Vortex shedding
Vortex shedding is caused when air flows past a blunt structure. The airflow past the object creates alternating low-pressure vortexes 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 resonant frequency of the structure, the structure will begin to resonate and the structure's movement can become self-sustaining. Some engineers believe that vortex shedding was one of the causes of the failure of the Tacoma Narrows Bridge in 1940.
References Failure of the Tacoma Narrows Bridge Another explanation of vortex shedding (includes a helpful diagram) A movie of the Tacoma Narrows Bridge failure
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