in weak
* form (up to inversion of M). The left hand side corresponds to an
* implicit Euler step with modified step size (right hand side
* given). Thus, the implementation of the theta scheme will use two
* Operator objects, one for the explicit, one for the implicit
* part. Each of these will use its own TimestepData to account for
* the modified step sizes (and different times if the problem is not
* autonomous).
*
* <h3>Usage of vectors in NamedData</h3>
*
* ThetaTimestepping uses NamedData for communicating vectors. With
* outer or inner Operator objects. It does not use itself the input
* vectors provided, but forwards them to the explicit and implicit
* operators.
*
* The explicit Operator #op_explicit receives in its input in first
* place the vector <tt>"Previous iterate"</tt>, which is the solution
* value after the previous timestep. It is followed by all vectors
* provided to ThetaTimestepping::operator() as input
* argument. #op_explicit is supposed to write its result into the
* first position of its output argument, labeled <tt>"Result"</tt>.
*
* The implicit Operator #op_implicit receives the result of
* #op_explicit in its first input vector labeled <tt>"Previous
* time"</tt>. It is followed by all vectors provided to
* ThetaTimestepping::operator() as input argument. The output of
* #op_implicit is directly written into the output argument given to
* ThetaTimestepping.
*
* <h3>Usage of ThetaTimestepping</h3>
*
* The use ThetaTimestepping is more complicated than for instance
* Newton, since the inner operators will usually need to access the
* TimeStepData. Thus, we have a circular dependency of information,
* and we include the following example for its use. It can be found
* in <tt>examples/doxygen/theta_timestepping.cc</tt>
*
* @dontinclude theta_timestepping.cc
*
* First, we define the two operators used by ThetaTimestepping and
* call them <code>Implicit</code> and <code>Explicit</code>. They
* both share the public interface of Operator, and additionally
* provide storage for the matrices to be used and a pointer to
* TimestepData. Note that we do not use a SmartPointer here, since
* the TimestepData will be destroyed before the operator.
*
* @skip class Explicit
* @until End of declarations
*
* These operators will be implemented after the main program. But let
* us look at how they get used. First, let us define a matrix to be
* used for our system and also an OutputOperator in order to write
* the data of each timestep to a file.
*
* @skipline main
* @until out.initialize
*
* Now we create objects for the implicit and explicit parts of the
* steps as well as the ThetaTimestepping itself. Notice how the
* TimestepData of ThetaTimestepping gets forwarded to the inner
* operators. There are two different data objects, because the
* timestep size is modified by #theta.
*
* @until set_output
*
* The next step is providing the vectors to be used. <tt>value</tt>
* is filled with the initial value and is also the vector where the
* solution at each timestep will be. Because the interface of
* Operator has to be able to handle several vectors, we need to store
* it in a NamedData object. Notice, that we need to create the
* intermediate pointer <tt>p</tt>. If we would use
* <code>&value</code> directly in the <code>add</code> function, the
* resulting object would be constant.
*
* @until add
*
* Finally, we are ready to tell the solver, that we are looknig at
* the initial timestep and run it.
*
* @until outdata
* @skip Explicit::initialize
*
* Now we need to study the application of the implicit and explicit
* operator. We assume that the pointer <code>matrix</code> points to
* the matrix created in the main program, and that
* <code>timestep_data</code> points to the correct data object of
* ThetaTimestepping.
*
* @skipline void
* @until vmult
* @until }
*
* @author Guido Kanschat
* @date 2010
*/
template <class VECTOR>
class ThetaTimestepping : public Operator<VECTOR>
{
public:
/**
* Constructor, receiving the
* two operators stored in
* #op_explicit and #op_implicit. For
* their meening, see the
* description of those variables.
*/
ThetaTimestepping (Operator<VECTOR> &op_explicit,
Operator<VECTOR> &op_implicit);
/**
* The timestepping scheme. <tt>in</tt>
* should contain the initial
* value in first position. <tt>out</tt>
*/
virtual void operator() (NamedData<VECTOR *> &out, const NamedData<VECTOR *> &in);
/**
* Register an event triggered
* by an outer iteration.
*/
virtual void notify(const Event &);
/**
* Define an operator which
* will output the result in
* each step. Note that no
* output will be generated
* without this.
*/
void set_output(OutputOperator<VECTOR> &output);
void declare_parameters (ParameterHandler ¶m);
void initialize (ParameterHandler ¶m);
/**
* The current time in the
* timestepping scheme.
*/
const double ¤t_time() const;
/**
* The current step size.
*/
const double &step_size() const;
/**
* The weight between implicit
* and explicit part.
*/
const double &theta() const;
/**
* The data handed to the
* #op_explicit time stepping
* operator.
*
* The time in here is the time
* at the beginning of the
* current step, the time step
* is (1-#theta) times the
* actual time step.
*/
const TimestepData &explicit_data() const;
/**
* The data handed to the
* #op_implicit time stepping
* operator.
*
* The time in here is the time
* at the beginning of the
* current step, the time step
* is #theta times the
* actual time step.
*/
const TimestepData &implicit_data() const;
/**
* Allow access to the control object.
*/
TimestepControl ×tep_control();
private:
/**
* The object controlling the
* time step size and computing
* the new time in each step.
*/
TimestepControl control;
/**
* The control parameter theta in the
* range <tt>[0,1]</tt>.
*/
double vtheta;
/**
* Use adaptive #theta if
* <tt>true</tt>.
*/
bool adaptive;
/**
* The data for the explicit
* part of the scheme.
*/
TimestepData d_explicit;
/**
* The data for the implicit
* part of the scheme.
*/
TimestepData d_implicit;
/**
* The operator computing the
* explicit part of the
* scheme. This will receive in
* its input data the value at
* the current time with name
* "Current time solution". It
* should obtain the current
* time and time step size from
* explicit_data().
*
* Its return value is
* <i>Mu+cAu</i>, where
* <i>u</i> is the current
* state vector, <i>M</i> the
* mass matrix, <i>A</i> the
* operator in space and
* <i>c</i> is the time step
* size in explicit_data().
*/
SmartPointer<Operator<VECTOR>, ThetaTimestepping<VECTOR> > op_explicit;
/**
* The operator solving the
* implicit part of the
* scheme. It will receive in
* its input data the vector
* "Previous time". Information on the
* timestep should be obtained
* from implicit_data().
*
* Its return value is the
* solution <i>u</i> of
* <i>Mu-cAu=f</i>, where
* <i>f</i> is the dual space
* vector found in the
* "Previous time" entry of the
* input data, <i>M</i> the
* mass matrix, <i>A</i> the
* operator in space and
* <i>c</i> is the time step
* size in explicit_data().
*/
SmartPointer<Operator<VECTOR>, ThetaTimestepping<VECTOR> > op_implicit;
/**
* The operator writing the
* output in each time step
*/
SmartPointer<OutputOperator<VECTOR>, ThetaTimestepping<VECTOR> > output;
};
template <class VECTOR>
inline
const TimestepData &
ThetaTimestepping<VECTOR>::explicit_data () const
{
return d_explicit;
}
template <class VECTOR>
inline
const TimestepData &
ThetaTimestepping<VECTOR>::implicit_data () const
{
return d_implicit;
}
template <class VECTOR>
inline
void ThetaTimestepping<VECTOR>::set_output (OutputOperator<VECTOR> &out)
{
output = &out;
}
}
DEAL_II_NAMESPACE_CLOSE
#endif
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