[TowerTalk] HyGain 155CA dampening rope

[Patrick Greenlee]

Adding weight inside the tube will do nothing for vortex shedding.  The 
mass loading will change the mechanical resonance properties, perhaps 
avoiding the natural frequencies excited by the vortex shedding.  A rope 
adds mass and an energy absorbent quality (inelastic collision sort of 
situation with frictional losses.)

Attacking the vortex shedding directly is a different approach. One 
approach is to put a spiral wrap of UV resistant cordage on the outside 
of the element.  (Fishing department at Walmart has UV resistant black 
cordage in various diameters.) Interior measures last much better than 
exterior ones.  Dampening the effects of vortex shedding can be as good 
as eliminating the vortex shedding itself.

See also...

https://en.wikipedia.org/wiki/Vortex_shedding#Mitigation_of_vortex_shedding_effects

http://www.spartaengineering.com/vortex-shedding-and-tall-structures/

Note the picture of a tall chimney at the first of the above URLs.  It 
shows the spiral wrap solution clearly.

The cause and effect are the same for a vertical or horizontal cylinder 
(element.)

I'm just guessing at how cable inside an element dampens oscillations 
but I put my money on friction between the cable and the ID of the 
tubing in addition to mass loading that lowers the frequency of the 
mechanical resonance of the element, perhaps moving the resonant 
frequency of the mass loaded element away from the natural frequency of 
the vortex shedding.

Any mechanical engineer vortex shedding experts out there care to 
comment.  I'm just guessing from a physicist point of view with what I 
hope makes some sense.

Patrick




On 6/22/2017 8:10 AM, Jim Thomson wrote:
> You need a bit of weight to kill the vortex shedding.