Characteristics of the Wind - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 487

Vertical Profiles of the Steady Wind
The mean horizontal wind speed is zero at the earth's surface and increases with
altitude in the atmospheric boundary layer. Instantaneous measurements of the horizontal
wind speed at various altitudes, u ( t,z ), would typically appear as illustrated in Figure 8-10.
The variation of wind speed with elevation is referred to as the vertical profile of the
wind speed or the wind shear. This instantaneous profile shows a number of peaks associated
with gusts or turbulent “eddies” of different sizes, whose locations and strengths are time-
dependent. Averaging is generally necessary to obtain the steady wind speed profile, U ( z ) .
Variations in the steady wind speed also occur in a horizontal direction, but these are
generally not significant over length scales important to wind turbines, except in complex
terrain. Turbulent fluctuations, on the other hand, can create essentially instantaneous,
short-lived variations in wind speed in both the horizontal and vertical directions, and these
will be presented later. Detailed discussions of spatial variations in wind speed within the
atmospheric boundary layer are given by Haugen [1973], Lumley and Panofsky [1964], and
Panofsky and Dutton [1984].
The variation of wind speed with elevation above ground has important influences on
both the assessment of wind energy resources and the design of wind turbines. Assessment
of wind energy resources over a wide geographical area normally requires that anemometer
data from a variety of sources be corrected to a common elevation. To do this, a model
of the variation of wind speed with altitude must be used. Rotor blade fatigue life will be
influenced by the cyclic loads resulting from rotation through a wind field that varies in the
vertical direction, especially when the rotor is not teetered. Power output is affected by the
height of the rotor above ground, so turbine designers must be able to trade off potential
gains in energy output against the costs of taller towers, for example.
Figure 8-10. Typical vertical profiles of the wind speed, both instantaneous and steady,
in the atmospheric layer where wind turbines operate. The steady profile shown repre-
sents “positive” wind shear, in which wind speed increases monotonically with altitude.