Relative motion between an object and a fluid is common occurrence. Simple examples are the motion of a plane in flight or the wind blowing on an obstacle. Obstacles disturb the flow and create particular shapes in their wakes. This phenomenon can be easily observed behind piers of a bridge, where eddies appear and are blown by the stream.

This wake might be complex depending on the shape of the obstacle. In order to understand this phenomenon, studies are done on a simple case: a two-dimensional flow past a circular cylinder. This case illustrates Strouhal instability and the particular wake known as Von Karman Vortex Street. It is a succession of eddies created close to the cylinder that break away alternatively from both sides of the cylinder. Vortex are emitted regularly and rotate in opposite senses.

Von Karman Vortex Street

These instabilities appear in nature but remain unnoticed. However an accident is known as the result of development of Strouhal Instability. It occurred on November 7, 1940 near Tacoma, Washington. Wind had blown a few days on Tacoma Narrows suspension bridge, built a few months before. This is not important in most cases, but this time, the wake of the bridge created vibrations at a particular frequency and structural vibrations got into resonance. The oscillations of the bridge amplified and it collapsed.

Tacoma Narrows Bridge Disaster

Another illustration of Von Karman Vortex Street is this phenomenon below, observed from space: the wind blowing on the sea is disturbed by an island and vortex are created. The clouds are like markers allowing us to see the wake.

Von Karman Vortex Street in the atmosphere

"The SeaWiFS image was provided courtesy of Orbimage, Inc., the SeaWiFS Project (Code 970.2) and the Distributed Active Archive Center (Code 902) at NASA's Goddard Space Flight Center, Greenbelt, MD, 20771."

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PASCAUD Stéphane
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