# Friction in YawDyn

Hi everyone,

I know YawDyn is no longer a priority but I have still a simple question.
I am currently running a free yaw simulation and I would like to quantify the effects of the friction on my yaw behaviour.
I read in the user’s guide that the yaw friction moment is a constant value. I would like to know why.
Do YawDyn neglect the friction from aerodynamics compared to the gravity one?

Thanks a lot for your attention.
Regards from belgium.

Thomas Duffey

Dear Thomas,

In addition to the aerodynamic damping implicit to the aero-elastic calculations, YawDyn has two additional yaw damping terms. The first is linear yaw damping, where the damping moment is linearly proportional to the yaw rate. The constant of proportionality, or damping coefficient, is input to YawDyn as “AV”. The second is sliding yaw friction, where the damping moment is rate-independent. This is based on the Coulomb model of sliding friction. The damping moment is input to YawDyn as “AF”.

I’m not sure what you mean by, “do YawDyn neglect the friction from aerodynamics compared to the gravity one?” Please clarify your question.

Best regards,

Once again, thank you Jason.

My meaning by "are the friction from aerodynamics neglects compared to the gravity one"is:

• YawDyn is using a coulomb friction Model,
• The main loads applied on the model are:
tilt moment due to shear wind, offset between aerodynamic centre of the rotor and yaw centre…,
• The thrust and the tilt moment are directly depending on the aerodynamic loads thus on the wind,
Then, according to me, the friction moment can not be steady, unless if only the weight of the nacelle is taken into account.

Is it the way to understand it or is my interpretation not correct?

Thomas Duffey.

Dear Thomas,

I agree that the yaw friction model used by YawDyn is simpler than the Coulomb friction model would suggest it should be, given the complications in the time-varying loads that you noted. The developers of YawDyn have assumed that the constant yaw friction model is a reasonable approximation of the Coulomb model (the Coulomb model is not perfect anyway).

Of course, YawDyn is an open source code, so, you could if you wish replace this simple friction model with a more sophisticated model of your own implementation.

Best regards,

I did not mean that the YawDyn friction was unefficient, I was more doubting about my interpretation!
I do not will change the YawDyn friction model because I am not able to do so and I also agree that constant friction could be a correct model.