Flow and pollution concentration large-Eddy simulation and transition conditions for different street canyons and wind speeds: Environmental pollution reduction approach

doi:10.1016/j.uclim.2020.100731

Urban Climate

Volume 35, January 2021, 100731

https://doi.org/10.1016/j.uclim.2020.100731 Get rights and content

Highlights

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Different ambient wind speeds have no effect on the structure of streamline fields (number of vortices)in the street canyon.
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The increase of vortices number along the depth of the canyon has very negative effects on the discharge of pollutants.
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In transition conditions, increasing of less than half a meter of the canyon width will drastically increase the ventilation.
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Transition condition phenomenon can be used to optimize canyon dimensions in designing urban areas for better ventilation.

Abstract

Effects of different ambient wind speeds and aspect ratios on turbulence and pollutant concentration in and above street canyons are numerically investigated and compared using parallelized LES model (PALM) and ((Transition Conditions)) are introduced for the first time. The results show that, increasing of ambient wind speed does not change the shape of streamlines but increases the magnitude of time- and area-averaged turbulent kinetic energy with the rate of square of speed ratios resulting in better ventilation of pollutants. As aspect ratio increases, the magnitude of turbulent kinetic energy and turbulent fluxes especially in lower depths of the canyon decrease sharply causing decreased pollution transport to higher levels. An applied case is between 2 and 4 aspect ratios in which there are two vortices along the depths of the canyon. As the width of the canyon increases, the rate of the upper vortex penetration increases and eventually at aspect ratio 2 makes a transition from two to one vortex; we call such conditions as transition conditions. In transition conditions increasing of less than half a meter of street canyon width will drastically decrease the pollution concentration inside the canyon; this is useful in optimizing canyon dimensions for better ventilation.

Introduction

Due to the rapid population growth and urbanization, the study of wind flow and air quality in built-up urban areas has become vital. There are many factors including the shape and density of buildings, ambient wind speed and direction, solar radiation and vehicles that affect the building scale flow and dispersion. Many studies have been conducted in this field as field measurements (Zajic et al., 2011; Hamlyn et al., 2007; Baik et al., 2000), wind tunnel experiments (Tolias et al., 2018; Ho and Liu, 2017) and computational fluid dynamics (CFD); each focusing on a specific aspect of the issue. Among all, CFD is of special importance because it prepares detailed information about the turbulent nature of building scale. Chatzimichailidis et al. investigated the pollution dispersion in street canyons qualitatively and quantitatively for three different Reynolds numbers; they found that domain height slightly affects street level concentration but source height had a major impact (Chatzimichailidis et al., 2019). Chew et al. surveyed the discrepancy between reduced-scale experiments and full-scale field measurements on the number of vortices at high aspect ratio street canyons of 1.5 ≤ aspect ratio ≤ 2; by conducting water channel experiments, they showed that the widely adopted critical Reynolds number (Re_c = 11,000) is not applicable for deep canyons i.e. aspect ratios greater than 1.5. They also confirmed that there is only one vortex in deep canyons at high Re (Chew et al., 2018). Li et al. simulated different deep street canyons of aspect ratios 3, 5 and 10; their study outcomes showed that there are three, five and eight vertically aligned primary vortices for the three cases respectively. They also found the local maxima of turbulence intensities at the interfaces of primary vortices and the shear layer (Li et al., 2009). Hang et al. introduced new parameters such as intake fraction (IF) i.e. inhaled average fraction of total emissions by each person, into their simulation to study the impact of different street layouts, viaduct settings and noise barriers on CO source-exposure correlation when realistic CO sources are defined; they showed that narrower street canyons experience larger IF and CO exposure. They also found that cases with viaduct, experience smaller IF than those without a viaduct (Hang et al., 2017). Zhang et al. conducted CFD simulations and outdoor field measurements to study the integrated impact of different aspect ratios (1, 3 and 5), viaduct settings, elevated building designs and height variations on the flow; their results showed much poorer ventilation and larger personal intake fraction (P-IF) for aspect ratio 5 compared to 1 and 3 (Zhang et al., 2019). Chen et al. used a digital building model of Tainan city to calculate roughness length and to estimate wind flow features and thermal conditions in urban areas, showing the importance of vertical scalar and radiation flux on thermal comfort and heat stress (Chen et al., 2017). Zhang et al. developed an Euler-Lagrangian method to study the influence of vehicles on air flow and turbulence within urban street-canyons indicating that vehicles induced turbulence at the lower portion and near the leeward wall of a street canyon is larger than those in other areas of a street canyon (Zhang et al., 2017). Kawaminami et al. investigated the geometrical impact of six types of urban-like arrays with uniform and non-uniform heights on the statistical features of wind speeds and scalar concentrations within urban canopy regions; they found the correlation between spatial distribution of scalar concentration and velocity magnitude distribution (Kawaminami et al., 2018). Li et al. summarized the findings of the thermal stratification effects on the transport of momentum, heat and pollutants in an urban street canyon in the skimming flow regime; they found that with increasing Richardson number (Ri) both the heat and pollutant transfer coefficients decrease toward a state where the transfer coefficients become zero at Ri ≈ 0.5 (Li et al., 2015). Keck et al. simulated the influence of new habitable areas being built around Macau᾽s coasts; they investigated the impact of planned reclamation areas within the city districts and showed the small impact of reclamation areas on the average ventilation within the city (Keck et al., 2014). Park et al. numerically simulated thermal effects on turbulent flow and dispersion in and above a street canyon with aspect ratio 1; they showed that heating of leeward street canyon wall or street bottom strengthens a primary vortex and heating of the windward wall induces a shrunken primary vortex and a winding flow between the vortex and windward wall compared to neutral case. They also showed that heating induces higher turbulent kinetic energy and stronger turbulent fluxes at the rooftop height (Park et al., 2012). Letzel et al. investigated pedestrian level ventilation in two neighborhoods of downtown of Hong Kong using LES; their results reveal the critical dependence of ventilation on urban morphology (Letzel et al., 2012). Wang et al. simulated the impact of solar radiation of different range on wind flow and pollutant dispersion in an urban street canyon of aspect ratio 1 and showed that heating building walls and ground lead to strong buoyancy forces as the air is heated by surfaces (Wang et al., 2011). Castillo et al. simulated a wind over explicitly cube arrays of buildings of aspect ratio 1 numerically to measure heat ventilation via bulk transfer coefficients; they showed that the proximity, orientation and horizontal projection of heating for each urban surface factor into momentum and heat exchange (Castillo et al., 2009). Santiago et al. numerically simulated a 3D air flow inside a regular array of cubes based on Reynolds-averaged Navier-Stokes equations (RANS) with standard k-ɛ closure; 3D analysis shows that stronger downward than upward motions are present within the canyon (Santiago et al., 2007).

