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)
Visit our Youtube channel for a short video explaining the phenomena of vortex shedding, with some amazing actual video of stacks that are vibrating due to vortex shedding here: http://youtu.be/hvg0QQt8Ifk. (If link doesn’t work, go to http://www.youtube.com/mecaenterprises )Since Meca specializes in Stack design, we frequently encounter problems with new and existing stacks experiencing vortex induced vibration. It’s the single biggest problem that faces stack designers. As a result, we began offering services to assist Engineers in solving vortex shedding problems. We can help you with evaluation of a specific stack vibration problem, but here are the common solutions as well as their advantages and disadvantages.
|Vortex Shedding Solution||Advantages||Disadvantages|
|Tuned Mass Damper (TMD)|
|Tuned Liquid Damper (TLD)|
|Continuous Helical Strakes|