Numerical Simulation of Ultrafine Particle Dispersion in Urban Street Canyons with the Spalart-Allmaras Turbulence Model

Abstract

The increased traffic emissions and reduced ventilation of urban street canyons lead to the formation of high particle concentrations as a function of the related flow field and geometry. In this context, the use of advanced modelling tools, able to evaluate particle concentration under different traffic and meteorological conditions, may be helpful.In this work, a numerical scheme based on the non-commercial fully explicit AC-CBS algorithm, and the one-equation Spalart-Allmaras turbulence model, was developed to perform numerical simulations of fluid flow and ultrafine particle dispersion in different street canyon configurations and under different wind speed and traffic conditions. The proposed non-commercial numerical tool was validated through a comparison with data drawn from the scientific literature.The results obtained from ultrafine particle concentration simulations show that as the building height increases the dispersion of particles in the canyon becomes weaker, due to the restricted interaction between the flow field in the street canyon and the undisturbed flow. Higher values of approaching wind speed facilitate the dispersion of the particles. The traffic effect has been evaluated by imposing different values of particles emission, depending on the vehicles type, with the lowest concentration values obtained for the Euro 6 vehicles, and the highest for High Duty Vehicles. A parametric analysis was also performed concerning the exposure to particles of pedestrians in different positions at the road level as a function of street canyon geometry, traffic mode, and wind speed. The worst exposure (1.25 × 10 6 part./cm 3 ) was found at the leeward side for an aspect ratio H/W = 1, wind speed of 5 m/s when High Duty Vehicles traffic was considered.

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

“…A two-dimensional (2D) street canyon is a simplification of an actual urban geometry. Numerous studies have focused on the impact of building characteristics (such as building heights, the shape roof of building, and the density of the building group) on the diffusion of pollutants in idealized and modeling urban street canyons (Li et al, 2008;Huang et al, 2009;Li et al, 2010;Buccolieri et al, 2011;Scargiali et al, 2011;Hang et al, 2012;Addepalli and Pardyjak, 2013;Scungio, 2013;Huang et al, 2015). Scungio et al (2015a) have used the detached eddy simulation to study the turbulent flow in isolated street canyons of different aspect ratios.…”

CFD Analysis of Urban Canopy Flows Employing the V2F Model: Impact of Different Aspect Ratios and Relative Heights

“…is parameter has already been investigated in other works [62,[72][73][74][75][76][77] for 2D ows, while useful insights into the e ects of building height variations for arrays of 3D buildings have recently been reported in [78][79][80]. e additional analysis we provide here focuses on the combined e ect of the variability of both building heights and AR.…”

Evaluating Dust Particle Transport Performance within Urban Street Canyons with Different Building Heights

“…In this deep canyon, the velocity is lower than 0.07u 0 at the bottom of the canyon. If H/b exceeds unity, the flow velocity in the canyon decreases; therefore, once PM enters the canyon, it is difficult to discharge and is consequently suspended in the air (Li et al, 2008;Scungio et al, 2013).…”

“…Turbulence models are the foundation of numerical simulations, and researchers have compared the performance of the renormalization group (RNG) k-ε model (Cheng et al, 2009), standard k-ε model, Reynolds stress model , and one-equation Spalart-Allmaras model (Scungio, 2013) in evaluating atmospheric pollutant dispersion in urban streets. Each turbulence model has advantages and limitations.…”