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Old 09-20-2012, 11:07 AM   #1 (permalink)
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recent article on vortex generators

Scientists discover second purpose for vortex generators

When a golf ball, an airplane, or any solid object moves through the air, a small layer of air called a boundary layer surrounds the object. Even the Earth has a planetary boundary layer, which consists of the lower atmosphere that extends a few hundred meters above the surface; we live most of our lives in this boundary layer. Since the boundary layer is somewhat viscous, or sticky, and slow-moving compared to the moving golf ball or flying plane, it falls behind and separates from the object, creating a wake. This wake creates drag on the object and slows it down, resulting in shorter drives for golfers and higher energy requirements, as well as the potential for loss of lift, for airplanes.

To combat this problem, the dimples on the golf ball and the vortex generators on an airplane's wings can delay the separation of the boundary layer by creating minor turbulence, which gives the boundary layer more energy and enables it to move a little faster to keep up with the object. For airplanes, the boundary layer separation is especially problematic when the wing is at a high angle of attack, such as during the take-off or landing. By keeping this air flow "attached" for as long as possible, the dimples and vortex generators make the boundary layer separate later and produce a smaller wake.

Now a team of researchers, Shahab Shahinfar, Sohrab Sattarzadeh, and Jens Fransson from the Linné Flow Centre at KTH Mechanics in Stockholm, Sweden, in cooperation with Alessandro Talamelli at the University of Bologna, Italy, have demonstrated that these simple vortex generators serve remarkably well at minimizing the boundary layer's drag by delaying its transition from a low-friction laminar flow to a high-friction turbulent flow. Their study is published in a recent issue of Physical Review Letters.

More information: Shahab Shahinfar, et al. "Revival of Classical Vortex Generators Now for Transition Delay." PRL 109, 074501 (2012). DOI: 10.1103/PhysRevLett.109.074501 </quote>

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