Civil Engineering Reference
In-Depth Information
3.3.7
Turbulence in a downdraft
The 'rear-flank' thunderstorm downdraft recorded by several towers near Lubbock,
Texas, on 4 June 2002, gave a unique opportunity to study the fluctuating wind
characteristics, near the ground, in a severe event of this type.
Figure 3.8 shows space-time histories from several anemometer towers at 10 mheight,
for this event. This plot shows a series of 'ridges' which indicates remarkable similarity
between time histories over a lateral distance of more than 1 km.
An individual time history from one tower is shown in Figure 3.9(a). By applying a
simple moving-average filter, a smoothed time history that shows the main features of the
event can be extracted. This is shown in Figure 3.9(b), in which a 40s moving average
has been applied; this record can be called a 'running mean'. Subtracting the 'filtered'
history from the original 'unfiltered' history results in a residual time history that is more
or less random in nature and can be described as 'turbulence' (Figure 3.9(c)). This is a
non-stationary time history, and the conventional 'turbulence intensity' (Section 3.3.1), as
defined for stationary synoptic winds, cannot be used here in the same way. However,
Figure 3.9(a) shows that the level of random fluctuation varies with the running mean
(Figure 3.9(b)), with an approximate 'intensity' of 10%. This is somewhat lower than the
level obtained in stationary boundary-layer winds at this height in open country (for
example, Table 3.3 gives a value of 18%), but is similar to that obtained in the Andrews
AFB downburst (Figure 1.9).
Data such as that shown in Figure 3.9 will need further analysis in the future to
understand the characteristics of this type of strong wind event, for applications such as
the dynamic response of structures (Chapter 5).
3.4
Modification of wind flow by topography
Mean and gust wind speeds can be increased considerably by natural and man-made
topography in the form of escarpments, embankments, ridges, cliffs and hills. These
effects were the subject of considerable research in the 1970s and 1980s, with
the incentive of the desire to exploit wind power and to optimize the siting of wind
turbines. This work greatly improved the prediction of mean wind speeds over shallow
topography. Less well defined are the speed-up effects on turbulence and gust
wind speeds and the effects of steep topography—often of interest with respect to
structural design.
