1. extend your exit area
2. use an internal/external analysis where your nozzle is in an external analysis and then you put a volume flow rate at the inlet to it and it expands outwards.
unless you use these methods, your flow trajectories stop at the outlet because that is the end of your computational domain
I hope you are trying to capture the flow behaviour when flow exits the C-D Nozzle and enters atmosphere.
Mostly flow tends to attain high velocity at exit of C-D Nozzle, Mach, 1.3>M<5 (in case of Supersonic flow).
As Conway suggested,
The exit area to be increased - To capture flow behaviour at atmosphere, also
The CV (Control Volume/ Control Domain) has to be extended in exit region of Nozzle
The Mesh settings has to be of very high refinement, inorder to capture High speed flows (like Mach > 1)