High angle of attack at large wind speed

Hi, everyone

I’m performing 10-min simulation with NREL 5MW turbine based on 5MW_Land_DLL_WTurb.fst with NTM condition in the wind speed range of 3m/s-25m/s. When the simulation is performed in large wind speed (e.g. 25m/s), I got the following warnings:

FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:RotCalcOutput:BEMT_CalcOut
put(node 5, blade 1):UA_CalcOutput:UA_BlendSteady:Temporarily turning off UA due to high angle of
attack or low relative velocity. 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(): 3.7688 s

FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:RotCalcOutput:BEMT_CalcOut
put(node 18, blade 2):UA_CalcOutput:Mach number exceeds 0.3. Theory is invalid. This warning will
not be repeated though the condition may persist.

The last two warning is mainly due to the start-up transient and does not repeat in the following time steps. However, for the first warning, the angle of attack at node 5, blade 1 is always high as in the following figure.

My question is:
(1) Are the 2nd and 3rd warning is indeed due to startup transient as I guessed?
(2) Is such high angle of attack normal? Does it match the realistic situation that in high wind speeds the A.o.A is such high?

Thanks in advance!

Dear @Tongzhou.Zhang,

Just a few comments:

  • You will get the first warning regarding “temporarily turning off UA” if either the angle of attack approaches UACutOut (DEFUALT = +/-45 degrees) (the warning is triggered 5-deg before the cut-out) or the relative inflow velocity approaches 0 m/s (the warning is triggered at 1 m/s).
  • You haven’t said what initial conditions you are using, but you can see if improving the initial conditions eliminates the 2nd and 3rd warnings. We typically recommend initializing the rotor speed and the blade-pitch angles to their expected (mean) values conditioned on the mean hub-height wind speed; are you doing that here? You can further improve the initial conditions by setting the initial blade and tower deflections to their expected (mean) values conditioned on the mean hub-height wind speed. Does that eliminate the 2nd and 3rd warnings?
  • I would expect large angles of attack at inboard stations and large variations in angle of attack at higher wind speeds (which likely have higher wind speed standard deviations than at lower mean wind speeds).

Best regards,

Dear Jason,

Thank you for your kindly reply!

Regarding your 2nd comment, the current initial condition I’m using is as follows. I only changed RotSpeed to a roughly selected value as 8 rpm, so that tower strike at startup transients could be avoided in some degrees.

I’m trying to modify the initial condition as suggested, but I don’t think I fully understand your 2nd comment, so I will state what I’m doing in the following text to see if that is right:

The OopDef1, IPDefl, BlPitch(1)-(3) and RotSpeed should be modified to a value determined by the following prodecure:
(1) The mean hub-height wind speed is specified as 8.5 m/s according to an IEC wind turbine class of IIB, which is the situation I’m focusing on.
(2) Given this mean hub-height wind speed, some 10-min simulations could be performed and the mean value of OopDef1, IPDefl, BlPitch(1)-(3) and RotSpeed could be used for the initial condition.

I hope my understanding is correct :disappointed_relieved:

Best regards,

Dear @Tongzhou.Zhang,

Yes, that is correct. In general, we recommend to set the initial rotor speed and initial blade-pitch angles to their expected (mean) values conditioned on the mean hub-height wind speed. Setting initial blade-tip or tower-top displacments could further reduce the initial transients. At 8.5 m/s for the NREL 5-MW baseline wind turbine, I would expect these to be around 9.6 rpm and 0 degree for RotSpeed and BlPitch.

Best regards,

Dear Jason,

Thank you for your reply.

I have modified the initial conditions as follows:

However, when I am performing multiple simulations at 25m/s wind speed with randomly selected random seeds in TurbSim, about 20% of the simulation are aborted due to the two following reasons:

  1. FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:SetDisturbedInflow:TwrInfl:Tower strike.

  2. FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:RotCalcOutput:BEMT_CalcOutput(node 18, blade 2): UA_CalcOutput:Mach number exceeds 1.0. Equations cannot be evaluated.

For the first case, the tower clearance reaches around 3m as shown in the following figure, which triggers the tower strike I guess. I also notice the abnormal behavior of blade pitch as shown below.

For the second case, the mach number becomes unstable and triggers the error.

I have no idea how I can further modify the model to avoid such situation.

Another situation is that, even though OpenFAST terminated normally, the load might be too little. For example, when I ran this model at 19m/s wind speed, the blade root flapwise bending moment may look like this:

The maximum of blade root flapwise bending is only 3000 kNm, which is much lower compared with existing studies. Is it because the turbine is not in the normal working condition?

Dear @Tongzhou.Zhang,

Again, the initial conditions should depend on the mean hub-height wind speed being simulated. I suspect your controller is not responding properly because of poor initial conditions. At 25 m/s for the NREL 5-MW baseline wind turbine, I would expect these to be around 12.1 rpm and 23.5 degree for RotSpeed and BlPitch.

Best regards,

Dear Jason,

I understand. I misunderstood the “mean hub-height wind speed” as the expected value of long-term wind speed where the turbine is working. The initial conditions are dependent on the 10-min average wind speed, which in my case should be the “URef” in TurbSim .inp file. I will try it later.

Thank you very much!

Best Regards,