[tt] advancednano: separation in unstable 3D flows

[Alejandro Dubrovsky]

(
http://web.mit.edu/newsoffice/2008/fluid-flow-0924.html
)

MIT solves 100-year-old engineering problem
Insights on fluid flow could impact fuel efficiency, more

Elizabeth A. Thomson, News Office
September 24, 2008

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 have reported 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.