UNC-CHAPEL HILL (US)—We know oil and water don’t mix. So why is the oil spewing out of the massive spill in the Gulf of Mexico forming underwater plumes that don’t rise to the top?

Experiments by fluid dynamics researchers at the University of North Carolina at Chapel Hill explain that it depends on whether a spill is released in the form of a turbulent jet or is under less pressure.

Density of the surrounding water, due to temperature and salinity, is also a factor.

“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,” says UNC-Chapel Hill professor Richard McLaughlin.

“In videos of the actual oil leak in the Gulf, the turbulent oil jet looks quite similar to our alcohol jet.” He adds that with the addition of dispersants, the effect would be further amplified.

In the two tests shown in the accompanying video, 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).

The mixture from 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.

McLaughlin and colleague Roberto Camassa 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 says.

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