[TowerTalk] suppressing vortex shedding

[jimlux]

On 5/1/18 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.
> 


I found some info on what are also called "helical strakes"

This source recommends 10-25% of diameter, with a pitch of 12-20 diameters.

They also mention some airfoil shaped things that you can slide over a 
pipe/mast.  the helical thing increases the drag significantly, while 
the streamlined, pivoting air foil reduces the drag.


There's also some info in a book called "Pressure Vessel Design Manual" 
which points to ASME STS-1-2000 "Steel stacks"  some googling might turn 
up other references.   This is oriented towards things like tall skinny 
reaction vessels, like at a refinery.

http://orbitalgas.com/wp-content/uploads/2017/01/Elimination_VIV_Sample_Probes_Thermowells.pdf

has some test data on fluid probes.


I think the take homes are that
a) you put the strake so that the direction isn't important, or, if the 
wind always comes from one direction, you can use a "trip wire" upstream 
to trigger turbulent flow.
b) you have the strake in different azimuths in different places, so 
that the entire thing doesn't shed vortices that are synchronized.  (a 
helix does this easily)
c) You could probably just have a few sections with the vortex disrupter 
- it's when the entire thing vibrates together that you have a problem.
d) the whole thing is about disrupting laminar flow, so if the wind 
speed is high enough, or the surface is rough enough, it's not going to 
be an issue.  A lattice tower could never have this problem (although 
the guy wires certainly could, and they could excite the resonance in 
the tower.





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