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The dual bell nozzle offers a simple and efficient altitude adaption through its contour inflection, which insures symmetrical and controlled separation at sea level and a large area ratio at high altitude. The understanding of the transition from one operating mode to the other is the key to its prediction. Intensive cold flow investigation has led to an empirical criterion for a precise transition prediction. However, the testing conditions may have a great influence on the actual transition. The temperature of the flow and the nozzle wall, especially when driving test series, can shift the nozzle pressure ratio leading to the transition. A test campaign in a high altitude chamber has also shown the dependence of the transition condition with the chamber pressure, i.e. with the density of the separated backflow. For a reliable prediction of the dual-bell nozzle flow behavior, both on test facility and in real flight conditions, all these influences have to be identified and taken into account.

Influence of the Test Environment on the Transition of Dual-Bell Nozzles