Jet-induced droplet breakup: Internal penetration and cascade channeling

Abstract

Localized gas-jet impingement produces droplet–deformation pathways that differ fundamentally from classical aerodynamic breakup under approximately uniform crossflow. Experiments and axisymmetric volume-of-fluid simulations are used to investigate millimetric water droplets exposed to gas jets whose diameters are smaller than or comparable with the droplet diameter. For sufficiently localized forcing, the jet produces an internal gas cavity or channel before large-scale disk-like deformation occurs. Three responses are distinguished within this localized-forcing range: nonpenetrating cavity formation, collapse-limited cascade channeling, and continuous single-shot piercing. Cascade channeling results from competition between axial channel-front advance and capillary–inertial closure of the windward opening. A pressure–capillary analysis provides a penetration criterion, while a curvature-induced momentum-turning correction leads to an effective axial Weber number that represents the reduction in penetration-driving momentum as the jet footprint extends over the curved droplet surface. The experimental and numerical results support an approximate crossover centered near Δ=Dj/Dd∼0.2, rather than a universal sharp threshold, above which internal channeling becomes progressively less dominant and the response approaches global, crossflow-like deformation. A two-mass Taylor analogy breakup-type model is developed to represent the relative motion of an effective channel-front mass and the bulk droplet. This reduced-order model captures the penetration threshold and principal trends for part of the localized-channeling range, but its velocity predictions are semi-quantitative and it does not describe three-dimensional fragmentation. The principal contribution is the identification and mechanistic interpretation of internal channeling, including its repeated collapse-and-reopening form, as a geometry-dependent response to localized aerodynamic forcing.