Calm:

38.5 %

< 3m/s

0246810121416182022242642.6

N

S

W E

Characterizing Inflow Conditions Across the Rotor Disk of a Utility-Scale W

Andrew Clifton1, Julie K. Lundquist1,2, Neil Kelley1, George Scott1, David Jager1, Scott S

1) National Renewable Energy Laboratory, Golden, Colorado. 2) University of Colorado at Boulder, Dept. of Atmospheric and Oceanic Scienc

Figure 1. A wind turbine at the National Wind Technology Center near Boulder, Colorado. The hub is 80 m above ground. The inflow monitoring tower (left, WNW from the turbine) is 135 m tall. Photo by 

Dennis Schroeder NREL/PIX 19013.

1. IntroductionMulti-megawatt utility-scale wind turbines operate in a turbulent, thermally-driven atmosphere where wind speed and air temperature vary with height. Changes in the wind profile across the rotor disk influence the power produced by the turbine, while turbulence can produce harmful transient loads. To characterize the inflow into utility -scale turbines at the National Wind Technology Center (NWTC) near Boulder, Colorado, NREL built two 135-meter inflow monitoring towers (Figures 1 and 2). Measurements are 

synchronized with turbine data systems by GPS time stamps, allowing us to investigate inflow conditions that cause high loads, alarms, or other unusual events.

Figure 2. Instrumentation on the 135-m tower measures mean and turbulent properties of flow into a utility-scale turbine. Photo by 

Dennis Schroeder NREL/PIX 19023.

131 m

Blade tip

100 m76 m50 m30 m15 m

3-D sonic 

anemometers

134 m

80 m

Hub

88 m26 m10 m

3 m

Cup anemometers, wind vanes

Temperature difference

Temperature, pressure, precipitation

2. Data ProcessingWe acquire data at 20 Hz from 73 instrument channels on the 135-m tower (Figure 2). That data is cleaned and 

converted into 10-minute summaries of the inflow conditions (Figure 3). These summaries include:• Quality measures• Average data from each instrument• Profiles of wind speed, direction, and temperature• Turbulence, including turbulent kinetic energy (TKE), 

turbulence intensity, and dissipation rates

• Stability measures, including sensible heat fluxes, 

Richardson Numbers, and Monin-Obukhov length.

20-Hz data

acquisition

•LabVIEW

•260 MB/day

Quality control

•MATLAB

Time 

averaging

•MATLAB

10-minute 

summaries

•MATLAB

•1 GB/day 

Figure 3. Processing 10-minute data from raw 20-Hz files to humanand machine-readable summaries.

0 5 10 15 20

0

50

100150

Hour of Day [UTC]

Frequency [x10 min]

Frequency of z/L (76 m)

0 5 10 15 20

0

50

100150

Frequency [x 10 min]

Frequency of Speed Ri (3:134 m)

Figure 5. The diurnal stability cycle for winds greater than 3 m/s in October 2011. Data are grouped into unstable (‘U’), neutral (‘N’), and stable (S1-3) conditions.

Stability has a clear diurnal cycle, changing from unstable during the day to stable at night (Figure 5).

Turbine directioprofiles Upwind over mFigure 7energy aOctober and incrnumber 4. CoInflow towersmonitoinflow tperformMeasumean fand staof wind turbine expectchangeneed foconditielevati

Local 

midnight

Local 

midday

1996-2010 Speed m s-1

U Unstable Ri < -0.01N Neutral |Ri| < 0.01S1 Slightly stable 0.01 ≤ Ri < 0.05S2 Stable 0.05 ≤ Ri < 0.25S3 Strongly stable Ri ≥ 0.25

Stability by Richardson number, Ri

U Unstable z/L< -0.01

N Neutral |z/L| < 0.01S Stable z/L > 0.01

Stability by Monin-Obukhov length, L

3 m

3. ResultsWinds at the NWTC are strongly seasonal, with frequent strong NW winds during the winter, and less common, weak southerly and northerly winds (Figure 4).

Calm:

21.3 %

< 3m/s

02468101214161820222426

N

S

W E

10.7.11-11.7.11

Figure 4. Wind roses at hub height. Left, 1996-2010 at the west end of the site. Right, October 2011 at a turbine test stand.Figure 6. TKE as a function of stability in October 2011. Data are for wind speeds greater than 3 m/s from all directions.

RiS range

-0.01

Unstable Stable

0.01 0.05 0.25