Flow Characteristics of Impinging Supersonic Oscillating Jets by Schlieren Visualization

Double-feedback fluidic oscillators offer a passive, no-moving-part method to generate self-sustained oscillating jets, making them attractive for robust flow control in harsh environments. While their behavior in subsonic regimes is well documented, understanding their operation under supersonic conditions, especially during surface impingement, remains limited. This study experimentally investigates supersonic oscillating jets produced by a double-feedback fluidic oscillator across nozzle pressure ratios (NPR) ranging from 6 to 14. A high-pressure air supply drives the flow, and jet dynamics are captured via high-speed Schlieren imaging at up to 12,800 fps using a collimated LED source, parabolic mirrors, and a knife edge. Key flow features, including shock evolution, jet deflection, and spreading behavior, are characterized. Proper Orthogonal Decomposition (POD) is applied to Schlieren sequences to extract dominant spatial modes and spectral content. The results reveal complex modal structures associated with Kelvin-Helmholtz instabilities, Mach disks, and recirculating wall jets. This work provides new insights into the physics of supersonic oscillating jets, emphasizing differences between time-averaged and instantaneous fields. These findings contribute to the experimental modeling of high-speed flow control systems and have potential implications for STOVL propulsion and aeroacoustic mitigation strategies.