Study on the Transient Starting Characteristics of Supersonic Nozzles in Free Jet Tests

This study investigates the transient start-up characteristics of a supersonic nozzle in a free jet. The nozzle profile was designed using the method of characteristics and analyzed through unsteady CFD simulations combined with free jet cold and hot tests. The evolution of the flow field during the nozzle start-up process was systematically revealed. Results show that the start-up process can be divided into two stages: cold pre-start and ignition hot start. The cold pre-start stage consists of four successive phases, namely gas filling in the initial convergence section, downstream migration of the shock wave train within the expansion section, ejection of the core flow from the nozzle exit, and a quasi-stable pressurization phase. The cold pre-start time was measured to be approximately 290 ms. The experimental pressure data were in good agreement with the numerical simulations, confirming the accuracy of the computational approach. This work provides deeper theoretical insights and experimental evidence for the design and application of supersonic nozzles, refines the theory of transient nozzle start-up, and contributes to optimizing nozzle performance and enhancing stability under practical operating conditions.