User:Val35/Vortex shedding

Vortex shedding behind a circular cylinder. Courtesy, Cesareo de La Rosa Siqueira.

Vortex Shedding is a phenomenon that can occur, in a fluid (gas or liquid),in the wake behind a non-streamlined object when there is relative motion between the object and the fluid. In its most pronounced form, the wake is a series of vortices (regions of low pressure) that form behind the object, and are "shed", alternately from one side then the other. It is sometimes called a Von Karman Vortex Street after the first researcher, Theodore von Karman. For vortex shedding to be strong enough to be made visible or detectable with sensors, there has to be special geometry of the object and special conditions of the velocity profile of the fluid. Von Karman used a circular cylinder mounted in a wind tunnel running at low Reynolds Numbers, so that the flow was laminar and the velocity uniform along the length of the cylinder. Under these conditions, the frequency is proportional to the velocity. The phenomenon can be used to measure volume flow rates in pipelines. Even weak vortex shedding can damage structures such as smoke stacks and bridges when the frequency is close to the resonant frequency of the structure.

Vortex Shedding and Flow Measurement

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Vortex Shedding and Damage to Structures

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.

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.