intermittent FF wind field with TurbSim

Wind & Water Inflow Turbulence
1

Hi Bonnie,
I have a FAST model of 5 kW HAWT; aiming to instigate the effect of turbulence and intermittency in the wind flow field on the fatigue and loading of the turbine blades.

I am feeding a single-point measured wind time history data into TurbSim (using TIMESR) and generated a full-field file (.bts) that goes into FAST’s AeroDyn subroutine. The simulated full-field seems to carry the turbulent feature of the original measured data, however, the intermittency (incremental velocity [deltau=u(t+tau)-u(t)] with ‘tau’ being time-lag of auto correlation) in the original wind set does not appear in the simulated FF flow field. I extracted the time-series of hub height wind speed from FF formatted file (.u) to check for the intermittency where the intermittent feature seems to be squashed out. Am I missing anything?

Much appreciate your suggestion!

2

Hi Anup,

You should get exactly the same time series output at the single point that you input; I would assume that you set that point to be at the hub. If you are not getting that output, check the time-series input file to make sure your measurement point is exactly on a grid point and that it’s reading the entire time series.

The other points in the simulated full-field file will have phases from a random uniform distribution, though the spatial correlation should make them a bit closer to the distribution from the time-series measurement that you input.

3

Hi Bonnie,
Thanks for the reply. I realized where the problem was. The original wind time series I fed into TurbSim was at different height than the hub height and I let TurbSim do the extrapolation using appropriate PLEXP value.

With revision, I extrapolated the wind data to the hub height outside TurbSim and applied the extrapolated time series to generate FF. Now,the hub-eight single point time series matches the extrapolated TIMESR values as you’ve mentioned in the reply, however the signs are different! Is this an issue?

The extrapolated values in TIMESR are the wind times series that are rotated to align with the mean wind speed (longitudinal, lateral and vertical components).

Much appreciate your suggestion!

4

Anup,

Is the wind direction at the hub 0 degrees? You should be able to see this in the TurbSim summary file.

If it is 0, I would not expect any difference in the wind time series output, including the sign.

5

Hi Bonnie,
I am getting the summary as below for a 12x12 m grid with 19x19 pts:

[code]
Profiles from User-Defined Time-Series Input:

Height Wind Speed Horizontal Angle Vertical Angle
(m) (m/s) (deg) (deg)

18.00 4.78 180.00 0.00

You have requested that the following file(s) be generated:

Aerogen_turbsim.bts (AeroDyn/TurbSim full-field wind file)
Aerogen_turbsim.u (formatted full-field U-component file)
Aerogen_turbsim.v (formatted full-field V-component file)
Aerogen_turbsim.w (formatted full-field W-component file)

Turbulence Simulation Scaling Parameter Summary:

Turbulence model used = User-input time series
Gradient Richardson number = 0.000
Monin-Obukhov (M-O) z/L parameter = 0.000
Monin-Obukhov (M-O) length scale = Infinite
Mean wind speed at hub height = 4.776 m/s

Wind profile type = Power law
Power law exponent = 0.143
Mean shear across rotor disk = 0.039 (m/s)/m
Assumed rotor diameter = 12.000 m
Surface roughness length = 0.030 m

Nyquist frequency of turbulent wind field = 5.000 Hz

Number of time steps in the FFT = 6000
Number of time steps output = 6000
Number of points simulated = 361

Mean Flow Angles:

Vertical = 0.0 degrees
Horizontal = 0.0 degrees

Mean Wind Speed Profile:

Height Wind Speed Horizontal Angle Vertical Angle U-comp (X) V-comp (Y) W-comp (Z)
(m) (m/s) (degrees) (degrees) (m/s) (m/s) (m/s)

