![Heard Island Karman vortex street](https://upload.wikimedia.org/wikipedia/commons/7/71/Heard_Island_Karman_vortex_street.jpg)
In
fluid dynamics
In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) an ...
, vortex shedding is an oscillating
flow that takes place when a fluid such as air or water flows past a bluff (as opposed to streamlined) body at certain velocities, depending on the size and shape of the body. In this flow,
vortices
In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...
are created at the back of the body and detach periodically from either side of the body forming a
Kármán vortex street
In fluid dynamics, a Kármán vortex street (or a von Kármán vortex street) is a repeating pattern of swirling vortices, caused by a process known as vortex shedding, which is responsible for the unsteady separation of flow of a fluid arou ...
. The fluid flow past the object creates alternating low-pressure
vortices
In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...
on the downstream side of the object. The object will tend to move toward the low-pressure zone.
If the bluff structure is not mounted rigidly and the frequency of vortex shedding matches the
resonance frequency
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillat ...
of the structure, then the structure can begin to
resonate
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillat ...
, vibrating with
harmonic oscillations driven by the energy of the flow. This vibration is the cause for overhead power line wires humming in the wind, and for the fluttering of automobile
whip radio antennas at some speeds. Tall
chimneys
A chimney is an architectural ventilation structure made of masonry, clay or metal that isolates hot toxic exhaust gases or smoke produced by a boiler, stove, furnace, incinerator, or fireplace from human living areas. Chimneys are typ ...
constructed of thin-walled steel tubes can be sufficiently flexible that, in air flow with a speed in the critical range, vortex shedding can drive the chimney into violent oscillations that can damage or destroy the chimney.
Vortex shedding was one of the causes proposed for the failure of the original
Tacoma Narrows Bridge
The Tacoma Narrows Bridge is a pair of twin suspension bridges that span the Tacoma Narrows strait of Puget Sound in Pierce County, Washington. The bridges connect the city of Tacoma with the Kitsap Peninsula and carry State Route 16 (know ...
(Galloping Gertie) in 1940, but was rejected because the frequency of the vortex shedding did not match that of the bridge. The bridge actually failed by
aeroelastic flutter
Aeroelasticity is the branch of physics and engineering studying the interactions between the inertial, elastic, and aerodynamic forces occurring while an elastic body is exposed to a fluid flow. The study of aeroelasticity may be broadly classif ...
.
A thrill ride, "
VertiGo
Vertigo is a condition where a person has the sensation of movement or of surrounding objects moving when they are not. Often it feels like a spinning or swaying movement. This may be associated with nausea, vomiting, sweating, or difficulties w ...
" at
Cedar Point
Cedar Point is a amusement park located on a Lake Erie peninsula in Sandusky, Ohio, United States. Opened in 1870, it is considered the second-oldest operating amusement park in the U.S. behind Lake Compounce. Cedar Point is owned and op ...
in
Sandusky, Ohio
Sandusky ( ) is a city in and the county seat of Erie County, Ohio, Erie County, Ohio, United States. Situated along the shores of Lake Erie in the northern part of the state, Sandusky is located roughly midway between Toledo, Ohio, Toledo ( wes ...
suffered vortex shedding during the winter of 2001, causing one of the three towers to collapse. The ride was closed for the winter at the time. In northeastern Iran, the
Hashemi-Nejad natural gas refinery's
flare stacks suffered vortex shedding seven times from 1975 to 2003. Some simulation and analyses were done, which revealed that the main cause was the interaction of the pilot flame and flare stack. The problem was solved by removing the pilot.
Governing equation
The frequency at which vortex shedding takes place for an infinite cylinder is related to the Strouhal number by the following equation:
:
Where
is the dimensionless
Strouhal number
In dimensional analysis, the Strouhal number (St, or sometimes Sr to avoid the conflict with the Stanton number) is a dimensionless number describing oscillating flow mechanisms. The parameter is named after Vincenc Strouhal, a Czech physicist who ...
,
is the vortex shedding frequency (s),
is the diameter of the cylinder (m), and
is the flow velocity (m s).
The Strouhal number depends on the
Reynolds number ,
but a value of 0.22 is commonly used. Over four orders of magnitude in Reynolds number, from 100 to 100000, the Strouhal number varies only between 0.18 and 0.22.
Mitigation of vortex shedding effects
![SchornsteinwendelSKL](https://upload.wikimedia.org/wikipedia/commons/7/7b/SchornsteinwendelSKL.jpg)
Fairings can be fitted to a structure to streamline the flow past the structure, such as on an aircraft wing.
Tall metal smokestacks or other tubular structures such as antenna masts or tethered cables can be fitted with an external corkscrew fin (a
strake
On a vessel's hull, a strake is a longitudinal course of planking or plating which runs from the boat's stempost (at the bows) to the sternpost or transom (at the rear). The garboard strakes are the two immediately adjacent to the keel on ea ...
) to deliberately introduce turbulence, so the load is less variable and resonant load frequencies have negligible amplitudes. The effectiveness of helical strakes for reducing vortex induced vibration was discovered in 1957 by Christopher Scruton and D. E. J. Walshe at the
National Physics Laboratory in Great Britain. They are therefore often described as Scruton strakes. For maximum effectiveness in suppression of vortices caused by air flow, each fin or strake should have a height of about 10 percent of the cylinder diameter. The pitch of each fin should be approximately 5 times the cylinder diameter.
[ ]
A
tuned mass damper
A tuned mass damper (TMD), also known as a harmonic absorber or seismic damper, is a device mounted in structures to reduce mechanical vibrations, consisting of a mass mounted on one or more damped springs. Its oscillation frequency is tuned ...
can be used to mitigate vortex shedding in stacks and chimneys.
A
Stockbridge damper
A Stockbridge damper is a tuned mass damper used to suppress wind-induced vibrations on slender structures such as overhead power lines, long cantilevered signs and cable-stayed bridges. The dumbbell-shaped device consists of two masses at the end ...
is used to mitigate
aeolian vibration
Aeroelasticity is the branch of physics and engineering studying the interactions between the inertial, elastic, and aerodynamic forces occurring while an elastic body is exposed to a fluid flow. The study of aeroelasticity may be broadly classif ...
s caused by vortex shedding on
overhead power line
An overhead power line is a structure used in electric power transmission and distribution to transmit electrical energy across large distances. It consists of one or more uninsulated electrical cables (commonly multiples of three for three-p ...
s.
See also
*
Aeroelastic flutter
Aeroelasticity is the branch of physics and engineering studying the interactions between the inertial, elastic, and aerodynamic forces occurring while an elastic body is exposed to a fluid flow. The study of aeroelasticity may be broadly classif ...
- vibration-induced vortices - by way of contrast
*
Vortex
In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...
*
Vortex-induced vibration
In fluid dynamics, vortex-induced vibrations (VIV) are motions induced on bodies interacting with an external fluid flow, produced by, or the motion producing, periodic irregularities on this flow.
A classic example is the VIV of an underwater c ...
*
Von Kármán vortex street
The term ''von'' () is used in German language surnames either as a nobiliary particle indicating a noble patrilineality, or as a simple preposition used by commoners that means ''of'' or ''from''.
Nobility directories like the ''Almanach de Go ...
References
{{Reflist
External links
Flow visualisation of the vortex shedding mechanism on circular cylinder using hydrogen bubbles illuminated by a laser sheet in a water channel. Courtesy of G.R.S. Assi.
Vortices
Fluid dynamics