Technical Note No.

5
WIND TUNNEL TESTING

This Technical Note is one of five on wind loading for the window and cladding industry. The series
comprises:
TN 2 Introduction to wind loading on cladding
TN 3 Wind loading on wall cladding and windows of low-rise buildings
TN 4 Issues relating to wind loading on tall buildings
TN 5 Wind tunnel testing
TN 6 Pressure equalisation

Introduction When wind tunnel tests are necessary

The estimation of wind effects on and around BS 6399: Part 2 states:

building structures is not an exact science and
the guidance given in BS 6399: Part 2 is a ‘Wind tunnel tests are recommended when the
conservative simplification of the pressures form of the building is not covered by the data
expected over a range of building structures. It within this Standard, when the form of the
is because of the unpredictability of the wind building can be changed in response to the test
and the difficulties of conducting full-scale results in order to give an optimised design, or
experiments, that the method of wind tunnel where loadings are required in more detail than
testing has evolved. These methods of are given in this Standard’.
assessment are considered to be of much greater
direct use to the designer and are the only A further situation where wind tunnel testing
practical way of determining wind loads on the might be considered is when the building has an
structure before construction. unusual site location, for example, where a
building is in close proximity to other tall
In many cases, the justification for model tests is buildings, or the topographical features are
to obtain data which allows the position and size capable of altering the upwind flow (this is
of the structural components to be optimised. explained in more detail in Technical Note 4
The degree of optimisation depends on how well Issues relating to wind loading on tall
and how early the tests are integrated into the buildings).
design process:
Properly conducted wind tunnel tests provide a

At worst - when the design is already tailored assessment of wind effects by
complete - tests only serve to verify the accurately modelling both the building and its
adequacy of the design; immediate surroundings to produce realistic
simulation of the local wind conditions. In

At best - when tests are made before the some situations this may be the only way that a
external shape is finalised - the data can be building can have assured structural and
used to modify the shape to reduce the wind environmental performance, and that economy
loading. of design may be realised.

The decision as to whether or not to undertake a

wind tunnel study will be clear in cases where
© CWCT 2000 January 2000
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Wind tunnel testing TN5

wind has a major influence on the design and/or features. Even when the basic form and
the building is outside of the scope of the orientation are fixed, there is still often scope
prevailing wind codes. In other situations the for minimising wind effects through minor
decision may be less clear and the potential changes or additions to the design of the facade.
benefits of testing must be justified, both
technically and economically. Wind tunnel techniques

Commissioning wind tunnel testing For all wind tunnel testing it is essential to
generate a mean velocity profile and scale and
Many engineers and architects are not familiar intensity of turbulence which are representative
with commissioning, and interpreting the results of those likely to be found at full-scale for the
of , wind tunnel tests. Hence, wind tunnel tests site under investigation. At full-scale, the
are often used as a last resort to provide design turbulence is determined by the type of terrain
data when adequate information can not be upstream from the site and the degree of natural
found from appropriate literature, historic data or man-made obstructions. A critical aspect of
or codified designs. The wind tunnel tests can, wind tunnel testing is the ability to model the
however, be a very powerful tool and can play a surrounding topography and structures to the
much more effective role in the design process required scale, so that pressures on the building
than simply quantifying the wind loads. The are accurately simulated. In the wind tunnel
full potential of wind tunnel studies can only be tests, turbulence is determined by means of:
fully realised when undertaken at the earliest
stages of design. The shape of the structure will
different size grids, to generate large-scale
affect the wind pressures on it, as will the eddies,
massing of buildings in the development.

walls, to slow the air down at ground level,
Furthermore, the shape of the structure will
and
affect the wind speed and direction of the flow
around the structure and predominately make
long stretches of small roughness elements
the difference between an environmentally- which generate the small-scale eddies.
friendly designed site which does not create
excessive wind speeds, to one which does, and It is the combination of these elements and their
dissuades people from using it as a result. The selected sizes which help simulate the air flow
flow chart shown in Figure 1 shows the that would occur under real site conditions. The
fundamental issues that need to be considered four criteria to consider at the design stage are:
when the structure falls outside of the scope of
the codified design guides.
Up wind topography

Local topography
Figure 1

Surrounding buildings
If wind tunnel studies are commissioned at the
conceptual design stage, the size and shape of
Building design/features
the building can be optimised to take full
Wind pressures can be measured using pressure-
advantage of the possibilities of avoiding high
tapped models which measure the localised
wind loads and wind speeds. The effects
pressure over an evenly distributed section of
associated with these high wind speeds around
the structure, as illustrated in Figure 2. Each
tall buildings and the close proximity of
pressure-tap consists of a pipe set into the
surrounding buildings can potentially increase
surface of the model which connects to a
loads, due to flow interference effects such as
pressure transducer, allowing measurement of
wake formation at the rear of buildings and
local wind pressures on the surface of the model
channel flow in confined spaces. The building
at the position of the tapping. The
orientation can also be tested in a number of
measurements from each pressure tap are
configurations to maximise the benefits of
statistically analysed to give appropriate full-
shelter, both from surrounding built and natural
scale equivalent data, and the results used to
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Wind tunnel testing TN5