 24.0        4.98            0.00            0.00          4.98         0.00         0.00
 23.3        4.96            0.00            0.00          4.96         0.00         0.00
 22.7        4.94            0.00            0.00          4.94         0.00         0.00
 22.0        4.92            0.00            0.00          4.92         0.00         0.00
 21.3        4.89            0.00            0.00          4.89         0.00         0.00
 20.7        4.87            0.00            0.00          4.87         0.00         0.00
 20.0        4.85            0.00            0.00          4.85         0.00         0.00
 19.3        4.83            0.00            0.00          4.83         0.00         0.00
 18.7        4.80            0.00            0.00          4.80         0.00         0.00
 18.0        4.78            0.00            0.00          4.78         0.00         0.00
 17.3        4.75            0.00            0.00          4.75         0.00         0.00
 16.7        4.72            0.00            0.00          4.72         0.00         0.00
 16.0        4.70            0.00            0.00          4.70         0.00         0.00
 15.3        4.67            0.00            0.00          4.67         0.00         0.00
 14.7        4.64            0.00            0.00          4.64         0.00         0.00
 14.0        4.61            0.00            0.00          4.61         0.00         0.00
 13.3        4.58            0.00            0.00          4.58         0.00         0.00
 12.7        4.54            0.00            0.00          4.54         0.00         0.00
 12.0        4.51            0.00            0.00          4.51         0.00         0.00
 [/code]

It seems to me like if the longitudinal wind components (u) time series are positive, then the hub height wind speed from TurbSim FF is negative and vice-versa. Is it because of the 180 deg horizontal angle?

6

Dear Anup,

Yes, it looks like you’ve defined the horizontal angle to be 180 degrees, which will flip the sign of the u and v components.

Best regards,

7

Hi Bonnie,
Thanks for your quick response.

Pardon me if I am stretching this a bit more. The obtained 180 degrees horizontal angle is not user-defined. TurbSim itself calculates the horizontal angle to be 180 or -180 regardless, and flips the sign of u and v components.

For a TIMESR input (rotated to align with mean wind speed) as below:

-------------------------------------------------------------------------------
3	 nComp - Number of velocity components in the file
1	 nPoints - Number of time series points contained in this file (-)
1	 RefPtID - Index of the reference point (1-nPoints)
Pointyi	Pointzi 	!nPoints listed in order of increasing height
(m) (m)
0	18
--------Time Series-------------------------------------------------------------
Elapsed Time	Point01u	Point01v	Point01w
(s)	 (m/s)	(m/s)	(m/s)
0.1	-5.806484029	0.100666024	-0.242708658
0.2	-6.301078609	0.290001668	-0.370310318
0.3	-6.75458697	-0.063503114	-0.389266782
0.4	-6.547569542	0.435988997	-0.80832477
0.5	-6.517950056	0.333349982	-0.807086687
0.6	-6.499797305	0.670912377	-0.913255718
0.7	-6.142139199	0.217008003	-1.005233562
0.8	-6.226535913	0.524925048	-1.115689118
0.9	-6.367628352	0.597918712	-0.907731109
1	-6.422405581	0.803196742	-1.016948583
1.1	-6.734515723	0.714260691	-0.923066852
1.2	-6.716362971	1.051823086	-1.029235881

Turbsim Settings:

--------Turbine/Model Specifications-----------------------
         19   NumGrid_Z       - Vertical grid-point matrix dimension
         19   NumGrid_Y       - Horizontal grid-point matrix dimension
        0.1   TimeStep        - Time step [seconds]
        600   AnalysisTime    - Length of analysis time series [seconds] (program will add time if necessary: AnalysisTime = MAX(AnalysisTime, UsableTime+GridWidth/MeanHHWS) )
      "ALL"   UsableTime      - Usable length of output time series [seconds] (program will add GridWidth/MeanHHWS seconds unless UsableTime is "ALL")
         18   HubHt           - Hub height [m] (should be > 0.5*GridHeight)
        12    GridHeight      - Grid height [m]
        12    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]

