CFD Simulation of Airflow and Pollutant Dispersion in Urban Street Canyons with Different Building Setback Configurations

Deymad, Zohreh; Jafari, Hamideh; Aram, Ali

doi:10.22034/ijumes.2026.2093831.1382

CFD Simulation of Airflow and Pollutant Dispersion in Urban Street Canyons with Different Building Setback Configurations

Document Type : Case Study

Authors

Zohreh Deymad ¹

Hamideh Jafari ²

Ali Aram ³

¹ Department of Urban Planning and Architecture, Yas.C., Islamic Azad University, Yasuj, Iran

² Department of Art and Architecture, ST.C., Islamic Azad University, Tehran, Iran

³ Department of Civil Engineering, Yas.C., Islamic Azad University, Yasuj, Iran

10.22034/ijumes.2026.2093831.1382

Abstract

Air quality in urban open spaces is significantly shaped by urban morphology, airflow behavior, and pollutant emission sources, and it plays a critical role in protecting public health and improving the thermal comfort of urban residents. Accordingly, this study addresses the problem of how building setback position and configuration influence airflow and traffic-related pollutant distribution in urban street canyons, with the aim of identifying the most effective geometric arrangements for improving outdoor air quality. Research methodology in this study is based on a computational fluid dynamic CFD-based numerical simulation to evaluate airflow behavior and pollutant dispersion under different street-canyon configurations. In this framework, building setback position and configuration were treated as the independent variables, while airflow velocity and pollutant concentration were considered the main indicators of air quality. The analysis included one reference canyon without setbacks and nine alternative cases with different setback arrangements on the opposing building facades. The findings show that the street-center region had the highest level of pollutant accumulation because of weaker ventilation conditions, whereas the windward-sidewalk region generally benefited from more favorable airflow patterns. The results also demonstrate that setback configuration plays a decisive role in air-quality performance. Among all examined cases, the best-performing configuration increased the overall mean airflow velocity by 52.4% and reduced the overall mean pollutant concentration by 25.8% compared with the reference case. In addition, this configuration achieved local pollutant reductions of 46.9% at the windward sidewalk and 36.8% at the street center.

Graphical Abstract

CFD Simulation of Airflow and Pollutant Dispersion in Urban Street Canyons with Different Building Setback Configurations

Highlights

• The street-center region exhibited the weakest ventilation and the highest pollutant accumulation.

• Windward-sidewalk regions showed more favorable airflow than leeward-sidewalk regions.

• Building setbacks on both façades provided the greatest improvement in airflow and pollutant dispersion.

• Longer setbacks (fewer setback segments) further enhanced natural ventilation and pollutant removal.

• The optimal configuration (Case 09) increased mean airflow velocity by 74% and reduced average NO₂ concentration by 26% compared with the reference canyon.

Keywords

Urban street canyon

Building setback configuration

Outdoor air quality

Airflow

Pollutant dispersion

Computational Fluid Dynamics (CFD)

· Allard, F., & Ghiaus, C. (Eds.). (2012). Natural ventilation in the urban environment: assessment and design. Routledge.

· Blocken, B. (2015). Computational Fluid Dynamics for urban physics: Importance, scales, possibilities, limitations and ten tips and tricks towards accurate and reliable simulations. Building and environment, 91, 219-245. doi.org/10.1016/j.buildenv.2015.02.015

· Cui, P. Y., Ji, R., He, L., Zhang, Z., Luo, Y., Yang, Y., & Huang, Y. D. (2023). Influence of GI configurations and wall thermal effects on flow structure and pollutant dispersion within urban street canyons. Building and Environment, 243, 110646. doi.org/10.1016/j.buildenv.2023.110646

· Deaves, D. M., & Harris, R. I. (1982). A note on the use of asymptotic similarity theory in neutral atmospheric boundary layers. Atmospheric Environment (1967), 16(8), 1889-1893. doi.org/10.1016/0004-6981(82)90376-6

· Hang, J., Sandberg, M., & Li, Y. (2009). Effect of urban morphology on wind condition in idealized city models. Atmospheric environment, 43(4), 869-878. doi.org/10.1016/j.atmosenv.2008.10.040

· Heidari, A. A., & Eskandari, H. (2022). Impact of inlet and outlet opening height variation on the air quality and ventilation efficiency in the on-top wind catcher buildings: A CFD simulation. Science and Technology for the Built Environment, 28(10), 1420-1438. doi.org/10.1080/23744731.2022.2089592

