Among the postprocessors that can be selected in the input parameter file (see Overview and Subsection: Postprocess / Visualization) are some that can produce files in a format that can later be used to generate a graphical visualization of the solution variables \(\mathbf u, p\) and \(T\) at select time steps, or of quantities derived from these variables (for the latter, see Visualization postprocessors).
By default, the files that are generated are in VTU format, i.e., the XML-based, compressed format defined by the VTK library, see http://vtk.org/. This file format has become a broadly accepted pseudo-standard that many visualization program support, including two of the visualization programs used most widely in computational science: VisIt (see https://visit.llnl.gov/) and ParaView (see http://www.paraview.org/). The VTU format has a number of advantages beyond being widely distributed:
It allows for compression, keeping files relatively small even for sizable computations.
It is a structured XML format, allowing other programs to read it without too much trouble.
It has a degree of support for parallel computations where every processor would only write that part of the data to a file that this processor in fact owns, avoiding the need to communicate all data to a single processor that then generates a single file. This requires a master file for each time step that then contains a reference to the individual files that together make up the output of a single time step. Unfortunately, there doesn’t appear to be a standard for these master records; however, both ParaView and VisIt have defined a format that each of these programs understand and that requires placing a file with ending
.visitinto the same directory as the output files from each processor. Overview gives an example of what can be found in the output directory.
You can select other formats for output than VTU, see the run-time parameters in Subsection: Postprocess / Visualization. However, none of the numerous formats currently implemented in deal.II other than the VTK/VTU formats allows for splitting up data over multiple files in case of parallel computattions, thus making subsequent visualization of the entire volume impossible. Furthermore, given the amount of data ASPECT can produce, the compression that is part of the VTU format is an important part of keeping data manageable.