I want to model nacelle rigidly fixed on top of tower by using YawSpr and YawDamp in FAST simulation. I chose very big values for the parameters YawSpr and YawDamp, like 9.999E20 and 9.999E16. But FAST aborted the simulation due to small angle violation. If I choose proper DT, the simulation will be very slow.

Which order big values could be chosen to parameters YawSpr and YawDamp to model the rigid nacelle for FAST model ? I tested the maximal values could be 2.999E10 and 3.999E8 for YawSpr and YawDamp respectively without aborting of FAST when DT is unchanged (like 0.00625 s). I am not sure if these values are big enough so that the nacelle are rigidly connected on top of tower in my model. I compared them with the values of 5MW onshore machine, they are 9.028E9 and 1.916E7, only three times and twice biger.

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).

Thanks. I read the FAST manual carefully for Nacelle Yaw again.

Is YawBrMass important to the Nacelle Yaw movement? I compared the results of two tests, one with YawBrMass=200kg, one with 0kg, it seems the results have no big difference.All the examples in CertTest of FAST do not have this mass. The 5MW machine has also YawBrMass=0kg.

How to choose NclMUxn (virtual nacelle inertial measurement unit) ? Could we just put this virtual measurement unit at any position of nacelle that we are interested in ?

The yaw bearing mass is often lumped into the mass of the nacelle. This is how all of the models in the FAST CertTest were made. You can separate out the masses if you wish, but it will have no impact on the dynamic response because the nacelle is modelled as a rigid body in FAST (that is, the yaw bearing mass and nacelle mass are always a fixed distance apart). The only outputs it will effect are the yaw bearing loads, which are calculated by assuming that the yaw bearing mass is lumped at the top of the tower (as opposed to the bottom of the naccelle), just below the point where the yaw bearing loads are calculated (that is, the acceleration/deceleration of the yaw bearing mass due to platform displacement/tower deflection is not measured in the yaw bearing loads).

The virtual nacelle IMU can be located anywhere in the nacelle via inputs NcIMUxn, NcIMUyn, and NcIMUzn. This makes it easy to compare the output of FAST with measurements taken from a real machine (that is, by locating the virtual nacelle IMU in FAST at the location where the IMU is in the real machine).

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

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.

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

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).

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