Flow Engineering - Vortex shedding

Vortex shedding

When airflow passes a cylinder it creates alternating low-pressure vortices on the side of the object.

This phenomenon is named Vortex Shedding.

Vortex Shedding can be seen by coloring the flow that passes a cylinder on the upper side of the cylinder red and lower side of the cylinder blue.

Video of vortex shedding:

The object will tend to move toward the low-pressure zone.

Eventually, if the frequency of vortex shedding matches the resonance frequency of the structure, the structure will begin to resonate and the movement can become extremely large.

Tall chimneys constructed of thin-walled steel tube can be sufficiently flexible that, in air flow with a speed in the critical range, vortex shedding can drive the chimney into violent oscillations that can destroy the chimney.

These chimneys can be protected in two ways:

One way is an Aerodynamic Stabilizer.

An aerodynamic stabilizer is a series of flat metal vanes that start at the top and run down the exterior of the chimney for approximately 30% of its length.

The bars are usually shaped in a helical pattern.

The aerodynamic stabilizer will prevent strong vortex shedding by creating smaller separate vortexes.

However adding these bars to the side of the chimney will have some disadvantages:

The building cost is very high because it is complicated to build.
When the chimney is already placed the mounting process of a aerodynamic stabilizer is very difficult and expensive.
An aerodynamic stabilizer will not help against the effects of aerodynamic interference caused by other nearby towers or chimneys.
The chimney will absorb twice as much energy from the wind because of the poor aerodynamically properties of a aerodynamic stabilizer. This means the foundation has to be made larger, and the chimney itself has to be reinforced.

The second way is a Tuned Mass Damper ( TMD )

Tuned mass dampers are mounted near the top of the chimney holding an extra mass, coupled to the chimney by an energy absorbing medium, which absorbs the wind induced energy.

These type of damper have the following advantages compared with an aerodynamic stabilizer.

Tuned mass dampers are relative cheap.
When the chimney is already placed or still under construction the mounting process of a TMD is very simple.
When this type of damper system is attached to a chimney it will increase its structural damping. The increase in structural damping will significantly reduce the crosswind and long wind vibrations including the effects of aerodynamic interference by other nearby towers or chimneys.
Because of their profile and small size, the associated increase in wind drag is minimized.

Therefore a tuned mass damper is a better and more economical solution.

Flow Engineering has developed 4 different types of tuned mass damper systems.

For more information about those dampers we refer to the page Damper Systems.