EXPERIMENTAL AND NUMERICAL INVESTIGATION ON SEPARATION CHARACTERISTICS OF SERPENTINE CONVERGENT-DIVERGENT NOZZLE

The serpentine convergent-divergent nozzle combines the high stealth and wide speed range characteristics required by future warplanes. However, it faces the same problems of flow separation and lateral load under over-expanded state as the axisymmetric convergent-divergent nozzle. Due to the special curved configuration and round-to-square cross-section design of the serpentine nozzle, there are complex pressure gradients and strong swirling flow characteristics inside the nozzle affecting the flow separation characteristics under over-expanded state. This paper constructs a design Mach number 2 serpentine convergent-divergent nozzle model, and carries out experimental and numerical investigation on the serpentine convergent-divergent nozzle with a nozzle pressure ratio(NPR) ranging from 1.4 to 3.0, with a view to obtaining the flow separation mechanism. The flow separation characteristics of the serpentine convergent-divergent nozzle were obtained experimentally using a schlieren system, PSI electronic pressure scanning valves and a six-component balance system. The internal flow within the serpentine convergent section is complex, causing the jet at the entrance of the divergent section to exhibit a vertically asymmetric and laterally non-uniform distribution. This leads to the jet in the divergent section deflecting upwards at low NPRs, and there are differences in the flow separation structures in different vertical planes within the divergent section. When a double "λ" shock structure appears, layered flow is observed. As the NPR increases, the flow separation structure transitions from an asymmetric upper restricted shock separation(RSS)-lower free shock separation(FSS) to an asymmetric FSS-FSS structure, and finally to a symmetric FSS-FSS structure, with the jet deflection disappearing. The separation shock within the nozzle exhibits a three-dimensional pattern, with double "λ" shock structures present in both vertical and horizontal planes. As the NPR increases, the thrust vector angle first increases and then decreases, while the axial thrust coefficient exhibits the opposite trend.