--------Meteorological Boundary Conditions-------------------
"TIMESR"      TurbModel       - Turbulence model ("IECKAI","IECVKM","GP_LLJ","NWTCUP","SMOOTH","WF_UPW","WF_07D","WF_14D","TIDAL","API","USRINP","TIMESR", or "NONE")
"PK_Ext_45_18112_Rot.inp"    UserFile  - Name of the file that contains inputs for user-defined spectra or time series inputs (used only for "USRINP" and "TIMESR" models)
"2"       	  IECstandard     - Number of IEC 61400-x standard (x=1,2, or 3 with optional 61400-1 edition number (i.e. "1-Ed2") )
"24"          IECturbc        - IEC turbulence characteristic ("A", "B", "C" or the turbulence intensity in percent) ("KHTEST" option with NWTCUP model, not used for other models)
"NTM"         IEC_WindType    - IEC turbulence type ("NTM"=normal, "xETM"=extreme turbulence, "xEWM1"=extreme 1-year wind, "xEWM50"=extreme 50-year wind, where x=wind turbine class 1, 2, or 3)
"default"     ETMc            - IEC Extreme Turbulence Model "c" parameter [m/s]
"IEC"     	  WindProfileType - Velocity profile type ("LOG";"PL"=power law;"JET";"H2L"=Log law for TIDAL model;"API";"USR";"TS";"IEC"=PL on rotor disk, LOG elsewhere; or "default")
"unused"      ProfileFile     - Name of the file that contains input profiles for WindProfileType="USR" and/or TurbModel="USRVKM" [-]
18		  	  RefHt           - Height of the reference velocity (URef) [m]
"default"     URef            - Mean (total) velocity at the reference height [m/s] (or "default" for JET velocity profile) [must be 1-hr mean for API model; otherwise is the mean over AnalysisTime seconds]
"default"     ZJetMax         - Jet height [m] (used only for JET velocity profile, valid 70-490 m)
"default"  	  PLExp           - Power law exponent [-] (or "default")
"default"	  Z0              - Surface roughness length [m] (or "default")

The summary file has:

Turbine/Model Specifications:

        19  Vertical grid-point matrix dimension
        19  Horizontal grid-point matrix dimension
     0.100  Time step [seconds]
   600.000  Analysis time [seconds]
   600.000  Usable output time [seconds]
    18.000  Hub height [m]
    12.000  Grid height [m]
    12.000  Grid width [m]
     0.000  Vertical flow angle [degrees]
     0.000  Horizontal flow angle [degrees]


Meteorological Boundary Conditions:

    TIMESR  User-input time series spectral model
       N/A  IEC standard
       N/A  IEC turbulence characteristic
       N/A  IEC turbulence type
       N/A  IEC Extreme Turbulence Model (ETM) "c" parameter [m/s]
       PL   Wind profile type
    18.000  Reference height [m]
     4.902  Reference wind speed [m/s]
       N/A  Jet height [m]
     0.143  Power law exponent
     0.030  Surface roughness length [m]


Non-IEC Meteorological Boundary Conditions:

    45.000  Site latitude [degrees]
     0.000  Gradient Richardson number
     0.307  Friction or shear velocity [m/s]
       N/A  Mixing layer depth [m]
       N/A  Mean hub u'w' Reynolds stress
       N/A  Mean hub u'v' Reynolds stress
       N/A  Mean hub v'w' Reynolds stress


Spatial Coherence Models:

   GENERAL  u-component coherence model
   GENERAL  v-component coherence model
   GENERAL  w-component coherence model
(   12.000, 0.353E-03)  u-component coherence parameters
(*********,  0.00    )  v-component coherence parameters
(*********,  0.00    )  w-component coherence parameters
     0.000  Coherence exponent


Profiles from User-Defined Time-Series Input:

  Height   Wind Speed   Horizontal Angle   Vertical Angle
   (m)        (m/s)          (deg)              (deg)    
  ------   ----------   ----------------   --------------
   18.00       4.90        -180.00               -0.00


You have requested that the following file(s) be generated:

