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## Re: [TowerTalk] 10 inch tube 100ft wind load? NN7J

 To: Red , towertalk@contesting.com Re: [TowerTalk] 10 inch tube 100ft wind load? NN7J "Dan Zimmerman N3OX" Thu, 31 Aug 2006 14:20:50 -0400
 ```On 8/31/06, Red wrote: > Hi, Dan; > > At what value does the Reynolds number, product of diameter and wind > velocity, increase and require a different coefficient of drag? Red, By my calculations the drag on pretty much any antenna element is going to be turbulent, and certainly so for a tens of miles per hour wind. The kinematic viscosity of air is around 1.3*10^-5 m^2/s... even with a tiny breeze of 2mph, and a 2 inch antenna boom the Reynolds number is, (courtesy of google calculator that does the unit conversions for you): ((2 miles per hour) * 2 inches) / ((1.3e-5 (m^2)) / s) = 3 493.78954 which is likely to be well developed turbulence...with a 10 inch tube in a 2mph breeze, the Reynolds number is about 17000. I think that it's very likely that all of the formulas you find here and there for antenna windload calculation assume fully developed turbulence. For example, a 1 inch rod in a 55mph wind gives: (55 miles per hour) * 1 inches) / ((1.3e-5 (m^2)) / s) = 48 039.6062 Another concern might be vortex shedding resonances... the 10 inch tube will shed vortices on alternate sides in a breeze... if the frequency at which it does so is near the natural modes of the structure, it could really get going. The spiral winding on car antennas is designed to break the vortex shedding up to avoid "singing" of the antenna... you can see the same thing on some tall smoke stacks... there's a spiral winding around them... but they're usually masonry and bricks and mortar aren't known for their flexibility or graceful failure mode. 73, Dan _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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