Environmental Engineering Reference
In-Depth Information
Equation 15.4 ignores any effect of uncertainty in the heights of the instruments.
The contribution of the height uncertainty can be estimated from the following
equation:
log
+
σ
r
,
h
)
log
h
2
/
(
1
α
obs,
h
=
α
h
1
(15.5)
This is very similar to Equation 15.4, except that the speed ratio uncertainty is replaced
by the height ratio uncertainty,
σ
r
,
h
, and there is an additional factor of the shear
exponent
α
is almost always much less than 1, this component of the uncertainty is usually much
smaller than that associated with the speed ratio. Nevertheless, its contribution can be
significant in some instances (such as when it is not possible to visit the tower to verify
the instrument heights), and where that is so, it should be added to the speed-related
uncertainty through the sum of the squares.
The second component, the uncertainty in the change in wind shear above mast
height, is more difficult to estimate and depends very much on the site. As a rule of
thumb, based on data from very tall towers, AWS Truepower estimates the uncertainty
in the shear exponent above the top of the mast to be 10-20% of the observed shear
depending on the complexity of the terrain and land cover. If the observed shear is
0.20 and the terrain is flat and open, an uncertainty of 0.02 might be assumed; if the
observed shear is 0.30 and the terrain is complex, the uncertainty could be as high as
0.06. (Note that in some cases it is possible to reduce the uncertainty through the use
of remote sensing.)
Combining the two components produces an overall uncertainty in the shear expo-
nent ranging from about 0.04 to 0.10. The corresponding uncertainty in the hub-height
speed is approximated by the following equation:
α
. Since heights can be determined quite accurately in the field, and since
100
h
h
h
2
1
α
σ
v,hh
=
−
(
%
)
(15.6)
The result is sensitive to the relative heights as well as, of course, the shear uncertainty.
For a mast height of 60 m and hub height of 80 m, the range of uncertainty in the
hub-height speed is 1.1-3.0%. For a mast height of 50 m and hub height of 90 m,
the upper bound increases to 6.3%.
15.5 WIND FLOW MODELING UNCERTAINTY
The wind flow modeling uncertainty
5
can be defined as the uncertainty in the average
wind speed at any point relative to the observed wind speeds at the site masts. The
5
This chapter focuses on the uncertainty associated with numerical wind flow models; however, the same
methods can be applied to other quantitative models as well.
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