I have been trying to simulate flow through a rocket nozzle for both underexpansion and overexpansion for a steady flow using 2D planar model. The residuals converged in the case of under-expansion, however they simply keep oscillating forever for the case of over-expansion. What could probably cause this to happen.

I am new to CFD . I even tried changing the CFL number and decreasing the under-relaxation factors, but to no use. A little bit of help would be much appreciated.