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RE: Vortex Shedding on Stacks - Vertical Strakes

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I contacted Bob Blevins (author of Applied Fluid Dynamics Handbook).  ....great book!!
Mr. Blevins confirmed that vertical strakes are ineffective inspite of their inclusion in API 560. 
However, helical strakes are very effective in eliminating vortex shedding and were even used on submarines as early as the 1930's. 

Regards, Harold Sprague


From: AD(--nospam--at)
To: seaint(--nospam--at)
Date: Wed, 23 Feb 2011 13:37:30 -0600
Subject: RE: Vortex Shedding on Stacks - Vertical Strakes



About 40 years ago, we called out for the manufacturer of 100’ tall light poles to design and fabricate these poles.  We only limited the deflection under 100 mph wind.  After a few summers of withstanding light steady southerly breezes around 8-10 mph, the light fixtures (circular cluster of lights) started falling down.  After a number of suggestions from us, they came up with three vertical strakes of 1”x”x1/4”? angles, the first one went a third of the way up,  the second was 120 degrees away and went up another third of the way and last one (another 120 degrees off) went up the rest of the way.  That completely eliminated the vortex shedding and we’ve never had any more problems.  Hope that helps.


Tony Diaz


From: Harold Sprague [mailto:spraguehope(--nospam--at)]
Sent: Tuesday, February 22, 2011 11:01 AM
To: seaint(--nospam--at)
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

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