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Have you ever taken a walk outside and wondered why some chimneys have spiral fins, or wings, near the top? The answer has to do with wind, but maybe not in the way one might think.
When air meets a uniform cylindrical object, it may form oscillating vortices behind the object. This formation is called a Kármán vortex street. In certain circumstances, this repeating airflow makes the chimney resonate – which is not good for it.
With the addition of fins to the outside of the object, the airflow is broken so that this oscillating pattern doesn’t appear. The chimney doesn’t start resonating and there’s a smaller risk of structural damage.
Engineers have designed different types of ways to break up the airflow around chimneys. A popular design is a fin that spirals down the side of the top of the chimney.
Another option would be to taper the chimney which is often not practical.
So, next time you see a chimney with spiral winds coming down the side, you can thank the engineers for coming up with a way to make it safer, sturdier – and more beautiful.