Analysis And Visualization Of The Perturbations Imposed On The Liquid Film By Crown Sheet Collapse Or Closure
D. Ribeiro (1), M. R. O. Panão (2), J. Barata (1), A. R. R. Silva (1)
(1) AEROG, LAETA, Aeronautics and Astronautics Research Center, University of Beira Interior, Covilhã Portugal
(2) ADAI, LAETA, Associação para o Desenvolvimento da Aerodinâmica Industrial, University of Coimbra, Coimbra, Portugal
Bubble encapsulation is a phenomenon that results from droplet impact on a liquid film for particular impact conditions. After splashing, the crown liquid sheet starts to bend inwards, and eventually, the jets at the top of the crown merge and form a perfect empty bubble. This bubble bursts due to the impingement of a secondary droplet that impacts the spherical dome or the thin dome film reaches its critical thickness. A few authors observed this phenomenon but barely studied it. Thus, this work focuses on understanding its dynamics and formation mechanisms. An experimental imaging study allows observing the phenomenon from two perspectives: 1) the common lateral perspective; 2) and a bottom perspective. The bottom shadowgraphs show the capillary waves and the perturbations imposed by the drop impact on the steady liquid film. Previous works observed that bubble encapsulation systematically occurs (p = 1) for 6263 < ReD < 8358, 4.776 < Oh_f×10^3 < 9.918, 0.4 ≤ δ=hf/D0 ≤ 1, and stochastically (p < 1) beyond the upper limits. One of the goals is to relate this phenomenon with any existent correlation in the literature. Departing from the Vander Wal et al. (2006) correlation for the spreading/splashing transition boundary but rearranging it to include the Ohnesorge number relating viscous forces with inertia and surface tension, this work develops a new criterion for the onset of bubble encapsulation. It uses drop characteristics in the Reynolds number to express the initial impact conditions and the liquid film’s characteristics in the Ohnesorge number because most of the crown material comes from the liquid film, as reported in the literature. The new criterion for the onset of bubble encapsulation is a ∊ [1.022, 1.142] = ln(34.5/ReD)/ln(Oh_f).
Bubble encapsulation events observed by other authors are within the region where the probability of occurrence is p < 0.5. However, despite being outside the validation range of the correlation, the values are close to the boundaries. There is scarce information about bubble encapsulation, but some works focus on the cavity underneath the bubble. We observed the cavity and concluded that the cavity shape is independent of the bubble encapsulation phenomenon. Finally, using the bottom shadowgraphs, the crown closure time measured for a specific range of impact conditions shows a sudden decrease between 0.5 < δ < 0.6. The reason is the subject of future work.