Experimental analysis of primary breakup of an ethanol jet using time-resolved ballistic imaging with wavelet-based optical flow velocimetry

The fluid mechanic forces that govern the breakup of liquid jets and sprays have long been of research interest, with recently developed experimental imaging methods and analysis techniques showing promise to provide velocity information in optically dense regions of the spray. This work uses three sequential ballistic images together with wavelet-based optical flow velocimetry (wOFV) to provide high resolution velocity and acceleration fields near to and on the liquid-gas interface (LGI) of an atomizing jet. These measurements were performed on an ethanol jet issuing from a 3 mm circular nozzle into quiescent air, with results presented at a series of distances from the nozzle for a constant injection pressure. The ballistic images reveal the formation of ligaments and primary drops within 3 diameters of the nozzle exit plane, with breakup around the LGI continuing farther downstream to form a highly complex spray structure. wOFV resolves the velocity and acceleration fields of the deforming liquid core and nearby drops, with a conservative estimate of velocity uncertainty relative to the bulk flow of ∼6.7%. Velocity and acceleration fields are analyzed to reveal distinct fluid mechanic behavior associated with the liquid core, ligaments, and drops as the jet evolves downstream. Analyses and images reveal the fluid mechanics associated with isolated breakup events, as well as the complexity associated with multiple events including ligament/drop collision, turbulent breakup, and aerodynamic drag that occur within the denser regions of the spray. Novelty and significance statement A combination of time-resolved ballistic imaging and wavelet-based optical flow velocimetry (wOFV) is used to provide high-resolution velocity and acceleration fields of liquid parcels associated with primary atomization of an ethanol jet. While previous work has reported velocity distributions associated with primary breakup, few works have resolved acceleration fields. Previous findings have been limited to cross-correlation methods with limited vector resolution. wOFV provides dense vector fields, enabling identification of coherent flow patterns associated with breakup of ligaments and primary drops. Measurements are applied to optically dense regions of a spray, where fluid mechanics have not been resolved experimentally.