draw contours of constant pressure over the Assessment of wind action

building surface.
Buildings and structures are immersed in the
Figure 2 Tapping of a model building to lower region of the earth’s atmosphere, and
measure air pressures within this region the wind is essentially
random, varying both in time and space due to
Wind tunnel models turbulence. This phenomena increases with
increased roughness of the ground and the
Models vary from simple block models used fluctuating wind generates pressures which vary
when investigating environmental flow around in both magnitude and position. A typical
structures through to pressure-tapped models pressure-time history plot of the wind would
which may incorporate several hundred taps. In look like that shown in Figure 3, which
all cases it is usual to model surrounding illustrates the variation of pressure on the
structures as simple block models in order to structure over a period of time. It is this
simulate any interactive effects which may fluctuating pressure that needs to be modelled if
occur between a building and its surroundings, realistic pressures are to be measured on the
and to make final adjustments to the boundary building.
layer.
Figure 3 Typical plot of the variation of
Flow visualisation pressure with time

Flow visualisation techniques may be used to With codified design, the fluctuating wind
give a true feeling of the dynamics of the wind pressures P are expressed in a non-dimensional
flow around the building under consideration time-average form as pressure coefficients (Cp)
and its surroundings. The principle aim of these where:
tests are to:
CP = P/½V2

Determine flow direction on horizontal and
vertical surfaces; where:

Determine windy and sheltered areas on

CP is the pressure coefficient
horizontal surfaces;

P is the measured pressure

Determine general flow patterns around
buildings and groups of buildings.

is the air density

V is the mean wind speed at a reference
Common visualisation techniques include:
height over the approach terrain

Particle erosion tests
The pressure coefficient values are zoned across

Smoke traces the front sides, rear and roof of the building for
various building shapes and roof types. The
The particle erosion tests principally work by codified zoned areas go some way to model the
covering the modelled area with appropriate fine distribution of pressure over a building, which
particles (e.g. ground almonds) before the test varies depending on the wind direction. BS
and then examining the area of the model after 6399: Part 2 goes further and allows for
the test to see where no particles remain. These modifications to the pressure coefficients to
zones can then be highlighted as potential windy account for funnelling where buildings are in
zones which may be analysed in more detail close proximity to each other and where
before the structure is finalised. Smoke traces excessive suction pressures are likely to occur
are used mainly for tracing the passage of smoke between the building.
or chemical effluents to assess their potential
effect on surrounding buildings, depending on It is because of the complexity of the wind flow
the direction of the wind. around buildings that wind codes and standards
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Wind tunnel testing TN5

have been derived exclusively from wind tunnel mean square value which gives a statistical
test data. Therefore, it is no surprise to learn method for indicating the pressure with the
that most wind loading codes allow the use of maximum likelihood of being experienced by
data from properly conducted wind tunnel the structure under consideration.
studies in lieu of codified design.
Conclusion
Assessment of wind pressure data
When using codes and design guides for
The problem with trying to analyse the wind assessing wind effects on structures there will
pressure over a building or cladding panel is to always be an element of uncertainty due to their
account for the large fluctuations of recorded inherent simplification and generalisation which
pressures, as illustrated by Figure 2. The cannot take into account local architectural and
fluctuations with time on the wind-ward face topographical features and surrounding
show the variation in pressure at one point on buildings. In general, codes give safe upper-
the building. The pressure taps are used to bound values for the majority of structures but
distinguish between local and global the level of uncertainty increases as the
coefficients which are expressed in non- building’s configuration deviates from the
dimensional terms to obtain appropriate codified design. Properly-conducted wind
pressure coefficient values for the purpose of tunnel tests provide a tailored assessment of
design. A typical trace of pressure coefficient wind effects by accurately simulating both the
plot (Cpe) is shown in Figure 4 and for the model and its immediate surroundings as well as
purpose of analysis: the local wind conditions.

CP indicates the mean value References

C’P indicates the route mean square value
BS 6399: Part 2: 1995, Code of practice for
(RMS)
wind loads, British Standards Institution,

CP indicates the maximum (most positive) London.
value
Cook, N J, 1990, The designer’s guide to wind

CP indicates the minimum (most negative) loading of building structures, Part 2 Static
value Structures, Building Research Establishment.
Figure 4 Typical trace of Cpe with time Wind, Rain, and the Building Envelope;
Invitational Seminar 1994. University of
In the case of analysing the loading actions, both Western Ontario.
the maximum (most positive) and minimum
(most negative) pressures need to be considered.
For the purpose of design, the load-time trace is
taken over a period of time - typically one hour -
from which the complete trace may be
considered in it entirety. However, cladding
design is based around a three second gust
© CWCT 2000
period and the question of what part of the
pressure-time plot should be used is common. It CWCT Technical Notes 1 – 30 have been part-funded
is considered inappropriate to design for the by the DETR under research contract 39/3/338 (CI 1354)
maximum positive pressure and the highest University of Bath, Claverton Down, Bath, BA2
suction pressures for economic reasons; hence it 7AY
is not appropriate to test for the peak value of Tel: 01225 826541; Fax: 01225 826556; email:
the pressure-time plot because this will be cwct@bath.ac.uk; website: www.cwct.co.uk
distorted by the fluctuations over the recorded
period. However, an alternative value that is
considered more statistically valid is the route
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