Civil Engineering Reference
In-Depth Information
relevant even further information is required with respect to the wind climate on the
construction site. Thus, mean wind statistics must be based on data covering numerous
meteorological observations over several years, as it is the values of
()
Vz
under a
f
large variation of weather conditions that are of interest (or ideally under any possible
weather condition at the site in question). Such statistics are usually performed on the
mean wind velocity at
z
10 m
T
10 min
=
and averaged over a period of
=
. A typi-
f
cal instantaneous wind velocity profile in the main flow direction is illustrated on the left
hand side of Fig. 3.1, together with the mean velocity and turbulence variation with
z
.
Fig. 3.1
The wind velocity and turbulence profiles
A theoretical approach renders a natural logarithmic profile for the height variation of
the mean wind velocity (first shown by Millikan [30])
z
⎧
⎛⎞
f
k
ln
when
z
z
⋅
>
⎪
⎜⎟
(
()
T
f
min
Vz
z
⎪
⎝⎠
10
f
0
=
(3.1)
⎨
V
10
z
⎛ ⎞
⎪
10
min
k
ln
when
z
z
⋅
≤
⎜ ⎟
⎪
T
f
min
z
⎝ ⎠
⎩
0
where the index 10 has been added to
V
, indicating an averaging period of 10 minutes,
while
z
are parameters characteristic to the terrain in question.
The height
mi
z
has been introduced because such a velocity profile has a limited va-
lidity close to the ground, where turbulence and directional effects prevail.
k
,
z
and
min
