Frequently, a walker who observes the water running behind bridge pillars can see vortices with opposite signs shed alternatively from either side of the obstacle. These vortices are called " Von Karman Street ". This regular pattern of vortices in the wake is called a Karman vortex street. It creates an oscillating flow at a discrete frequency that is correlated to the Reynolds number of the flow. The periodic nature of the vortex shedding phenomenon can sometimes lead to unwanted structural vibrations, especially when the shedding frequency matches one of the resonant frequencies of the structure.This is a complex phenomenon on which a lot of studies have been made and published, but it can be explain easily with notion of swirling sheets instabilities.
I.2 Historical Outline of the Phenomenon
In the fifteenth century, Leonardo da Vinci sketched a row of vortices in the wake of a piling in a stream. In 1878, Strouhal found that the Aeolian tones generated by a wire in the wind were proportional to the wind speed divided by the wire thickness. He noticed that the sound greatly increased when the natural tones of the wire coincided with the Aeolian tones. In 1879, Lord Raleigh found that a violin string in a chimmey draft vibrated primarily across the flow, rather than with the flow. The periodicity of the wake of a cylinder was associated with vortex formation by Benard in 1908 and with the formation of a stable street of staggered vortices by von Karman in 1912.
I.3 Why study this phenomenon ?
Those Karman vortex streets are generally harmfull to the lift of a sail or a sreamlined body. They specially can be dangerous for offshore oil rigs safety, when pilars slipstream induced vibrations get resonances with the structural vibrations. In consequence, pilars are particularly strengthen to avoid their destruction. Some accidents happened on bridges in the past (the most famous example is the breakup of the Tacoma Narrows Bridge over Puget Soundon November 7, 1940. Even though the odd motions of the bridge were recorded in detail, including film footage that documented the final moments of the structure, technical experts still disagree on the exact nature of the phenomena which led to its destruction. One aspect of the failure remains quite clear, however. The Tacoma Narrows Bridge, despite the embarrassment it caused those involved with its design, has played a significant role in creating more sophisticated analytical tools for engineers to use.
The Collapse of the Tacoma Narrows Bridge