[TowerTalk] Fwd: Tower Permitting Spec's

[Grant Saviers]

The reference about power line cable oscillations is interesting, I 
think more applicable to wire antennas rather than masts.  I found this 
thesis which delves into the issues for masts and how to use FEA tools 
to analyze them (it's an MS level thesis so is at least superficially 
readable by me w/o delving into the partial differential equations, etc.)

https://engagedscholarship.csuohio.edu/cgi/viewcontent.cgi?article=1627&context=etdarchive

One observation I have is the TT discussion is conflating spoilers which 
reduce the magnitude of the wind forces by spoiling the air flow with 
dampers which absorb the energy of oscillations.  From the thesis:

a. Spoilers: which tend to change fluid dynamic characteristics of 
structures in such
a way as to interfere with and weaken the exciting force resulting from 
vortex
shedding. Some examples for spoilers are helical strakes <the car 
antenna spiral>, shrouds, slates, fairing,
splitter plate, and flags.
b. Dampers: which provide a mechanism for dissipation of energy and that 
leads to
an increase in the structural damping of the mast and thus reducing the 
amplitudes
of forced vibration resulting from vortex shedding and finally reducing the
possibility of structural damage or failure. Some examples for dampers 
are tune
mass damper, tune liquid damper, impact damper <rope inside TH7DX elements>.

Some clever ME probably has one design to perform both functions.

Grant KZ1W


On 5/1/2018 5:45 AM, jimlux wrote:
> On 5/1/18 12:31 AM, J Chaloupka via TowerTalk wrote:
>> Wondering, couldn't you wrap heavy gauge wire in a coarse spiral 
>> around the mast, similar to the wire wrapped around the proverbial 
>> automobile receiving antenna mast, in an attempt to dampen the 
>> vibration?  (Aluminum wire on an Aluminum mast)(look at the 2013 
>> Chrysler Town and Country van as example)
>
>
> You can - and it can be plastic, or rope, or almost anything. What I 
> don't know off hand is how big that spiral has to be. Obviously, 
> wrapping a AWG 20 wire isn't going to do it.
>
> One thing to think about, though, is that the vibration may be excited 
> somewhere else (guys?) and the tower just happens to be the resonator.
>
> The other thing is to make it stiffer (raise the resonant frequency) 
> so it's not excited, or to make the diameter different in different 
> places.
>
> This is a pretty complicated phenomenon - you don't see aluminum 
> flagpoles having the problem.
>
>
> Interestingly, the phenomenon is probably more severe in moderate 
> winds, rather than high winds (turbulence in the wind inhibits the 
> effect)
>
> Here's the formula:
>
> f = 0.185 * V/D
>
> f in Hz
> V in m/s
> D in meters
>
> 1 mi/hr = 0.48 m/s
> 1 " = 0.025 m
>
> so a 2" mast in a 10 mi/hr wind (0.05 m, 4.8m/s) would tend to vibrate 
> at 1.78 Hz.
>
> On the other hand a 1/4" guy wire will be oscillating at 8 times that, 
> around 14 Hz.
>
> I'd guess that the guy wires are higher Q than the mast.
>
>
> More info at:
>
>  http://www.tdee.ulg.ac.be/userfiles/file/Vibrations_eoliennes_intro.pdf
>
> http://sites.ieee.org/pes-resource-center/files/2015/08/PES-TR17-Aeolian-Vibration-of-Single-Conductors-Final-08-17-2015.pdf 
>
>
>
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