Capability of air exchange rate to predict ventilation of three-dimensional street canyons

Zuzana Kluková,Štěpán Nosek,Vladimír Fuka

doi:10.1051/epjconf/201921302042

Zuzana Kluková, Štěpán Nosek + Show 1 more

Open Access

https://doi.org/10.1051/epjconf/201921302042

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Abstract

As most of the world’s population lives in cities, it is critical to understand dispersion processes of pollutants in urban areas. This study focuses on so called air exchange rate (ACH) index, which is frequently used by numerical studies to determine ventilation of street canyons without a simulation of a pollution source. These studies applied the ACH on idealised 2D street canyons, where the ventilation acts only through the one opening roof top. There are two pertinent questions: i) is the ACH really capable to predict the ventilation of a street canyon without knowing of a pollutant source; and ii) how much the ACH differs between 2D and 3D street canyons? To answer these questions, we performed large-eddy simulations of pollution of complex 3D street canyons from ground-level line sources. We computed ACHs and spatially-average concentrations for three different street canyons and compared these quantities with those from previous studies. Results clearly demonstrate that these quantities strongly depend not only on street-canyon geometry but also on geometry of surrounding buildings. It is also shown that 2D canyon gives unrealistic result for retention of pollutant within an urban street canyon. The ACH might lead to significant underestimation of the street-canyon ventilation if a source would be outside the canyon.

Highlights

Owing to buildings, people are protected from unfavourable meteorological conditions in cities
The computed positive air exchange rate for all investigated canyons is presented in Fig. 2 separately for the right (ACH+R), left (ACH+L), and top (ACH+T) opening and for the sum of these three parts
80 % of air exchange rate” (ACH)+ is at the top opening of that canyon, while it is more than 85 % in the case of the canyons A1-R and A2-L

Summary

Introduction

People are protected from unfavourable meteorological conditions (e.g., high or low temperatures, precipitation, and high wind speed) in cities These same buildings might interfere in the ventilation of the streets [1,2,3]. The accumulation of pollutants in the street network might worsen the indoor air quality of the surrounding buildings [4,5,6] This is a typical environmental problem as most of the world’s population lives in cities. The street canyons have been subjected to numerous experimental and numerical studies in order to understand the urban ventilation processes Numerical methods such as computational fluid dynamics (CFD) are widely used because they do not suffer from similarity criteria as reduced-scale experiments, provide entire velocity and pollution fields, and they are fully controllable in comparison with field experiments. CFD models need to be validated before interpreting their results

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