# YawDyn: FREE Analysis

Hi everyone…
I hope someone of you is still remembering the good old YawDyn
I have a question about the FREE analysis option, as opposed to FIXED Yaw. I inserted the following data:

``````FREE	Yaw Model: FREE or FIXED yaw system
0.0	Initial yaw angle (deg)
0.0	Initial yaw rate (deg/sec)
53860.	Mass moment of inertia about yaw axis (kg-m2)
0.	YawStiff, stiffness of yaw spring (N-m/rad)
0.	YawDamp, yaw damping coefficient (N-m-sec)
50000.	YawFriction, constant friction moment at yaw axis (N-m)
``````

In my intentions, this should mean that when the Yaw torque, in any direction, is bigger than 50000. N*m, than the turbine begins to yaw, subjected to the difference between the actual aerodynamic yaw moment, and 50000.

The case I am trying to simulate is a brake failure, with EWM 50 years + pitch to feather. I use a slow rotation of the blades (they are actually parked) and make the wind “turn” around the turbine at 1deg/sec. I expected to see the turbine finding an equilibrium position, and then following the wind. Instead of this, it oscillates wildly, and what is worse, it continue to give tha same answer even if I use a much bigger YawFriction, and if I use a much lower wind than the EWM50.

What am I doing wrong?
thanks a lot for your attention
Claudio Pedrazzi

Claudio,

I’ve never used this feature, but my guess is that the lack of damping is making the yaw go unstable. I think the huge moment you are applying is enough to cause YawDyn to change the sign on the acceleration every time step. I think YawFrict is intended to be used to model a friction ring rather than a brake.

We really don’t do much with YawDyn any more. I haven’t used it in years. Have you considered using FAST for this sort of thing?

Marshall

Claudio,

I looked breifly into the YawDyn source code and discovered that the implementation of the model for YawFriction is too simple to handle the transition from fixed yaw to slipping yaw. In fact, the model is numerically unstable (as you describe) during the transition. The YawFriction model is only valid when the yaw mechanism continously slips, in which case, YawFriction provides the rate-independent yaw moment.

Unfortunately, FAST does not currently have a rate-independent friction model for yaw. This is a feature we plan to add in the future, but haven’t gotton around to yet.

Jason and Marshall,
thanks for your answers. Of course I am thinking about using FAST, (actually I do have a FAST model of my turbine), and I know YawDyn is not so much used… but as I said before, it is a good tool for quick and dirty simulations.
Now I know that this particular usage is not meaningful.
Thanks a lot
Best regards from Italy