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Outdoor air pollution is now one of the most important major environmental issues. Urban areas are characterized by a wide range of pollutant sources such as building heating, ventilation and air conditioning (HVAC) systems, traffic, industries, and even certain tree species that promote allergenic risks. Urban air pollution is associated with a wide range of acute and chronic health effects.
Wind ventilation of urban areas is critical to outdoor and indoor air quality. Outdoor air quality can be improved by wind when it helps dilute and remove pollutants. This is why we generally observe pollution peaks in cities in the absence of wind.
Outdoor air quality also affects indoor air quality through mechanical and/or natural ventilation, where indoor air is replaced by outdoor air. On the other hand, in highly ventilated areas, the wind comfort and wind safety of pedestrians can be negatively affected. Therefore, in every practical situation, a compromise has to be found between urban ventilation for outdoor and indoor air quality on the one hand and pedestrian wind comfort and wind safety on the other.
Urban wind flux is strongly related to urban morphology as a combination of urban density, the layout of buildings, their shape and their individual dimensions.
Urban pollution flows can be assessed by large-scale measurements.
The following video presents a sample of CFD projects related to particulate dispersion and flux modeling. These projects cover a range of applications focusing on exhaust emissions, gas dispersion and particle flow. The main purpose of many of these analyses is to monitor specified minor species (gases, dust, etc.) and to ensure that they are not being drawn in by other systems or for fresh office air.
The nature of the dispersion of cold or hot chemical atmospheric pollution, the release of dust from all sources is influenced by the wind regime of the territory.
When the wind circulates around an urbanized site or with high roughness, its speed and direction change : swirls at different scales can be formed; for each wind direction, local phenomena are formed where the wind slows down or accelerates, others where the wind vortices are stronger or weaker, where the concentration of pollutants is more or less high. For these reasons, the propagation parameters of the pollutants are not the same for different wind directions.
The wind is unstable and is characterized by an uneven repeatability of directions and speeds generally illustrated by the wind rose. These data can be known for each locality from the results of long-term weather observations. The combination of the results of the calculation of the wind fluxes for all wind directions, taking into account the frequency of these directions, can show the uneven distribution of the average annual concentrations of pollutants downstream of the emission zones.
The analysis of these factors helps to address the following issues:
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