In article , John Larkin
wrote:
On Sun, 8 Feb 2015 19:10:02 +0000 (UTC), "Danny D."
wrote:
John Larkin wrote, on Sun, 08 Feb 2015 09:39:53 -0800:
Don't all those trees sway (asynchronously!) in the wind?
You could occasionally get the equivalent of a rogue wave, a big spike
of acceleration.
I'm not sure how they sway in the wind, but, we decoupled the 75-foot
long bridge from the middle trees after that last storm, and we're
hoping to see the bridge still there after the one that is raging as
we type.
https://c4.staticflickr.com/8/7394/1...b70b7c5d_c.jpg
Yeah, we're getting some wild gusts today in the Alemany Gap, a break
in the coast hills between the ocean and the bay, our own local wind
tunnel.
https://dl.dropboxusercontent.com/u/...MtSanBruno.JPG
One thing we noticed is that, in the wind, the bridge actually *rises*
a few inches, like what happens with an airplane wing or a sailboat sail.
Since the bottom is dirtier, with respect to laminar flow, we suspect
the air flows faster over the top than over the bottom:
https://c1.staticflickr.com/9/8647/1...7c0b7fba_b.jpg
The whole bridge is roughly the size of an airplane wing:
https://c1.staticflickr.com/9/8642/1...972b0444_c.jpg
Yikes. Suspension bridges have all sorts of aerodynamic issues.
Galloping Gertie and all that.
https://www.youtube.com/watch?v=j-zczJXSxnw
It's called flutter in aerodynamics. This is what happens when the
frequency of the aeolian tone matches the deck-twist resonant
frequency.
When it happens to an airplane wing, the airplane is usually lost.
Look at some of the film clips.
http://en.wikipedia.org/wiki/Aeroelasticity#Flutter
https://www.youtube.com/watch?v=kQI3AWpTWhM
https://www.youtube.com/watch?v=fl2E...ZiYkfdJzm_UaYq
vWwY4xeR7hil&index=7
One can greatly reduce the effect by spacing the boards apart by say an
inch, which will equalize the pressure. The world is full of rope
bridges over chasms, erected by people who never heard of aerodynamics.
http://en.wikipedia.org/wiki/Aeolian_harp
The whole idea of rigid decks spanning flexible trees seems destined to
fail.
Joe Gwinn