Aerodynamic breakup of drops at moderate Weber numbers

Aerobreakup of drops is essential to many industry applications such as fuel injection. Due to?the highly complex nature and the wide range of spatial scales involved, both experimental and numerical approaches have their own limitations. In this study, we have investigated the deformation of a water droplet in a uniform air stream through direct numerical simulation. Particular attention is paid at the regime of moderate Weber numbers, for which the drop deforms to a forward bag before breakup eventually occurs.?The open-source?Basilisk solver has been used for simulations. The sharp interface is resolved using a?mass-momentum consistent VOF method on an adaptive octree mesh. When the drop is suddenly exposed to the gas stream,?the drop first deforms to a disk and then an inflating bag with a peripheral rim. The Rayleigh-Taylor instability (RTI) plays a critical role in the bag formation and growth. The competition between baroclinic torque and the capillary retraction from the periphery rim determine whether the bag will be atomized. Since RTI?is closely associated with the drag on the drop and the resulting acceleration of the interface, it is important to accurately resolve the flow around the drop, such as the turbulent wake, so that the unsteady drag and the drop deformation. As the bag undergoes inflation, the liquid sheet thickness decreases over time rapidly. The high-fidelity simulation results showed that rupture of the bag is initiated through hole nucleation which gradually expands and gets collected into a rim which surrounds the hole. The hole-rim is unstable due to another RTI when the hole on the curved sheet expands, forming fingers on the rim from which small drops are detached.