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Introduction
A feature of the new Princesshay development in Exeter is an open air plaza-style cafeteria area, on a raised platform (see left). However, it is planned to install some underground toilet facilities beneath this area, with the exhaust pipes situated on the edge of the plaza area. Consequently, it is required that the exhaust fumes will be drawn upwards and away from the plaza area.
Flow Analysis were commissioned to assess the risk of the plaza area being contaminated under a variety of wind conditions. Being fairly tightly enclosed on three sides, it was clear from the outset that complex re-circulations could cause problems. CFD simulations were conducted using realistic inflow profiles to examine the flows in detail.
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South-Westerly Wind Conditions
Since local weather data for the area indicate that the wind is from a south-westerly direction for about 50% of the time, these simulations are the most important. Further, the climate is likely to be more clement under this wind direction and so it is more likely that people will be sitting outside.
The figure to the right show the results for a wind-speed of 3 m/s. Referring to the figure, the wind direction is approximately from the left. The dominant flow feature is a horizontal re-circulation which initially draws the exhaust fumes back over the plaza area, but then sweeps it rapidly upwards and away over the church to the rear.
Importantly, the three exhaust streamlines (see the figure to the right) do not dip below the height of the top of the exhaust stacks, and so the risk of contamination is small.
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Other Wind Conditions
For a north-westerly wind, the flow patterns were found to be the most complex and constituted the highest risk of contamination. The figure to the left illustrates the problem. On the figure, the wind direction is from the left.
Although the exhaust from the two leeward stacks is rapidly dispersed, the exhaust from the more sheltered windward stack tends to get 'trapped' and re-circulates over the plaza area. Minor adjustments to the stack height and location made little difference.
When the wind is from the north-east, the flowfield is more discernable. The figure below shows the path-lines that the exhaust from the three stacks follow in steps of 5 seconds for each frame. The wind direction is from the right in the figure.
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The results showed that the fast moving air stream to the east of the plaza tends to draw the air in a re-circulating 'corkscrew' pattern away from the area.
When the wind is from the south-east, the local flow is in a northerly direction, which immediately draws the exhaust up and away from the plaza area.
Conclusions
The effects of a variety of wind speeds and direction on the fume dispersal characteristics have been
analysed by Flow Analysis Ltd. It is concluded that there is only a small risk of contamination of the plaza area, and that this results from a north-westerly wind. The wind is expected to be from this direction for only about 15% of the time. Changes to the heights and positions of the exhaust stacks make no appreciable difference to the dispersion characteristics.
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