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“On the Breakup and Transport of Crude Oil by Surface Waves and Subsurface Plumes”
Presented by Professor Joseph Katz
Department of Mechanical Engineering, Johns Hopkins University
Nearly a decade after the Deepwater Horizon massive oil spill, this presentation summaries a series of laboratory studies aimed at characterizing interfacial phenomena affecting the breakup of surface slicks and subsurface plumes of crude oil. After entrainment of slicks by surface waves, the droplet size distributions agree with classical turbulence-based scaling, and their subsequent temporal evolution can be modeled by combining effects of turbulent diffusion and buoyant rise. Pre-mixing the crude oil with dispersant, which drastically reduces the oil-water interfacial tension, causes tip streaming, an interfacial phenomenon that decreases the droplet sizes to the micron range. Aerosolization of oil is caused by the initial splash and by subsequent bubble bursting. Premixing the oil with dispersant increases the concentration of airborne nano-droplets by one to two orders of magnitude, raising health concern. In contrast, the dispersant causes a reduction in concentration of volatile organic compounds. Extended mixing of oil with seawater generates poly-dispersed water-in-oil emulsions with two-orders-of-magnitude higher viscosity. Dispersant partially separates the water, but the viscosity of the remaining sparser emulsion is still higher than that of the original oil. Micro emulsions also form as oil droplets rises and cross an oil-water interface. These droplets do not mix with the bulk oil since they remain coated by submicron water films that persist long after crossing. These films eventually break up owing to droplet deformation induced by electrostatic forces. Below the surface, fragmentation of a vertical buoyant oil jet is elucidated by refractive index matching. Compound oil droplets containing water droplets, some with smaller oil droplets, form regularly. Their fraction increases with droplet diameter, reaching 78% for 2mm droplets. While the exterior surfaces of the oil droplets are deformed by the high shear field, the interior interfaces remain spherical, indicating quiescent domains. In the presence of cross flow, entrainment of small droplets into the core of the counter-rotating vortex pair defines the lower boundary of the plume while large droplets escape and define the upper boundary. Hence, reduction of droplet sizes by dispersant increases the fraction of oil entrained into the vortex pair and lowers the upper boundary of the plume.
Joseph Katz received his B.S. degree from Tel Aviv University, and his M.S. and Ph.D. from California Institute of Technology, all in mechanical engineering. He is the William F. Ward Sr. Distinguished Professor of Engineering, and the director and co-founder of the Center for Environmental and Applied Fluid Mechanics at Johns Hopkins University. He is a Member of the National Academy of Engineering, as well as a Fellow of the American Society of Mechanical Engineers (ASME) and the American Physical Society. He has served as the Editor of the Journal of Fluids Engineering, and as the Chair of the board of journal Editors of ASME. He has co-authored more than 380 journal and conference papers. Dr. Katz research extends over a wide range of fields, with a common theme involving experimental fluid mechanics, and development of advanced optical diagnostics techniques for laboratory and field applications. His group has studied laboratory and oceanic boundary layers, flows in turbomachines, flow-structure interactions, swimming behavior of marine plankton in the laboratory and in the ocean, as well as cavitation, bubble, and droplet dynamics, the latter focusing on interfacial phenomena associated with oil spills.