I would like to run simulations for the NREL 5-MW baseline with FAST, using different wind fields with average windspeeds from 3m/s to 25 m/s. When I use the AeroDyn tower influence model (Bak-model), FAST shows the following warning at higher average windspeeds:
„WARNING: tower model temporarily disabled due to possible tower strike. This message will not“ (unfortunately it doesn´t show the end of the sentence)
The windfields are 10min fullfield wind fields created with TurbSim (NTM).
I couldn´t find specific information about this warning in the AeroDyn theory guide or the user guides.
It can be avoided by increasing the flap and edge stiffness factor of the blades (AdjFlSt and AdjEdSt) to 1.2 or setting the blade precone angle to 8deg.
Are there other ways to work around this problem besides altering the turbine configuration?
The full warning reads, “Tower model temporarily disabled due to possible tower strike. This message will not be repeated though the condition may persist.” This error is produced when a blade aerodynamic analysis node lands within the space occupied by the tower in the tower influence model. Obviously, this implies a tower strike and a breakdown of the tower influence model.
That said, I have ran many cases with the NREL 5-MW wind turbine and with the tower influence model and I have never seen this error before. The NREL 5-MW turbine was designed to have plenty of tip-to-tower clearance to avoid tower strikes. So, I am curious as to why your model shows otherwise. You shouldn’t have to increase the blade stiffness or the clearance to achieve this.
My suspicion is that you have disabled the pitch controller. In high winds (i.e., higher than the rated wind speed of 11.4 m/s), the pitch controller acts to shed power to enable constant power and speed. Along with the shedding of power, the pitch controller also sheds thrust, which should lead to a decrease in the mean blade deflection with increasing wind speed above rated. With this, there should never be a tower strike. If the pitch controller were disabled, thrust and blade deflection (and power) would continually increase with wind speed; but this is not a realistic situation.
the pitch controller is working properly, but i figured out that problem disappears when I use proper initial values. So i think the tower strike was just a problem of the turbine start-up, especially because the tip deflection during the run afterwards doesn´t come even close to the tower.
What might be interesting is that the initial blade azimuth makes the difference in my case. Starting with an initial angle of 0 deg there is a tower strike, with an angle of 180 deg there is none.
Based on the additional information you provided, I agree with you that the problem is a result of the model start-up transient and not an flaw in the turbine design. To minimize the start-up transients, I always recommend that–at a minimum–the initial pitch angles and rotor speed be set appropriotely for the given wind speed. Even in this case, it is common for the start-up transient to last for about 30 s, so, I also recommend that the first 30 s of simulation not be used in post processing.
To follow up on Jason’s comment, you should always look at your time series to see when the start-up transient ends. It can vary. I’ve have had to wait nearly a minute in some cases.
I am running the NREL 5MW Monopile (FAST v8), I am only changing the hub-height wind speeds (a range between 6 to 25 mps), Hs and Tp, and random seeds (simulations of 20 min, 2nd order wave kinematics disabled), the rest of input files are kept the same as it is provided with FAST. The wind fields are generated in TurbSim V2.0 (the generation of *.bts was OK without any error or warning).
For simulations considering low wind speeds (let’s say 6 mps, 12 mps, etc.) the FAST produce very good results but, when I have tried to run simulations for wind speeds near the cut-out it appears a lot of errors of type “Small angle assumption violated”, “Tower strike” in the first seconds of simulations. Then I read in previous posts that it could be a problem of transient model start-up and it could be solved by setting some initial values properly.
My questions are:
a) The initial pitch and rotor speed (‘BldPitch()’ and ‘RotSpeed’ in ElastoDyn input file) are the only initial values I should change?
b) Which could be the strategy to get proper values to avoid the “Tower strike”/“Small angle assumption violated”?
I attach some *.txt files of what I get when I run my simulations for near cut-out wind speeds.
I thank you in advance for any help/comment you can provide.
Here are my answers to your questions:
a) Yes.
b) To minimize the start-up transient, you should set the initial pitch angle and rotor speed to be the mean value expected for the given mean hub-height wind speed.
I am getting the following error when I run the analysis, I have changed the tower geometry. Could you please advice on this.
[code]FAST_Solution0:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:SetInputs:TwrInfl:getLoca
lTowerProps:Tower strike.
SolveOption2:SrvD_CalcOutput:DLL_controller_call:Running with torque and pitch control of the
NREL offshore 5MW baseline wind turbine from DISCON.dll as written by J. Jonkman of NREL/NWTC for
use in the IEA Annex XXIII OC3 studies.
FAST encountered an error during simulation initialization.
Simulation error level: FATAL ERROR
You’d get this error if a blade node falls within the “volume” defined by the tower geometry in the AeroDyn v15 input file. This must be related to whatever it is you changed in the tower geometry (made the diameter wider?).
I changed the diameter which it was in mm before (It was my mistake), thanks for that. After changing the diameter to meter, I am getting the following error.
[code]FAST_Solution:FAST_AdvanceStates:ED_ABM4:ED_AB4:ED_RK4:ED_CalcContStateDeriv:SetCoordSy:Small
angle assumption violated in SUBROUTINE SmllRotTrans() due to a large platform displacement
(ElastoDyn SetCoordSy). The solution may be inaccurate. Simulation continuing, but future
warnings from SmllRotTrans() will be suppressed.
