Modeling the Airflow and Particle Dispersion in Street Canyons under Unsteady Thermal Environment with Sinusoidal Variation

Abstract

Unstable temperature stratification conditions have a considerable influence on pollutant diffusion inside street canyons. In this study, we tried to model the air flow and particle dispersion in street canyons under unsteady thermal environment. The sinusoidal variation was used to model the atmosphere temperature on the basis of the solar radiation cycle that occurs over a day. The two-dimensional model of step-up building layouts was applied as the research object and an RNG turbulence model was applied to study the dynamic characteristics of instantaneous airflow, the dimensionless air exchange rate (ACH) and turbulent kinetic energy (TKE) in the different canyons. A series of numerical simulations were performed for different Richardson numbers (Ri). The results demonstrated that the stream function within the street canyons exhibited a periodic shift over a day, and the flow morphology gradually evolved from paralleled bilateral vortexes into a row of vortexes, particularly when the ground temperature increased. Moreover, the local PM concentrations at different times were obtained, and they were affected by the flow field patterns. The total PM mass in the street canyon decreased as the air velocity increased. Furthermore, the dimensionless ACH and TKE exhibit a noticeable fluctuation with time at higher Ri and stronger buoyancy. As the Ri decreases, the enhanced forced convection causes the ACH and TKE to remain constant over time. From these results, inhabitants should be advised to adopt preventative measures aimed at the PM pollution according to the time and location where they live.

Effect of radiation interaction and aerosol processes on ventilation and aerosol concentrations in a real urban neighbourhood in Helsinki

“…The particle concentrations are the lowest near the surface in the middle of the street with a decrease of 46.3%. Idealised simulations conducted by Xie et al (2005), Nezis et al (2011), Mei et al (2016 and Mei et al (2017) concluded that pollutant transport to the leeward side (southwest in our case) increases concentrations on this side due to the enhanced vortex, and overall pollutant concentrations are reduced in the canyon when the leeward wall is heated, whereas in our case there is an overall decrease at both sides, but the decrease is not as strong at the southwestern side.…”

Effects of void deck on the airflow and pollutant dispersion in 3D street canyons

“…Therefore, many studies have been conducted on airflow and pollutant dispersion inside the street canyons to reduce these adverse effects of traffic pollution emission on people over the past 30 years (Antoniou et al ( 2019 ); Baik et al ( 2012 ); Ricci et al ( 2019 )). Factors affecting the airflow and pollutant distribution inside the street canyon include wind direction and speed (Huang et al ( 2019 ); Zhang et al ( 2018b )), street canyon structure (Cui et al ( 2021 ); Reiminger et al ( 2020 )) (symmetry, asymmetry), building structure (roof (Huang et al ( 2009 ); Huang et al ( 2014 ); Llaguno-Munitxa et al ( 2017 )), balcony (Cui et al ( 2020 ); Cui et al ( 2021 )), void deck (Huang et al ( 2020 ); Huang et al ( 2021 )), solar radiation (Chew et al ( 2018 ); Mei et al ( 2017 )), and green infrastructure (Huang et al ( 2019 ); Jeanjean et al ( 2015 )). Among these factors, the street canyon structure has a great influence on the airflow and pollutant dispersion inside the canyon (Li et al 2006 ).…”

Impacts of specific street geometry on airflow and traffic pollutant dispersion inside a street canyon

“…CFD simulations, under the premises of validation by the first two methods, are more and more applied to study the dispersion of traffic pollutants in street canyons because of their economical, comprehensive data and less constrained by external conditions. Previous numerical simulation studies focused on the investigation of effects of street-canyon geometry (Assimakopoulos et al 2003;Llaguno-Munitxa et al 2017), ambient wind direction/speed (Jeanjean et al 2015;Kwak et al 2016;Zhang et al 2018;Huang et al 2019a), traffic-induced turbulence (Sahlodin et al 2007;Solazzo et al 2016;Wang et al 2019), thermal conditions due to solar radiation (Allegrini et al 2014;Mei et al 2017;, and green infrastructure (Jeanjean et al 2015;Huang et al 2019b) on the flow and pollutant dispersion in street canyons.…”