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Re: Midterm Question - Sort of



Leigh Palmer wrote:
>Well, that's a bit like saying that the steady bowing of a violin
string is an example of resonance. There are certainly *normal modes
of oscillation* for the bridge (and the violin string), and these
modes can be excited by an external source of energy in a nonresonant
interaction. Resonance I take to refer to the excitation of a normal
mode by an *applied* periodic forcing function, clearly not the case
encountered here. The applied forcing function is, more or less, DC.
That's exactly what I was saying, Leigh. The bridge and the violin string both resonate because they are excited by an applied periodic force. The periodic force, of course, is in addition to the DC force being applied. In the case of the bridge, the periodic force is the 3rd-law reactive force owing to the vortex flipping. For the string, the periodic force is the 3rd-law reactive force owing to flipping of the rosin-coated bow between static and dynamic friction against the string.
It was not the cables of the bridge that shed vortices at the bridge
normal mode oscillation frequency, but rather the bridge deck itself.
That is, indeed, the mechanism responsible for aeolian oscillation.
I believe the bridge actually flexed in more than one of its normal
modes. I won't check that just now, but maybe someone else can help
me. I seem to have no allies in this discussion. Have I slaughtered
a sacred cow?
As usual, you have straightened out my thinking about what was actually shedding the vortices.
As I re-read your piece, I believe that our mutual misunderstanding stems, perhaps, from not being explicit about which applied force causes the resonant condition. I think we agree that it's an AC force, not a DC force.

poj
Collin County College