Postprocessing

By default, oofem produces output in the form of a readable text file, called the output file. At the same time, oofem can produce output in other formats that are more suitable for postprocessing in third-party tools. This is done by configuring one or more export modules in the input file. In this short tutorial, we will cover VTK postprocessing.

Configuring vtk export

VTK is a widely used format for storing simulation results. VTK files can be opened in many visualization tools. The export of results in VTK format can be done using the vtkxml export module.

The number of export modules is declared in the analysis record using the nmodules keyword, and the export module records must immediately follow the analysis record. The detailed syntax of the vtkxml record is described in the oofem input manual.

As an example, consider the 3-point bending test input file (located in tests/regression/benchmark/sm/concrete_3point.in), which we modify to add the vtk export. The original file already declares one module (errorcheck, which makes it a regression test), so nmodules grows from 1 to 2 and the new record is inserted next to it:

concrete_3point.out
Test: 3-point bending, triangular elements, 1 loaded point
StaticStructural nsteps 20 solverType "calm" stepLength 0.025 rtolf 1e-4 Psi 0.0 MaxIter 200 reqIterations 80 HPC 2 1 2 stiffmode 1 nmodules 2
vtkxml tstep_all domain_all primvars 1 1 vars 2 1 4
errorcheck
domain 2dPlaneStress
...

The export has been requested for all solution steps (tstep_all) and all domains (domain_all).

The primary variables exported consist of the displacement vector: primvars 1 1, where primvars is the keyword for primary variable export; what follows is an array of primary variable IDs (defined in src/core/unknowntype.h), the first number being the array size.

The export of secondary (internal) variables consists here of the stress and strain tensors (vars 2 1 4, where the internal variable codes are defined in src/core/internalstatetype.h; 1 is IST_StressTensor and 4 is IST_StrainTensor).

Two further keywords are often useful: cellvars, which exports quantities per element rather than smoothed to the nodes, and ipvars, which exports the raw integration point values with no smoothing at all. Note that the selectors are those of an export module, not of the OutputManager: the dofman_all and element_all keywords are not accepted here, and regionsets is what restricts the export to part of the model.

After running the modified, extended input, the solver will produce one vtu file per solution step, named after the output file with the module number and the step number appended:

concrete_3point.out.m0.1.vtu
concrete_3point.out.m0.2.vtu
concrete_3point.out.m0.3.vtu
...

Postprocessing in paraview

In this example, we will use Paraview (an open-source, multi-platform data analysis and visualization application, www.paraview.org). After installation of this tool, simply launch paraview and open one or all of the vtu files produced. Paraview recognizes the trailing numbers as a series, so opening the group lets you step through the solution in time.

Other export modules

vtkxml is the most commonly used export module, but it is not the only one. The modules below are all configured the same way — a record following the analysis record and counted in nmodules — and several of them can be active at the same time:

vtkhdf5

VTK data in a single HDF5 file instead of one file per step, which is preferable for analyses with many steps.

gnuplot

Load-displacement diagrams and similar curves, ready to plot.

hom

Homogenized (volume-averaged) quantities over the model or over selected sets, one row per step.

gpexportmodule

Raw integration point values, for custom processing.

matlab

Results as a Matlab script.

poi

Values interpolated at user-defined points of interest.

dm

Selected dof manager (nodal) values.

output

The standard text output file, exposed as a module so that its selectors can be configured like any other module’s.

errorcheck

Compares results against reference values embedded in the input file. This is the mechanism behind the regression test suite (see Installation) and it is also a practical way to guard your own models against unintended changes.

The full list, with the parameters of each module, is in the oofem input manual.