Civil Engineering Reference
In-Depth Information
Figure 7.1
Sketch of W.C.Kernot's 'blowing
machine' of 1893.
7.2.2 Open-circuit type
The simplest type of wind-tunnel layout is the open-circuit or NPL type. The main
components are shown in Figure 7.2. The contraction, usually with a flow straightener
and fine mesh screens, has the function of smoothing out mean flow variations and
reducing turbulence in the test section. For modelling atmospheric boundary-layer flows,
which are themselves very turbulent, as described in Chapter 3, it is not essential to
include a contraction, although it is better to start with a reasonably uniform and smooth
flow before commencing to simulate atmospheric profiles and turbulence.
The function of the diffuser, shown in Figure 7.2, is to conserve power by reducing the
amount of kinetic energy that is lost with the discharging air. Again this is not an
essential item, but omission will be at the cost of higher electricity charges.
Figure 7.2 shows an arrangement with an axial-flow fan downstream of the test
section. This arrangement is conducive to better flow, but, as the function of the fan is to
produce a pressure rise to overcome the losses in the wind tunnel, there will be a pressure
drop across the walls and floor of the test section that can be a problem if leaks exist. An
alternative is a 'blowing' arrangement in which the test section is downstream of the fan
(see Figure 7.5). Usually a centrifugal blower is used, and a contraction with screens is
essential to eliminate the swirl downstream of the fan. However, in this arrangement the
test section is at or near atmospheric pressure.
Both the arrangements described above have been used successfully in wind
engineering applications.
Figure 7.2
Layout of an open-circuit wind
tunnel.
