Environmental Engineering Reference
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Figure 8-12. Examples of vertical profiles of the steady wind speed modeled by the loga-
rithmic/linear law.
Data are averages and ranges of measurements on 16 consecutive days,
(a) Well-mixed neutral or near-neutral atmospheric conditions at noon (b) Uncoupled stable
conditions at 0500 hours [Data from Sisterson and Frenzen 1978]
Power Law for Vertical Profiles of Steady Wind
A
power law
is commonly used in wind engineering for defining vertical wind profiles
because it is simple and direct. The basic equation of the wind shear power law is
U
=
U
r
(
z
/
z
r
)
a
(8-11)
where a
= empirical wind shear exponent
In general, the exponent a
is a highly variable quantity, often changing from less than
1/7 during the day to more than 1/2 at night over the same terrain, as is the case for the
profiles in Figure 8-12. In early wind energy work, it was recognized that a
varies with
elevation, time of day, season of the year, nature of the terrain, wind speed, temperature,
and various thermal and mechanical mixing parameters [Golding 1955, 1977], Implicit in
a
must be a dependence on atmospheric stability [DeMarrais 1959, Cramer
et al.
1972,
Touma 1977, Panofsky and Dutton 1984, Wade and Walker 1988]. Relationships have been
suggested for calculating a
from the parameters in the logarithmic/linear law, which is
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