The transition to aeration in two-phase mixing in stirred vessels

We consider the mixing of a viscous fluid by the rotation of a pitched blade turbine inside an open, cylindrical tank, with air as the lighter fluid above. To examine the flow and interfacial dynamics, we utilise a highly-parallelised implementation of a hybrid front-tracking/level-set method that employs a domain-decomposition parallelisation strategy. Our numerical technique is designed to capture faithfully complex interfacial deformation, and changes of topology, including interface rupture and dispersed phase coalescence. As shown via transient, three-dimensional direct numerical simulations, the impeller induces the formation of primary vortices that arise in many idealised rotating flows as well as several secondary vortical structures resembling Kelvin-Helmholtz, vortex breakdown, blade tip vortices, and end-wall corner vortices. As the rotation rate increases, a transition to `aeration' is observed when the interface reaches the rotating blades leading to the entrainment of air bubbles into the viscous fluid and the creation of a bubbly, rotating, free surface flow. The mechanisms underlying the aeration transition are probed as are the routes leading to it, which are shown to exhibit a strong dependence on flow history.Comment: 14 pages, 9 figure

Dynamics of retracting surfactant-laden ligaments at intermediateOhnesorge number

International audienceThe dynamics of ligaments retracting under the action of surface tension occurs in a multitude of natural and industrial applications, such as inkjet printing and atomisation. We perform fully three-dimensional, two-phase direct numerical simulations of the retraction dynamics with soluble surfactants. A full parametric study is performed using a hybrid interface-tracking/level-set method, which is utilised to treat the interface; this method is capable of capturing faithfully the topological transitions that are a feature of the flow over a certain range of ligament aspect ratios and Ohnesorge numbers. Our results demonstrate the delicate interplay between capillarity, modulated by the presence of surfactants, surfactant-induced Marangoni stresses, inertial and viscous effects. Particular attention is paid to the formation of vortices, which accompany the retraction process, and the influence of surfactant on the vortex dynamics