Hi everybody.

For my master thesis, I am evaluating the consequences of using a yaw bearing at the tower base instead of the normal position at the tower top. Can I analyze such a structure with FAST? Is it necessary to do some changes to the source code of FAST to allow for the yaw bearing at the tower base?

Best regards

Jacob K. Pedersen

Dear Jacob,

Without customizing the source code, it is not possible in FAST to model the yaw bearing at the tower base instead of the tower top. However, you may be able to get around this by one of two methods:

- If your tower is axisymmetric, rotation of the tower about its centerline will not change its response. Thus, you may be able to get by simply with modeling the yaw bearing at the tower-top and post-processing the results differently. For example, you may want to consider a tower coordinate system that rotates with the tower, but FAST won’t actually rotate the tower, so, you’ll simply need to apply the rotations to the deflections/loads output by FAST in a post-processing step.
- If you’re yaw angles are not expected to be large (i.e. stay within 20 degrees of zero), you could disable the nacelle-yaw DOF and all of the platform DOFs except the platform-yaw rotation DOF of the ElastoDyn module of FAST v8 (or the structural model of FAST v7), and use the platform-yaw rotation DOF as a tower-base yaw DOF. However, FAST is not currently set up to allow the user to actively control the platform-yaw DOF if you intend to apply active yaw control in your model.

I hope that helps.

Best regards,

Dear Jason.

Thank you for your response. My idea is to make a Tower, which is not axisymmetric (circular) but rather elliptic in order to reduce the material costs. Would it be possible for you to explain a little more about why the Yaw angle must be equal to or smaller than 20 degrees? I will try to follow your advice on disabling the Yaw dof. Without an active yawing control it will then probably not be possible to analyse the load cases in IEC61400-3, which involve a change in Wind direction over a short duration of time?

Thank you.

Best regards

Jacob Pedersen

Dear Jacob,

The platform pitch, roll, and yaw rotational DOFs in the ElastoDyn module of FAST v8 (or the structural model of FAST v7) employ small-angle approximations (with no sequencing of rotations). While nonlinear corrections are included to maintain orthogonality, the platform rotational transformation loses considerable accuracy for rotation angles greater than about 20 deg. (The nacelle-yaw DOF has no such limitation and large angles are allowed.)

Normally yaw-control systems are slow to avoid inducing gyroscopic loads, so, I wouldn’t expect much (if any) actively driven nacelle-yaw motion during IEC-style discrete direction-change events. Regardless, while FAST is not set-up to allow the user to actively control the platform-yaw DOF, you could modify the FAST source code to apply active platform-yaw control if you need that.

Best regards,

Dear Jason,

I have been writing my own multibody dynamics code using Kane’s dynamics like you did for FAST. In your formulation, I have observed that you have small angle approximations for the platform rotation and blade and tower rotations which makes the order of rotation unimportant. If I don’t want to use this approximation for my model, my transformation matrix will be full of sin and cosine functions. Then, how will I account for the order of rotation? And how does the order of rotation affect my numerical results?

It would be really helpful if you could guide me on this issue.

Thank you,

Saptarshi

Dear Saptarshi,

Going beyond small angle approximations where rotation sequence is unimportant will require choosing an appropriate rotational parameterization and ensuring proper relationships between the rotational parameters, rotational velocities, and rotational accelerations. Examples of rotational parameterizations include Euler angles (in various sequences), quaternions, Rodrigues parameters, etc. In BeamDyn, we’ve used Wiener-Milenkovic rotational parameters. You can find descriptions of these approaches in the literature.

Best regards,