On the dynamics of air disk contraction

Abstract

Contraction of an air disk in liquid was investigated by direct numerical simulations of the Navier–Stokes equations in an axisymmetric framework. Five mechanisms of the first breakup were revealed: End-pinching, center-opening, cross-splitting, vertical splitting and no-breakup, and a parameter space as a function of the Ohnesorge number Oh and the aspect ratio Γ was established. Similar to air film contraction of other geometries, the contraction velocity of the air disk increases from rest to a maximum velocity, then decreases. Theories involving volume conservation, hydrodynamic drag, or added mass were proposed to predict the contraction velocity at different stages. Interaction between complicated morphological changes and vortex dynamics was studied. Generation and shedding of vortices around the bubble were responsible for the complex contraction dynamics.