5.2. Spinodal Decomposition¶

5.2.1. Spinodal decomposition in a binary fluid¶

It is assumed that the code has been compiled in the parallel version for -D_D3Q19_.

Copy the input file from docs/tutorial/test2/input to your current working directory. The input sets up a small three-dimensional system and uses the symmetric free energy. The initial composition is set to spinodal which introduces a small-amplitude noise in the otherwise uniform initial field to initiate spinodal decomposition.
Run the code (we assume in the current working directory) using, e.g.,
```
$ mpirun -np 4 ./Luwdig.exe
```
Statistics are reported at every 500 time steps, and output files are produced at 1000 step intervals for 10,000 steps. The compositional (scalar) order parameter appears in files phi-. At the end of execution there should be 10 time steps worth of data. Velocity and density files are also produced.
As requested in the input, the format for these files is binary. In general, this is a more efficient format (than ASCII) in terms of both file size, and time taken to read and write the files. The metadata data files are always ASCII and can still be viewed as normal.

5.2.2. Manipulating the output files¶

While it is possible to view the binary (raw) data without further processing, in some cases it may be necessary to process the raw output data to get a meaningful picture. This is done with the extract utility, which is found in the util directory.

For this example, we process the data to produce VTK format files which can then be read into, for example, Paraview. [Paraview]
To process a single data file, the extract utility is run with the data file as the sole (positional) argument and the -k option:
```
$ ./extract -k phi-000010000.001-001
...
Writing result to phi-000010000.vtk
Complete processing for phi-000010000.001-001
```
The -k is an instruction to re-write the data in VTK format. This is an ASCII file whoich can be inspected. As reported, the resultant file has extension .vtk to replace .001-001. [vtkOptions]
To process a series of files, it can be convenient to use the loop feature of the shell, e.g.,
```
$ for f in phi-000*.001-001; do ./extract -k ${f}; done
```
(Be careful not to include phi-metadata.001-001 in the glob by mistake.) Repeat the process for the velocity field files vel-000*.001.001.

5.2.3. Using Paraview¶

VTK format files may be conveniently read into Paraview.

From the GUI, clip the open file icon (or File->Open from the pull-down menu) and select a individual file, or group of files from the file dialogue.
Click on the group of files in the pipeline browser to select, and apply a contour filter by selecting Filters->Common->Contour from the pull-down menu or by clicking on the contour icon in the toolbar

The contour icon in the paraview toolbar is highlighted

Apply the contour with contour value 0.0. You should see a rendering of the $\phi = 0$ contour which represents the interface between the two components (usually with $\phi^\star = \pm 1$).
Load the corresponding velocity field files. Clip on the group of velocity files in the pipeline browser, and apply a glyph filter from the Filters->Common->Glyph pull down or using th icon in the toobar

In the glyph properties click Apply, and then in the Coloring section set the solid color menu to velocity_field to give a velocity magnitude colour scale.
One can step thorough the different time steps using the current time controls in the topmost toolbar