convergence problem?

Max

I have a problem with convergence during my simulation. At the iteration of sth around 13000 it suddenly diverges (after converging smoothly over 5000 iterations). I am simulating a supersonic jet. This jet is released fom a pipe with the diameter of 0.014m into the envitonment with the M=3. The B.C. are as follows: inlet (the otlet of the tube):

Pressure inlet

P0=Ps(1+(gama-1)/2 * Much)^(...)

Ps=900bar

Ts=600K Outlet (ambient):

the ordinary condition of the ambient.

I have set my CFL even 0.01 but it always diverge (without relation to CFL or initialization).

any help will be appreciated,

Best Regards,

Max