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• This study presents the findings of a liquid jet in crossflow (LJIC) experiment conducted under varying flow conditions across different breakup regimes, including bag, multimode, and shear breakup. Both conventional shadowgraphy and Optical Connectivity (OC) techniques were employed to investigate the characteristics of the liquid jet during the primary breakup process. • The OC technique reduced the optical disturbance of the continuous liquid jet by the surrounding droplet clouds present on the shadowgraphic images and allowed the measurement of the disintegration of the LJIC. Observing the liquid jet geometry from two different views offered stereoscopic interfacial information. The OC results from the front view revealed that the liquid jet deforms into a liquid film during ejection, highlighting the significant role of intense surface stripping in shaping the jet cross-section during disintegration, particularly in the multimode and shear breakup regimes. • The dominant wavelengths of the waves on the liquid jet interface were extracted using POD analysis and classified into two distinct types. Wavelength λ m which has a longer value close to the breakup point and can be found under all operating conditions with a roughly constant value. Wavelength λ s which emerges immediately after the nozzle exit with a shorter value. λ s can only be observed for high W e G numbers and its magnitude changes with the corresponding non-dimensional number. Contrary to previous literature, which links surface waves to R-T instability, the generation of surface waves in the current experiment was shown to be associated with a shear-type instability related to the bending of the liquid jet towards the airflow. The surface wave appeared only when the liquid jet begins to deflect towards the crossflow and the aerodynamic shear on the interface becomes sufficiently strong. The local shearing conditions may ultimately determine the measured wavelengths, which differs from values estimated from the theory for Rayleigh-Taylor and Kelvin-Helmholtz instabilities. • A method combining POD-reconstructed images and OFV measurements has been developed to quantify the velocities of different surface structures on the LJIC. It was found that larger, more dominant features, such as interfacial waves with longer wavelengths, exhibit lower velocities on the windward surface in the jet ejection direction compared to smaller-scale structures observed for less energetic modes. Based on these findings, a plausible new mechanism for the breakup process of a liquid jet was proposed, involving the superposition of interfacial waves with varying wavelengths that travel at different velocities. When the crests of these waves overlap, they cause constructive interference leading to a sudden increase in amplitude, forming randomly occurring high-amplitude waves (referred to as ‘rogue waves’) on the liquid jet. This mechanism can explain the random nature of the liquid jet, which may break up during the random formation of a 'rogue wave' due to aerodynamic interactions. The detailed interfacial characteristics of the liquid jet in crossflow (LJIC) are studied using Optical Connectivity (OC) across a range of gas Weber numbers (14.9 to 112.6) and liquid-to-gas momentum ratios (2.1 to 36.4). The liquid jet undergoes primary breakup in three regimes: bag, multimode and shear breakup, depending on the flow parameters. OC enables the visualisation of interfacial features of LJIC from two different viewing angles, trying to reconstruct the instantaneous three-dimensional characteristics of the liquid jet surface during breakup. The strengths of the OC technique compared to conventional shadowgraphy allow more accurate characterisation of the breakup parameters and capture informative images of the cross-sectional liquid jet view, leading to a better understanding of the breakup process of LJIC. Additionally, the dynamic features of the liquid jet, including the oscillating motion and windward convective interfacial waves, are extracted using Proper Orthogonal Decomposition (POD). From the POD modes, the dominant wavelength of the interfacial instability waves varies under different flow conditions and may increase with distance from the nozzle exit. The interfacial waves are believed to be dominated by a shear-type instability associated with the liquid jet trajectory. Meanwhile, Optical Flow Velocimetry (OFV) is used to quantify the interfacial motion of the LJIC from the OC images reconstructed by selected POD modes. In this way, the velocities of the interfacial structures identified by the selected POD modes are measured. It was found that interfacial structures with different wavelengths exhibit different propagation velocities along the interface, which allow them to form constructive interference within the breakup length and generate randomly high amplitude wave structures. The formation of such structures suggests a new non-linear ‘rogue wave’ type mechanism that explains the random nature of the primary breakup of a LJIC and the deviation of the measured breakup characteristics from the classical Kelvin-Helmholtz and Rayleigh-Taylor instabilities.
Wang et al. (Tue,) studied this question.