Numerical Studies on Flow Fields Around Buildings in an Urban Street Canyon and Cross-Road

The questions on how vortices are constructed and on the relationship between the flow patterns and concentration distributions in real street canyons are the most pressing questions in pollution control studies. In this paper, the very large eddy simulation (VLES) and large eddy simulation (LES) are applied to calculate the flow and pollutant concentration fields in an urban street canyon and a cross-road respectively. It is found that the flow separations are not only related to the canyon aspect ratios, but also with the flow velocities and wall temperatures. And the turbulent dispersions are so strongly affected by the flow fields that the pollutant concentration distributions can be distinguished from the different aspect ratios, flow velocities and wall temperatures.

Effects of Street-Bottom and Building-Roof Heating on Flow in Three-Dimensional Street Canyons

Using a computational fluid dynamics （CFD） model, the effects of street-bottom and building-roof heating on flow in three-dimensional street canyons are investigated. The building and street-canyon aspect ratios are one. In the presence of street-bottom heating, as the street-bottom heating intensity increases, the mean kinetic energy increases in the spanwise street canyon formed by the upwind and downwind buildings but decreases in the lower region of the streamwise street canyon. The increase in momentum due to buoyancy force intensifies mechanically induced flow in the spanwise street canyon. The vorticity in the spanwise street canyon strengthens. The temperature increase is not large because relatively cold above-roof-level air comes into the spanwise street canyon. In the presence of both street-bottom and building-roof heating, the mean kinetic energy rather decreases in the spanwise street canyon. This is caused by the decrease in horizontal flow speed at the roof level, which results in the weakening of the mean flow circulation in the spanwise street canyon. It is found that the vorticity in the spanwise street canyon weakens. The temperature increase is relatively large compared with that in the street-bottom heating case, because relatively warm above-roof-level air comes into the spanwise street canyon.

Numerical Study on the Impact of Ground Heating and Ambient Wind Speed on Flow Fields in Street Canyons

The impact of ground heating on flow fields in street canyons under different ambient wind speed conditions was studied based on numerical methods. A series of numerical tests were performed, and three factors including height-to-width （H/W） ratio, ambient wind speed and ground heating intensity were taken into account. Three types of street canyon with H/W ratios of 0.5, 1.0 and 2.0, respectively, were used in the simulation and seven speed values ranging from 0.0 to 3.0 m s-1 were set for the ambient wind speed. The ground heating intensity, which was defined as the difference b-ween the ground temperature and air temperature, ranged from 10 to 40 K with an increase of 10 K in the tests. The results showed that under calm conditions, ground heating could induce circulation with a wind speed of around 1.0 m s-i, which is enough to disperse pollutants in a street canyon. It was also found that an ambient wind speed threshold may exist for street canyons with a fixed H/W ratio. When ambient wind speed was lower than the threshold identified in this study, the impact of the thermal effect on the flow field was obvious, and there existed a multi-vortex flow pattern in the street canyon. When the ambient wind speed was higher than the threshold, the circulation pattern was basically determined by dynamic effects. The tests on the impact of heating intensity showed that a higher ground heating intensity could strengthen the vortical flow within the street canyon, which would help improve pollutant diffusion capability in street canyons.

Using the OSPM Model on Pollutant Dispersion in an Urban Street Canyon

An observational campaign was conducted in the street canyon of Zhujiang Road in Nanjing city in 2007. Hourly mean concentrations of PM10 were measured at street and roof levels. The Operational Street Pollution Model （OSPM） street canyon dispersion model was used to calculate the street concentrations and the results were compared with the measurements. The results show that there is good agreement between measured and predicted concentrations. The correlation coefficient R2 values （R2 is a measure of the correlation of the predicted and measured time series of concentrations） are 0.5319, 0.8044, and 0.6630 for the scatter plots of PM10 corresponding to light wind speed conditions, higher wind speed conditions, and all wind speed conditions, respectively. PM10 concentrations tend to be smaller for the higher wind speed cases and decrease rapidly with increasing wind speed. The presentations of measured and modelled concentration dependence on wind direction show fairly good agreement. PM10 concentrations measured on the windward side are relatively smaller, compared with the corresponding results for the leeward side. This study demonstrates that it is possible to use the OSPM to model PM10 dispersion rules for an urban street canyon.

