[Obad]

Hey guys!

I'm currently working on the simulation of the flow behind a supersonic nozzle. I tried it for 3 weeks with different versions of the Maccormack scheme, finally I thought I have the answer to my problem by using the Maccormack scheme with the equations in conservation form.

Now my problem is, that the calculation blows up
For the calculation of the time steps I use the CFL criterion by keeping the Courant number constant. I use the following equation:

delta_t = C*delta_x/(a + V)

Where C is the Courant number, a the speed of sound and V the magnitude of the velocity.

The values for delta_t are around 10^-5 at the beginning but decrease to values around 10^-147.
I think that the cause for the small values is, that my x component of the velocity becomes really large, but I don't know why. The location of those high values is right after the nozzle exit near the symmetry plane. (see my attached picture)

I also tried to make my simulation more stable by adding artificial viscosity, but it made it even worse...

Does anyone of you have an hint, what could be wrong?
I would be really grateful if someone could help me.