· Heidari, A., Peyvastehgar, Y., Eskandari, H., & Mansourian, E. (2025). The effect of balcony configurations on ACH and pollutant concentration in symmetrical and asymmetrical street canyons. Scientific Reports, 15(1), 8894. doi.org/10.1038/s41598-025-93736-5

· Jo, W. J., & Sohn, J. Y. (2009). The effect of environmental and structural factors on indoor air quality of apartments in Korea. Building and Environment, 44(9), 1794-1802. doi.org/10.1016/j.buildenv.2008.12.003

· Lelieveld, J., Evans, J. S., Fnais, M., Giannadaki, D., & Pozzer, A. (2015). The contribution of outdoor air pollution sources to premature mortality on a global scale. Nature, 525(7569), 367-371. doi.org/10.1038/nature15371

· Mansourian, E., Eskandari, H., & Heidari, A. (2025). The effect of balcony geometry on improving indoor air quality in mid-rise buildings adjacent to urban highways: A case study of Shahid Chamran Highway, Tehran. Haft Hesar Environmental Studies, 14(51), 25–38. (In Persian).

· Oke, T. R. (1988). Street design and urban canopy layer climate. Energy and buildings, 11(1-3), 103-113. doi.org/10.1016/0378-7788(88)90026-6

· Richards, P. J., & Hoxey, R. P. (1993). Appropriate boundary conditions for computational wind engineering models using the k-ϵ turbulence model. Journal of wind engineering and industrial aerodynamics, 46, 145-153. doi.org/10.1016/0167-6105(93)90124-7

· Santamouris, M. (2013). Using cool pavements as a mitigation strategy to fight urban heat island—A review of the actual developments. Renewable and Sustainable Energy Reviews, 26, 224-240. doi.org/10.1016/j.rser.2013.05.047

· Sin, C. H., Cui, P. Y., Luo, Y., Jon, K. S., & Huang, Y. D. (2023). CFD modeling on the canyon ventilation and pollutant exposure in asymmetric street canyons with continuity/discontinuity balconies. Atmospheric Pollution Research, 14(1), 101641. doi.org/10.1016/j.apr.2022.101641

· Soulhac, L., Salizzoni, P., Cierco, F. X., & Perkins, R. (2011). The model SIRANE for atmospheric urban pollutant dispersion; part I, presentation of the model. Atmospheric environment, 45(39), 7379-7395. doi.org/10.1016/j.atmosenv.2011.07.008

· Tehran Municipality, Technical and Civil Engineering Deputy. (2017). Traffic data of Tehran highways. Tehran: Office of Studies, Planning and Programming. (In Persian).

· Tominaga, Y., & Stathopoulos, T. (2013). CFD simulation of near-field pollutant dispersion in the urban environment: A review of current modeling techniques. Atmospheric environment, 79, 716-730. doi.org/10.1016/j.atmosenv.2013.07.028

· Tominaga, Y., Mochida, A., Yoshie, R., Kataoka, H., Nozu, T., Yoshikawa, M., & Shirasawa, T. (2008). AIJ guidelines for practical applications of CFD to pedestrian wind environment around buildings. Journal of wind engineering and industrial aerodynamics, 96(10-11), 1749-1761. 10.1016/j.jweia.2008.02.058

· World Health Organization (WHO). (2016). WHO treatment guidelines for drug-resistant tuberculosis. World Health Organization.

· Yazid, A. W. M., Sidik, N. A. C., Salim, S. M., & Saqr, K. M. (2014). A review on the flow structure and pollutant dispersion in urban street canyons for urban planning strategies. Simulation, 90(8), 892-916. doi.org/10.1177/0037549714528046

· Zabardast, E., & Riazi, H. (2015). Built environment indicators and their effects on air pollution: A case study of the areas surrounding fourteen air quality monitoring stations in Tehran. Fine Arts: Architecture and Urban Planning, 20(1), 55–66. (In Persian). doi.org/10.22059/jfaup.2015.56371

· Zhang, Y., Gu, Z., & Yu, C. W. (2020). Impact factors on airflow and pollutant dispersion in urban street canyons and comprehensive simulations: A review. Current pollution reports, 6(4), 425-439. doi.org/10.1007/s40726-020-00166-0

· Zheng, X., Montazeri, H., & Blocken, B. (2022). Impact of building façade geometrical details on pollutant dispersion in street canyons. Building and Environment, 212, 108746. doi.org/10.1016/j.buildenv.2021.108746