Pollution Dispersion in Urban Street Canyons with Green Belts

In this study,numerical simulations were used to explore the effects of roadside green belt,urban street spatial layout,and wind speed on vehicle exhaust emission diffusion in street canyon.The diffusion of different sized particles in the street canyon and the influence of wind speed were investigated.The individual daily average pollutant intake was used to evaluate the exposure level in a street canyon microenvironment.The central and leeward green belts of the road were the most conducive to the diffusion of pollutants,while the positioning of the green belts both sides of a road was least conducive to the diffusion of pollutants.Pollutant levels increased with increasing canopy height,canopy width,and decreasing tree spacing,with optimal values of 12 m,7 m,and 0.4 H,respectively.This provides protection from pollution for low-rise residents and pedestrians.The results presented here can be used to improve the air quality of the street microenvironment and provide a basis for the renovation of old street buildings.

Numerical and Experimental Studies on Flow and Pollutant Dispersion in Urban Street Canyons

在这研究，数字模拟和水坦克实验被用来在一个城市的街峡谷调查流动和污染物质分散。峡谷几何学的二种类型被测试。研究显示在一个加速槽口峡谷(峡谷的下游的方面上的更高的大楼)，在上游的更低的大楼的高度和形状在流动模式和污染物质分散起一个重要作用，当时在一个电压降低的槽口峡谷(下游的方面上的更低的大楼)，下游地更低的大楼几乎没有小影响。研究也证明为峡谷的一个方面上的平行六面体大楼的高塔的替换可以提高污染物质集中由机动车辆射出的街通风和减少。

Evaluation of Local Scale PM Pollution Levels in Typical Street Canyon in Riga

The present study describes long term PM10 and PM2.5 changes in typical street canyon with particular emphasis on seasonal, diurnal variations in context with meteorological data. In order to understand PM10 pollution sources during 28 April 2007-31 December 2007, chemical composition measurements were done with particular emphasis on heavy metals (As, Cd, Ni, and Pb), crustal material (Ca, Mg, Na, and K) and anions (sulphates, nitrates, chlorides). Meteorological data used for this evolutional analysis were measured close to traffic related stations and several meteorological parameters were analyzed in relation to particulate measurements. Keep in mind that atmospheric aerosols are generally hydroscopic. Relative humidity which plays very important role in rain/snow and humidity impact are analyzed.

Coupling Relationship Between Summer Microclimate and Spatial Layout of Street Canyons in the White Stupa Temple Area of Beijing

With the rapid urbanization, many high-rise buildings and new districts have been built continuously. However, the old town of cities has gradually been forgotten by people and its environment is becoming increasingly harsh. The old town usually has diversified space and function. As an important part of the old town, the street canyon that is closely related to human settlements is a public space where people communicate and do activities. Therefore, research on microclimate-based improvement of the street canyon in the old town is of great significance. Six typical street canyons in the White Stupa Temple area of Beijing were selected for research to explore spatial characteristics of the street canyon. Microclimate factors such as temperature, humidity, wind speed, and solar radiation were measured on the spot in summer. These factors were combined with two microclimate assessment indexes of physiological equivalent temperature(PET) and wet bulb globe temperature(WBGT) to evaluate the microclimate of the street canyon. In the analysis of the measured data, the mean value comparison method was used to analyze the average values of the microclimate factors in different time periods. Spatial layout of microclimate included the orientation of the canyon, the ratio of the canyon height to canyon width, and green coverage of the canyon, and an in-depth study was made on the coupling relationship between the spatial layout and the microclimate of the canyon. Research results can provide an optimization strategy for the transformation design of the street canyon in White Stupa Temple area, and provide a scientific reference for the research on spatial layout and microclimate improvement in the old town, so as to improve the living quality of residents in the old town.

