Dear Dr. Jason,

Thank you for your reply. We have implemented it according to your suggestion. I have another question, though.

The problem is about the definition of the coordinate system for the airfoil-shaped tower sections, see an example in the above attached figure. In this example, the x and y components of the tower aerodynamic force can be written as:

Fx=Fd*cos(alpha)-Fl*sin(alpha)

and

Fy=Fd*sin(alpha)+Fl*cos(alpha)

respectively. Well, this definition conflicts with the Hansen’s definition in his book (“Aerodynamics of wind turbine”, second edition, M.O.L. Hansen, 2008, page 48, see below attached screenshot) that

Fx=Fl*sin(alpha)-Fd*cos(alpha) (PT, Eq.6.11)

and

Fy=Fl*cos(alpha)+Fd*sin(alpha), (PN, Eq.6.10)

with a difference of sign in the Fx component.

In addition, the direction of pitching moment also has something strange. In the present first figure, the positive pitching moment direction is pointing downward along negative z direction (based on the right-hand rule) if viewed from the tower top to the ground level (up-to-down).

I am not sure how the coordinate system of tower section is defined (or by default) in FAST. In order to make sense of it, should I change the positive direction of the y-axis (or x-axis ?) to the opposite, or change the normal direction of the pitching moment to make it point upward along the positive z direction?

I would very much like to know your opinions.

Thank you and best wishes,

Yingyi Liu