Why chimneys have spiral fins at its outer side ?

        We may have seen that some chimneys have spiral shaped fin outside its cylindrical wall. This spiral is seen, especially, in tall chimneys.

Chimney with spiral fins

The main reason behind the use of this spiral fin is to avoid the vibration of it, thereby avoiding the failure of the chimney.

Here is the animation of the von karman vortex street, when fluid passes through the cylindrical object.

See the von karman vortex street during flow past the cylinder

 In this animation we can see that there is the vortex formations in the wake regions. And it is changing. This flow is due to symmetry of the object. If symmetry is not there the vortex get vanished. When  Eddy formation at two sides of the cylinder, interacts with each other, then this phenomenon happens. There is fluctuation of force acting on the chimney due to change in position of the wake, this leads to the vibration of the chimney.

To avoid the fluctuating force, we have to avoid interaction of the two eddies.. To avoid interaction of the eddies, we have to remove the cause it - symmetry of the object. So we have to make the cylinder asymmetric.  By attaching the spiral fins we are making it asymmetric.Spiral fins effectively create  asymmetric 3D flow, thereby discouraging the alternate shedding of vortices.

That's why tall chimneys have spiral fins attached to it.