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A
model is made of the development and surrounding area, typically at
a scale of 1:200 to 1:400. This is then placed in the 'working section'
of the wind tunnel. The surrounding terrain is simulated upstream,
so that the air flow passing over the model represents the appropriate
wind characteristics, at a scale matching that of the buildings. The
wind characteristics represented include the variation in mean velocity
with height above the ground, as well as the turbulence or 'gustiness'
characteristics and the length scale. Thus the unsteady aerodynamic
interaction of wind flows with buildings is represented.
The wind tunnel model is mounted on a turntable, to allow wind
flows from any direction to be tested.
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Structural and cladding
design
In the case of structural and cladding design, the designer generally
needs to know peak or gust loads. Fluctuating wind pressures are measured
by fitting the model with 'pressure-taps' - brass tubes that end flush
with the surface and are connected to pressure transducers. These
transducers respond to wind pressures acting on the surface of the
model. The measured gust pressures are analysed further to provide
more detailed information than BS 6399*.
* British Standard BS 6399: Part 2: 1997 Loading for buildings, Part
2. Code of practice for wind loads.
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Pedestrian environment
To assess pedestrian environment, 'time-averaged' and 'gust' wind
speeds are measured using velocity probes mounted at pedestrian height.
Flow visualisation tests identify regions where high wind speeds are
likely to occur at pedestrian level. Quantitative measurements are
carried out at a number of locations where high wind speeds are anticipated,
and where a comfortable environment is required (for example, parks,
squares and pedestrian walkways). Measurements are analysed with local
meteorological wind data in order to predict the frequencies with
which different wind speeds will occur. Safety and comfort criteria
are used to assess whether the wind speeds are acceptable. If they
are found to be unacceptable, wind engineers work with the designer
to develop and test remedial measures.
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Ventilation
Time-averaged measurements of surface pressures are carried out using
techniques similar to those employed for measuring wind loads. Pressures
are measured at locations coinciding with ventilation openings and
exhausts. The pressures may be used as boundary conditions to predict
air change rates; typically, the flow paths through a building (windows,
cracks) are represented as a network of flow resistances. Pressures
may also be used in CFD models to predict detailed velocity and temperature
distributions within a room.
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Pollutant dispersion
The pollutant source is represented using tracer gases or smoke, which
is released into the flow with the appropriate momentum and buoyancy
characteristics. Measurements of tracer gas concentration are taken
at key locations to assess dispersion patterns and pollutant concentrations.
Acceptability will depend on a variety of issues including the toxicity
of the pollutant emitted and odour. Any assessment is therefore carried
out with reference to the impact of the pollutant, for which there
may be guidelines or legislation.
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