Modifying the Input File

The previous two examples (the Shock tube tests in 1D, and 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.

Changing the grid resolution

The grid resolution used in the previous example is not so 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.

> 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