We extend the work of Theofanous and Li [“On the physics of aerobreakup,” Phys. Fluids 20, 052103 (2008)] on aerobreakup physics of water-like, low viscosity liquid drops, to Newtonian liquids of any viscosity. The scope includes the full range of aerodynamics from near incompressible to high Mach number flows. The key physics of Rayleigh–Taylor piercing (RTP, first criticality) and of shear-induced entrainment (SIE, second and terminal criticality) are verified and quantified by new viscosity- and capillarity-based scalings for fluids of any viscosity. The relevance and predictive power of linear stability analysis of the Rayleigh–Taylor and Kelvin–Helmholtz problems (both including viscosity) is demonstrated for the RTP and the SIE regimes, respectively. The advanced stages of breakup and of the resulting particle-clouds are observed and clear definition and quantification of breakup times are offered.

We extend the work of Theofanous and Li [Phys. Fluids 20, 052103 (2008)] on aerobreakup physics of water-like, low viscosity liquid drops, and of Theofanous et al. [Phys. Fluids 24, 022104 (2012)] for Newtonian liquids of any viscosity, to polymerthickened liquids over wide ranges of viscoelasticity. The scope includes the full range of aerodynamics from near incompressible to supersonic flows and visualizations are recorded with μs/μm resolutions. The key physics of Rayleigh-Taylor piercing (RTP, first criticality) and of Shear-Induced Entrainment (SIE, second criticality) are verified and quantified on the same scaling approach as in our previous work, but with modifications due to the shear-thinning and elastic nature of these liquids. The same holds for the onset of surface waves by Kelvin-Helmholtz instability, which is a key attribute of the second criticality. However, in the present case, even at conditions well-past the first criticality, there is no breakup (particulation) to be found; instead the apparently unstable (extensively stretched into sheets) drops rebound elastically to reconstitute an integral mass. Such a resistance to breakup is found also past the second criticality, now with extensive filament formation that maintain a significant degree of cohesiveness, until the gas-dynamic pressure is high enough to cause filament ruptures. Thereby we define the onset of a third criticality peculiar to viscoelastic liquids-SIER, for SIE with ruptures. Past this criticality the extent of particulation increases and the characteristic dimension of fragments generated decreases in a more or less continuous fashion with increasing dynamic pressure. We outline a rheology-based scaling approach for these elasticity-modulated phenomena and suggest a path to similitude (with polymer and solvent variations) in terms of a critical rupture stress that can be measured independently. The advanced stages of breakup and resulting particle clouds are observed and a clear definition and quantification of breakup time is offered. C 2013 American Institute of Physics. Theofanous, Mitkin, and Ng Phys. Fluids 25, 032101 (2013) informs measures and concepts of operation under such an attack, which is a principal motivator of the present work. Notable early work is due to Wilcox et al. 7 and Matta et al. 8 As the solvents utilized in these efforts suggest, 6 in both cases interest derived from the subject of targeted atmospheric dissemination (high-speed delivery of Newtonian liquids yields fine mist which tends to remain airborne, evaporates significantly, and likely fails to impact the target). Wilcox dissolved poly-isobutyl methacrylate (PIBMA), polyvinyl acetate (PVA), or nitrocellulose (NC) in bis (2-ethyl hexyl) hydrogen phosphide (BIS) or di-butyl phthalate (DBP). Matta worked with poly-methyl methacrylate (PMMA) in diethyl malonate (DEM) as a solvent. Working with a shock tube Wilcox documented the essential role of viscoelasticity: "retardation" or "inhibition" of breakup at "concentrations as low as 0.1%. . . ...

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An experimental and computational investigation of the primary breakup of nonturbulent and turbulent round liquid jets in gas crossflow is described. Pulsed shadowgraph and holograph observations of jet primary breakup regimes, conditions for the onset of breakup, properties of waves observed along the liquid surface, drop size and velocity properties resulting from breakup and conditions required for the breakup of the liquid column as a whole, were obtained for air crossflows at normal temperature and pressure. The test range included crossflow Weber numbers of 0-2000, liquid/gas momentum ratios of 100-8000, liquid/gas density ratios of 683-1021, Ohnesorge numbers of 0.003-0.12, jet Reynolds numbers of 300-300,000. The results suggest qualitative similarities between the primary breakup of nonturbulent round liquid jets in crossflows and the secondary breakup of drops subjected to shock wave disturbances with relatively little effect of the liquid/gas momentum ratio on breakup properties over the present test range. The breakup of turbulent liquid jets was influenced by a new dimensionless number in terms of liquid/gas momentum ratio and the jet Weber number. Effects of liquid viscosity were small for present observations where Ohnesorge numbers were less than 0.4. Phenomenological analyses were successful for helping to interpret and correlate the measurements.

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A computational study of the deformation and surface wave properties of nonturbulent round liquid jets in gaseous crossflow is described. The objective of the study was to consider effects of liquid viscosity, liquid/gas density ratio, and crossflow Weber number that are representative of practical sprays. Three-dimensional computations of the deformation of round liquid jets in gaseous crossflow were carried out using FLUENT software utilizing its Volume of Fluid (VOF) module. The computations were evaluated satisfactorily based on earlier measurements of the properties of nonturbulent round liquid jets in crossflow (liquid jet deformation and surface waves) and revealed three-dimensional properties of the surface waves that could not be observed by previous measurements that were taken from the side of the jet.

The deformation of laminar liquid jets in gaseous crossflow before the onset of primary breakup is studied motivated by its application to fuel injection in jet afterburners and agricultural sprays, among others. Three crossflow Weber numbers that represent three different liquid jet breakup regimes; column, bag, and shear breakup regimes, were studied at large liquid/gas density ratios and small Ohnesorge numbers. In each case the liquid jet was simulated from the jet exit and ended before the location where the experimental data indicated the onset of breakup. The results show that in column and bag breakup, the reduced pressures along the sides of the jet cause the liquid to move to the sides of the jet and enhance the jet deformation. In shear breakup, the flattened upwind surface pushes the liquid towards the two sides of the jet and causing the gaseous crossflow to separate near the edges of the liquid jet thus preventing further deformation before the onset of breakup. It was also found out that in shear breakup regime, the liquid phase velocity inside the liquid jet was large enough to cause onset of ligament formation along the jet side, which was not the case in the column and bag breakup regimes. In bag breakup, downwind surface waves were observed to grow along the sides of the liquid jet triggered a complimentary experimental study that confirmed the existence of those waves for the first time.