Structure and Dynamics of Impinging Supersonic Jets via Schlieren Flow Visualization and Modal Analysis

This study examines the unsteady structure of impinging supersonic jets using high-resolution Schlieren flow visualization combined with proper orthogonal decomposition (POD) analysis. Two converging-diverging nozzle configurations were tested to study the effect of nozzle geometry and relative standoff distance (Z/D = 1–3) on jet behavior. Experiments were conducted over a wide range of nozzle pressure ratios (NPR = 3-19), covering regimes from slightly overexpanded to highly underexpanded jets. Results reveal clear transitions in shock-cell structures, Mach disk positioning, and shear-layer development with increasing NPR. For both nozzles, increased Z/D resulted in longer, more stable shock-cell systems, whereas closer standoff distances promoted stronger shock-surface interactions and more complex reflection patterns. POD analysis showed that a few dominant coherent modes capture the primary flow dynamics, with mode energy and spatial distribution varying systematically with NPR and Z/D. Notably, the larger nozzle exhibits stronger shock-vortex interactions and higher modal energy in the near field, indicating intensified impingement unsteadiness.