In spite of the above and many other researches done in this area, less attention has been paid to simultaneous comparison of buildings pure mechanical effects on turbulent wind flow and pollution concentration for different speeds and different aspect ratios in and above street canyons. Changes in the wind speed and aspect ratio changes turbulent structure, mean flow and therefore pollutants concentration in and above street canyons; this can be taken into account in designing buildings dimensions and urban buildings density. In this research, we will study these matters using LES.

Section snippets

Model description and simulation setup

In this study the Parallelized LES Model (PALM) is chosen as the suitable numerical model (Raasch and Schröter, 2001; Maronga et al., 2015). The model is based on the non-hydrostatic, filtered, incompressible Navier-Stokes equations in Boussinesq-approximated form Eq. 1, continuity equation Eq. 2 and the equations for the conservation of energy Eq. 3 and passive scalars Eq. 4; $\frac{\partial \hat{u_{i}}}{\partial t} = - \frac{\partial}{\partial x_{k}} \hat{u_{k}} \hat{u_{i}} - \frac{1}{ρ_{0}} \frac{\partial \hat{p^{*}}}{\partial x_{i}} - (ε_{ijk} f_{j} \hat{u_{k}} - ε_{i 3 k} f_{3} u_{gk}) + g \frac{\hat{θ^{*}}}{θ_{0}} δ_{i 3} - \frac{\partial}{\partial x_{k}} τ_{ki}$ $\frac{\partial \hat{u_{i}}}{\partial x_{i}} = 0$ $\frac{\partial \hat{θ}}{\partial t} = - \frac{\partial}{\partial x_{k}} \hat{u_{k}} \hat{θ} - \frac{\partial}{\partial x_{k}} \hat{u_{k}^{"} θ^{"}}$ $\frac{\partial \hat{c}}{\partial t} = - \frac{\partial}{\partial x_{k}} \hat{u_{k}} \hat{c} - \partial$

Mean flow

Fig. 2, Fig. 3 has been drawn using time- and spanwise-averaged vertical and streamwise velocity components $\bar{w}$ and $\bar{u}$ for different ambient wind speeds and different ARs. In order to calculate the time- and spanwise-averaged of a quantity, first the time averaging operation is performed over a two-hour period and then spatial averaging is applied to the time-averaged quantity in spanwise direction. As shown, the magnitude of r_mn $(= \frac{r_{m}}{r_{n}})$ at a certain AR for different ambient wind speeds is

Summary and conclusions

Using a parallelized LES model (PALM) pure mechanical effects of buildings on turbulent flow and dispersion in and above idealized street canyons of different aspect ratios for four different ambient wind speeds were numerically investigated. Twenty one numerical simulations were performed; four simulations of four different wind speeds for each aspect ratio (=1, 2, 4 and 8) as well as five more for aspect ratios 2.66, 2.50, 2.35, 2.16 and 2.05 at the same speed. The followings are the key

Statement

In order to reduce air pollution in urban environments, accurate and comprehensive knowledge of the effects of different parts of urban topography on air flow and pollutants is inevitable. A new and accurate study of the effects of each of these topographic parts on air flow and pollutants will help significantly in predicting pollutants concentration during the design of cities. The most important part of urban environments are the street canyons. Although many studies have been done on street

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Part of this research has been done at Leibniz University Hannover, Institute of meteorology and climatology. Special thanks to Prof. Dr. Siegfried Raasch and PALM group at the Institute of meteorology and climatology of Leibniz University Hannover, Germany that this research would not have been possible without their vital cooperation and guidance.

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View full text

Flow and pollution concentration large-Eddy simulation and transition conditions for different street canyons and wind speeds: Environmental pollution reduction approach

Highlights

Abstract

Introduction

Section snippets

Model description and simulation setup

Mean flow

Summary and conclusions

Statement

Declaration of Competing Interest

Acknowledgments

Atmos

Atmos. Environ.

J. Wind Eng. Ind. Aerodyn.

Atmos. Environ.

Atmos. Environ.

Atmos. Environ.

J. Wind Eng. Ind. Aerodyn.

Sci. Total Environ.

A laboratory model of urban street-canyon flows

J. Appl. Meteorol.

Heat ventilation efficiency of urban surfaces using large-eddy simulation

Ann. J. Hydraul. Eng.

Qualitative and quantitative investigation of multiple large eddy simulation aspects for pollutant dispersion in street canyons using OpenFOAM

Atmos

Flows across high aspect ratio street canyons: Reynolds number independence revisited

Environ. Fluid Mech.

Stratocumulus-capped mixed layers derived from a three-dimensional model

Boundary-Layer Meteorol.