# FAST Simulation in yawed flow conditions

To whom it may concern,
I need to run a FAST simulation in yawed flow conditions in order to see the resultant perturbations in the AoA. I tried to either change the parameter PropagationDir and HFlowAng in the InflowWind subfunction, but the former results in really large values while the latter has almost no influence, even with large yaw angles. I also tried to disable the YawDOF and setting a constant NacYaw value (in the ElastoDyn subfunction), but again, it seems to have no influence on the perturbations of the AoA. Could anybody please tell me how to perform such a simulation? Probably there is something i’m doing wrong.

with Kind Regards,
Piero.

Dear Piero,

I would think all three of these changes would result in an oscillatory variation of AoA. So, I’m not sure why you are seeing “almost no influence” in two out of the three approaches. Are you rerunning FAST after changing each input and post-processing the correct output files?

Just one comment: It would better to change PropagationDir in InflowWind rather than HFlowAng in TurbSim. The former rotates the entire wind domain (full-field wind planes) while the latter only adds a transverse component to the wind vectors, leaving the full-field planes to march downwind at the zero-degree wind direction (the Taylor’s frozen turbulence assumption is bit off when changing HFlowAng).

Best regards,

Dear Jason,
Thank you for the response. Yes, i am post-processing the correct output files. My problem is that by setting a PropagationDir value of 25 (which i am ssuming correspondent to a yaw misaligment angle of 25°) the standard deviation of the AoA (e.g. for a wind speed of 17 m/s) is around 13° for a radial position of r/R = 0.4 and about 4° at r/R = 0.95. By performing the difference between the Maximum AoA and the Minimum AoA over a revolution, the situation gets worst, because the values are around 35° for r/R = 0.4 and 11° for r/R = 0.95. Both cases with a descending trend in the intermediate radial positions.

These values seem to be large to me, can i please have your opinion? (I am running the NREL5MW wind turbine for 360s and i am excluding from the analysis the first 180s of outputs to avoid transient behavior)

On the other hand, by changing the HFlowAng and NacYaw values even with 45°, the standard deviation of AoA was less than 1°. This is why i am a bit confused of which value actually represents the Yaw Angle in the simulations and i feel there is something i am doing wrong.

Kind Regards,
Piero.

Dear Piero,

I’m not sure what you are doing differently, but I just ran three 60-s cases of the land-based NREL 5-MW turbine as follows:

1. Test18 unchanged (includes turbulence at 12 m/s mean wind speed, but without a mean yaw error)
2. Test18 with PropogationDir = 20deg
3. Test18 with YawDOF=False and NacYaw=-20deg

With these simulations, I get an AoA range of about 10deg for (1) and 18deg for (2) and (3) at about 40% span and an AoA range of about 8deg for (1) and 10deg for (2) and (3).

Best regards,

Dear Jason.Jonkman

without mean yaw error, is that mean the turbine follow the wind direction without error?
how it realized? the turbulence wind at 12 m/s change the turblence to 5%, the wind direction change fast

``"5"            IECturbc        - IEC turbulence characteristic ("A", "B", "C" or the turbulence intensity in percent) ("KHTEST" option with NWTCUP, not used for other models)``

seen as the figure

Dear Wenting,

By “without mean yaw error”, I don’t mean that the yaw angle instantaneously follows the wind direction. Instead, I mean that the time-invariant mean yaw angle and mean wind direction are the same (this means that NacYaw and PropogationDir have the same magnitude, but opposite signs, because the sign convention differs between InflowWind and ServoDyn).

Best regards,

Dear Jason
In the Test.18 file “DervoDyn” there is a “NacYawF”

```-- NACELLE-YAW CONTROL ------------------------------------- 4 YCMode - Yaw control mode {0: none, 3: user-defined from routine UserYawCont, 4: user-defined from Simulink/Labview, 5: user-defined from Bladed-style DLL} (switch) 0 TYCOn - Time to enable active yaw control (s) [unused when YCMode=0] 0 YawNeut - Neutral yaw position--yaw spring force is zero at this yaw (degrees) 9.02832E+09 YawSpr - Nacelle-yaw spring constant (N-m/rad) 1.916E+07 YawDamp - Nacelle-yaw damping constant (N-m/(rad/s)) 9999.9 TYawManS - Time to start override yaw maneuver and end standard yaw control (s) 0.5 YawManRat - Yaw maneuver rate (in absolute value) (deg/s) 0 NacYawF - Final yaw angle for override yaw maneuvers (degrees)```

and in the inflowWind file there is " PropagationDir"

## [code]------- InflowWind v3.01.* INPUT FILE ------------------------------------------------------------------------- 12 m/s turbulent winds on 31x31 FF grid and tower for FAST CertTests #18, #19, #21, #22, #23, and #24

False Echo - Echo input data to .ech (flag)
3 WindType - switch for wind file type (1=steady; 2=uniform; 3=binary TurbSim FF; 4=binary Bladed-style FF; 5=HAWC format; 6=User defined)
0 PropagationDir - Direction of wind propagation (meteoroligical rotation from aligned with X (positive rotates towards -Y) – degrees)
1 NWindVel - Number of points to output the wind velocity (0 to 9)
0 WindVxiList - List of coordinates in the inertial X direction (m)
0 WindVyiList - List of coordinates in the inertial Y direction (m)
90 WindVziList - List of coordinates in the inertial Z direction (m)[/code]

In the outfut file there is a “NacYaw”

Do you mean that the controller is to control the “NacYaw” in the output file to the opposite of “PropogationDir” , and it is a constant？
Best wishes

Dear Wenting,

NacYawF is the final yaw angle during an override yaw maneuver; you have not enabled the override yaw maneuver (because TYawManS > TMax), so, you shouldn’t need to set NacYawF.

PropogationDir is set to 0, so, to have zero mean yaw error means that NacYaw in the output file should have a time-series mean of 0. I see that you’ve enabled yaw control from Simulink (YCMode = 4), so, I can’t tell how you are changing the yaw angle. Most commonly though, yaw control is disabled (YCMode = 0), in which case to have zero mean yaw error means that both YawNeut (in ServoDyn) and NacYaw (in ElastoDyn) should be set to -PropogationDir (in InflowWind), which in your case is 0.

Best regards,

Dear Jason