Possible high COVID-19 airborne infection risk in deep and poorly ventilated 2D street canyons

Despite the widespread assumption that outdoor environments provide sufficient ventilation and dilution capacity to mitigate the risk of COVID-19 infection,there is little understanding of airborne infection risk in outdoor urban areas with poor ventilation.To address this gap,we propose a modified Wells-Riley model based on the purging flow rate(QPFR),by using computational fluid dynamics(CFD)simulations.The model quantifies the outdoor risk in 2D street canyons with different approaching wind speeds,urban heating patterns and aspect ratios(building height to street width).We show that urban morphology plays a critical role in controlling airborne infectious disease transmission in outdoor environments,especially under calm winds;with deep street canyons(aspect ratio>3)having a similar infection risk as typical indoor environments.While ground and leeward wall heating could reduce the risk,windward heating(e.g.,windward wall~10 K warmer than the ambient air)can increase the infection risk by up to 75%.Our research highlights the importance of considering outdoor infection risk and the critical role of urban morphology in mitigating airborne infection risk.By identifying and addressing these risks,we can inform measures that may enhance public health and safety,particularly in densely populated urban environments.

NUMERICAL STUDIES ON AIRFLOW AND POLLUTANT DISPERSION IN URBAN STREET CANYONS FORMED BY SLANTED ROOF BUILDINGS

Based on the CFD technique, fifteen cases were evaluated for the airflows and pollutant dispersions inside urban street canyons formed by slanted roof buildings. The simulated wind fields and concentration contours show that W/H, W/h and h/H （where W is the street width, and Hand h are the heights of buildings at the leeward and windward sides of the street, respectively） are the crucial factors in determining the vortex structure and pollutant distribution within a canyon. It is concluded that （1） in a symmetrical canyon, at W/H =0.5 two vortices （an upper clockwise vortex between the slanted roofs and a lower counter-clockwise one） are developed and pollutants accumulate on the windward side of the street, whereas at w/H=2.0 only one clockwise vortex is generated and thus pollution piles up on the leeward side, （2） in a step-up canyon with W/H=0.5 to 2.0 （at h/H =1.5 to 2.0）and a step-down canyon with W/h=1.0 （at h/H =0.5 to 0.667）, the pollution level close to the lower building is higher than that close to the taller building since a clockwise vortex is generated in the step-up canyon and a counter-clockwise one in the step-down canyon, （3） in a narrow step-down canyon with W/h=0.5 （at h/H =0.667） very poor ventilation properties is detected, and inside a wider step-down canyon with W/h=2.0 the vortex structure and consequently pollutant distribution varies greatly with h/H.

Numerical Investigation of the Impact of Different Configurations and Aspect Ratios on Dense Gas Dispersion in Urban Street Canyons

The dispersion of chlorine gas in urban street canyons was numerically simulated using the fire dynamics simulator, a code developed by the National Institute of Standards and Technology of USA, which uses large eddy simulation coupled with the Smagorinsky sub-grid scale model. The unsteady flow fields were computed by solving the filtered incompressible Navier-Stokes equations under low Mach number approximation by the finite difference method. The studies analyzed the influence of different street canyon configurations and aspect ratios on the flow and chlorine gas dispersion. The geometric configuration and aspect ratio both affect the vortices and the local concentration distributions in street canyons.

TRAFFIC EMISSION TRANSPORTATION IN STREET CANYONS

Spatial distributions of traffic-related pollutants in street canyons were investigated by field measurements and Computational Fluid Dynamics （CFD）. Two typical street canyons were selected for field monitoring, and a three-dimensional numerical model was built based on Reynolds-averaged Navier-Stokes equations equipped with the standard κ-ε turbulence models for CFD simulations. The study shows that the pollutant concentrations of vehicle emission correlate well with the traffic volume variation, wind direction and wind speed. The wind direction and speed at the roof level determine overwhellmingly the flow field and the distributions of pollutant concentrations in the street canyon. When the wind speed is equal to zero, the pollutant concentrations on the breath height of the both sides of the street canyon are almost the same. When the wind direction is perpendicular to the street, one main vortex is formed with a shape depending on the building structure on both sides of the street, the pollutant is accumulated on the leeward side, and the pollutant concentrations at the breath height on the leeward side are 2 to 3 times as those at the breath height on the windward side. If the wind direction makes some angles with the street canyon, the pollutant concentration will be higher on the leeward side because one main vortex will also be formed in the vertical section of the canyon by the perpendicular component of the wind. But pollutant concentrations decrease in the canyon because pollutants are dispersed along the axis of the street. Pollutants at different heights of the vertical section decrease with height, i.e. there are concentration gradients in the vertical section, and the pollutant concentrations on the leeward side of the upstream building are much higher than those on the windward side of the downstream building.

