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show the typical decrease in turbulence intensity with increasing
wind speed. Clearly, insufficient numbers of 10minute data
sets are available to allow statistically significant comparisons,
but the trends are evident. The low deviations for the single
cycle statistics are much more germane to the current analysis.
For literally thousands of cycles, standard deviations for V∞ are
less than 5% for mean velocities between 5 m/s and 25 m/s. As
one would expect, the sonic anemometers show higher
deviations than the cup or propeller instruments due to
bandwidth. Standard deviations significantly exceed 5% below
5 m/s mean velocity and above 25 m/s. At low mean velocities,
this is due to division by low mean velocities. At high mean
velocities, this is caused by the tendency of sonic anemometers
to give false readings from dust and debris. 0.40 Turbulence Intensity 0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0 10
20
Wind Speed (m/s) Phase II (Prop Vane)
Phase IV (Sonic)
Phase IV (Cup or Bivane)
Phase V (Sonic) 30 Phase II 10min. (Prop Vane)
Phase IV 10min. (Sonic)
Phase IV 10min (Cup)
Phase V (Cup or Bivane) Figure 5: Turbulence intensity versus average wind speed. Figure 4: Number of cycles as a function of mean velocity (V∞)
and yaw error (ϕ). Fig. 5 contains inflow turbulence intensity plotted for
all cycles in two different ways. First, standard deviations were The same format and trends are apparent in ϕ (Fig. 6).
Yaw error data collected from mechanical bivanes are in
excellent agreement with the values for ϕ calculated from the
sonic anemometer. For the single cycle statistics, yaw error
standard deviations are less than 5% for all mean wind speeds
between 7 m/s and 25 m/s. Thus, binning aerodynamic
performance on cycle mean averaged V∞ and ϕ provide a 5 reasonable global overview of blade performance across a wide
range of parametric changes. 25
20
15 3 10
5
0
0 10 20 30 Wind Speed (m/s)
Phase II (Prop Vane)
Phase IV (Sonic)
Phase IV (Cup or Bivane)
Phase V (Sonic) Normal Force Coefficient (Cn) Yaw Error Standard Deviation 30 portion of the chord pointing radially toward the tip with little
deviation.
If the inflow direction produces significant yaw error,
or if the inflow magnitude creates a local angle of attack near
stall, the resulting flow effect can be extremely dynamic. Cyclic
threedimensional unsteady separation and reattachment from
both tower shadow and yawed inflow can occur within a single
rotation cycle. This threedimensional separation in nearstall
and poststall flow conditions is responsible for the large
aerodynamic loads that exceed static stall values. 2
1.5
1
0.5
0 10 Phase II 10min. (Prop Vane)
Phase IV 10min. (Sonic)
Phase IV 10min (Cup)
Phase V (Cup or Bivane) 0 10 20 30 40 0.5
Angle of Attack (deg)
Phase II, 30% Span
Phase II, 80% Span
Phase IV, 47% Span
Phase IV, 80% Span Figure 6: Yaw error standard deviation versus average wind
speed.
COMPARISON WITH TWODIMENSIONAL RESULTS
Normal force coefficients (CN) for both blades and all
span locations are shown as a function of local α in Fig. 7.
There is excellent agreement with twodimensional wind tunnel
data at α’s lower than static stall (α=15.2°). Above stall, data
corresponding to 30% span show the most radical departure,
with CN values reaching values of nearly 3.0. For comparison,
Cp on the suction surface of an S809 at a Reynolds number of
500,000 never decreases below a value of 5.0. The peak CN
occurs at approximately 0.9 [2]. At a Reynolds number of
2,000,000, the peak CN increases to 1.1, again with Cp minima
remaining above 5.0 [9].
Notably, in near and poststall flow, CN values for all
span locations greatly exceed twodimensional wind tunnel
data. Associated Cp minima decrease substantially below static
minima, as documented below. Visualization using fine thread
tufts fixed to the upper blade surface shows attached and twodimensional flow until the onset of stall. As separation initiates
at the trailing edge and propagates forward with increasing α,
flow over the upper surface becomes immediately t...
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 Spring '11
 O.Uzol

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