Flow characteristics of supersonic oscillating jet impinging by Schlieren visualization

Flow characteristics of supersonic oscillating jets impinging upon a perpendicular flat plate were experimentally studied. The effects of nozzle-pressure-ratios (NPRs) and nozzle-to-wall distance variations were investigated by Schlieren visualization. The frequency of oscillation and deflection angle of the oscillating jet were compared with the subsonic oscillating jet using a smoke visualization technique. Key variables included NPRs ranging from 6 to 14 and two nozzle-to-wall distances (H = D-h and H = 2D(h)). Flow characteristics were also studied by a proper orthogonal decomposition (POD) method. The results showed the independence of oscillation frequency from NPR while demonstrating significant variations in deflection angles and shock structures with increasing NPR. At higher NPR values, complex flow features such as D-shaped shock cells, secondary shock formations, and prominent expansion fans emerged and were affected by nozzle-to-wall separation. POD analysis categorized flow modes into symmetric and antisymmetric groups, showing multiscale interactions including Kelvin-Helmholtz instabilities, vortex streets, and Mach disk oscillations. In addition, the interplay between large-scale coherent structures and smaller turbulent features was emphasized across POD modes. Schlieren and POD analysis revealed periodic oscillation of the flow structures such as shock cells, stand-off shock wave, and wall jet boundary driven by unsteady flow motion.