Multiple long-time solutions and helical wakes with drop spinning for nitrobenzene drops falling in water

We report three-dimensional numerical simulations of freely falling nitrobenzene drops in water, for a range of drop size (2 - 4 mm diameter), covering several flow regimes, from steady, axisymmetric, laminar flows with a wake without flow separation, to transitional flows with highly complex flows that are unsteady, and three-dimensional. Beyond an initial transient, a 4-mm diameter drop falls essentially vertically and "spins" about a vertical axis approximately coinciding with the drop center-of-mass. The spinning motion is accompanied by a helical-like wake. For a 3.8-mm drop, different prescribed initial transients give rise to two long-time solutions: an axisymmetric drop with time-periodic vortex shedding and shape oscillations, and a spinning drop with a helical-like wake, similar to the 4-mm drop. The wake structure is discussed and possible mechanisms for the spinning motion are discussed.