Aeroelasticity is a complex field of expertise, in wind energy the field of wind turbine aeroelasticity it is a relatively new field of expertise with only a hand full of people that really understand this field. On this page of our website we will try to give a short introduction to this field. There is a short general introduction and there are some mini-lectures that can be downloaded which provide a more detailed explanation of some (interesting) phenomena.
If you would like to find out if you already know something about wind turbine aeroelasticity, you may enjoy looking at our quiz.
A first distinction that has to be made is the distinction between resonance and instabilities. A resonance occurs when there is an excitation frequency that is close to some natural frequency of the system. Every wind turbine has infinitely many natural frequencies corresponding to infinitely many modes. Only the modes at the lowest frequencies for the different components are relevant as the higher modes have a lot of structural damping resolving any possible issues with resonance or instabilities.
On a complete turbine there will of course be interaction between the different components, so a blade mode will show up on a complete turbine as a combination of all blades deforming combined with some tower deformation. So if there is some external force at a frequency close to a natural frequency of the turbine, there can be resonance. From linear vibration theory it is known that this can lead to an infinite amplitude if the frequencies are equal and there is no damping (figure above), to smaller amplification factors for cases where there is some damping and/or some difference between the excitation frequency and the natural frequency (right figure below).
An excitation frequency can come from the rotational speed (tower shadow, rotational sampling of the turbulence,...) or perhaps from pitching actions or electronics. In calculations one has a lot more possibilities in creating an excitation frequency, for example one could have a sinusoidal wind speed. On a real turbine the excitation is usually related to the RPM and its multiples and possibly from the controls.
An aeroelastic instability is not caused by any excitation frequency. In case of an aeroelastic instability the shape of a natural mode at the corresponding natural frequency adds energy to the vibration from the aerodynamics. In one cycle the mode shape changes the aerodynamic forces in such a way that energy is extracted from the air to increase the vibration of the structure. So for instabilities the shape is the most important property, and usually changing the model such that this shape changes is the solution to the instability. This also illustrates why focusing purely on the frequencies and avoiding resonance is not enough to provide an aeroelastically sound design, one needs to ensure that all natural modes on the turbine are well damped. Such that some sudden change in for example the wind speed, results in a vibration that is quickly damped, as illustrated below.
For more information on wind turbine aeroelasticity, please refer the the following literature:
Chapter 5 in: BOOK: Advances in wind turbine blade design and materials
There are of course several other publications available concerning this subject, for example in the wind energy journal.
Also we have some mini-lectures provided below.
In this mini-lecture we explain the frequency shift from rotating frame to non-rotating frame: why, when and how.
Our second mini-lecture, this provides more information on classical flutter.
Why edgewise frequency at 6P could perhaps be a good idea...
Modal dynamics of structures with bladed isotropic rotors and its complexity for two-bladed rotors Morten Hansen Wind Energy Science 1-2016
Modal properties and stability of bend–twist coupled wind turbine blades Alexander Stäblein et al. Wind Energy Science 2-2017
PhD thesis Aeroelastic modal dynamics of wind turbines including anisotropic effects, Peter Fisker Skjoldan, 2011
Multiblade Coordinate Transformation and Its Application to Wind Turbine Analysis G. Bir, 2008 - NREL
Field validation of the Stability Limit of a Multi MW turbine Bjarne S. Kallesøe, The science of making torque from wind 2016