Sitting at a traffic stop on a windy day, you may have noticed the beam holding the traffic lights shaking steadily up and down. This phenomenon is called vortex-induced vibration. When the wind flows over the beam, it looks something like the flow animation shown below.
Airflow follows the shape of the beam until near the backside, where the air separates from the surface and creates a vortex that sloughs off into the beam’s wake. These vortices form asymmetrically on the beam – first on one side, then the other. This creates unequal pressures on either side of the beam, and those pressure differences create a force that moves the beam. Because vortices are being steadily shed off the beam, it will keep moving back and forth as long as the wind is strong enough.
VIV - read & learn:http://web.mit.edu/13.42/www/handouts/reading-VIV.pdf
Reference:http://www.sciencedirect.com/science/article/pii/S0889974604000350
h/t FYFD
Animation:
Numerical simulation of VIV due to the flow around a circular cylinder
Credit: Aphex82
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