Instability modes of millimeter-scale supersonic jets

The appearance of instability modes on subsonic and supersonic impinging jets is a phenomenon of great interest that has been observed in jets produced by nozzles of different sizes. The present study describes these instabilities in millimeter-sized supersonic jets produced by convergent and convergent–divergent nozzles (\(D_\mathrm{throat} = 2 \hbox { mm}\)). The smaller nozzle sizes allowed for a comprehensive parametric study involving 378 impingement distance values between \(H{/}D=0.5\) and \(H{/}D=9.9\), as well as 30 nozzle pressure ratio values for both nozzles, totaling a set of more than 22,000 experiments. The detailed resolution on the impingement distance parameter allowed for the observation of acoustic resonant tones that alternate between two or three distinct frequency bands. It was observed that very small changes in the impingement distance (\(\Delta H{/}D \le 0.5\)) are sufficient for resonant mode switching, which is related to the instability mode switching phenomenon. The complementary high-speed schlieren images, captured for a set of 36 cases, allowed for the effective observation and classification of the jet resonant modes via feature tracking. A correlation between the resonant mode shape (helical or axisymmetric) and the resonant tone was clearly observed and quantified. Further experiments with high-resolution schlieren images showed the shape of the acoustic waves produced, connecting the acoustic wave packets with the production of shear layer instabilities. In the helical instability mode, it was possible to observe what appears to be a helix-shaped acoustic wave.