Flying High
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Flying HIGH
TAU engineers develop an innovative method for improving aerodynamic
performance
Have you ever wondered, as you dined at 37,000 feet, what keeps an airplane
aloft? It is the “lift” created by the difference in pressure between the fast air
flow above the surface of the wings and the slower air below.
Working against lift, however, is friction; when the rapidly moving air flows
near the surface of the wing, a thin "boundary layer” of slower-moving air is
created. At steep flight angles this flow detaches from the surface of the wing
and slides directly downstream, causing energy loss, airplane “drag,” and
“stall" (loss of lift).
A technique developed by TAU scientists may eliminate this major cause of
aerodynamic inefficiency by substantially delaying stall, enhancing lift and
reducing drag. Developed by Prof. Israel Wygnanski, Dr. Avi Seifert, and
their students at TAU’s Department of Fluid Mechanics and Heat Transfer,
Fleischman Faculty of Enginnering, the technique re-energizes the sluggish
boundary layer of air and in some cases forces the flow to reattach to the
wing.
The method involves mounting an electro-mechanical device inside the wing
that blows pulses of air onto the surface of the wing. A combination of weak,
steady blowing with strong periodic puffs creates vortices that mix the faster
air current above with the slower air just below. The jolts of fast air stimulate
the boundary layer and delay or even prevent its separation.
Experiments conducted at TAU’s Meadow-Knapp Wind Tunnel on a variety
of wing types have confirmed that the method-- known as “oscillatory
blowing”-- is an effective, low energy solution to boundary layer separation.
Furthermore, tests conducted at the Illinois Institute of Technology at
velocities of half the speed of sound indicate that the TAU method could
increase the performance of airplanes and helicopters in high speed or
maneuvering situations.
The investigators have successfully tested the technology, which has already
generated three patents, in remote-piloted vehicles (RPVs) in cooperation with
the AD&D company of Rehovot and the Israeli Ministry of Defense. Flight
tests are in progress. NASA is also interested in the technology and
experiments with the participation of the Israeli investigators are planned for
1997.
Vortices in separated (left figure) and reattached
flow (right figure) as measured by a particle image velocimeter over the “generic flap.”
The inclined line indicates the flap surface. (Data, A.Darabi.)
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