# Examples¶

This section provides an example of typical simulation postprocessing routine and some advanced chaining patterns (see Command Chains for basic information on chaining commands together).

## Two Stream Instability¶

Following are the results of a two-stream instability simulation [input file] with one configuration space coordinate and one velocity space coordinate (1X1V).

The info command (info) can be used as a first step to get more information

pgkyl -f two-stream_elc_0.bp info

Dataset #0
- Time: 0.000000e+00
- Number of components: 8
- Number of dimensions: 2
- Dim 0: Num. cells: 64; Lower: -6.283185e+00; Upper: 6.283185e+00
- Dim 1: Num. cells: 32; Lower: -6.000000e+00; Upper: 6.000000e+00
- Maximum: 1.676015e+00 at (31, 18) component 0
- Minimum: -4.698334e-01 at (31, 19) component 2


It is a $$64\times 32$$ simulation with 8 expansion coefficients in each cell (2D second order Serendipity basis).

Next step is to plot the initial and final states. For this, both files can be loaded simultaneously, interpolated to an uniform mesh (interpolate; second order -p2 modal Serendipity basis -b ms), and plotted (plot).

pgkyl -f two-stream_elc_0.bp -f two-stream_elc_100.bp interpolate -p2 -b ms plot


For better understanding of chaining and interpolation, it is interesting to take a look how data information changes in the process

\$ pgkyl -f two-stream_elc_0.bp info interpolate -p2 -b ms info

Dataset #0
- Time: 0.000000e+00
- Number of components: 8
- Number of dimensions: 2
- Dim 0: Num. cells: 64; Lower: -6.283185e+00; Upper: 6.283185e+00
- Dim 1: Num. cells: 32; Lower: -6.000000e+00; Upper: 6.000000e+00
- Maximum: 1.676015e+00 at (31, 18) component 0
- Minimum: -4.698334e-01 at (31, 19) component 2

Dataset #0
- Time: 0.000000e+00
- Number of components: 1
- Number of dimensions: 2
- Dim 0: Num. cells: 192; Lower: -6.283185e+00; Upper: 6.283185e+00
- Dim 1: Num. cells: 96; Lower: -6.000000e+00; Upper: 6.000000e+00
- Maximum: 9.498275e-01 at (95, 55)
- Minimum: -6.751242e-04 at (97, 62)


During the two-stream instability the free kinetic energy of the counter-streaming beans is converted into electric field energy. Gkyl stores the domain integrated field energies into the sequence of history files two-stream_fieldEnergy_X.bp which can be loaded in a similar way to the frame data. However, the files contains multiple components which correspond to $$E_x^2,$$ $$E_y^2,$$ $$E_y^2,$$ $$B_x^2,$$ $$B_y^2,$$ and $$B_z^2.$$ Only $$E_x$$ is growing in this case, so we might want to limit the plot only to the first component using select (select).

The decrease of kinetic energy can be seen in the previous figures as the beams are getting closer together for $$x=0$$. To emphasize this, line-outs for $$x=0$$ (select --c0 0.) could be plotted on top of each other (plot -f0).

Alternatively, the collect command (collect) could be used to plot the whole time evolution (stacking the line-outs in time).

However, the previous plot shows only a localized information. In order to get a global view, the x-dimension can be integrated out instead of selecting a line-out (integrate).