Civil Engineering Reference
In-Depth Information
turbulence was observed (Wilson, 1979). This was considerably more intense than the
'mechanical turbulence' seen closer to the ground and was associated with the passage of
bands of rain clouds.
Table 3.3
Longitudinal turbulence intensities for rural terrain
(
z
0
=0.04m)
Height, z (m)
I
u
2
0.26
5
0.21
10
0.18
20
0.16
50
0.14
100
0.13
Turbulence intensities in thunderstorm downburst winds are even less well defined
than for tropical cyclones. However, the Andrews Air Force Base event of 1983 (Figure
1.9) indicates a turbulence 'intensity' of the order of 0.1 (10%) superimposed on the
underlying transient flow (see also Section 3.3.7).
3.3.2
Probability density
As shown in Figure 3.1, the variations of wind speed in the atmospheric boundary layer
are generally random in nature and do not repeat in time. The variations are caused by
eddies or vortices within the air flow, moving along at the mean wind speed. These
eddies are never identical, and we must use statistical methods to describe the gustiness.
The probability density,
f
u
(u
0
),
is defined so that the proportion of time that the wind
velocity,
U(t),
spends in the range
u
0
+d
u
is
f
u
(u
0
)
· d
u
. Measurements have shown that the
wind velocity components in the atmospheric boundary layer follow closely the Normal
or Gaussian probability density function, given by:
(3.21)
This function has the characteristic bell shape. It is defined only by the mean value,
Ū,
and standard deviation, σu(see also Section C3.1 in Appendix C).
Thus, with the mean value and standard deviation, the probability of any wind velocity
occurring can be estimated.
