Aerobreakup of Newtonian and Viscoelastic Liquids

Abstract

In this review, we consider and unify all aspects of the dynamics of Newtonian and viscoelastic liquid drops in high-speed gas flows, including shock waves. The path to understanding is opened by novel, laser-induced fluorescence visualizations at spatial resolutions of up to 200 pixels for millimeter and exposure times as low as 5 ns. The central role of the competition between Rayleigh-Taylor and Kelvin-Helmholtz instabilities is assessed in the frame of rich aerodynamics, from low subsonic to supersonic, and the multitude of characteristic length scales and timescales at play with varying liquid properties. Acceleration and liquid redistribution (drop deformation) early in the evolution set the stage for this competition, and we insist on an interpretation of the drag coefficient that is physically meaningful. Two principal breakup regimes (patterns of bodily loss of coherence) are identified depending on whether the gas finds its way through the liquid mass, causing gross disintegration, or goes around to induce, through shear, a surface-layer peeling-and-ejection action. Corresponding criticalities are quantified in terms of key physics, consistent with experiments. This covers in a unified fashion all liquids, independent of viscosity and elasticity, and the potential role of direct numerical simulations in supporting further advances is forecast. The resulting particle-size distributions (in a final equilibrium cloud) depend crucially on the pattern of breakup, although in this respect the role of elasticity obtains a special significance in terms of the underlying entangled-polymer-chain dynamics. From a more general perspective, we explain the canonical significance of this fundamental problem and summarize the wide range of its practical relevance, including the recently renewed interest in predicting shock-induced fluidization (or high-speed, atmospheric dissemination) of large masses of liquid agents (so-called weapons of mass destruction).