How Do Underwater Oil Plumes Form?





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Published on May 20, 2010

This video shows two experiments that help explain why and how an oil spill on the seafloor, similar to the current BP leak in the Gulf of Mexico, could form underwater oil plumes that do not rise to the surface.

The reasons include:

•whether a spill is released in the form of a turbulent jet, or is under less pressure
•the density of the surrounding water, due to temperature and salinity.

In both tests, the tank contains salt water that's denser at the bottom than at the top (bottom 1.06 g/cc, top 1.015g/cc). The first video shows a green-colored alcohol/water mixture being released into the tank; the second experiment shows red gauge oil being released. Both the oil and alcohol mixtures are about the same density (approximately 0.9g/cc) and are released at the same rate (about 0.8 gallons/minute).

As you can see, the first more turbulent jet is trapped underwater in a horizontal plume when it reaches the level where the surrounding water density changes; the second less turbulent jet is not trapped, and the oil rises to the surface.

The experiments were conducted by Richard McLaughlin, Ph.D., and Roberto Camassa, Ph.D., fluid dynamics experts in the mathematics department at the University of North Carolina at Chapel Hill

"We've been thinking that the recent news about underwater oil plumes is very reminiscent of these jet experiments, in which the effect of the strong turbulence is creating an emulsion which can lead to an underwater trapping," McLaughlin said. "In videos of the actual oil leak in the Gulf, the turbulent oil jet looks quite similar to our alcohol jet." He added that with the addition of dispersants, the effect would be further amplified.

He and Camassa along with students in their summer lab have also analyzed video of the current spill to try to determine how much oil is leaking into the Gulf.

"We estimated the flow rate to be about 56,000 barrels per day, quite a bit higher than BP's estimate of 5,000 barrels per day, and closer to the estimate recently in the news of 70,000 barrels per day," Camassa said.

Both researchers are professors in the UNC College of Arts and Sciences. Camassa is also director of the Carolina Center for Interdisciplinary Applied Mathematics.

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