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Figure 5-16 shows the local axial induction factors measured behind the rotor at various
radial stations at a tip-speed ratio of 8, compared with factors predicted using modified strip
theory. Predicted induction factors show good agreement with test data for the inboard sta-
tions out to about 80 percent of the blade's span, but they are higher near the tip. However,
numerical integration of the local axial induction factors yields an overall rotor thrust coef-
ficient that is consistent with the measured thrust coefficient, as shown in Table 5-5. Differ-
ences between measured and predicted power coefficients listed in this table fall within the
reported range of experimental error (± 2.5percent). On the basis of wind tunnel tests such as
these on small-scale HAWTs and field tests of larger turbines, we conclude that performance
calculations based on strip theory adequately model test results. The tip loss model requires
further refinement.
Figure 5-16. Comparison of experimental and theoretical axial induction factors for
a 3.5-m, two-bladed HAWT tested in a wind tunnel.
[data from Anderson
et al.
1982]
Table 5-5.
Comparison of Predicted and Measured Rotor Parameters for a 3-m HAWT
1
Tip-speed
ratio
l
Axial Induction
Factor,
a
2
Thrust Coefficient,
C
T
Power Coefficient,
C
P
Strip
theory
Wind tunnel
test
Strip
theory
Wind tunnel
test
Strip
theory
Wind tunnel
test
8.0
0.24
0.23
0.69
0.70
0.44
0.42
1
Test data from [Anderson
et al
. 1982]
2
Average for 0.25 < r/R < 0.95
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