Environmental Engineering Reference
In-Depth Information
Figure 11-11 shows a plot of modal damping ratios, as calculated with NExT, versus
turbine rotation rate for the blade flatwise modes. The damping ratios of these modes gener-
ally increase significantly with turbine rotation rate. For example, the first flatwise damping
ratio increases from 2 percent to 7 percent. The notable exception is the second blade flatwise
mode that drops between 15 rpm and 28 rpm. Such a drop in damping ratio is likely due to
modal coupling to a more lightly damped mode, since modal coupling varies with rotation
speed. This data validates the need to understand the variation in parameters in operating
conditions including rotation rate effects, especially in variable-speed machines or during
start-up/shut-down.
Figure 11-11. Modal damping versus rotor rotation rate for the Sandia/DOE 34-m
VAWT.
[James
et al
. 1993]
Application of NExT to HAWT Systems
Table 11-4 shows the results of comparison between impact excitation and natural ex-
citation on a parked HAWT. In this case the machine was an
Atlantic Orient Corporation
(AOC) 15/50, which is a three-bladed turbine with a 15-m rotor mounted downwind of the
tower. The results show a very close agreement between the measured frequencies and a
typical agreement for the damping values [Rumsey
et al.
1997].
A controlled yaw or upwind-rotor HAWT is the most direct application of NExT to a ro-
tating turbine, as the kinematics of the system are primarily limited to rotation of the blades.
This class of turbine shows the same dynamic characteristics as the rotating VAWT, including
distinct natural frequencies, environment-dependent damping, and rotation-dependent forced
harmonics. NExT has been applied to field results from a
Northern Power Systems
100-kW
HAWT, which has a two-bladed teetering rotor, 17.8 m in diameter [James 1994]. Figure 11-
12 displays some representative data from this analysis. In this case the damping in the 5.34 Hz
edgewise mode is plotted versus wind speed. The damping is seen to have a clear and increasing
trend as the wind speed increases.
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