Vortex Shedding

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 Vortex Shedding     Helical Strakes 

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Every stack has a critical wind speed at which vortex shedding occurs.  Vortex Shedding is the instance where alternating low pressure zones are generated on the downwind side of the stack, as shown in the figure.  These alternating low pressure zones cause the stack to move towards the low pressure zone, causing movement perpendicular to the direction of the wind.  When the critical wind speed of the stack is reached, these forces can cause the stack to resonate where large forces and deflections are experienced.  The critical wind speed can be calculated as follows:

Vc = f * D / S

F – Natural Frequency of stack

D – Diameter at the top

S –  0.2  (unless other structures are nearby)