Simulation of flow from exit of Nozzle
 

I am finding it difficult to simulate the flow from the nozzle exit to the ambient conditions.... I had tried different domains and boundary condition in FLUENT.... would like to know if i had missed anything

Trim/extend the nozzle, ambient b/c is possible.

I have tried giving the ambient boundary condition after extending the domain behind the nozzle exit... And also at temperatures above 500k the simulation is not working