Computational investigation of non-trivial breakup morphologies observed during critical secondary fragmentation

In our previous work on the threshold of Droplet fragmentation under impulsive acceleration, we discovered that in addition to the trivial backward bag breakup normally observed for water-air analogous droplet systems, other breakup morphologies such as forward bag and backward bag-plume may also be observed. These non-trivial morphologies form at critical Weber numbers only for specific combination of parameters dictating the fragmentation process: density ratio (ρ), and drop (Oh_d) and outside (Oh_o) Ohnesorge numbers. Full 3D Volume of Fluid based direct numerical simulations for two representative cases are performed to capture all the physics pertaining to the two fragmentation morphologies under consideration. Features such as forward pancake formation and flipping of bag orientation for forward bag breakup; generation of plume on the upstream pole and its unstable growth for backward bag-plume breakups; generation of non-axisymmetric instabilities in the inflated bag; and the corresponding internal flows are explored in detail. This allows us to propose physically consistent fragmentation mechanisms for forward and backward bag-plume breakups.