[Editors] MIT solves 100-year-old engineering problem

[Teresa Herbert]

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MIT solves 100-year-old engineering problem
--Insights on fluid flow could impact fuel efficiency, more
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For Immediate Release
THURSDAY, SEP. 25, 2008

Contact: Teresa Herbert, MIT News Office
E: therbert at mit.edu, T: 617-258-5403

Photo, Video and Graphic Available


CAMBRIDGE, Mass.--As a car accelerates up and down a hill then slows  
to follow a hairpin turn, the airflow around it cannot keep up and  
detaches from the vehicle. This aerodynamic separation creates  
additional drag that slows the car and forces the engine to work  
harder. The same phenomenon affects airplanes, boats, submarines, and  
even your golf ball.

Now, in work that could lead to ways of controlling the effect with  
potential impacts on fuel efficiency and more, MIT scientists and  
colleagues report new mathematical and experimental work for  
predicting where that aerodynamic separation will occur.

The research solves “a century-old problem in the field of fluid  
mechanics,” or the study of how fluids — which for scientists include  
gases and liquids — move, said George Haller, a visiting professor in  
the Department of Mechanical Engineering. Haller’s group developed the  
new theory, while Thomas Peacock, the Atlantic Richfield Career  
Development Associate Professor in the same department, led the  
experimental effort.

Papers on the experiments and theory are being published in the Sept.  
25 issue of the Journal of Fluid Mechanics and in the September issue  
of Physics of Fluids, respectively.

Fluid flows affect everything in our world, from blood flow to  
geophysical convection. As a result, engineers constantly seek ways of  
controlling separation in those flows to reduce losses and increase  
efficiency. One recent accomplishment: the sleek, full-body swimsuits  
used at the Beijing Olympics.

Controlling fluid flows lies at the heart of a wide range of  
scientific problems, including improving the performance of vehicles,  
Peacock said.

For example, picture air flowing around, over and past an object.  
“Instead of flowing smoothly past the object, the air tends to  
dramatically part from the surface, or separate,” said Peacock. Like  
the wake behind a boat, the water doesn’t automatically reconfigure  
into a single stream. Rather, the region is quite turbulent. “And that  
adversely affects the lift [or vertical forces] and drag [or  
horizontal forces] of the object.”

In 1904, Ludwig Prandtl derived the exact mathematical conditions for  
flow separation to occur. But his work had two major restrictions:  
first, it applied only to steady flows, such as those around a car  
moving at a  constant low speed. Second, it only applied to idealized  
two-dimensional flows.

“Most engineering systems, however, are unsteady. Conditions are  
constantly changing,” Haller said. “For example, cars accelerate and  
decelerate, as do planes during maneuvers, takeoff and landing.  
Furthermore, fluids of technological interest really flow in our three- 
dimensional world,” he added.

As a result, ever since 1904 there have been intense efforts to extend  
Prandtl’s results to real-life problems, i.e., to unsteady three- 
dimensional flows.

A century later, Haller led a group that did just that. In 2004 Haller  
published his first paper in the Journal of Fluid Mechanics explaining  
the mathematics behind unsteady separation in two dimensions. This  
month, his team reports completing the theory by extending it to three  
dimensions. Haller’s coauthors are Amit Surana, now at United  
Technologies; MIT student Oliver Grunberg; and Gustaaf Jacobs, now on  
the faculty at San Diego State University.

Equally important, this month Peacock and colleagues report important  
experimental work. Said Peacock, “while we fully trust George’s new  
mathematical results, the engineering community is usually skeptical  
until they also see experimental results.” Haller added, “while giving  
a beautiful validation of the 2D theory, Tom’s work also gives strong  
experimental backing to our new 3D theory.” Coauthors on the  
experimental work are Haller, Jacobs, Matthew Weldon, now at Penn  
State; and Moneer Helu, now at the University of California at Berkeley.

The research was initially supported by an internal source, the MIT  
Ferry Fund. Currently the work is supported by the Air Force Office of  
Scientific Research and the National Science Foundation.

The researchers said it’s too soon to quantify the level of  
improvement in performance of cars and planes that might stem from the  
work, noting that more work must be done before it can be applied to  
commercial technologies. “This is the tip of the iceberg, but we’ve  
shown that this theory works,” Peacock said.

--END--

Written by Elizabeth Thomson, MIT News Office

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