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>4. What mechanism started and supported the oscillations of that bridge?
The bridge deck, initially more or less having the wind velocity
in-plane, got a little bit out of plane and the wind reinforced
this distortion. A vortex was shed, allowing the distortion to
relax and, it turns out, overshoot its equilibrium position, at
which time the opposite polarity distortion is driven to
increase by the wind until another vortex is shed, the deck
relaxes past equilibrium, usw. When the wind was gentler, this
oscillation did not grow in amplitude to bridge failure, which
gave the bridge its nickname of "Galloping Gertie".