Environmental Engineering Reference
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frequency or damping results from NExT. It should also be noted that the higher modes
(3.65 Hz, 3.73 Hz, and 3.88 Hz) have NExT-estimated damping ratios that are lower than the
specified damping ratios. The amplitudes of these modes are low compared to the noise level,
which affected the estimates. This finding suggested the need for longer time histories to
improve the accuracy. Also, this comparison shows the validity of natural wind excitation as
a tool for wind turbine testing, as well as pointing out the issues (such as period elongations)
associated with numerical integration schemes used in analysis codes.
Comparison of NExT to Non-Rotating Step Relaxation Excitation
Another insightful comparison is between NExT processing using wind excitation and
traditional analysis using step-relaxation excitation. For this comparison, a
FloWind Cor-
poration
19-m VAWT in Altamont Pass, CA, was tested using step-relaxation modal testing
techniques during quiescent daytime winds. NExT was then used during periods of more
substantial nighttime winds (above 7 m/s or 16 mph). The turbine was parked (nonrotat-
ing) during all testing. Accelerometers were used to measure the response at predetermined
locations on the turbine. This allowed a comparison between modal parameters estimated
by NExT and modal parameters estimated using conventional testing techniques (step-
relaxation testing).
Table 11-3 compares the modal frequencies and modal damping ratios of the 19-m
VAWT as determined from conventional testing and from NExT. The two methods pro-
duced estimates of the modal frequencies that are in good agreement, particularly in view of
the temperature difference between day and night. The average difference for the ten modes
is only 0.5 percent. Also, the modal damping ratios of all six of the tower modes (rotor twist,
tower in-plane, tower out-of-plane) are very similar. However, the modal damping ratios of
all four blade flatwise modes (flatwise symmetric and flatwise antisymmetric) are substan-
tially higher from NExT estimates.
Table 11-3.
Comparison of NExT With Step-Relaxation Excitation
[James
et al
. 1993]
Mode
Frequency (Hz)
Damping (%)
Step Relax
NExT
Step Relax
NExT
1st Rotor Twist
2.37
2.38
0.2
0.1
1st Flatwise Antisymmetric
2.48
2.49
0.2
1.3
1st Flatwise Symmetric
2.51
2.51
0.1
1.4
1st Tower Out-of-Plane
2.72
2.76
0.4
0.4
1st Tower In-Plane
3.11
3.15
0.4
0.4
2nd Tower Out-of-Plane
4.53
4.53
0.1
0.1
2nd Flatwise Antisymmetric
5.30
5.31
0.3
0.8
2nd Flatwise Symmetric
5.64
5.65
0.1
0.6
2nd Rotor Twist
6.59
6.62
0.1
0.1
2nd Tower In-Plane
6.64
6.71
0.3
0.6
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