   Aerogen_turbsim.bts (AeroDyn/TurbSim full-field wind file)
   Aerogen_turbsim.u (formatted full-field U-component file)
   Aerogen_turbsim.v (formatted full-field V-component file)
   Aerogen_turbsim.w (formatted full-field W-component file)


Turbulence Simulation Scaling Parameter Summary:

   Turbulence model used                            =  User-input time series
   Gradient Richardson number                       =    0.000
   Monin-Obukhov (M-O) z/L parameter                =    0.000
   Monin-Obukhov (M-O) length scale                 =  Infinite
   Mean wind speed at hub height                    =    4.902 m/s

   Wind profile type                                =  Power law
   Power law exponent                               =    0.143
   Mean shear across rotor disk                     =    0.040 (m/s)/m
   Assumed rotor diameter                           =   12.000 m
   Surface roughness length                         =    0.030 m

   Nyquist frequency of turbulent wind field        =    5.000 Hz

   Number of time steps in the FFT                  =     6000
   Number of time steps output                      =     6000
   Number of points simulated                       =      361


Mean Flow Angles:

   Vertical   =   0.0 degrees
   Horizontal =   0.0 degrees

Mean Wind Speed Profile:

   Height    Wind Speed   Horizontal Angle  Vertical Angle  U-comp (X)   V-comp (Y)   W-comp (Z)
     (m)        (m/s)         (degrees)       (degrees)       (m/s)        (m/s)        (m/s)   
   ------    ----------   ----------------  --------------  ----------   ----------   ----------
     24.0        5.11            0.00            0.00          5.11         0.00         0.00
     23.3        5.09            0.00            0.00          5.09         0.00         0.00
     22.7        5.07            0.00            0.00          5.07         0.00         0.00
     22.0        5.04            0.00            0.00          5.04         0.00         0.00
     21.3        5.02            0.00            0.00          5.02         0.00         0.00
     20.7        5.00            0.00            0.00          5.00         0.00         0.00
     20.0        4.98            0.00            0.00          4.98         0.00         0.00
     19.3        4.95            0.00            0.00          4.95         0.00         0.00
     18.7        4.93            0.00            0.00          4.93         0.00         0.00
     18.0        4.90            0.00            0.00          4.90         0.00         0.00
     17.3        4.88            0.00            0.00          4.88         0.00         0.00
     16.7        4.85            0.00            0.00          4.85         0.00         0.00
     16.0        4.82            0.00            0.00          4.82         0.00         0.00
     15.3        4.79            0.00            0.00          4.79         0.00         0.00
     14.7        4.76            0.00            0.00          4.76         0.00         0.00
     14.0        4.73            0.00            0.00          4.73         0.00         0.00
     13.3        4.70            0.00            0.00          4.70         0.00         0.00
     12.7        4.66            0.00            0.00          4.66         0.00         0.00
     12.0        4.63            0.00            0.00          4.63         0.00         0.00

and the corresponding HH u and v components (after simulation):

t	u	v	w
0.1	5.808	-0.101	-0.243
0.2	6.299	-0.29	-0.37
0.3	6.756	0.064	-0.389
0.4	6.546	-0.436	-0.808
0.5	6.52		-0.333	-0.807
0.6	6.498	-0.671	-0.913
0.7	6.144	-0.217	-1.005
0.8	6.225	-0.525	-1.116
0.9	6.369	-0.598	-0.908
1	6.421	-0.803	-1.017

Would appreciate your suggestion!

8

Your TIMESR input is rotated 180 from the mean wind direction. The mean u component should almost always be positive, but you’ve defined them to all be negative in your input file.

TurbSim’s output is actually equivalent to your input file, though. It uses the convenction that u is positive in the mean wind direction, so if you rotate by the direction in the summary file, you will get what you input.

9

Thanks a lot, Bonnie, for clearing up the confusion!!!
Really appreciate your suggestions!
Best,
Anup