Outdoor thermal comfort: Analyzing the impact of urban configurations on the thermal performance of street canyons in the humid subtropical climate of Sydney

The quality of outdoor space is becoming increasingly important with the growing rate of urbanization.Visual,acoustic,and thermal balance degradation are all negative impacts associated with outdoor comfort in dense urban fabrics.Urban morphology thus needs assessment and optimization to ensure favorable outdoor thermal comfort(OTC).This study aims to evaluate the thermal performance of streets in residential zones of Liverpool,NSW,Australia,and tries to improve their comfort index(Physiological Equivalent Temperature)to reveal optimum urban configurations.This evaluation is done by investigating the following urban design factors affecting OTC using computational simulation techniques:street orientation,aspect ratio,building typology,and surface coverage.Our findings reveal that street canyon orientation is the most influential factor(46.42%),followed by aspect ratio(30.59%).Among the influential meteorological parameters(air temperature,wind speed,humidity and solar radiation),wind velocity had the most significant impact on the thermal comfort of the outdoor spaces in this coastal region,which typically experiences intense airflow.The results of our analysis can be utilized by multiple stakeholders,allowing them to understand and extract the most vital design factors which contextually influence the thermal comfort of outdoor spaces.Outdoor thermal comfort has a direct effect on the health and wellbeing of occupants of outdoor spaces.

OPTIMIZING LAYOUT OF URBAN STREET CANYON USING NUMERICAL SIMULATION COUPLING WITH MATHEMATICAL OPTIMIZATION

Optimizing the layout of the urban street canyon to achieve the maximum environmental benefits should become a new idea for modem urban street design and planning. This paper aims to find out the optimized street canyon from a viewpoint of environmental protection by using the two-dimensional numerical simulation model with the κ-ω turbulence model, coupling with the mathematical optimization method. The total pollutant concentration within and at top of the specific street canyons was taken as the objective function, and the height of one side of the canyon as the constrained condition. A nonlinearly improved constrained variable metric solver was used. The effect of the height of the leeward building and windward building on the integrated pollutant dispersion was studied to achieve the most beneficial configuration of the urban geometry. The optimization of layout for an asymmetrical street canyon was obtained. It is further found that the step-down street canyon with a large height difference is generally a good layout favoring to reduce the concentration accumulation in the street canyon.

Field synergy analysis of pollutant dispersion in street canyons and its optimization by adding wind catchers

The microenvironment,which involves pollutant dispersion of the urban street canyon,is critical to the health of pedestrians and residents.The objectives of this work are twofold:(i)to effectively assess the pollutant dispersion process based on a theory and(ii)to adopt an appropriate stratigy,i.e.,wind catcher,to alleviate the pollution in the street canyons.Pollutant dispersion in street canyons is essentially a convective mass transfer process.Because the convective heat transfer process and the mass transfer process are physically similar and the applicability of field synergy theory to turbulence has been verified in the literature,we apply the field synergy theory to the study of pollutant dispersion in street canyons.In this paper,a computational fluid dynamics(CFD)simulation is conducted to investigate the effects of wind catcher,wind speed and the geometry of the street canyons on pollutant dispersion.According to the field synergy theory,Sherwood number and field synergy number are used to quantitatively evaluate the wind catcher and wind speed on the diffusion of pollutants in asymmetric street canyons.The results show that adding wind catchers can significantly improve the air quality of the step-down street canyon and reduce the average pollutant concentrations in the street canyon by 75%.Higher wind speed enhances diffusion of pollutants differently in different geometric street canyons.

