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Abstract
The multiscale dynamics of a shock–droplet interaction is crucial in understanding the atomisation of droplets due to external airflow. The interaction phenomena are classified into wave dynamics (stage I) and droplet breakup dynamics (stage II). Stage I involves the formation of different wave structures after an incident shock impacts the droplet surface. These waves momentarily change the droplet's ambient conditions, while in later times they are mainly influenced by shock-induced airflow. Stage II involves induced airflow interaction with the droplet that leads to its deformation and breakup. Primarily, two modes of droplet breakup, i.e. shear-induced entrainment and Rayleigh–Taylor piercing (RTP) (based on the modes of surface instabilities) were observed for the studied range of Weber numbers $(We\sim 30\text{--}15\,000)$ . A criterion for the transition between two breakup modes is obtained, which successfully explains the observation of RTP mode of droplet breakup at high Weber numbers $(We\sim 800)$ . For $We > 1000$ , the breakup dynamics is governed by the shear-induced surface waves. After formation, the Kelvin–Helmholtz waves travel on the droplet surface and merge to form a liquid sheet near the droplet equator. Henceforth, the liquid sheet undergoes breakup processes via nucleation of several holes. The breakup process is recurrent until the complete droplet disintegrates or external drag acting on the droplet is insufficient for further disintegration. At lower Weber numbers, the droplet undergoes complete deformation like a flattened disk, and a multibag mode of breakup based on RTP is observed.
Highlights
The atomisation of liquid droplets by aerodynamic forces is of great importance in many engineering and industrial applications
The present study addresses the full interaction dynamics of a liquid droplet interacting with a shock wave of varying Mach numbers ( Ms ∼ 1.1 to 1.8)
The flow dynamics could not be obtained experimentally due to the requirement of a high-resolution and high-frame-rate particle image velocimetry system, which necessitates the usage of a high-power pulse-burst laser system (Beresh et al 2015)
Summary
The atomisation of liquid droplets by aerodynamic forces is of great importance in many engineering and industrial applications. Kelvin–Helmholtz (KH) waves are augmented by the action of shear on the liquid–air interface This mechanism of droplet breakup at high Weber number (We > 1000) is discussed in many works, an actual attempt to compare the formed KHI waves with the theoretical analysis is still limited (Marmottant & Villermaux 2004; Kim et al 2006; Theofanous et al 2012; Jalaal & Mehravaran 2014) and most of which are primarily numerical (Kim et al 2006; Jalaal & Mehravaran 2014) or seminumerical (Theofanous et al 2012), while the experimental works are related to liquid–air interaction in a coaxial jet system (Marmottant & Villermaux 2004). At higher Weber numbers (We > 1000) observations are primarily made through numerical simulations (Jalaal & Mehravaran 2014; Guan et al 2018; Liu et al 2018; Biasiori-Poulanges & El-Rabii 2019; Dorschner et al 2020) Most of these works lack the accurate prediction of the atomisation stage, requiring high spatiotemporal resolutions.
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