Vortex shedding is an unsteady flow that takes place in special flow velocities (according to the size and shape of the cylindrical body). In this flow, vortices

Vortex

A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...

 are created at the back of the body and detach periodically from either side of the body. See Von Kármán vortex street

Von Kármán vortex street

A Kármán vortex street is a term in fluid dynamics for a repeating pattern of swirling vortices caused by the unsteady separation of flow of a fluid over bluff bodies...

.

Vortex shedding is caused when a fluid flows past a blunt object. The fluid flow past the object creates alternating low-pressure vortices

Vortex

A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...

 on the downstream side of the object. The object will tend to move toward the low-pressure zone.

Eventually, if the frequency of vortex shedding matches the resonance frequency

Resonance

In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...

 of the structure, the structure will begin to resonate and the structure's movement can become self-sustaining. Tall chimneys constructed of thin-walled steel tube 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. These chimneys can be protected from this phenomenon by installing a series of fences (sometimes called strakes or spoilers) at the top and running down the exterior of the chimney for approximately 20% of its length. The fences are usually located in a helical pattern. The fences prevent strong vortex shedding with low separation frequencies. The optimal pitch for vortex shedding is a 5D pitch (5 x the diameter of the stack).

Vortex shedding was one of the causes proposed for the failure of the original Tacoma Narrows Bridge (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.

A thrill ride, "Vertigo

Vertigo (Cedar Point)

Vertigo was a thrill ride at Cedar Point Amusement Park in Sandusky, Ohio. It opened in August 2001, and was open for only four months. The ride was designed by S&S Power.-The Ride:The ride structure consisted of three placed in a triangular arrangement...

" at Cedar Point

Cedar Point

Cedar Point is a 364 acre amusement park located in Sandusky, Ohio, United States on a narrow peninsula jutting into Lake Erie. Cedar Point is the only amusement park with four roller coasters that are taller than...

 in Sandusky, Ohio

Sandusky, Ohio

Sandusky is a city in the U.S. state of Ohio and the county seat of Erie County. It is located in northern Ohio and is situated on the shores of Lake Erie, almost exactly half-way between Toledo to the west and Cleveland to the east....

 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.

Governing equation

The frequency at which vortex shedding takes place for a cylinder is related to the Strouhal number by the following equation:
Where is the Strouhal number

Strouhal number

In dimensional analysis, the Strouhal number is a dimensionless number describing oscillating flow mechanisms. The parameter is named after Vincenc Strouhal, a Czech physicist who experimented in 1878 with wires experiencing vortex shedding and singing in the wind...

, is the vortex shedding frequency, is the diameter of the cylinder, and is the flow velocity.

The Strouhal number depends on the body shape and on the Reynolds number.

Mitigation of vortex shedding effects

Modern tall smokestacks usually have a corkscrew fin (a strake

Strake (aviation)

In aviation, a strake is an aerodynamic surface generally mounted on the fuselage of an aircraft to improve the airflow and hence the flight characteristics.In general a strake is longer than it is wide, in contrast to a winglet or a moustache....

) to deliberately introduce turbulence, so that the load is less variable and resonant load frequencies have negligible amplitudes.

See also

• Aeroelastic Flutter - vibration-induced vortices - by way of contrast
• Vortex
  Vortex
  A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...
  
  
• Vortex induced vibration
  Vortex Induced Vibration
  In fluid dynamics, vortex-induced vibrations are motions induced on bodies interacting with an external fluid flow, produced by – or the motion producing – periodical irregularities on this flow.A classical example is the VIV of an underwater cylinder...
  
  
• Von Kármán vortex street
  Von Kármán vortex street
  A Kármán vortex street is a term in fluid dynamics for a repeating pattern of swirling vortices caused by the unsteady separation of flow of a fluid over bluff bodies...
  
  

External links

• Failure of the Tacoma Narrows Bridge
• Another explanation of vortex shedding (includes a helpful diagram)
• A movie of the Tacoma Narrows Bridge failure
• 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.