Environmental Engineering Reference
In-Depth Information
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
6.00
7.00
8.00 9.00
Mean wind speed at 57 m (m/s)
10.00
11.00
12.00
Figure 11-2.
Scatter plot of wind shear versus mean wind speed at 57 m, showing an outlying
mast where the shear is higher than expected for the speed. In this case, a reduction in the shear
could reasonably be applied.
Source:
AWS Truepower.
of thermal stability (negative buoyancy), a more complicated form with an additional
parameter, the stability length, should be used. The effect of this additional parameter
is to increase the shear. Unfortunately, the stability length must be estimated from
temperature data at multiple heights, which is rarely available.
Out of convenience, therefore, most analysts rely only on the neutral form of the
equation. This is important because if the roughness is determined from the vegetation
(or other land cover) characteristics alone, there is a tendency to substantially under-
estimate the wind shear, especially in temperate climates like that of North America.
Thus,
z
0
must usually be treated as an empirical parameter that is fit to the data, much
like the shear exponent. Table 11-1 indicates values of
z
0
corresponding to values of
α
for shears calculated between 40 and 60 m height.
An important question is how the two methods compare when projecting speeds
to hub height. Maintaining a constant value of
z
0
is equivalent to reducing the shear
exponent with increasing height. In going from 60 m to 80 m,
is reduced by about
5-11% in the shear range 0.14-0.35. The impact on the hub-height mean speed ranges
from about
α
.
Thus, the logarithmic approach is the more conservative. However, while there
are certainly many sites where the shear exponent decreases with height, there are
many others where it holds steady or increases. For example, in the Great Plains of
the United States, the well-known nocturnal jet phenomenon (caused by a decoupling
of the lower atmosphere from surface roughness under stable nighttime conditions)
often produces an increasing shear exponent with height. The limited research that has
been published comparing the power law and logarithmic methods does not establish
clearly which is the more accurate under most circumstances.
−
0
.
2%
(α
=
0
.
14
)
to
−
1
.
1%
(α
=
0
.
35
)
Search WWH ::
Custom Search