Large deformation of the tower

Wind & Water Structural Analysis
1

I am interested in studying how large deformation of the tower may affect the turbine aerodynamics, which may in turn affect the loading on the tower. Therefore, the conventional two-step approach of getting the loading from FAST and then using a FE code will not work.

  1. Will it be possible to customize FAST so that the tower part is not based on the modal dynamics but full transient dynamics? If so, can anyone tell me the functions that should be replaced?

  2. When does NWTC expect to publish a FAST theory manual and FAST developer’s manual? I understand it is not easy to have a good estimate, given how busy you are with other works, but any rough estimate will be helpful.

Thanks for your attention.

2

Dear Sungmoon,

I’m not sure what exactly you mean by “large deformation”, but the theory of FAST is based on a number of nonlinearities that allow for what I would call “medium deformation”. Examples of nonlinearities already included in FAST include:

*Corrections for coordinate system orthogonality
*Motions include radial shortening terms (geometric nonlinearity)
*Inertial loads include nonlinear centrifugal, Coriolis, & gyroscopic terms

With these nonlinearities, the tower (and blade) model in FAST permits deflections up to about 15 degrees slope; the model looses considerable accuracy when the slopes greatly exceed 20 degrees. Most wind turbine towers don’t reach nearly these limits (the limits are usually reached first by the blades). So, the conventional approach should be appropriate to most towers.

More inform on the FAST theory basis is available in the following forum topic: Coupled blade modes in FAST.

I’m sure with a bit of work it is possible to add a direct finite-element-based option for the tower. In fact, we at NREL are already currently working on adding such an option (more for the blades than the tower), although such a release is still a ways off.

Best regards,

3

Jason,

By large deformation I meant both material and geometric nonlinearity. The motivation is not really for analyzing the conventional towers but to research if towers built with innovative materials would be possible. It is a purely research topic at this point, but such towers may be composed of somewhat ductile materials. I’m also interested in ultimate strength of the tower (earthquakes, hurricanes). To study these cases, I need to implement the stiffness matrix that is constantly updated.

Thanks for pointing to the theory related materials. If you could elaborate on “with a bit of work it is possible to add a direct finite-element-based option” that would be even better but your link is already helpful.

Sungmoon