The building height ratio of a street canyon significantly affects airflow and pollutant dispersion. Height-asymmetric street canyons exhibit distinct characteristics in terms of pollutant diffusion compared to symmetric street canyons. However, little attention has been given to height-asymmetric street canyons, particularly rarely been considered with different lengths. Based on our previous wind tunnel experiment, this study conducted numerical simulations to study the effects of four wind directions (α = 30°, 45°, and 90°) and three different lengths to height ratios (LHR = 5, 10, and 15) in both step-up and step-down canyons.The ventilation capacity of the canyons was evaluated using average concentration (<InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="10652_2025_10037_Article_IEq1.gif" Format="GIF" Height="19" Rendition="HTML" Resolution="72" Type="Linedraw" Width="50" /> </InlineMediaObject> <EquationSource Format="TEX">${{C}^{}}_{\_avg}$</EquationSource> <EquationSource Format="MATHML"><math> <mmultiscripts> <mrow> <mmultiscripts> <mrow> <mi>C</mi> </mrow> <mrow /> <mrow> <mrow /> <mo>∗</mo> </mrow> </mmultiscripts> </mrow> <mrow> <mi>_</mi> <mi>a</mi> <mi>v</mi> <mi>g</mi> </mrow> <mrow /> </mmultiscripts> </math></EquationSource> </InlineEquation>) and the dimensionless net escape velocity (NEV). The results indicated that longer step-up canyons result in higher concentration of pollutants on the leeward side compared to other sides. Moreover, an updraft occurs in the step-up canyon, causing pollutants to accumulate on the top of the central facade.Short step-down canyons (LHR = 5) do not exhibit reverse flow, while long step-down canyons (LHR = 10,15) experience reverse flow, leading to increased pollutant concentrations. Both short and long canyons demonstrate reverse flow under the influence of vertical wind influence, with the intensity of reverse flow increasing with α. The ventilation capacity of a step-down canyon is lower than that of a step-up canyon due to the generation of reverse flow. The NEV on the windward side of the step-up canyon is consistently higher than on the step-down canyon, regardless of wind direction. With increasing LHR, NEV decreases in the step-up canyon, reaching a maximum value of 76.54 at α = 60° for short canyons with LHR = 5. Conversely, the ventilation capacity of the leeward and windward sides of the step-down canyon significantly decreases. For short canyons (LHR = 5), NEV on the leeward side decreases with increasing α, reaching a minimum at α = 60° and slightly increasing at α = 90° (17.7% at LHR = 5). However, for long canyons, the leeward NEV slightly increases with α. These results indicate that step-up canyons have better ventilation capacity compared to step-down canyons. Therefore, urban planning should avoid constructing long step-down canyons with an LHR greater than 5 due to strong reversed airflow, and designing buildings as step-up canyons is a more reasonable approach. Since the dispersion of pollutants in the urban environment is primarily influenced by the airflow around complex building structures, enhancing the ventilation of street canyons is crucial in urban planning. The results of this study provide valuable insights for urban designers aiming to create sustainable cities with high ventilation capacity.

Effect of different canyon lengths and wind conditions on ventilation and pollutant dispersion inside asymmetric street canyons

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发表日期：

2025年4月23日

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Meng-xin Chai,
Kwang Song Jon,
Chung Hyok Sin,
Peng-yi Cui,
Yang Luo,
Yuan-dong Huang

摘要

The building height ratio of a street canyon significantly affects airflow and pollutant dispersion. Height-asymmetric street canyons exhibit distinct characteristics in terms of pollutant diffusion compared to symmetric street canyons. However, little attention has been given to height-asymmetric street canyons, particularly rarely been considered with different lengths. Based on our previous wind tunnel experiment, this study conducted numerical simulations to study the effects of four wind directions (α = 30°, 45°, and 90°) and three different lengths to height ratios (LHR = 5, 10, and 15) in both step-up and step-down canyons.The ventilation capacity of the canyons was evaluated using average concentration ( ${{C}^{*}}_{\_avg}$ ${C^{*}}_{_a v g}$ ) and the dimensionless net escape velocity (NEV^*). The results indicated that longer step-up canyons result in higher concentration of pollutants on the leeward side compared to other sides. Moreover, an updraft occurs in the step-up canyon, causing pollutants to accumulate on the top of the central facade.Short step-down canyons (LHR = 5) do not exhibit reverse flow, while long step-down canyons (LHR = 10,15) experience reverse flow, leading to increased pollutant concentrations. Both short and long canyons demonstrate reverse flow under the influence of vertical wind influence, with the intensity of reverse flow increasing with α. The ventilation capacity of a step-down canyon is lower than that of a step-up canyon due to the generation of reverse flow. The NEV^* on the windward side of the step-up canyon is consistently higher than on the step-down canyon, regardless of wind direction. With increasing LHR, NEV^* decreases in the step-up canyon, reaching a maximum value of 76.54 at α = 60° for short canyons with LHR = 5. Conversely, the ventilation capacity of the leeward and windward sides of the step-down canyon significantly decreases. For short canyons (LHR = 5), NEV^* on the leeward side decreases with increasing α, reaching a minimum at α = 60° and slightly increasing at α = 90° (17.7% at LHR = 5). However, for long canyons, the leeward NEV^* slightly increases with α. These results indicate that step-up canyons have better ventilation capacity compared to step-down canyons. Therefore, urban planning should avoid constructing long step-down canyons with an LHR greater than 5 due to strong reversed airflow, and designing buildings as step-up canyons is a more reasonable approach. Since the dispersion of pollutants in the urban environment is primarily influenced by the airflow around complex building structures, enhancing the ventilation of street canyons is crucial in urban planning. The results of this study provide valuable insights for urban designers aiming to create sustainable cities with high ventilation capacity.