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.

Note that the amplitude of a vibration that is damped follows an exponential function, in case the vibration is linear. In the same way an instability will have increasing amplitudes that follow an exponential function, while a resonance for an undamped mode will be linearly increasing, as illustrated earlier.

Many website pages could be filled with all the knowledge available on wind turbine aeroelasticity, so this short introduction stops here for now. Explaining what whirling modes are, why the change in frequency occurs when looking at stand still frames of reference is explained in the mini lecture that can be downloaded below. Also a short description concerning classical flutter is provided in another mini-lecture. Other important knowledge for wind turbine designers would include at least: which modes have the least damping, which instabilities we know of, how to perform a classical flutter speed calculation and how one can increase the classical flutter speed and so on and so on. However the experience is that it takes more than simply reading the material to get to grips with this complex subject. A more active approach which includes discussion and assignments, is much more effective. And time, to let it all sink in. One option is to follow our training in the field of wind turbine aeroelasticity.

researchgate link

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.

There are many articles dealing with the subject, but the following links are only to reports, articles and papers that are freely accessable. Most links will directly download the pdf.

PhD thesis Jessica Holierhoek, 2008

Aeroelastic stability of idling wind turbines Kai Wang et al. Wind Energy Science 2-2017

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

Wind Energy Science: a very good open access journal

Wind Energy - Wiley: a very good journal with more and more open access articles being published

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