Asymmetry-regulated atomization in laminar impinging microjets: Aerodynamic and upper-sheet-perforation-induced waves

This study experimentally examines the breakup dynamics and droplet statistics of laminar impinging microjets with asymmetric pre-impingement lengths. Dual-view high-speed imaging, combined with proper orthogonal decomposition (POD) and droplet size measurements, is applied over a wide range of jet Weber numbers (We_j). Two distinct atomization modes are identified. Under low-asymmetry conditions, breakup is governed by the aerodynamic-wave mode, in which aerodynamic waves developing at the trailing-edge drive ligament formation and droplet shedding. Under high-asymmetry conditions, a new upper-sheet-perforation-induced mode emerges, characterized by periodic perforations in the upper sheet and rim–jet interactions that establish a self-sustained breakup cycle and periodic atomization. Breakup length increases with We_j, but the scaling differs: nonlinear under low asymmetry and nearly linear under high asymmetry. POD analysis confirms the separation of modal responses and shows that geometric asymmetry intensifies oscillations and accelerates sheet destabilization. Droplet sizes follow lognormal distributions in all cases. For low asymmetry, the distribution width is largely insensitive to We_j and the median droplet size remains nearly constant. For high asymmetry, the distribution width decreases as We_j increases, while the median size changes little, resulting in narrower spectra and more uniform sprays. These findings clarify the distinct modal pathways of asymmetry-regulated atomization and provide guidance for the design and optimization of impinging-jet atomizers.