K?rm?n vortex street

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Von Km vortex street off the Chilean coast near the Juan Fernandez Islands.

Km vortex street off the coast of Rishiri Island in Japan.
A Km vortex street is a term used in fluid dynamics for a repeating pattern of swirling vortices caused by the unsteady separation of flow of a fluid over bluff bodies. It is named after the engineer and fluid dynamicist, Theodore von Km and is responsible for such phenomena as the "singing" of suspended telephone or power lines, the vibration of a car antenna at certain speeds.
Contents
1 Analysis
2 Engineering problems
3 Formula
4 Insect flight
5 See also
6 References
7 External links
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Analysis
A vortex street will only be observed over a given range of Reynolds numbers (Re), typically above a limiting Re value of about 90. The Reynolds number is a measure of the ratio of inertial to viscous forces in the flow and is defined as:

Where:
d = diameter of the cylinder (or some other suitable measure of width of non-circular bodies)
V = steady velocity of the flow upstream of the cylinder
= The kinematic viscosity of the fluid.
The range of Re values will vary with the size and shape of the body from which the eddies are being shed, as well as with the kinematic viscosity of the fluid. Over a large Re range (47When a single vortex is shed, an asymmetrical flow pattern forms around the body and changes the pressure distribution. This means that the alternate shedding of vortices can create periodic lateral (sideways) forces on the body in question, causing it to vibrate. If the vortex shedding frequency is similar to the natural frequency of a body or structure, it causes resonance. It is this forced vibration which, at the correct frequency, causes suspended telephone or power lines to "sing" and the antenna on a car to vibrate more strongly at certain speeds.
Engineering problems
Periodic forcing set up in this way can be highly undesirable and hence it is important for engineers to account for the possible effects of vortex shedding when designing a wide range of structures, from submarine periscopes to industrial chimneys.
External images
Volkswagen Fuba antenna
In order to prevent the unwanted vibration of such cylindrical bodies, a longitudinal fin can be fitted on the downstream side, which, providing it is longer than the diameter of the cylinder, will prevent the eddies from interacting and consequently they remain attached. Obviously for a tall building or mast, the relative wind could come from any direction. For this reason, helical projections which look like large screw threads are sometimes placed at the top, which effectively create unsymmetrical three-dimensional flow thereby discouraging the alternate shedding of vortices; this is also found in some car antennas, such as late 1990s/early 2000s Fuba (Fuba Communications Systems) amplified antenna masts, used on the Volkswagen Golf Mk4, Jetta Mk4 and New Beetle.
Even more serious instability can be created in concrete cooling towers for example, especially when built together in clusters. Vortex shedding caused the collapse of three towers at Ferrybridge power station in 1968 during high winds.
Formula

Animation of the phenomenon. Courtesy, Cesareo de La Rosa Siqueira.
When considering a long circular cylinder, the frequency of vortex shedding is given by the empirical formula

Where:
f = vortex shedding frequency
This formula will generally hold true for the range 250 < Re < 2 105. The dimensionless parameter fd/V is known as the Strouhal number and is named after the Czech physicist, Vincenc Strouhal (1850-1922) who first investigated the steady humming or singing of telegraph wires in 1878.
Insect flight
Recent studies have shown that insects such as bees borrow energy from the vortices that form around their wings during flight. Vortices inherently create drag. Insects can recapture some of this energy and use it to improve speed and maneuverability: They rotate their wings before starting the return stroke, and the wings are lifted by the eddies of air created on the downstroke. The high frequency oscillation of insect wings means that many hundreds of vortices are shed every second. However, this leads to a symmetric vortex street pattern,...(and so on) To get More information , you can visit some products about satellite receiver card, Motorola Signal Booster, . The USB2.0 Satellite Box products should be show more here!