Strong wind characteristics and turbulence - Wind Loading of Structures - page 67

A typical value of I u at 10 m height in open country is 0.2. Then, taking T equal to 600
s and s equal to 2 s, Equation (3.24) gives a value of gust factor of 1.57. A study by
Krayer and Marshall (1992) of four US hurricanes gave a similar value of 1.55. These
values are based on tropical cyclone winds with a wide range of wind speeds, to values as
low as 10 m/s.
An analysis by Black (1992), which appeared to be based on higher wind speeds in
hurricanes, gave a higher value of 1.66 for the gust factor, Û 2s,10m /Ū 10min,10m .
3.3.4 Wind spectra
The probability density function (Section 3.3.2) tells us something about the magnitude
of the wind velocity, but nothing about how slowly or quickly it varies with time. In order
to describe the distribution of turbulence with frequency, a function called the spectral
density, usually abbreviated to 'spectrum', is used. It is defined so that the contribution to
the variance ( or square of the standard deviation), in the range of frequencies from n
to n +d n, is given by S u (n) · d n, where S u (n) is the spectral density function for u(t) . Then,
integrating over all frequencies,
(3.25)
There are many mathematical forms that have been used for S u (n) in meteorology and
wind engineering. The most common and mathematically correct of these for the
longitudinal velocity component (parallel to the mean wind direction) is the von Karman-
Harris form (developed for laboratory turbulence by von Karman (1948) and adapted for
wind engineering by Harris (1968)). This may be written in several forms; Equation
(3.26) is a commonly used non-dimensional form:
(3.26)
where ℓ, is a turbulence length scale.