Nacelle Yaw Parameter (YawSpr and YawDamp)

[Ran.Tu]

Dear Jason,

I am confused by YawDof in Elastodyn. Could you please correct, if my understanding is wrong.

1. In my understanding, nacelle is rigidly connected with tower top in terms of translation in x axis, translation in y axis, translation in z aix, rotation around x axis, rotation around y axis. YawDof will determine the relative rotation around z axis between nacelle and tower top. So if I set YawDof = False, nacelle will be rigidly connected with tower top for all 6 DOFS.

2. If YawDof = False and PtfmYDOF =False, that means tower bottom is rigidly fixed. Both tower and nacelle can not rotate around z axis.

3. If YawDof = False and PtfmYDOF =True, that means both tower and nacelle can rotate around z axis. But as nacelle is still rigidly connected with tower top. There is no relative rotation between tower and nacelle during calculation. The rotational stiffness of tower is offered by substructures (i.e. floating platform, mooring system defined in Moordyn, foundation defined in subdyn).

4. Only when YawDof = True, can YawSpr in Servodyn start to work. There could exist relative rotation between tower and nacelle during calculation.

Regards,
Ran

@Jason.Jonkman

Hi Yinping Yang,

To model the nacelle-yaw bearing as a rigid joint, simply disable the yaw DOF (YawDOF = False). Otherwise, setting YawSpr to a very large number will introduce a very high natural frequency in the system model. High natural frequencies require small time steps to avoid numerical instabilities in the solution (which often lead to small angle assumption violation warnings and eventual simulation crash).

I hope that helps,