Have you ever wondered why sometimes the wind turbines (and other similar tall structure) sometime vibrate?
Under some conditions the wind blowing on the tower create vortices.
These vortices appears regularly on both sides of the tower, creating low pressure zones first on one side and then in the other.
For these reason the tower will start moving perpendicularly to the wind, first toward one side and after toward the opposite.
The tower has a very low structural damping – when the oscillation start its reduction is very slow because the steel tower has a limited capacity to absorb the kinetic energy.
It also has a very low natural frequency (the frequency at which the tower will tend to vibrate when subjected to external forces).
The vortices created by the wind will appear at a frequency that depends on the speed of the wind and the diameter of the tower.
The formula to calculate this frequency is very simple:
f = St · U / D
f is the vortex shedding frequency
St is a value called Strouhal number (in our case it is around 0,2)
U is the wind speed
D is the diameter of the tower
At a certain wind speed the vortices will appear and shed at a frequency equal to the natural frequency and the tower will resonate. The wind speed that start the resonance is called the critical speed.
If the wind blow at the critical speed for enough time the amplitude of the vibration will increase and you will see the tower oscillating.
This is a simplistic summary of a very complex phenomenon. It is however a limiting factor for the use of higher steel tower. In additional to the risk of catastrophic failure the wind induce vibrations will also generate additional fatigue loads, shortening the life of the tower.
It is also interesting to observe that a structure has more than a natural frequency.
Vibrations in the lowest natural frequency (first mode) will have this shape:
However, sometime a turbine can vibrate in what is called “the second mode” the second lowest natural frequency):
Following this link you can see a real world example of how is a tower vibrating in the second mode.
What can we do to avoid these dangerous vibrations?
Some solutions are structural - basically aimed at increasing the damping of the tower or changing the way the mass is distributed.
It is not easy to change the mass distribution in a wind turbine tower (basically its an inverted pendulum).
Nevertheless it is possible (and it is becoming increasingly common) to install dampers in the wind turbines, either only temporarily during the installation or as a permanent feature.
Other solutions are aerodynamic - the idea is to change the shape of the tower adding elements that disrupt (or "spoil") the vortices. Conceptually they are similar to the spoilers used in cars or planes, in the sense that they are intended to mitigate an unwanted aerodynamic effect.