Hi @Jason.Jonkman:

I have set YawDOF to True and PtfmYDOF is also set to True.

Regards,

AOAW

Hi @Jason.Jonkman:

I have set PtfmYDOF to False and the simulation starts but I get a series of errors as shown below.

**FAST_Solution:FAST_AdvanceStates:ED_ABM4:ED_CalcContStateDeriv:SetCoordSy:Small angle assumption**

**violated in SUBROUTINE SmllRotTrans() due to a large blade deflection (ElastoDyn SetCoordSy). The**

**solution may be inaccurate. Simulation continuing, but future warnings from SmllRotTrans() will**

**be suppressed.**

** Additional debugging message from SUBROUTINE SmllRotTrans(): 6.50000E-02 s**

**FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:AD14_CalcOutput:ELEMFRC:BeddoesModel:ATT**

**ACH: Blade #2 element #37 is supersonic! Other elements are likely supersonic as well.**

**Supersonic mach nos. will be set to 0.7 to attempt continuation.**

**AD14_CalcOutput:ELEMFRC:BeddoesModel:ATTACH: Supersonic condition has subsided with Blade #3 element #4. **
FAST_Solution:CalcOutputs_And_SolveForInputs:ED_CalcContStateDeriv:LAPACK_DGETRF: U( 16,
16)=0. Factor U is exactly singular.
CalcOutputs_And_SolveForInputs:SolveOption2:AD14_CalcOutput:ELEMFRC:BeddoesModel:ATTACH: Blade #2
element #22 is supersonic! Other elements are likely supersonic as well. Supersonic mach nos.
will be set to 0.7 to attempt continuation.
AD14_CalcOutput:ELEMFRC:BeddoesModel:ATTACH: Supersonic condition has subsided with Blade #3
element #4.
CalcOutputs_And_SolveForInputs:SolveOption1:ED_SD_HD_BD_Orca_InputOutputSolve:ED_CalcContStateDeri
v:LAPACK_DGETRF: U( 16, 16)=0. Factor U is exactly singular.
** FAST encountered an error at simulation time 0.2 of 3600 seconds. Simulation error level: FATAL ERROR**

It seemed as if if the deflections on the blades were too large, so I increased the blade structural damping but this does not solve he problem.

Regards,

AOAW

Dear @Andre.White,

ElastoDyn does not consider the inertia of the tower about its centerline. So, if both `PtfmYDOF`

and `YawDOF`

= TRUE, you must have some nonzero `PtfmYIner`

set (e.g., to represent the tower inertia about its centerline), otherwise the yaw inertia about the platform will be zero and ill conditioned, resulting in NaNs or a singularity. This is discussed more in online SubDyn documentation–see: 4.2.5.5. Modeling Considerations — OpenFAST v3.2.0 documentation.

Moreover, if you’ve set a nonzero initial nacelle-yaw angle (`NacYaw`

in ElastoDyn) while keeping the neutral yaw position in ServoDyn to `YawNeut`

= 0deg, this will cause the nacelle-yaw angle to quickly snap back to 0deg and result in unrealistic loads and deflections. If you want to keep the nacelle at a nonzero yaw angle and keep the nacelle-yaw DOF enabled (`YawDOF`

= TRUE), then set `YawNeut`

= `NacYaw`

.

Best regards,

Hi @Jason.Jonkman:

Thanks for the clarification. However, I want to clarify some things further:

- Regarding PtffmYDOF and YawDOF =TRUE and PtfmYIner set to non-zero value, is this only considered when yaw misalignment is considered? Such that under normal conditions, say, when YawDOF=False, PtfmYIner=0 is satisfactory.
- When YawDOF=True, Can PtfmYDOF=False and that would solve the problem for the yaw misalignment case? I am asking because I did this and was no longer getting the error for 15 to -15 degree yaw angle. However, angles above 15 deg gave an error.
- Also, when YawDOF = TRUE, do you have to set YawNeut=NacYaw even though YCMode=0 or should YCMode = non-zero value, say 5, when running yaw misalignment case?
- In general, when running the yaw misalignment case, should I set YawNeut=NacYaw?

