Reference documentation for deal.II version Git 36ab825 2016-05-25 21:19:30 +0200
|virtual void||compute_derived_quantities_scalar (const std::vector< double > &uh, const std::vector< Tensor< 1, dim > > &duh, const std::vector< Tensor< 2, dim > > &dduh, const std::vector< Point< dim > > &normals, const std::vector< Point< dim > > &evaluation_points, std::vector< Vector< double > > &computed_quantities) const|
|virtual void||compute_derived_quantities_vector (const std::vector< Vector< double > > &uh, const std::vector< std::vector< Tensor< 1, dim > > > &duh, const std::vector< std::vector< Tensor< 2, dim > > > &dduh, const std::vector< Point< dim > > &normals, const std::vector< Point< dim > > &evaluation_points, std::vector< Vector< double > > &computed_quantities) const|
|virtual std::vector< std::string >||get_names () const =0|
|virtual std::vector< DataComponentInterpretation::DataComponentInterpretation >||get_data_component_interpretation () const|
|virtual UpdateFlags||get_needed_update_flags () const =0|
|Public Member Functions inherited from Subscriptor|
|Subscriptor (const Subscriptor &)|
|Subscriptor (Subscriptor &&)|
|Subscriptor &||operator= (const Subscriptor &)|
|Subscriptor &||operator= (Subscriptor &&)|
|void||subscribe (const char *identifier=0) const|
|void||unsubscribe (const char *identifier=0) const|
|unsigned int||n_subscriptions () const|
|void||list_subscribers () const|
|template<class Archive >|
|void||serialize (Archive &ar, const unsigned int version)|
|Static Public Member Functions inherited from Subscriptor|
|static::ExceptionBase &||ExcInUse (int arg1, char *arg2, std::string &arg3)|
|static::ExceptionBase &||ExcNoSubscriber (char *arg1, char *arg2)|
This class provides an interface to compute derived quantities from a solution that can then be output in graphical formats for visualization, using facilities such as the DataOut class.
For the (graphical) output of a FE solution one frequently wants to include derived quantities, which are calculated from the values of the solution and possibly the first and second derivatives of the solution. Examples are the calculation of Mach numbers from velocity and density in supersonic flow computations, or the computation of the magnitude of a complex-valued solution as demonstrated in step-29. Other uses are shown in step-32 and step-33. This class offers the interface to perform such postprocessing. Given the values and derivatives of the solution at those points where we want to generated output, the functions of this class can be overloaded to compute new quantities.
A data vector and an object of a class derived from the current one can be given to the DataOut::add_data_vector() function (and similarly for DataOutRotation and DataOutFaces). This will cause DataOut::build_patches() to compute the derived quantities instead of using the data provided by the data vector (typically the solution vector). Note that the DataPostprocessor object (i.e., in reality the object of your derived class) has to live until the DataOut object is destroyed as the latter keeps a pointer to the former and will complain if the object pointed to is destroyed while the latter still has a pointer to it. If both the data postprocessor and DataOut objects are local variables of a function (as they are, for example, in step-29), then you can avoid this error by declaring the data postprocessor variable before the DataOut variable as objects are destroyed in reverse order of declaration.
In order not to perform needless calculations, DataPostprocessor has to provide information which input data is needed for the calculation of the derived quantities, i.e. whether it needs the values, the first derivative and/or the second derivative of the provided data. DataPostprocessor objects which are used in combination with a DataOutFaces object can also ask for the normal vectors at each point. The information which data is needed has to be provided via the UpdateFlags returned by the virtual function get_needed_update_flags(). It is your responsibility to use only those values which were updated in the calculation of derived quantities. The DataOut object will provide references to the requested data in the call to compute_derived_quantities_scalar() or compute_derived_quantities_vector() (DataOut decides which of the two functions to call depending on whether the finite element in use has only a single, or multiple vector components; note that this is only determined by the number of components in the finite element in use, and not by whether the data computed by a class derived from the current one is scalar or vector valued).
Furthermore, derived classes have to implement the get_names() function, where the number of output variables returned by the latter function has to match the size of the vector returned by the former. Furthermore, this number has to match the number of computed quantities, of course.
Deriving from the current class allows to implement very general postprocessors. For example, in the step-32 program, we implement a postprocessor that takes a solution that consists of velocity, pressure and temperature (dim+2 components) and computes a variety of output quantities, some of which are vector valued and some of which are scalar. On the other hand, in step-29 we implement a postprocessor that only computes the magnitude of a complex number given by a two-component finite element. It seems silly to have to implement four virtual functions for this (compute_derived_quantities_scalar() or compute_derived_quantities_vector(), get_names(), get_update_flags() and get_data_component_interpretation()).
To this end there are two classes DataPostprocessorScalar and DataPostprocessorVector that are meant to be used if the output quantity is either a single scalar or a single vector (here used meaning to have exactly dim components). When using these classes, one only has to write a constructor that passes the name of the output variable and the update flags to the constructor of the base class and overload the function that actually computes the results.
This is the main function which actually performs the postprocessing. The last argument is a reference to the postprocessed data which has correct size already and must be filled by this function.
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.
normals will always be an empty vector when working on cells, not on faces.
This function is called when the original data vector represents scalar data, i.e. the finite element in use has only a single vector component.
This functions returns information about how the individual components of output files that consist of more than one data set are to be interpreted.
For example, if one has a finite element for the Stokes equations in 2d, representing components (u,v,p), one would like to indicate that the first two, u and v, represent a logical vector so that later on when we generate graphical output we can hand them off to a visualization program that will automatically know to render them as a vector field, rather than as two separate and independent scalar fields.
The default implementation of this function returns a vector of values DataComponentInterpretation::component_is_scalar, indicating that all output components are independent scalar fields. However, if a derived class produces data that represents vectors, it may return a vector that contains values DataComponentInterpretation::component_is_part_of_vector. In the example above, one would return a vector with components (DataComponentInterpretation::component_is_part_of_vector, DataComponentInterpretation::component_is_part_of_vector, DataComponentInterpretation::component_is_scalar) for (u,v,p).
Return, which data has to be provided to compute the derived quantities. This has to be a combination of
update_hessians. If the DataPostprocessor is to be used in combination with DataOutFaces, you may also ask for a update of normals via the