Vortex shedding

Vortex shedding

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

Vortex shedding is caused when air flows past a blunt object. The airflow past the object creates alternating low-pressure vortices on the downwind 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 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 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.

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 [K. Billah and R. Scanlan (1991), "Resonance, Tacoma Narrows Bridge Failure, and Undergraduate Physics Textbooks", American Journal of Physics, 59(2), 118--124 [http://www.ketchum.org/billah/Billah-Scanlan.pdf (PDF)] ] .

A thrill ride "Vertigo" at Cedar Point in Sandusky, Ohio Suffered the fate of vortex shedding during the winter of 2001, one of the three towers collapased. The ride was closed for the winter at the time.

Governing equation

The frequency at which vortex shedding takes place for a cylinder can be derived using the following equation::St = frac{fD}{V}Where St is the Strouhal number, f is the frequency, D is the diameter of the cylinder, and V is the flow velocity.

For Reynolds numbers greater than 1000, the Strouhal number is approximately equal to 0.21.

See also

* Vortex
* Vortex induced vibration
* Von Kármán vortex street


* [http://www.tech.plym.ac.uk/sme/Interactive_Resources/tutorials/FailureCases/sf1.html Failure of the Tacoma Narrows Bridge]
* [http://www.mecaenterprises.com/vortex_shedding.htm Another explanation of vortex shedding] (includes a helpful diagram)
* [http://www.civeng.carleton.ca/Exhibits/Tacoma_Narrows/ A movie of the Tacoma Narrows Bridge failure]

External links

* [http://www.youtube.com/watch?v=_AJgEa2dbJU 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.]

Wikimedia Foundation. 2010.

Игры ⚽ Поможем сделать НИР

Look at other dictionaries:

  • vortex-shedding meter — kliūtinis sūkurinis srautmatis statusas T sritis Standartizacija ir metrologija apibrėžtis Srautmatis, turintis stačiabriaunį kūną, nuo kurio iš abiejų pusių pakaitomis sklinda sūkurių sekos. Tam tikrose tėkmės vietose sūkurių sklidimo dažnis yra …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

  • Vortex induced vibration — Vortex induced vibrations (VIV) are motions induced on bodies facing an external flow by periodical irregularities on this flow. The classical example is the VIV of an underwater cylinder. You can see how this happens by putting a cylinder into… …   Wikipedia

  • Vortex — A vortex (pl. vortices ) 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. The speed and rate of rotation of the… …   Wikipedia

  • Kármán vortex street — A Kármán vortex street is a repeating pattern of swirling vortices caused by the unsteady separation of flow over bluff bodies. They are named after the engineer and fluid dynamicist, Theodore von Kármán [Theodore von Kármán: Aerodynamics]… …   Wikipedia

  • Tacoma Narrows Bridge (1940) — This article is about the first Tacoma Narrows Bridge, which collapsed in 1940. For the article on the current bridges, see Tacoma Narrows Bridge. Tacoma Narrows Bridge The original Tacoma Narrows Bridge roadway twisted and vibrated violently… …   Wikipedia

  • Galloping Gertie — This article is about the first Tacoma Narrows Bridge, which collapsed in 1940. For the article on the current bridge, see Tacoma Narrows Bridge. Infobox Bridge bridge name = Tacoma Narrows Bridge (Galloping Gertie) caption = The first Tacoma… …   Wikipedia

  • Lift (force) — For other uses, see Lift. Boeing 747 8F landing A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the …   Wikipedia

  • building construction — Techniques and industry involved in the assembly and erection of structures. Early humans built primarily for shelter, using simple methods. Building materials came from the land, and fabrication was dictated by the limits of the materials and… …   Universalium

  • Morison equation — Flow forces according to the Morison equation for a body placed in a harmonic flow, as a function of time. Blue line: drag force; red line: inertia force; black line: total force according to the Morison equation. Note that the inertia force is… …   Wikipedia

  • Gurney flap — The Gurney Flap (or wickerbill) is a small flat tab projecting from the trailing edge of a wing. Typically it is set at a right angle to the pressure side surface of the airfoil,cite journal author = Van Dam, C.P. coauthors = Yen, D.T.; Vijgen, P …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”