Coherent structures in a parallel planar gas–liquid mixing layer

In this work, we thoroughly examine the main dynamics of a two-phase mixing layer formed by the interaction of two parallel planar jets. Using data-driven modal decomposition techniques applied to a direct numerical simulation database, the coherent structures governing the flow dynamics are identified. This study provides a novel and comprehensive characterization of the associated phenomena to the best of the authors' knowledge. The studied problem exhibits two main dynamics: interfacial wave propagation and flapping events associated with the emergence of small-scale interfacial structures linked to atomization. These two dynamics have been individually isolated and characterized. The coherent structures that govern the flow's dynamics are identified by means of higher order dynamic mode decomposition and spatiotemporal Koopman decomposition. Two characteristic modes of this problem dominate the dynamics of the system. The propagation of the interfacial wave is characterized by a wave packet, while flapping events result from the interaction of structures which move in the spanwise direction and others which are spanwise-stationary. Additionally, the analysis reveals the presence of other modes, arising from the nonlinear interaction between these two fundamental dynamics. The identification and characterization of these modes deepen our understanding of the underlying physics and also offer potential avenues for flow control.