Vortex Shedding

[mcrossley]

Tim Long, a chap I know through astro software development has a blog post relevant to this group...

http://www.tigranetworks.co.uk/blog/pre ... al-strake/

[uncle_bob]

Interesting stuff!

[robynfali]

That's brilliant stuff, and let's face it, SO easy to do, even if you have the pole mounted higher, you could always use a spare piece of cable lying around to create the vortex

[beteljuice]

But ........

Is there a 'sweet' angle and how much of a 'fin' before it creates turbulence around your sensors ?

If it's so easy and obvious why hasn't it been done for roadside weather kit and CCTV masts ?

[mcrossley]

But ........

Is there a 'sweet' angle and how much of a 'fin' before it creates turbulence around your sensors ?

Good questions, anyone know or able to find the answers?

If it's so easy and obvious why hasn't it been done for roadside weather kit and CCTV masts ?

I'd suspect because this is normally in the realm of 'big stuff' - architects and structural/civil engineers - along with aeolian dampers etc. and the 'small scale stuff' is put together by workshop engineers?

[beteljuice]

Found this:

Helical strakes are aerodynamic stabilizers which are sometimes used to reduce the forces and deflections of the stack experienced due to vortex shedding. Strakes consist of three (3) vanes which can be wrapped in a helical pattern on the upper 1/3 of the stack. They have the appearance of a 'Snake' which spirals around the stack.

When strakes are added the drag coefficient of the stack is increased greatly. A smooth cylinder will have a shape factor of 0.7, while the same stack with strakes will increase its shape factor to 1.4. Consequently the load on the top 1/3 of the stack is doubled. Since a stack is like a large cantilever beam, increasing the forces at the top by a factor of 2 will increase the loads at the base by approximately 1.5. Similarly, the deflections will also increase significantly.

Advantages
==> Anyone can build them
==> In some rare cases they provide the most economical solution

Dis-Advantages
==> Double the wind load on the upper 1/3 of the stack, increasing stack weight significantly
==> Nearly always more expensive than a vibration damper
==> Complex geometry make fabrication difficult
==> Can interfere with other attachments (i.e. ladders, platforms, etc.)
==> Foundation and Anchor Bolts usually increase in size
==> Do not work in the case of interfering stacks

[mcrossley]

I guess we need to know how big the strakes need to be on small poles rather than large stacks. I see references to car aerials having a spiral wound into them to prevent this vibration - I have noticed this before and just thought they were being cheap and leaving the wire visible inside the outer sheath.

Another reference has a 16mm rope being effective on 250mm diameter yachts masts.

An alternative that presumably doesn't induce much more drag is to have a downwind fin, for a pole that is subject to wind from any direction this would have to be arranged so it could swivel around the pole! Good animations here http://en.wikipedia.org/wiki/K%C3%A1rm% ... tex_street

As an aside, we get some lovely vortex shedding whirlpools in the sea from some small islands/large rocks off the coast of Anglesey (the 'Mice'). Great fun in the RIB!

[Gina]

I've often wondered what those spiral bits round the tops of chimneys on chemical plants etc. were for - now I know

As for my own weather station mast, it sways in the wind a bit but I haven't noticed any vibration.