Kármán vortex street
[1] It is named after the engineer and fluid dynamicist Theodore von Kármán,[2] and is responsible for such phenomena as the "singing" of suspended telephone or power lines and the vibration of a car antenna at certain speeds.
This is actually the reason for which the most precise sources for airfoil and channel flow data specify the reference length at the Reynolds number.
The reference length can vary depending on the analysis to be performed: for a body with circle sections such as circular cylinders or spheres, one usually chooses the diameter; for an airfoil, a generic non-circular cylinder or a bluff body or a revolution body like a fuselage or a submarine, it is usually the profile chord or the profile thickness, or some other given widths that are in fact stable design inputs; for flow channels usually the hydraulic diameter about which the fluid is flowing.
On the other hand, for fairings and struts the given parameter is usually the dimension of internal structure to be streamlined (let us think for simplicity it is a beam with circular section), and the main target is to minimize the drag coefficient or the drag/lift ratio.
The range of Re values varies with the size and shape of the body from which the eddies are shed, as well as with the kinematic viscosity of the fluid.
When 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.
The flow of atmospheric air over obstacles such as islands or isolated mountains sometimes gives birth to von Kármán vortex streets.
[13] In low turbulence, tall buildings can produce a Kármán street, so long as the structure is uniform along its height.
In urban areas where there are many other tall structures nearby, the turbulence produced by these can prevent the formation of coherent vortices.
[14] Periodic crosswind forces set up by vortices along object's sides can be highly undesirable, due to the vortex-induced vibrations caused, which can damage the structure, 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 and skyscrapers.
[15] To prevent vortex shedding and mitigate the unwanted vibration of cylindrical bodies is the use of a tuned mass damper (TMD).
When a tuned mass damper is installed on a cylindrical structure, such as a tall chimney or mast, it helps to reduce the vibration amplitudes caused by vortex shedding.
In many cases, the spring is replaced by suspending the mass on cables such that it forms a pendulum system with the same resonance frequency.
Engineers carefully analyze the structural dynamics and characteristics of the vortex shedding phenomenon to determine the optimal parameters for the tuned mass damper.
For this reason, helical projections resembling large screw threads are sometimes placed at the top, which effectively create asymmetric three-dimensional flow, thereby discouraging the alternate shedding of vortices; this is also found in some car antennas.
Although named after Theodore von Kármán,[19][20] he acknowledged[21] that the vortex street had been studied earlier by Arnulph Mallock[22] and Henri Bénard.
[23] Kármán tells the story in his book Aerodynamics:[24] [...] Prandtl had a doctoral candidate, Karl Hiemenz, to whom he gave the task of constructing a water channel in which he could observe the separation of the flow behind a cylinder.
'In his autobiography, von Kármán described how his discovery was inspired by an Italian painting of St Christopher carrying the child Jesus whilst wading through water.