Additional debugging message from SUBROUTINE SmllRotTrans(): 0.375 s
FAST_AdvanceStates:SolveOption2c_Inp2AD_SrvD:InflowWind_CalcOutput:CalcOutput:IfW_UniformWind_Calc
Output:GetWindSpeed:Height must not be negative. Error calculating the wind speed at position
(-267.23, -6799.4, -2529.3) in the wind-file coordinates
FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 5, blade 3):BEMT_UnCoupledSolve:There
is no valid value of phi for these operating conditions: Vx = 3.30951E+06, Vy = -3.51617E+07,
rlocal = 10.16, theta = 0.39483, geometric phi = 3.0477. This warning will not be repeated though
the condition may persist. (See GeomPhi output channel.)
FAST encountered an error at simulation time 0.25 of 90 seconds.
Simulation error level: FATAL ERROR
Now your model appears to have suffered a numerical instability, typically caused by changes to the mass, stiffness or geometry without associated changes to the time step. Similar problems have been discussed many times on this forum.
Dear Jason
I have already got the mex file ,thx for your help! And then I build a simnlink model, but when i run it , it has the error as below :
block ‘Test01_SIG/FAST Nonlinear Wind Turbine/S-Function’: 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(): 9.23 s
In Run_Test01_SIG (line 35)
Warning: SkewedWakeCorrection encountered a large value of chi (94.719 deg), so the yaw
correction will be limited. This warning will not be repeated though the condition may persist.
See the AD15 chi output channels, and consider turning off the Pitt/Peters skew model (set
SkewMod=1) if this condition persists.
Time: 10 of 60 seconds. Estimated final completion at 22:15:31.
FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 17, blade 3):BEMT_UnCoupledSolve:There is no valid value of phi for these operating conditions: Vx = 1.6084, Vy = -74.078, rlocal = 54.07, theta = -1.5598, geometric phi = 3.1199. This warning will not be repeated though the condition may persist. (See GeomPhi output channel.)
OpenFAST completed.
Error reported by S-function ‘FAST_SFunc’ in ‘Test01_SIG/FAST Nonlinear Wind Turbine/S-Function’:
FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:SetInputs:TwrInfl:getLocalTowerProps:Tower strike.
I just build a model with simulink and change the inflow wind at 12m/s with turbsim .I didn’t change the other parameters.
I try to find a solution to the problem in the forum，then i change DT from 0.0125 to 0.005 , set the initial blade pitch to 2.5 and the initial rotspeed to 12.1,but it does not work.
Where did I set it wrong? I’m sorry to trouble you again and again.
We’ll, it sounds like your model has gone numerically unstable, but you haven’t provided enough information for me to know why. A few clarifying questions:
Are you running one of the sample models provided by NREL (in the CertTest or r-test) or something else? If you are running a sample model provided by NREL, what input parameters did you change? Did the model run as expected before changing input parameters?
You say you are running in Simulink. Are you implementing a controller in Simulink or are you running open loop, such that you’d get the same answer if running with the executable apart from Simulink?
Have you set good initial conditions in the model? You say that you have changed the wind speed, and normally we recommend setting the initial rotor speed and initial blade-pitch angle to be their expected mean value for a given mean wind speed.
Thanks for your reply.
1)Yes,I am running Test #24: NREL 5.0 MW Baseline Wind Turbine with OC3 Hywind Configuration in the r-test.And I build the model on the basis of Test01_SIG.mdl.I only change DT from 0.0125 to 0.005 . After i set PCMode and VSControl to 4 , there was an error in the simulation.
2) I have already change the initial rotor speed and initial blade-pitch angle.
Will it be the reason why my model is unreasonable?
You shouldn’t need to change the time step to be smaller than 0.0125 s for the OC3-Hywind model , unless you change the structural properties (mass, stiffness, or geometry).
If PCMode = VSContrl = 5 works well while PCMode = VSContrl = 4 does not, this sounds like a problem in how you’ve defined the controller in Simulink, or the lack of any control in Simulink if you’ve not set that.
I am running analysis for 15MW turbine for IEC load case DLC 6.3 (EWM01) in IECWind, I have considered NacYaw = 8deg, and I am getting the following error (attached the input files).
[code]FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 49, blade
1):UA_UpdateStates:UA_UpdateDiscOtherState:ComputeKelvinChain:Mach number exceeds 0.3. Theory is
invalid. This warning will not be repeated though the condition may persist.
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(): 0.485 s
Time: 1 of 90 seconds. Estimated final completion at 11:03:58.
IEC load case DLC 6.3 involves the modeling of a parked/idling turbine, not an operational wind turbine. It looks like you are applying a very high wind speed, but are keeping the rotor speed, blade-pitch, and controller operational. I would think you’d want to set up your model in a parked/idling condition without active control.
I have run the analysis by considering parked condition. I am getting following error and I have attached the input files.
[code]FAST_InitializeAll:FAST_Init:FAST_ReadPrimaryFile:1/VTK_fps is not an integer multiple of DT.
FAST will output VTK information at 15.385 fps, the closest rate possible.
FAST_InitializeAll: IfW_Init:IfW_UniformWind_Init: Only 1 line in uniform wind file. Steady,
horizontal wind speed at the hub height is 56 m/s.
Time: 0 of 90 seconds.
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(): 6.00000E-02 s
FAST_Solution:FAST_AdvanceStates:SolveOption2c_Inp2AD_SrvD:InflowWind_CalcOutput:CalcOutput:IfW_Un
iformWind_CalcOutput:GetWindSpeed:Height must not be negative. Error calculating the wind speed
at position (-55.591, 143.95, -127.67) in the wind-file coordinates
FAST encountered an error at simulation time 6.00000E-02 of 90 seconds.
Simulation error level: FATAL ERROR
