Abstract
Studies are still required to understand how rural/marine wind remove
ground-level pollutants released uniformly in street networks of
high-rise urban areas. The link between building height variability and
pollutant removal process still remains unclear. Several idealized
urban-like neighbourhoods made of 9-row and 18-row small-scale high-rise
square arrays (building width B = street width W, building packing
density lambda(p) = 0.25) were first numerically studied with a parallel
approaching wind and neglecting thermal effects. Normalized pollutant
transport rates and pedestrian purging flow rate were applied to
quantify the contribution of pollutant removal by mean flow and
turbulent diffusion and their net purging capacity. Results show that
the prediction of isothermal turbulent flows agreed generally well with
wind tunnel data. For 9-row arrays with building height variations
(standard deviation of 0-573\%) and the same average canopy height (H-0
= 2.33W), pollutant removal mainly depends on mean flows. Larger
standard deviations tend to induce better pedestrian ventilation. In
comparison to small and large standard deviations, medium values of
14.3-42.9\% may experience smaller purging capacity by horizontal mean
flows but significantly enhance that by vertical mean flows. For arrays
with uniform heights, lowering aspect ratios (H/W = 2.33 and 2.67-1.5)
or increasing street lengths (9-row to 18-row) may enhance the
contribution of removing pollutants by turbulent diffusions across
canopy roofs which may be similarly important as that by mean flows.
Although further investigations are still required, this paper clarifies
the relationship between building layouts, height variability and
removal potential of ground-level pollutants in high-rise urban-like
geometries. (C) 2012 Elsevier Ltd. All rights reserved.
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