Environmental Engineering Reference
In-Depth Information
Correlation of Model Load Calculations with Test Data
A spectrum of lapwise cyclic bending loads measured at two blade stations on the DOE/
NASA Mod-2 2.5 MW HAWT shown in Fig. 3-37 [Boeing 1982] was used for preliminary
veriication of this R-S turbulence model. Equation (2-35d) was used to scale the Clayton
VPA data to the Mod-2 hub elevation of 61.0 m and tip radius of 45.7 m. The two blade sta-
tions at which load measurements were made were located at 21 percent and 65 percent of the
tip radius. Good correlation was found between calculated blade loads and the 50
th
percentile
measured loads in the spectrum at both stations [Spera 1995]. The conclusion drawn is that
an R-S harmonic wind model like that in Equations (2-35) is a useful tool for predicting aver-
age blade fatigue loads in the lapwise direction, as the wind input in a structural-dynamic
computer code. The 99.9
th
percentile cyclic loads at the two measurement stations on the
Mod-2 blade were found to be approximately 3 times this average.
Sample Application of R-S Turbulence Model
To illustrate the application of R-S turbulence modeling of the wind input, consider the
hypothetical onshore and offshore vertical wind proiles whose wind power densities are
shown in Figure 2-23. Steady wind speeds in both of these two proiles at an elevation of 100
m are assumed to be 7.63 m/s. However, the offshore proile has a much smaller wind shear,
which should lead to lower R-S wind turbulence and lower lapwise blade loads. We can use
Equations (2-35) to compare the expected harmonic amplitudes at the onshore and offshore
locations, as a basis for the future calculation of rotor fatigue loads. For this example, the
turbine rotor dimensions are taken to be those of a Mod-2 2.5 MW HAWT, with the site data
listed in Table 2-6.
Table 2-8 lists calculated parameters for the wind ields at the offshore and onshore sites,
followed by a comparison of the irst six harmonic turbulence amplitudes at the tip circumfer-
Table 2-8.
Comparison of Estimated Amplitudes of Wind Turbulence Harmonics at Offshore
and Onshore Sites with the Same High-Level Winds.
The sample rotor swept area at
both sites is that of the 2.5 MW Mod-2 HAWT
Parameter
Wind Vertical Proile
Ratio:
Offshore to Onshore
Onshore Site
Offshore Site
Hub elevation,
H
61.0 m
Same
Rotor tip radius,
R
45.7 m
Same
Annual average wind speed at 100 m
7.63 m/s
Same
0.2141
0.0561
0.0026
Wind shear exponent, a
Annual average wind speed at
reference elevation (10 m),
U
R
4.66 m/s
6.71 m/s
1.44
Annual average wind speed at hub
elevation,
U
0
6.86 m/s
7.43 m/s
1.08
Vertical wind shear across disk, D
U
2.63 m/s
0.79 m/s
0.30
Scale factor,
S
2.21
1.21
0.55
First harmonic amplitude,
A
1
-1.448
-0.630
0.44
Second harmonic amplitude,
A
2
-0.752
-0.446
0.59
Third harmonic amplitude,
A
3
0.555
0.329
“
Fourth harmonic amplitude,
A
5
-0.447
-0.265
“
Fifth harmonic amplitude,
A
5
-0.379
-0.225
“
Sixth harmonic amplitude,
A
6
-0.330
-0.196
“
Search WWH ::
Custom Search