Civil Engineering Reference
In-Depth Information
3.3.1
Turbulence intensities
The ratio of the standard deviation of each fluctuating component to the mean value is
known as the
turbulence intensity
of that component.
Thus,
I
u
=
σ
u
/Ū
(longitudinal)
(3.15)
I
υ
=
σ
υ
/Ū
(lateral)
(3.16)
I
w
=
σ
w
/Ū
(vertical)
(3.17)
Near the ground, in gales produced by large-scale depression systems, measurements
have found that the standard deviation of longitudinal wind speed,
σ
u
,
is equal to 2.5
u
*
to
a good approximation, where
u
*
is the friction velocity (Section 3.2.1). Then the
turbulence intensity,
I
u
,
is given by:
(3.18)
Thus, the turbulence intensity is simply related to the surface roughness, as measured by
the roughness length,
z
0
. For a rural terrain, with a roughness length of 0.04 m, the
longitudinal turbulence intensities for various heights above the ground are given in
Table 3.3.
Thus, the turbulence intensity decreases with height above the ground.
The lateral and vertical turbulence components are generally lower in magnitude than
the corresponding longitudinal value. However, for well-developed boundary-layer
winds, simple relationships between standard deviation and the friction velocity
u
*
have
been suggested. Thus, approximately the standard deviation of lateral (horizontal)
velocity,
σ
υ
,
is equal to 2.20
u*
, and for the vertical component,
σ
w
is given approximately
by 1.3−1.4
u*
. Then equivalent expressions to Equation (3.18) for the variation of
I
υ
and
I
w
with height can be derived:
(3.19)
(3.20)
The turbulence intensities in tropical cyclones (typhoons and hurricanes) are generally
believed to be higher than those in gales in temperate latitudes. Choi (1978) found that
the longitudinal turbulence intensity was about 50% higher in tropical cyclone winds
compared to synoptic winds. From measurements on a tall mast in north-western
Australia during the passage of severe tropical cyclones, convective 'squall-like'
