Documentation/Tutorial/Input File

The previous two examples (Sod shocktube in 1D hydrodynamics, and Orszag-Tang vortex in 2D MHD) used the default input files in the /athena/tst directories. Suppose, however, one would like to change the grid resolution, or output files, or any other parameter of the runs. This is accomplished by editing the input files.

Changing the Grid Resolution

For example, the grid resolution in the 2D Orszag-Tang vortex test can be changed by editing the input file using the following steps.

First, make sure there is an executable for the Orszag-Tang problem in the /athena/bin directory. If necessary, repeat step 1 from the 2D MHD tutorial
```
 % make clean
 % configure --with-problem=orszag-tang --with-order=3
 % make all
```

Next, copy the input file into the /bin directory.

 % cd bin
 % cp ../tst/2D-mhd/athinput.orszag-tang  athinput.new

Now, edit the new input file to change the grid resolution. This is set by the Nx1 and Nx2 parameters in the <domain1> block. Therefore, change the lines:

 ...
 Nx1             = 192       # Number of zones in X1-direction
 ...
 Nx2             = 192       # Number of zones in X2-direction
 ...

to any other values you like. Make the resolution larger will make the code run slower, so try decreasing the resolution to

 ...
 Nx1             = 128       # Number of zones in X1-direction
 ...
 Nx2             = 128       # Number of zones in X2-direction
 ...

Run the code using this new input file
```
 % athena -i athinput.new
```
This should generate the same data files as before, but on a grid with resolution 128^2^. The code should have run about four times faster compared to the default resolution of 192^2^

Adding a New Output

Suppose you would like to output the data in the 2D Orszag-Tang vortex test in vtk, instead of binary, format, and you would like to add output to make a movie of the magnetic energy. You need to edit existing, and add new, <output> blocks in the input file. Use the following steps.

If you haven’t already, configure and compile the code, and copy the input file to the /bin directory.

 % make clean
 % configure --with-problem=orszag-tang --with-order=3
 % make all
 % cd bin
 % cp ../tst/2D-mhd/athinput.orszag-tang  athinput.new

Next, edit the <output2> block in the input file to change the output format from:

 out_fmt = bin               # Binary data dump

 out_fmt = vtk               # vtk data dump

Add a new <output> block for ppm images of the magnetic energy. A useful time interval is one that creates several hundred images (so the time evolution is smooth). Since we don’t know what the min/max of the magnetic energy will be, we can use autoscaling of the images. Thus, the new output block might be
```
 <output5>
 out_fmt = ppm
 dt      = 0.004
 out     = ME
 id      = ME
 palette = heat
```
Now increase the number of output blocks to be read, by changing the maxout parameter in the <job> block to
```
 maxout       = 5            # Read output blocks numbered from 1 -> maxout
```

Now run the code. The last few lines of output (when run on a 192^2^ grid) should be something like

 cycle=655 time=9.999868e-01 next dt=1.320914e-05 last dt=1.449403e-03
 cycle=656 time=1.000000e+00 next dt=0.000000e+00 last dt=1.320914e-05
    
 terminating on time limit
   tlim= 1.000000e+00   nlim= 100000
   time= 1.000000e+00  cycle= 656
    
 zone-cycles/cpu-second = 2.251027e+05
    
 elapsed wall time = 1.074434e+02 sec.
    
 zone-cycles/wall-second = 2.250746e+05
 Global min/max for P: 0.0100164 0.755822
 Global min/max for d: 0.0442782 0.609765
 Global min/max for ME: 8.51744e-08 0.411589
    
 Simulation terminated on Thu Apr 29 11:56:58 2010

Note the global min/max of ME are reported. If you animate the OrszagTang.*.ME.ppm images, they will seem to flicker (especially near the beginning, where the magnetic energy is evolving rapidly) since each image uses a different scaling (min/max). So edit the <output5> block to add appropriate values for the min/max of ME reported above:

 <output5>
 out_fmt = ppm
 dt      = 0.004
 out     = ME
 id      = ME
 dmin    = 0.0
 dmax    = 0.4
 palette = heat

Run the code again, and now the movie of the ME should be smooth. Try editing the <output5> block to use different palettes, and see what the resulting images are like.

Note the code no longer outputs .bin files, but these have been replaced by .vtk files. Athena vtk files can be read directly by VisIt. Try plotting the density, pressure, velocity, etc using VisIt to read the .vtk files. You can also make movies of any quantity you like from the files using VisIt.

Try modifying other parameters of the runs by editing the input files.