Pedestrian Comfort in an Outdoor Shopping Mall

Introduction

The main shopping mall area for the Princesshay development in Exeter is arranged around a walkway 'crossroads'. At an early stage, it was identified that the design could lead to a 'funnelling' effect under the prevailing south-westerly wind which could lead to high local wind speeds and thus cause discomfort to pedestrians.

A design feature is the incorporation of a curved canopy of approximately 45m in length which shelters a portion of the walkway from the elements. From the outset, it was unclear whether or not this canopy, plus some other flat canopies, would have any adverse aerodynamic effects.

Flow Analysis were commissioned to perform both an environmental wind analysis of the site and a thermal analysis of the canopy.

Modelling Approach

The whole of the site, plus some surrounding buildings were modelled to obtain as realistic a simulation as possible. Inflow profiles suitable for an atmospheric boundary layer over the type of terrain in the area were used. The prevailing south-westerly wind direction plus the north-westerly direction were simulated for several realistic wind speeds.

The thermal environment beneath the curved canopy was assessed using both dynamic thermal simulation and CFD. The thermal properties of the glazing and the surrounding building materials were fed into the dynamic thermal simulation software, which predicts hourly surface temperatures using real weather data. This information then formed the basis of the boundary conditions for the CFD simulation, which included a radiation model for predicting resultant (comfort) temperature.

Wind Loading Results

Flow Analysis found that the air flow patterns are highly complex and they are perhaps sometimes counter-intuitive. The surrounding buildings play a highly significant role in influencing the air movement in the regions of interest.

A study of the prevailing south-westerly wind found the flow beneath the canopy in the occupied zone is in fact largely reversed. The wind whips round the southerly corner of the development at fairly high speed, but this occurs mainly above the occupied zone.

For a north-westerly wind, the model revealed a complex interaction between two dominant air streams. However, the net effect is that the high-speed flow is deflected upwards and out of the occupied zone.

Thermal Analysis Results

The three main factors that determine a person's thermal comfort in any environment are the air temperature, the air velocity and the radiant temperature. These three factors are combined into a single parameter called the resultant temperature, which is, in effect the temperature that a person 'feels' in the particular environment.

In this instance, it was not clear whether the canopy would cause an increase in radiative temperature due to a 'greenhouse' effect, or a decrease due to direct solar absorption and reflection. In fact, it was demonstrated by Flow Analysis that the latter was the dominant effect.

A surprising amount of buoyancy-driven air movement was found to be caused solely by surface temperature imbalances. This indicated that in this instance, satisfactory comfort temperatures in the occupied zone, even under conditions of zero wind, were achievable. (see the animation to the right).

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