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

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 transversely injected liquid jets, which are a popular mixing strategy for high-Reynolds 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 to Mach 0.7) and an expanding Mach 2 gaseous crossflow with plain-orifice injectors of 0.5, 1, and 2 mm diameter. The subsonic test section is rectangular with a 50 cm length and a 23 × 17 cm cross section; a contoured insert is installed in this rectangular test section to generate the supersonic flow path with a 2.54 × 23 cm throat area. 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 4,000 and 52,000. A high-speed camera is used with backlit, shadowgraph, and schlieren imaging systems to collect transmittance contours of the spray at 50 kHz. Unique ‘dagger’ and ‘hump’ shaped spray features are identified and correlated with the shear breakup regime of turbulent and semi-turbulent liquid jets and the catastrophic breakup regime. The same features are identified as ‘advection modes’ with the POD along with a ‘breathing mode’ corresponding to a low frequency oscillation of the mean penetration, 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 that places the three breakup regimes in specific groupings of Weber number and momentum flux ratio.