A Modal Analysis of Spray Structures From a Transversely Injected Liquid Jet in Subsonic and Supersonic Crossflows

Abstract Flow instabilities in high-Reynolds flows are difficult to characterize because the increasingly chaotic interactions of fluid particles allow for larger variety and superpositions of flow patterns. These are especially underexplored in liquid jets in crossflows (LJIC) that are a popular mixing strategy for high-Weber gaseous flows. This study utilizes proper orthogonal decomposition (POD) to identify periodic flow patterns in experimental spray images. A liquid ethanol jet is injected into both a linear subsonic (Mach 0.25–Mach 0.7) and an expanding Mach 2 gaseous crossflow with plain-orifice injectors of 0.5, 1, and 2 mm diameter. Across both regimes, crossflow mass flows vary from 0.5 to 12 kg/s with injectant momentum flux ratios between 0.3 and 180. Reynolds numbers scaled by the injector diameter varied between 4000 and 52,000. A high-speed camera is used with extinction imaging systems to collect transmittance contours of the spray at 50 kHz. Unique “dagger” and “hump” shaped spray features are identified and correlated with a newly ascribed wave-crest and the well-known catastrophic LJIC breakup modes, respectively. The same features are identified as “advection modes” with the POD, and the mode images provide an accurate representation of the phase-averaged size and shape of these periodic spray structures. A characteristic length is used to normalize the peak POD frequencies and produces a mapping of the wave-crest and catastrophic breakup modes into regimes of Weber number and momentum flux ratio.