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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".
This also makes great sense to me that the entire bridge deck is acting as
an aeolian harp. These vortex pushes strike me as giving pulses that match
the natural frequency of the bridge deck. As I read my H&R, my prime
source, this is resonance. Why is it not?
The vortex shedding is not driving the motion. The frequency of
vortex shedding is determined by the frequency of the torsional
oscillation of the bridge; it is not inherent in the wind. If
bridge were not oscillating the frequency would be different.
This is not the phenomenon usually referred to as (von Karmann)
vortex shedding.
Leigh