Rainfall and the Deepwater Horizon Oil Spill: A numerical approach to high Weber number drop impacts on liquid pools coated with a thin film of immiscible fluid

Abstract

The dynamics of drop impact and resulting splash is critical to many processes, including soil erosion, rainfall and spray painting. The phenomenon has been investigated thoroughly by researchers, beginning with Arthur Worthington’s 1896 qualitative catalog of droplet impact. Specifically, the splash dynamics of two-phase droplet impact, where a water droplet impacts on a water surface is now considered to be well-understood in the low to mid Weber number regimes, We < 1000. However, the impact of a water droplet onto a surface coated by a film of immiscible liquid not yet been studied numerically in the We > 2000 range. This process occurs during rainfall on surfaces such as oil slicks at sea and lipid rich ocean surfaces, and can often eject oily aerosols into the atmosphere. This project has obtained a simulation that qualitatively agrees with experimental data under the same conditions and regimes for both the two and three phases cases, but however, the dynamics of the sheet of fluid ejected above the surface of the bulk fluid accelerates as the resolution of the simulation is increased, rendering any concrete quantitative results unreliable until grid resolution is achieved. The dynamics of the crown ejected above the surface of the fluid are much more complicated, and the exact quantitative details of the crown remain grid-dependent, potentially due to the high density gradient between the air and water phases and the failure of an axisymmetric simulation to capture all of the details of the splashing event. However, as the level of refinement is increased in the two-phase case, the crown dynamics appear to show preliminary signs of convergence. The effects of the oil thickness, drop shape and immiscible fluid properties on the crown dynamics were also investigated and shown to correspond with experimental data. As droplet shape is varied from horizontally to vertically elongated, the average amount of material ejected above the surface increases, implying that ”wider” drops have the potential to create a greater amount of aerosols than ”taller” drops. Additionally, as the Weber number of the oil layer increases, a narrower, taller crown is formed due to the effects of the decreased surface tension on the momentum transfer of the crown. Finally, the Reynolds number of the system is also shown to affect the atomization of the crown, with drop ejection from the crown not being observed when the viscosity of the simulation is increased by a factor of ten.