Regards,

AOAW

Dear @Andre.White,

Here are my responses:

- Regardless of the load case, you can always set
`PtrmYIner`

to represent the otherwise unconsidered rotational inertia of the undeflected tower about its centerline. But it is necessary to set`PtfmYIner`

nonzero when`PtfmYDOF`

=`YawDOF`

= TRUE. - I’m not sure which “error” you are referring to. But I would not expect that you would set
`YawDOF`

or`PtfmYDOF`

differently for cases with and without yaw misalignment. - When
`YawDOF`

= True, I would always recommend setting`YawNeut`

=`NacYaw`

unless you are purposely trying model a case where the nacelle-yaw snaps back to its neutral state. - See 3. This applies for cases with and without yaw misalignment.

Best regards,

Hi @Jason.Jonkman:

Noted. Regarding the response to question 1, two more questions.

- How would I find non-zero value for PtfmYIner.
- Also, would I have to set YCMode = 5 to set YawNeut=NacYaw.

Regards,

AOAW

Hi @Jason.Jonkman:

Regarding the value for PtfmYIner, the section on SubDyn from the theory manual states the following:

“we recommend setting the platform-yaw inertia (**PtfmYIner** ) in ElastoDyn equal to the total rotational inertia of the undeflected tower about its centerline”.

What I really would like to find out is if a value has been given for the rotational inertia of the tower to use for PtfmYIner?

Regards,

AOAW

Dear @Andre.White,

For which wind turbine are you seeking this rotational inertia of the tower for?

Best regards,

Dear @Andre.White,

I have not calculated the rotational inertia of the tower for the DTU 10-MW turbine. But you should be able to calculate it yourself based on the documented diameter and thickness distribution and mass density.

Best regards,

Hi @Jason.Jonkman:

Noted. Thank you.

I have sent you an email. I would be grateful if you would provide a response.

Regards,

AOAW

Hi @Jason.Jonkman:

Is the fore-aft moments at the base of the tower (TwrBsMyt) usually greater than the side-side moments at the base (TwrBsMxt). I did a simulation of an idle turbine using the DTU 10MW reference wind turbine and the maximum fore-aft moment over the simulation is less than max side-side moment. I find this strange as I was expecting the opposite to happen, based on what I saw from a paper. When I simulated the operating condition at rated wind speed, the maximum fore-aft moment was greater than the maximum side-side moment, as expected. Maximum here is the absolute maximum.

Please let me have your feedback as I am not sure if I am doing something wrong causing the fore-aft moments to be less than they should be for the idle turbine.

Regards,

AOAW

Dear @Andre.White,

During operation, I would expect the tower-base fore-aft moment to exceed the tower-base side-side moment because the thrust times tower height typically exceeds the generator torque.

During idling with active yaw control and the blades feathered, I would expect aerodynamic loads on the rotor to be small, and aerodynamic loads on the tower to dominate.

During idling with the blades feathered but loss of yaw control, the loads are likely heavily dependent on yaw-misalignment and you have to watch out for blade-edgewise / tower side-side instabilities.

Best regards,

Hi @Jason.Jonkman:

Regarding the wind veer parameter, it states that it is given in a counterclockwise direction. Does this mean I have to change the sign to make degrees negative or do I have to subtract from 360 degrees?

When I run the DTU model and include the veer angles it causes the deflection on the blades to decrease. Trying to find out why this is so and how I could fix it.

Regards,

AOAW

Hi @Jason.Jonkman besides the blade structural damping, is there any other parameter that could result in lower blade tip deflection than expected?

Regards,

AOAW

Dear @Andre.White,

Regarding veer, using a positive sign for counter-clockwise angles from above and use a negative sign for clockwise angles from above.

I’m not sure I understand your question regarding blade deflection.

Best regards

Thank you @Jason.Jonkman. What I was asking is if the blade structural damping value can alter the blade tip deflections. If this is so (or not), are there other parameters that could alter blade tip deflections.

Regards,

AOAW

Dear @Andre.White,

Certainly the blade structural damping could influence the deflection because damping can temper resonance or instabilities. Blade mass, stiffness, and aerodynamic loads would also influence the deflection.

Best regards,

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