Helical Strakes

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 Vortex Shedding 

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
==> Some codes do not allow their use for Scruton Number < 8 (which covers most stacks with vibration issues)