Strong wind characteristics and turbulence - Wind Loading of Structures - page 63

Figure 3.4 Velocity components in a tornado.
This model does not define the radial, U r, or vertical, U v , velocity components, but
empirical values of these are shown in Figure 3.4.
Several alternative theoretical models are discussed by Lewellen (1976).
3.3 Turbulence
The general level of turbulence or 'gustiness' in the wind speed such as that shown in
Figure 3.1 can be measured by its standard deviation or root-mean-square. First we
subtract out the steady or mean component (or the slowly varying component in the case
of a transient storm, like a thunderstorm), then quantify the resulting deviations. As both
positive and negative deviations can occur, we first square the deviations before
averaging them, and finally the square root is taken to give a quantity with the units of
wind speed. Mathematically, the formula for standard deviation can be written as:
(3.14)
where U(t) is the total velocity component in the direction of the mean wind, equal to
Ū+u(t), where u(t) is the 'longitudinal' turbulence component, i.e. the component of the
fluctuating velocity in the mean wind direction.
Other components of turbulence in the lateral horizontal direction denoted by υ(f) and
in the vertical direction denoted by w(t) are quantified by their standard deviations σ υ and
σ w , respectively.