Gas jets formed by rectangular nozzles submerged in water were studied using a non-invasive photographic technique which allowed simultaneous measurements of the entire interface. Three aspect ratios were considered corresponding to 2, 10, and 20 with all nozzles sharing a common width. As far as the authors know this study represents the first time the effects of aspect ratio and Mach number on a submerged gas jet have been studied. The results indicate aspect ratio and Mach number play a large role in dictating both the unsteadiness of the interface and the penetration of the gas jet into the surrounding liquid medium. The jet pinch-off is shown to have a logarithmic decay with increasing Mach number and when appropriately scaled by the total viewing length and a geometric length scale (LQ) is relatively constant across all aspect ratio nozzles. The location of pinch-off is also a function of aspect ratio, with the subsonic aspect ratio 2 nozzles showing maximum pinch-off at y/LQ ≈ 23–26 while sonic and supersonic Mach numbers have peaks over the range y/LQ ≈ 11–14. The AR 10 and 20 nozzles show no dependence on Mach number with the maximum number of pinch-off events observed over the interval y/LQ ≈ 3–5. Jet spreading which is indicative of liquid entrainment is also shown to increase with Mach number and aspect ratio. The jet penetration also increases with increasing Mach number and aspect ratio. The spatial instability growth rate was deduced from the downstream evolution of the interfacial unsteadiness and it is shown that the nozzle with aspect ratio of 2 follows a different trend than the aspect ratio 10 and 20 nozzles, suggesting a fundamentally different mechanism dominates the stability of large aspect ratio rectangular gas jets.

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