Hi @Jason.Jonkman:
I am getting the below error In TurbSim when I change value of Cohexp to make it greater than 0.35; so for 0.40 I got the error. I would like to use a Cohexp value of 0.85. I believe the error has something to do with my grid size, say. If I use the following model parameters I get no error with a Cohexp of 0.85 .
--------Turbine/Model Specifications-----------------------
31 NumGrid_Z - Vertical grid-point matrix dimension
31 NumGrid_Y - Horizontal grid-point matrix dimension
0.05 TimeStep - Time step [seconds]
3600 AnalysisTime - Length of analysis time series [seconds] (program will add time if necessary: AnalysisTime = MAX(AnalysisTime, UsableTime+GridWidth/MeanHHWS) )
3600 UsableTime - Usable length of output time series [seconds] (program will add GridWidth/MeanHHWS seconds unless UsableTime is “ALL”)
119.0 HubHt - Hub height [m] (should be > 0.5GridHeight)
210 GridHeight - Grid height [m]
210 GridWidth - Grid width [m] (should be >= 2(RotorRadius+ShaftLength))
0 VFlowAng - Vertical mean flow (uptilt) angle [degrees]
0 HFlowAng - Horizontal mean flow (skew) angle [degrees]
However, if I use the following model parameters I get the error:
--------Turbine/Model Specifications-----------------------
31 NumGrid_Z - Vertical grid-point matrix dimension
31 NumGrid_Y - Horizontal grid-point matrix dimension
0.05 TimeStep - Time step [seconds]
3600 AnalysisTime - Length of analysis time series [seconds] (program will add time if necessary: AnalysisTime = MAX(AnalysisTime, UsableTime+GridWidth/MeanHHWS) )
3600 UsableTime - Usable length of output time series [seconds] (program will add GridWidth/MeanHHWS seconds unless UsableTime is “ALL”)
150.0 HubHt - Hub height [m] (should be > 0.5GridHeight)
275 GridHeight - Grid height [m]
750 GridWidth - Grid width [m] (should be >= 2(RotorRadius+ShaftLength))
0 VFlowAng - Vertical mean flow (uptilt) angle [degrees]
0 HFlowAng - Horizontal mean flow (skew) angle [degrees]
TurbSim Error
Reading the input file “150m_88mps_wind_shearJCS1_test.inp”.
Calculating the spectral and transfer function matrices:
u-component matrices (general coherence model)
CalcFourierCoeffs:CalcFourierCoeffs_General:Error in Cholesky factorization: LAPACK_SPPTRF:
Leading minor order 111 of A is not positive definite, so Cholesky factorization could
not be completed.
The error occurred in the u-component coherence matrix at frequency 1 (2.76817E-04 Hz) Check the input file for invalid physical properties or modify the coherence.
I think the error is due to the size of my grid. However, I need a sufficiently large grid width to prevent blades from deflecting outside of the grid and causing an error in OpenFAST.
Please let me know if you have any ideas on how to solve this problem.
Thank you.
Regards,
AOAW
Hi @Jason.Jonkman:
I solved the problem by using a coarser grid. Instead of 31 x 31, I used 21 x 21. This coarser grid allows me to use Cohexp of 0.85 without getting an error.
Regards,
AOAW
Hi @Andre.White,
Makes sense. A Cholesky factorization error is typically caused by the use of too high coherence for the given grid spacing.
Best regards,
1 Like
Thanks @Jason.Jonkman. Another thing learnt.
Regards,
AOAW
Hi @Jason.Jonkman:
I have one query.
If the turbine is modelled for the wave only case, what is it that causes shear force and moments at the base of the tower? I had thought, if there were no wind, (whether, parked, idle or operational) then the shear force and moments at the base caused by the wind blowing on the turbine would be negligible. However, in the output I see quite significant values for moment and shear force at base of the tower for the wave only case.
Would be grateful for some clarification.
Thank you.
Regards,
AOAW
Dear @Andre.White,
In the wave-only case, there is no wind-induced effect. However, the waves can induce tower-base shear forces and moments as a result of the structural motion caused by the waves excitation. Gravitational loading (in the absence of wind or wave forcing) will also generate nonzero tower-base loads if there rotor-nacelle assembly CM is offset from the tower centerline.
Best regards,
Thank you @Jason.Jonkman .
Regards,
AOAW
Hi @Jason.Jonkman:
I am trying to run what I thought would have been a straightforward simulation for DTU 10MW Reference wind turbine. I am running the idle case for a turbulent wind speed of 65.8 m/s and yaw misalignment of 30 degrees but I get the following error:
FAST_Solution0:CalcOutputs_And_SolveForInputs:SolveOption1:ED_SD_HD_BD_Orca_InputOutputSolve:LAPAC
K_SGETRF: U( 473, 473)=0. Factor U is exactly singular.
FAST encountered an error during simulation initialization.
Simulation error level: FATAL ERROR
Please let me know if you can assist. When I run the simulation for said case but without yaw misalignment there is no error.
Regards,
AOAW
Dear @Andre.White,
Can you clarify how you are setting the yaw misalignment? Are you enabling the nacelle-yaw degree of freedom (YawDOF = True)? Does your model also have PtfmYDOF = True?
Best regards,
Hi @Jason.Jonkman:
I have set YawDOF to True and PtfmYDOF is also set to True.
Regards,
AOAW
Hi @Jason.Jonkman:
I have also set PCMode and YCMode to 0 in ServoDyn.
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 v3.3.0 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
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,
Hi @Jason.Jonkman:
The DTU 10MW Reference wind turbine.
Regards,
AOAW
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,