The serpentine nozzle demonstrates significant application potential for advanced military Unmanned Aerial Vehicles (UAVs) due to its low-observable stealth capabilities. Obtaining the high-resolution flow field data and elucidating the complex fluid dynamic mechanisms of serpentine nozzle jets are prerequisites for conducting stealth optimization design. This study employs synchronized multi-view Background-Oriented Schlieren Tomography (BOST) to investigate three-dimensional flow features of under-expanded jets from a convergent serpentine nozzle with an Aspect Ratio (AR) of 2 under Nozzle Pressure Ratios (NPR) of 3, 4, and 5. Results demonstrate strong agreement between BOST reconstructions and both schlieren imaging (for shock structure localization) and numerical simulations (with a 5.1 % average relative error along the jet centerline). Quantitative three-dimensional density fields and shock structures reveal non-axisymmetric flow evolution, characterized by pocket-shaped low-density regions originating from the nozzle exit corners and incident shock reflections propagating in the major axis plane. The serpentine configuration induces wall and corner vortices at the exit, enhancing transverse jet-ambient mixing and producing strong density gradients in the minor axis plane. As the NPR increases, the shock cell spacing expands while the major axis dominance induced by nozzle exit geometric constraints diminishes, ultimately triggering axis switching at x/D_e (equivalent diameter) = 3.66 for NPR = 5. The asymmetric shock development between major/minor axes further highlights geometry-driven flow instabilities. These findings provide critical insights into the hydrodynamic mechanisms of serpentine nozzle jets, directly supporting the optimization of the next-generation low-observable propulsion system.

The high-efficiency Shock Vectoring Control Serpentine Nozzle (SVCSN) takes into account both thrust vectoring and infrared stealth, and significantly improves the comprehensive performance of the aero-engines through an additional auxiliary duct. In this paper, the schlieren photographs at the exit of the high-efficiency SVCSN and the wall static pressure distributions were obtained by experiments, and the numerical results were used to enrich the thrust vectoring characteristics. The effects of the auxiliary injection were analyzed first to reveal the advantages of the high-efficiency SVCSN compared to the conventional SVCSN. Then, the aerodynamic parameters and the structural parameters of the high-efficiency SVCSN were investigated, including the Nozzle Pressure Ratio (NPR), the Secondary flow Pressure Ratio (SPR), the secondary flow relative area and the secondary flow injection angle. Finally, the coupling performance of the high-efficiency SVCSN is studied by using the approximate modeling technology. Results show that the auxiliary injection increases the range between the two shock legs of the “λ” shock wave induced by the secondary flow, then causes the separation zone and high-pressure boss of the down wall to expand upstream, and finally results in a prominent increase in the thrust vectoring performance. The thrust vectoring angle and Vectoring Efficiency (VE) of the high-efficiency SVCSN are about 61.6% and 75.7%, respectively, higher than those of the conventional SVCSN at NPR = 6. The effects of the NPR and the SPR on the thrust vectoring performance of the high-efficiency SVCSN are coupled with each other. A larger NPR matched with a smaller SPR shows better thrust vectoring performance. The maximum fluctuations in thrust vectoring angle and VE caused by the NPR and SPR are about 22% and 64%. The VE decreases monotonously with the increase of the secondary flow relative area. Smaller secondary flow injection angle shows better thrust vector performance, and the thrust vectoring angle and VE of the secondary flow injection angle of 90° are about 20% higher than those of the secondary flow injection angle of 110° at NPR = 6. Therefore, the secondary flow relative area of 0.06 and the secondary flow injection angle of 90° are recommended.

The aerodynamic profile design of multi-dimensional deflected serpentine nozzles with high aerodynamic performance and complex configurations is one of the crucial technologies for stealth aircraft with integrated wing-body layouts. By improving the Lee curve method and multiparametric coupled method, we propose a design method for multi-dimensional deflected serpentine nozzles with an abnormal exit, and realize the multi-dimensional deflected serpentine centerline design with vector angles and the cross-sectional transition from the circular inlet to the abnormal exit. This method can be used for the aerodynamic profile design of multiple serpentine nozzles with conventional and space-confined layouts, such as multi-dimensional deflected serpentine nozzles, serpentine nozzles with vector angles, serpentine nozzles with unsymmetrical abnormal exits, and serpentine nozzles for single and twin aeroengine layouts. The proposed design method is verified by the numerical simulation method and the small turbojet engine test. The results show that the discharge and thrust coefficients of the ultra-compact serpentine nozzle designed with the space-confined layout reach 0.982 and 0.989, respectively, under the critical condition, and are no smaller than 0.984 and 0.992, respectively, under the overcritical condition. Those of the multi-dimensional deflected serpentine nozzle designed with the space-confined layout reach 0.980 and 0.986, respectively, under the critical condition and are no smaller than 0.981 and 0.990, respectively, under the overcritical condition. The total thrust of engine after installing the juxtaposition serpentine nozzles designed with the twin engine layout is no more than 4.58% lower than that with the original nozzle. The designed serpentine nozzles have better aerodynamic performance.