Civil Engineering Reference
In-Depth Information
3
Strong wind characteristics and turbulence
3.1
Introduction
As the earth's surface is approached, frictional forces play an important role in the
balance of forces on the moving air. For larger storms such as extra-tropical depressions,
this zone extends up to 500-1000 m height. For thunderstorms, the boundary layer is
much smaller—probably around 100 m(see Section 3.2.6). The region of frictional
influence is called the 'atmospheric boundary layer' and is similar in many respects to the
turbulent boundary layer on a flat plate or airfoil at high wind speeds.
Figure 3.1 shows wind speeds recorded at three heights on a tall mast at Sale in
southern Australia (as measured by sensitive cup anemometers, during a period of strong
wind produced by gales from a synoptic depression (Deacon, 1955)). The records show
the main characteristics of fully developed 'boundary-layer' flow in the atmosphere:
• the increase of the average wind speed as the height increases;
• the gusty or turbulent nature of the wind speed at all heights;
• the broad range of frequencies in the gusts in the air flow;
• there is some similarity in the patterns of gusts at all heights, especially for the more
slowly changing gusts, or lower frequencies.
The term 'boundary layer' means the region of wind flow affected by friction at the
earth's surface, which can extend up to 1 km. The Coriolis forces (Section 1.2.2) become
gradually less in magnitude as the wind speed falls near the earth's surface. This causes
the geostrophic balance, as discussed in Chapter 1, to be disturbed, and the mean wind
vector turns from being parallel to the isobars to having a component towards the low
pressure, as the height above the ground reduces. Thus, the mean wind speed may change
in direction slightly with height, as well as magnitude. This effect is known as the
Ekman
Spiral
. However, the direction change is small over the height range of normal structures
and is normally neglected in wind engineering.
The following sections will mainly be concerned with the characteristics of the mean
wind and turbulence, near the ground, produced by severe gales in the higher latitudes.
These winds have been studied in detail for more than 40 years and are generally well
understood, at least over flat homogeneous terrain. The wind and turbulence
characteristics in tropical cyclones (Section 1.3.2) and thunderstorm downbursts (Section
1.3.5), which produce the extreme winds in the lower latitudes, are equally important, but
are much less well understood. However, existing knowledge of their characteristics is
presented in Sections 3.2.5 and 3.2.6. Tornadoes are rare events, but can produce
significant damage in some parts of the world. A simple horizontal profile of wind
components in a tornado vortex is discussed in Section 3.2.7.
