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[TowerTalk] Antenna element vibration damping - How

 To: [TowerTalk] Antenna element vibration damping - How jimlux@earthlink.net (Jim Lux) Mon Aug 11 17:03:10 2003
 ```At 11:19 AM 8/10/2003 +0200, Jiri Sanda wrote: >To later responses. >Jan - I would like to see some designs that DOES NOT vibrate at all. Until >now I have not seen such a beast !!! Any round element will vibrate in the wind...simple mechanics and aerodynamics (shed vortices), and forms the basis of such things as aeolian harps. The vibration might not be noticable, or it might be quite significant, and in a high Q system (which many mechanical systems are) the vibration might build up to a significant amplitude. This is a classic fluid mechanics problem... f = S*U/D f = frequency in Hz S = Strouhal number (function of Reynolds number and cross section U = velocity in m/sec D = diameter in meters Anything in aerodynamics is going to have something related to Reynolds numbers, so it's worthwhile to compute it so we know what regime we're in. Re = Reynolds number = U*D/nu U = velocity D = Characteristic length (diameter of tubing in this case) nu = fluid kinematic viscosity: 1.51E-5 m^2/sec @ 20C Say you've got a 2" diameter element (0.05 meters) in a 20 m/sec (45 mi/hr) wind Re = 20 * .05 / 1.51E-5 ... Re= 66E3 (scaling with size and wind speed..) It looks like we're in the big medium Re regime (1000 to 1 million) For 1E3To kill the vibration with some rope or wire is fairly easy and does not >represent any significant cost or problem to put in. ```
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