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full scale, by using anemometers to measure wind speeds and identify areas where the wind speed is acceptable and determine which activities can take place. Combining comfort assessments

Figure 2: Wind patterns around the Wakefield Hospital building.

external resting, training, and cafe areas. The environment surrounding these spaces is one of the most important characteristics that relates to all these areas.

Comfort scales For many years in the UK the Lawson comfort

criteria has been used for the assessment of pedestrian comfort around buildings. This can be classified as one variable assessment and comfort index. It is based on the assessment of the frequency data of wind speeds measured at a meteorological station and the calculated/measured speeds in a boundary layer wind tunnel. The yearly and seasonal frequencies are

linked to the modelled wind speeds calculated by using a boundary layer wind tunnel and/or CFD. In the UK practitioners use at least two variations of this code, where the main differences are related to the way the wind speed exceedences are used for the detailed assessments.3, 4 Most of the work done for outdoor

environments is related to supporting the design of public realm spaces. The Lawson criterion and other indices such as PET, PT and UTCI can be used for this. A detailed description of all indices is perhaps beyond the scope of this article. However, we have included a discussion below of the most relevant aspect of these types of studies. The public realm final design is likely to have an effect on the points raised above.5-10 The different analyses carried out to

evaluate the external and semi-external conditions include solar analysis and wind assessments as well as combination of both or more variable into one single assessment, that look at the energy that is received from solar radiation, local temperature, wind speed and humidity. For a number of years consultants have

used CFD (Computational Fluids Dynamics) techniques for this work. The BRE (BRE 1994) and others have assessed the potential of CFD


for prediction of wind speeds around a complicated urban environment. CWE (Computational Wind Engineering) has been used to resolve planning issues related to the effect of massing buildings within city landscapes.

Modelling techniques Urban wind simulation (impact of wind on

natural ventilation): The combination of macro scale and micro scale (close to buildings) urban wind simulation is required to fully understand the microclimate surrounding a particular development. This is of primary importance for the modelling of wind, pollution dispersion, etc, that can affect natural ventilation strategies within a building. The turbulent boundary layer of the atmosphere is modelled by an approximation of surface roughness. A number of methods exist for assessing

human comfort – many developed independently by different researchers during the 1970s and Melbourne (Melbourne 1978) found that there was remarkable agreement between them. The Building Research Establishment (BRE) has performed a review of this area and BRE Digest 390 (BRE 1994) summarises the techniques used to assess wind speed and methods for relating wind speed to human comfort. It found that people generally accept occasional high wind speeds as a fact of life, but object to particularly accelerated winds at specific locations. For certain activities, such as sitting at a

café, the absolute wind speed is most important and in these cases the number of days during which a critical wind speed will be exceeded may be the most useful measure of serviceability. The techniques most commonly employed

at present are CFD and wind tunnel testing in a Boundary Layer Wind Tunnel or flow visualisation techniques (most commonly scouring methods). Other techniques assess

and the use of thermal indices: There has been an increased interest in the development of comfort indices to assess outdoor environments. In the last decade, several methodologies have been developed and used in a number of projects to aid design strategies and to ensure users are comfortable. One thing that these indices have in common is the number of variables included as part of the methodology/analysis. Wind, temperature, mean radiant temperature and humidity are becoming the key ‘assessment variables’. A full set, or a combination of two or more of these variables is frequently used when assessing

comfort using different methodologies. The assessment variables can be used to estimate how comfortable these outdoor environments are for their designated activities. The number of models and indices used to

assess outdoor environments is huge. Indices still under development, however, are being used by practitioners and can be found currently supporting design work. Most of them show shortcomings when thermal- physiology and heat exchange theories are combined. The ideal would be to work towards a unified comfort index for both internal and external spaces, but currently the closest to this is called the Universal Thermal Climate Index (UTCI). Within indices or models, the number of

environmental assessment variables used has increased with time and the latest models tend to include a link between external environmental variables (mean wind speed, temperature, mean radiant temperature, humidity) and human body thermal- physiology (mean skin temperature, sweat rate, evaporative heat loss, wetness of the skin, insulation value of clothing). The most common indices linked to semi-external and

external spaces are as follows: • PMVS + (based on the Predicted Mean

• Vote) • PT Perceived Temperature. PET Physiological Equivalent

• Temperature. OUT_SET Outdoor Standard Effective

• Temperature. WCT Wind Chill Temperature.

Some steps have been taken to unify some of the above by the COST Action 730 WG into a more general application model called Universal Thermal Climate Index (UTCI), which combines the advanced multi-node ‘Fiala’ thermo-regulation model within a reference environment for a given set of environmental variables. The UTCI equivalent temperature for a given combination of wind,


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