Environmental Engineering Reference
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Rotationally-Sampled Wind Turbulence
The static and dynamic loads acting on a wind turbine strongly affect its initial capital
cost, its operations and maintenance costs, and its reliability. The structural dynamic behav-
ior of wind turbines subject to aerodynamic, inertial, and gravity loads is discussed in detail
in Chapters 10, 11, and 12. An important category of aerodynamic loading is that resulting
from non-uniform wind speeds within the swept area of the rotor. If the wind ield is envi-
sioned as a mixture of local regions of higher and lower wind speeds as illustrated in Figure
8-16, a section of a rotor blade following a circular path through these regions will experience
a repetitively time-varying wind velocity. The rotary motion of the turbine blade converts
steady winds that are non-uniform in space into a wind speed at a given blade section that var-
ies with time. This time variation is referred to as rotationally-sampled turbulence [Veholek
1978, Connell 1981, Kristensen and Brandsen 1982, Connell and George 1983a and 1983b,
George 1984, and Connell 1995].
The basic method for measuring rotationally-sampled (R-S) turbulence is illustrated in
Figure 2-24 [Verholek 1978]. Mounted on a line of instrument towers perpendicular to the
prevailing wind, a set of anemometers is arranged in a circular pattern in a vertical plane,
with center elevation H and radius R (in this case, 24.4 m and 12.2 m, respectively). This set
of towers and anemometers is called a vertical plane array or VPA, and represents the path
Figure 2-24. Schematic diagram of a vertical plane array (VPA) of anemometers for
measuring wind turbulence experienced by a rotating HAWT blade. Sampling of sig-
nals from anemometers 2 to 9 is synchronized with the passage of the hypothetical blade
section A. [Verholek 1978]
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