.. _step2_section:


STEP 2 : Perform simple changes 
================================


Modify the source file as well as the **INITGAZ.DAT** file to easely change the initial condition. Implement the next:

  - Solution of an uniform flow : intial conditions constant in pressure, density and velocity (even zero velocity is admissible), but take care of the boundary conditions...

.. hint:: 

 Save the solution of the shock tube problem in a file  named fo exemple *solref* : after every modification in the 1D code the plots must exactly recover this solution. Do not hesitate to open the **PLOT** file and compare exact values with *solref* : sometimes small differences in values are not visible on the plots but they may indicate unwanted or uncontrolled changes...


At the end of this *step 2* you should know:

- What are the unknowns and in which vectors they are stored.
- Where are the fluxe computed **This is not useful to know how the Riemann solver works!**.
- Where and when are plotted the solution. 
- Where is the finite volume relation given :eq:`eq_godunov` in the code.
- How is defined the mesh.