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Re: Vortex Shedding on Stacks - Vertical Strakes

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Harold,
 
        I have specified helical strakes several times where the Canadian analysis procedures indicated that vortex shedding would be a problem and I have never had a vortex shedding problem with a stack equipped with strakes.
 
        I did once have a problem with a set of three 8" pipe exhaust stacks for generator sets. These stacks were cantilevered some twenty feet (more or less as I recall some 30 years ago).  They vibrated unacceptably at a 15 mph wind put were rock steady at 50 mph wind exactly as the Canadian code procedures predicted.  We solved that problem by making the supporting tower a few feet taller.
 
        My understanding of the applicable formulae is that the frequency of vortices shed is a function of both the exposed "width" and "length" of the stack.  My further understanding is that helical strakes cause the apparent width and length of the stack to vary (but out of sync with each other) thereby causing the shed vortices to become a local phenomenon rather that a global phenomenon.  I would, therefore, expect that short vertical strakes rotated in increments of 15 or 30 degrees roughly following the trajectory traced by helical strakes actually would be effective.  That said, I have not tested my hypothesis nor do I know of any literature that discussed this.
 
        I hope these thoughts are helpful.
 
Regards,
 
H. Daryl Richardson
----- Original Message -----
Sent: Tuesday, February 22, 2011 10:01 AM
Subject: Vortex Shedding on Stacks - Vertical Strakes

Those familiar with vortex shedding will note that ASME STS 1 discusses strakes as does API 560.  Both API 560 and ASME STS 1 have provisions for helical strakes.  API 560 allows for vertical strakes ASME STS 1 does not. 
 
Has anyone ever studied whether or not vertical strakes are effective?  Are there any published papers?
 
My own personal experience is that vertical strakes do not work.  But since the vertical strakes are cheaper (especially in repairs), there is a desire to use them.

Regards, Harold Sprague