Three-dimensional flow features of under-expanded jets from serpentine nozzle using background-oriented schlieren tomography

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.