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

Abstract

In developing cities, buildings with different heights obstruct the diffusion of pollutants. In this study, dust particles transported within urban street canyons were simulated using Reynolds-averaged Navier-Stokes (RANS) method, and airsolid two-phase flow fields were obtained on a regional scale. Four typical street building models were used in this study: (a) low-rise buildings (H/b = 1), (b) step-up building arrangements (H/b = 1, 2, 3, 4, 5), (c) step-down building arrangement (H/b = 5, 4, 3, 2, 1), and (d) high-rise buildings (H/b = 5). The particle volume fraction distribution in the four models reflected the basic properties of particle transportation in the canyons. Vortices were observed on the roofs of street canyons, which prevented particles from being transmitted into the canyons, and the vortex regions were characterized as low particle concentration.To evaluate the dust particle transport performance in the models, three indices, namely particle transport efficiency, suspension fraction and suspension density, were defined. These concepts were based on the particle number concentrations, which were obtained using the Lagrange approach. The high-rise building model (H/b = 5) demonstrated the lowest transport efficiency among all four models, and it also had the lowest suspension density in the street canyons. This implied that the high-rise buildings hindered the particles from being transporting further and that the particles could not enter the deep canyons easily. In the step-down building arrangement model (H/b = 5, 4, 3, 2, 1), the particle concentration level and suspension density in the canyons were lower than those observed in the step-up building arrangement model (H/b = 1, 2, 3, 4, 5), indicating that a step-down building height arrangement is appropriate for creating construction plans for developing cities.Finally, the variation of the streamwise air velocity, vertical velocity and fluctuating velocity on the roof of canyons along the x direction were separately examined. Notably, the fluctuating velocity was the dominant mechanism of the particle suspension in the canyons.

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

“…The step-up arrangement is usually selected for urban residences which are adjacent to lake and in villa districts. By studying the effects of buildings arrangements in the cities on the particle dispersion, the particle concentration in the step-up street canyon was the most than that of other building arrangements (Mei et al, 2016). So the step-up building model is used as object model to study the particle temporal distribution.…”

A Computational Fluid Dynamic (CFD) Simulation of PM10 Dispersion Caused by Rail Transit Construction Activity: A Real Urban Street Canyon Model

“…Based on turbulent kinetic energy and rate of turbulent dissipation transport equations, the flow structure will be formulated by κ - ε closure scheme. The κ - ε equation is described below [ 35 ]: …”

“…Recently, a number of studies have been undertaken to assess flow and pollutant dispersion in symmetric canyon models, and the aspect ratios are the most significant parameters. A street canyon with an aspect ratio of 0.5, 1.0 and 2.0 could be employed, and benchmarking the simpler cases gives confidence in the more complicated street canyon case so that the pollutant distribution and concentration characteristics could be investigated [ 34 , 35 ]. Similarly, field-size canyon models with various length to height ratio (L/H) and roof shapes were analyzed to obtain the characteristics of pollutant transport and diffusion [ 36 ].…”

“…Existing studies predominately have focused on the street canyon configuration, such as the aspect ratio and height ratio, in order to examine the impact of street geometry on the flow structure, the vortexes and the pollutant dispersion [ 19 , 33 , 34 , 35 , 38 , 39 ]. Most of these studies are based on idealized modeling in a CFD approach, overlooking factors that exist in real street canyons, such as irregular streets and the existing gaps in adjacent buildings [ 34 , 43 ].…”

Spatial Distribution of Fine Particulate Matter in Underground Passageways

“…The effects of diverse forces such as Saffman, drag, and gravity can be assessed. The dynamic equations of particulate transport can be expressed as follows [25]:$\frac{\pi d_p^3{\rho }_P}{6}\frac{d{u}_P}{dt}=F_{drag}+F_{gravity}+F_{saffman}$ $F_{drag}=-\frac{1}{6}\pi d_P^3{\rho }_P\frac{1}{\tau }(u_P-u)$ $F_{gravity}=\frac{1}{6}\pi d_P^3({\rho }_P-\rho )g_i{\delta }_i$ $F_{saffman}=\frac{1}{6}\pi d_P^3{\rho }_P\frac{5.188v^{0.5}{d}_{ij}}{S{d}_P{false(d_{lk}-d_{kl}false)}^{0.25}}false(u_P-ufalse)$ where u P is the particle velocity; u is the fluid velocity; ρ P is the particle density; ρ is the fluid density; d P is the particle diameter; S is the density ratio between a particle and adjacent fluid; ν is the kinematic viscosity; δ i is the unit delta function; g i is the hydrodynamic viscosity; τ is the particle relaxation time; and d ij = ( u ij + u ji )/2 is the deformation rate tensor.…”