Modifying the Input File
The previous two examples (the shock tube tests in 1D, and the Orszag-Tang vortex in 2D) used the default input files in the /athena/inputs 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.
The grid resolution used in the default 2D Orszag-Tang vortex test is not very high. Let us double the resolution.
1. Edit the input file.
Because the input file is just a plain text file, you can modify it using standard text editors. It is recommended to create a new copy of the input file for each simulation.
> cd ~/work
> cp athinput.orszag_tang athinput.orszag_tang_high
Then open it with your favorite editor.
<mesh>
nx1 = 200 # Number of zones in X1-direction
x1min = 0.0 # minimum value of X1
x1max = 1.0 # maximum value of X1
ix1_bc = periodic # inner-X1 boundary flag
ox1_bc = periodic # outer-X1 boundary flag
nx2 = 200 # Number of zones in X2-direction
x2min = 0.0 # minimum value of X2
x2max = 1.0 # maximum value of X2
ix2_bc = periodic # inner-X2 boundary flag
ox2_bc = periodic # outer-X2 boundary flag
To increase the resolution, change nx1 and nx2.
<mesh>
nx1 = 400 # Number of zones in X1-direction
...
nx2 = 400 # Number of zones in X2-direction
You do not have to reconfigure or rebuild the code if you already have an executable binary configured for the Orszag-Tang problem. If not, reconfigure and make the code.
2. Run the simulation using the new input file.
> ~/athena/bin/athena -i athinput.orszag_tang_high
RootGrid = 1 x 1 x 1
MeshBlock 0, rank = 0, lx1 = 0, lx2 = 0, lx3 = 0, level = 0
is=2 ie=401 x1min=0 x1max=1
js=2 je=401 x2min=0 x2max=1
ks=0 ke=0 x3min=-0.5 x3max=0.5
Setup complete, entering main loop...
cycle=0 time=0.00000000000000e+00 dt=4.51495485504003e-04
...
cycle=2774 time=1.00000000000000e+00 dt=3.40143584065472e-04
Terminating on time limit
time=1.00000000000000e+00 cycle=2774
tlim=1.00000000000000e+00 nlim=-1
cpu time used = 8.07699951171875e+02
zone-cycles/cpu_second = 5.49511000000000e+05
3. Visualize the result and compare it with the previous simulation.
Suppose you would like to output the data in the 2D Orszag-Tang vortex test not only in primitive variables but also in conservative variables. You need to add a new <output> block in the input file. Use the following steps.
1. Add a new <output> block.
Open the input file and add another output block for conservative variables.
<output3>
file_type = vtk # VTK data dump
variable = cons # variables to be output
dt = 0.01 # time increment between outputs
This will work but the default file names do not say what the output variable is. You can specify the output ID to change the file names.
<output2>
file_type = vtk # VTK data dump
variable = prim # variables to be output
id = prim # file ID
dt = 0.01 # time increment between outputs
<output3>
file_type = vtk # VTK data dump
variable = cons # variables to be output
id = cons # file ID
dt = 0.01 # time increment between outputs
2. Run the simulation with the new input file.
The code will output OrszagTang.block0.prim.?????.vtk and OrszagTang.block0.cons.?????.vtk files.