smoke stack

I was told that it was to improve the air flow around the stack and thus improve dispersion.

They are sometimes called 'spiral strakes'.

Consider a flagpole positioned in a steady wind. As the wind passes the flagpole a sequence of vortex eddies is caused downstream of the flagpole, first to one side and then the other. At certain speeds the frequency of the eddies approaches the natural frequency of oscillation of the flagpole, and the flagpole starts to move from side to side in the airstream in response. The same sort of thing happens with ropes and cables on the masts of sailing boats, and on a vessel with a hollow mast and a loose rope the effect can be heard as the rope hits the mast regularly and the whole lot rings.

The same vortex shedding phenomenon can happen to a plain chimney. Being a hollow structure it has a lower strength-per-unit-length than a rigid body of the same size, so the tendency to oscillate can be greater. In the limit it could shake itself apart.

So as well as the above feature of dispersion, the purpose of the strakes is to disrupt the airflow past the chimney so that regular vortex eddies do not occur, and in turn the chimney does not oscillate to the point of self-disassembly.

<At certain speeds the frequency of the eddies approaches the natural frequency of oscillation of the flagpole, and the flagpole starts to move from side to side in the airstream in response. >

The (in)famous suspension bridge, 'Galloping Gertie' is worthy of study while on this topic, as it's a similar phenomenon horizontally polarised, though the bridge deck's aerofoil shape contributed to the failure. http://en.wikipedia.org/wiki/Galloping_Gertie

Yes: I understand the problem of vortex shedding and the vibration it causes. However for typical chimney diameters, even in winds of the order of a 100 miles per hour the vibration frequency will be only of the order of tens of Hertz. I would imagine the natural frequency would be well above this for such stacks. Perhaps the "strake" as you call it, would reduce the net bending moment of the wind trying to tople the chimney by changing the direction of the aerodynamic drag along the height of the structure. Wonder what the ideal ratio between the width of the strake and the diameter of the chimney is. It must be large enough to be aerodynamically significantly and not large enough to cause too high a drag.

I read of a similar case once involving a suspension bridge (not Tacoma Narrows) that was experiencing severe vibration in the main cable - but only in rainstorms. After investigation it was discovered that the rain running down the underside of the cable was causing a resonant vibration. The solution was a very small cable wrapped around the main cable in a helix to force the rain to shed instead of running down the entire length.

Thanks for a power answer. I didn't know why myself.

Would you happen to know when the use of these strakes on chimneys was first started?

I was of the understanding that this was a "heat sink" installed to keep a uniform temperature along the perifery. This to prevent warping and refractery failure.

In addition to be mentioned and prescribed by friends,

Helical rib(s) on chemical plant smoke stacks is necessary to withstand both circumferential (hoop) and longitudinal stresses due to wind loads. Helical rip substitutes for longitudinal and circumferential ribs. Also, we prefer that type to avoid accumulation of condensates, rain, ice, ... etc. around the stack, which causes additional loads, in addition to avoiding corrosion occurs at these supports.

Having been atop say 1-2 hundred of these stacks I have heard all the above plus one.

The helix helps create a 'lifting effect' in windier conditions. (insert emoticon shrugging his shoulders here)

cr3