The propelling nozzle directs the exhaust gas aft and regulates the gas stream flow. It must be designed to obtain the correct balance of pressure and velocity to prevent flow separation. That’s why most of the times it is converging, so that it can induce a decrease in pressure and temperature, and an increase in exhaust gases velocity (kinetic energy) resulting in a higher thrust. The acceleration of the flow by the propelling nozzle can somehow disturb the airflow, so it is positioned at a distance from the jet pipe where the disturbances won’t cause a flow separation.