5.2 Convert .fld / .chk files into Paraview, VisIt or Tecplot format

To convert the Nektar++ output binary files (.chk and .fld) into a format which can be read by two common visualisation softwares: Paraview (.vtu format), VisIt (.vtu format) or Tecplot (.dat or .plt format) the user can run the following commands:

• Paraview or VisIt (.vtu format)
  FieldConvert test.xml test.fld test.vtu
• Tecplot (.dat format)
  FieldConvert test.xml test.fld test.dat
• Tecplot or VisIt(.plt format)
  FieldConvert test.xml test.fld test.plt

where FieldConvert is the executable associated to the utility FieldConvert, test.xml is the session file and test.vtu, test.dat, test.plt are the desired format outputs, either Paraview, VisIt or Tecplot formats.
When converting to .dat or .plt format, it is possible to enable output with double precision, which is more accurate but requires larger disk space. For example, double precision output in plt. format can be produced with the command:

5.2.1 Using the VTK library for output

While the basic VTK output detailed above (.vtu unstructured grid files) works without the third party VTK library, Nektar++ can also optionally be linked with the VTK library. By enabling VTK library support the creation of the VTK output files is offloaded to the VTK library rather than being manually written by Nektar++.

If you want to build a version of Nektar++ with integrated VTK library support, you need to turn on the NEKTAR_USE_VTK switch in cmake. This will then look for a compatible VTK library on the system, VTK version 8.0.0 or higher is recommended for full functionality because this is the minimum version which allows for high-order VTK output. Versions older than this are supported but the high-order VTK output will be disabled.

Using the VTK library automatically enables file compression on the output .vtu file which can substantially reduce file sizes. This can be turned off with the command line option uncompress when using FieldConvert.

If Nektar++ has been compiled with NEKTAR_USE_VTK enabled then there are certain situations in which the legacy VTK output (manually writing rather than using the VTK library) must be used. The current VTK library output only supports 1D, 2D, 3D and 3DH1D (Quasi-3D) simulations. Any simulations with expansions other than those mentioned, such as 2DH2D will need to be converted using the legacy option to invoke the manual Nektar++ VTK output.

The VTK library also adds two new output types that can be specified by the command line when choosing to output a .vtu format file, these are detailed below.

5.2.1.1 High-order output

High-order output can be enabled using the option highorder which first runs the mesh through the equispacedoutput module to get a truly equispaced mesh and then constructs a .vtu file using arbitrary-order Lagrange polynomials. This gives a more precise representation of the curvature of elements and allows for polynomial refinement using non-linear subdivisions. This should result in smaller output files as refinement of the mesh geometry can be computed on the fly. This allows for faster postprocessing in software such as Paraview due to the reduced memory requirements. High-order output is currently only implemented for non-homogenous expansions and does not support pyramid elements. A minimum VTK version of 8.0.0 is required for the high-order output, any VTK versions older than this will result in the high-order output being disabled.

5.2.1.2 Multi-block output

Multi-block output can be enabled using the option multiblock which splits the mesh in to separate blocks, one block for each domain, boundary, and any other composite. This allows each block to be toggled in the visualisation software independently.

Multi-block output uses the composite/boundary/domain names defined in the session file to label each block, so the ID numbers above will be replaced by the name if specified. It is also necessary to include the CONDITIONS section in the session file supplied to FieldConvert because this is where the boundary definitions are stored. Multi-block output is currently only implemented for non-homogenous expansions i.e. 1D, 2D, and 3D.

An example multi-block tree structure is shown in figure 5.1, with the default output on the left and the output with the optional defined names on the right.

5.2.1.3 Improved FieldConvert filter

An additional advantage of using the VTK library is that when a filter is set up with FieldConvert to output a .vtu file, the mesh is now cached inbetween timesteps and only the new expansion data is written each filter call. This can provide significant speedups by keeping the geometry in memory. An example filter specifying a high-order and uncompressed .vtu output is shown below: