Shock induced aerobreakup of a droplet
Journal of Fluid Mechanics, Vol. 929, 2021
S. Sharma , A.P. Singh, S. Rao S. , A. Kumar , S. Basu
The multiscale dynamics of a shock–droplet interaction is crucial in understanding the atomisation of droplets due to external airflow. The interaction phenomena is 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 ∼ 30–15 000). 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 ∼ 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.
Journal of Fluid Mechanics [Front Cover Article], Vol. 918, 2021
S. Sharma , A. P. Singh and S. Basu
Interaction of droplets with vortical structures is ubiquitous in nature, ranging from raindrops to a gas turbine combustor. In this work, we elucidate the mechanism of co-axial interaction of a droplet with a vortex ring of different circulation strengths (Γ = 45–161 cm2 s−1). We focus on both the droplet and the vortex dynamics, which evolve in a spatio-temporal fashion during different stages of the interaction, as in a two-way coupled system. Vortex rings of varying circulation strengths are generated by injecting a slug of water into a quiescent water-filled chamber. Experimental techniques such as high-speed particle image velocimetry, planner laser-induced fluorescence imaging and high-speed shadowgraphy are used in this work. In the droplet dynamics, different regimes of interaction are identified, including deformation (regime-I), stretching and engulfment (regime-II) and breakup of the droplet (regime-III). Each interaction regime is explained using existing theoretical models that closely match the experimental data. In the vortex dynamics, we compare the interaction’s effect on different characteristics of the vortex rings, such as pressure and the vorticity distribution, circulation strength, total energy and enstrophy variation with time. It is found that the interaction leads to a reduction in these parameters
Science Advances, Vol. 7, N0. 10, 2021
S. Sharma , R. Pinto , A. Saha , S. Chaudhuri , S. Basu
Face masks prevent transmission of infectious respiratory diseases by blocking large droplets and aerosols during exhalation or inhalation. While three-layer masks are generally advised, many commonly available or makeshift masks contain single or double layers. Using carefully designed experiments involving high-speed imaging along with physics-based analysis, we show that high-momentum, large-sized (>250 micrometer) surrogate cough droplets can penetrate single- or double-layer mask material to a significant extent. The penetrated droplets can atomize into numerous much smaller (<100 micrometer) droplets, which could remain airborne for a significant time. The possibility of secondary atomization of high-momentum cough droplets by hydrodynamic focusing and extrusion through the microscale pores in the fibrous network of the single/double-layer mask material needs to be considered in determining mask efficacy. Three-layer masks can effectively block these droplets and thus could be ubiquitously used as a key tool against COVID-19 or similar respiratory diseases.
Experiments in Fluids, Vol. 64 (4), p. 65, 2023
S. Sharma , S. Rao, N.K. Chandra, A. Kumar , S. Basu, C. Tropea
The two-sensor depth from defocus technique for the measurement of drop sizes in a spray is further developed to achieve higher spatial and temporal resolution, to improve estimates of spatial size distribution and number concentration, and to provide additional guidelines for the calibration and design of the optical system for a specific application. The technique and these improvements are demonstrated using the case of secondary atomization when a shock wave interacts with a single drop. This is an application in which both high spatially and temporally resolved number density and size distributions of secondary droplets generated in the wake of the original drop are necessary.