Abstract
Techniques for improving the removal of pollution from urban canyons are crucial for air quality control in cities. The removal mainly occurs at the building roof level, where it is supported by turbulent mixing and hampered by roof shear, which tends to isolate the internal canyon region from the atmospheric flow. Here, a modification of roof infrastructures is proposed with the aim of increasing the former and reducing the latter, overall enhancing the removal mechanisms. The topic is investigated by numerical experiment, using large-eddy simulation to study the paradigmatic case of a periodic square urban canyon at Re=2 times 10^4. Two geometries are analyzed: one with a smooth building roof, the other having a series of solid obstacles atop the upwind building roof. The pollutant is released at the street level. The simulations are successfully validated against laboratory and numerical datasets, and the primary vortex displacement detected in some laboratory experiments is discussed. The turbulence triggered by the obstacles destroys the sharp shear layer that separates the canyon and the surrounding flow, increasing the mixing. Greater vertical turbulent mass fluxes and more frequent ejection events near the upwind building (where pollution accumulates) are detected. Overall, the obstacles lead to a reduction in the pollution concentration within the canyon of about 34%.
Highlights
More than half of the world’s population lives in urban settlements and the process of urbanization is estimated to progress further in the upcoming decades (United Nations 2018)
Air quality in a city context has a critical impact on human health: 7.6% of total global deaths are attributable to air pollution and the ambient PM2.5 produced by combustion processes
The time-average velocity in two x − y planes is shown: the behind-obstacle plane, that cuts an obstacle at the mid-width, and the behind-gap plane, that passes through the gap centre
Summary
More than half of the world’s population lives in urban settlements and the process of urbanization is estimated to progress further in the upcoming decades (United Nations 2018). Concerning the present topic, LES studies have mostly dealt with a simplified geometry of two-dimensional square canyon: Liu and Barth (2002) simulated passive scalar transport using the Smagorinsky SGS model and wall functions (Smagorinsky 1963) They found that the 97% of pollutant is retained in the canyon during the simulated period, and that turbulent diffusion was the prevailing mechanism for scalar removal. Simulations reproduce a periodic and infinitely long urban canyon, with and without solid obstacles on the upwind building roof The study of such an archetypal configuration aids in understanding the fundamental physical processes that rule the system and analyzing in detail the effects of the obstacles on the overall fluid dynamics. The paper is organized as follow: in Sect. 2 the cases under examination are described; in Sect. 3 the simulation methodology and settings are presented; in Sect. 4 the numerical results are validated against laboratory experiment and numerical datasets; in Sect. 5 the effects of the roof obstacles are analyzed and the enticement in pollutant removal is estimated, and in Sect. 6 the final comments are given
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