Civil Engineering Reference
In-Depth Information
The following sections consider natural growth methods requiring long test sections,
methods used for wind tunnels with short test sections and methods developed for
simulating only the inner or surface layer of the atmospheric boundary layer. Finally,
some possibilities for simulations of strong winds in tropical cyclone and thunderstorm
conditions are discussed. Laboratory modelling of these phenomena is still in an early
stage of development, but some ideas on the subject are presented in Section 7.3.4.
7.3.1
Similarity criteria and natural growth methods
The 'ideal' neutral atmospheric boundary layer has two characteristic length scales—one
for the outer part of the flow which depends on the rate of rotation of the earth and the
latitude and on a velocity scale, and one for the flow near the surface itself which
depends on the size and density of the roughness on the surface. The region near the
surface, which is regarded as being independent of the effects of the earth's rotation, has
a depth of about 100 m and is known as the
inner
or
surface layer
.
The first deliberate use of boundary-layer flow to study wind pressure on buildings
was apparently by Flachsbart (1932). However, the work of Martin Jensen in Denmark
provided the foundation for modern boundary-layer wind-tunnel testing techniques.
Jensen (1958) suggested the use of the inner layer length scale or roughness length,
z
0
(see Section 3.2.1), as the important length scale in the atmospheric boundary-layer flow,
so that for modelling phenomena in the natural wind, ratios such as building height to
roughness length
(h/z
0
)
—later known as the Jensen number—are important. Jensen
(1965) later described model experiments carried out in a small wind tunnel in
Copenhagen, in which natural boundary layers were allowed to grow over a fetch of
uniform roughness on the floor of the wind tunnel. In the 1960s, larger 'boundary-layer'
wind tunnels were constructed and were used for wind engineering studies of tall
buildings, bridges and other large structures (Davenport and Isyumov, 1967; Cermak,
1971). These tunnels are either of closed-circuit design (Section 7.2.3) or of open circuit
of the 'sucking' type, with the axial-flow fan mounted downstream of the test section
(Section 7.2.2). In more recent years, several open-circuit wind tunnels of the 'blowing'
type have been constructed with a centrifugal fan upstream of the test section, supplying
it through a rapid diffuser, a settling chamber containing screens and a contraction. As
discussed in Section 7.2.2, the latter system has the advantage of producing nearly zero
static pressure difference across the wind-tunnel walls at the end of the boundary-layer
test section.
A naturally grown rough-wall boundary layer will continue to grow until it meets the
boundary layer on the opposite wall or roof. In practical cases, this equilibrium situation
is not usually reached, and tests of tall structures are carried out in boundary layers that
are still developing, but are sufficient to envelop the model completely. In most cases of
structural tests, more rapid boundary-layer growth must be promoted by a 'tripping'
fence or grid at the start of the test section. Dimensional analysis indicates that the full
height of the atmospheric boundary layer depends on the wind speed and the latitude.
However, the typical height is about 1000m. Assuming a geometric scaling ratio of
1/500, this means that a
minimum
wind-tunnel height of 2 m is required to model the full
atmospheric boundary layer. Usually a lower boundary-layer height is accepted, but the
turbulent boundary-layer flow should completely envelop any structure under test.
