Environmental Engineering Reference
In-Depth Information
Assuming s
0
,z
/
U
*
= 1.3,
U
*
/U =
k /ln(
z
/
z
o
), and k = 0.4, the turbulence intensities for
neutral atmospheric conditions at elevations less than 600 m can be expressed as
s
0,
x
U
0.52
ln(
z
/
z
o
)
(0.177 + 0.00139
z
)
-0.4
=
(8-19a)
s
0,
y
U
0.52
ln(
z
/
z
o
)
(0.583 + 0.00070
z
)
- 0.8
=
(8-19b)
s
0,
z
U
=
0.52
ln(
z
/
z
o
)
,
z
o
< <
z
< 600
m
(8-19c)
The effect of surface roughness on longitudinal turbulence intensity is shown in Figure 8-18
where the elevation is held constant at 30 m. Both the test data (compiled by Counihan
[1975]) and Equation (8-19a) indicate a strong effect of surface roughness on turbulence
intensity. Test data showing the dependence of turbulence intensity on elevation have been
correlated with Equations (8-19), and the general trend and magnitude are in agreement,
although the data scatter is relatively large.
Figure 8-18. Effect of surface roughness on turbulence intensity.
[data: Counihan 1975]
Influence of Atmospheric Stability on Turbulence Spectra
In general, the critical wind forces on a wind turbine are those that occur at high wind
speeds for which neutral atmospheric conditions can normally be assumed. However, winds
containing significant turbulent energy may also persist for long hours under both stable and
unstable atmospheric conditions at many turbine sites. Good design practice would there-
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