Numerical representation of film rupture in bag breakup

A droplet in a relatively gentle airstream may undergo bag-breakup. There are two components to the breakup: a thin film rupture (the bag), and the subsequent disintegration of the remaining rim. In numerical simulations, it is especially challenging to capture the thin film fragmentation, which yields the finest fragments, because of how the rupture process is represented by the numerical mesh. This problem is indeed more general than bag-breakup, as it appears in many different types of fragmentation. In particular it is difficult to be certain of the physicality of numerical representations of the process. Here, we propose an approach to address this problem from three angles. First, film rupture is initiated with a controlled perforation technique, as reported in previous work. Second, the numerical parameters which control this perforation are linked to the physical problem by interpreting them in terms of a perforation efficiency. Third, given the large separation of time scales governing bag-breakup, a specialised adaptive-mesh-refinement solution is adopted to keep the problem computationally tractable. Some indicative results are presented, and we assess the remaining challenges in numerical representations of thin-film breakup.