Real-time measurements of PM_(2.5),PM_(10-2.5),and BC in an urban street canyon

A continuous dichotomous beta gauge monitor was used to characterize the hourly content of PM2.5, PM10-2.5, and Black Carbon （BC） over a 12-month period in an urban street canyon of Hong Kong. Hourly vehicle counts for nine vehicle classes and meteorological data were also recorded. The average weekly cycles of PM2.5, PM10-2.5, and BC suggested that all species are related to traffic, with high concentrations on workdays and low concentrations over the weekends. PM2.s exhibited two comparable concentrations at 10：00-11：00 （63.4 μg/m3） and 17：00-18：00 （65.0 p.g/m3 ） local time （LT） during workdays, correspond- ing to the hours when the numbers of diesel-fueled and gasoline-fueled vehicles were at their maximum levels： 3179 and 2907 h-1, respectively. BC is emitted mainly by diesel-fueled vehicles and this showed the highest concentration （31.2μg/m3） during the midday period （10：00-11：00 LT） on workdays. A poor correlation was found between PM2.s concentration and wind speed （R= 0.51, P-value 〉 0.001 ）. In contrast, the concentration of PM10-2.s was found to depend upon wind speed and it increased with obvious statistical significance as wind speed increased （R = 0,98, P-value 〈 0.0001 ）.

Wind tunnel simulation of pollutant dispersion inside street canyons with galleries and multi-level flat roofs

In this study, the pollutant dispersion within street canyons is studied by experiments conducted in an environmental wind tunnel. The vehicular exhaust emissions are modeled using a line source. The pollutant （smoke） concentrations inside the canyons are measured based on a light scattering technique. The pollutant concentrations within the four different street canyons containing the galleries and the three-level flat-roofs under both the isolated and urban environments are obtained and discussed. For each of the four canyon configurations investigated, it is found that there is an obvious discrepancy between the pollutant dispersion patterns under the isolated environment and the urban environment. The three-level fiat roof is found to significantly influence the pollutant distribution pattern in a street canyon. In order to clarify the impacts of the wedge-shaped roofs on the pollutant dispersion inside an urban street canyon of an aspect ratio of 1.0, the pollutant distributions inside urban street canyons of three different wedge-shaped roof combinations are measured and analyzed. It is revealed that the pollutant distribution pattern inside the urban street canyon of an aspect ratio of 1.0 is influenced greatly by the wedge-shaped roof, especially, when an upward wedge-shaped roof is placed on the upstream building of the canyon. Images from this study may be utilized for a rough evaluation of the computational fluid dynamics （CFD） models and for helping architects and urban planners to select the canyon configurations with a minimum negative impact on the local air quality.

Numerical investigation on fire-induced indoor and outdoor air pollutant dispersion in an idealized urban street canyon

Fire-induced pollutant dispersion under the influence of buoyancy in urban street canyons has attracted wide attention given its adverse impact on human health.This study analyzes the influences of fire source location and crossflowing wind(perpendicular to the canyon centerline)on indoor and outdoor air pollutant dispersion in an idealized urban street canyon by employing large eddy simulation.Three fire scenarios are defined according to the transverse location of the fire source:near the windward building(scenario 1),in the middle of the canyon(scenario 2),and near the leeward building(scenario 3).Results show that a re-entrainment phenomenon appears when the wind velocity reaches a critical value in scenarios 1 and 2,but it doesn't occur in scenario 3.Fire source location significantly influences the critical re-entrainment velocity.The critical velocity in scenario 1 is approximate 1.2-1.5 m/s larger than that in scenario 2.When the heat release rate is large,the critical Fr numbers are less sensitive to changes in HRR,and remain approximately constant with values of 0.47(scenario 1)and 0.37(scenario 2).When the wind velocity is large,more compartments are expected to be affected in the upper floors in all of the three scenarios,and smoke is distributed in an inverted triangle within the buildings.The indoor/outdoor temperature and pollutant concentrations are also analyzed.Our findings can provide valuable information for both human and property safety in relation to urban street canyons and their surrounding buildings.

Numerical study of reactive pollutants diffusion in urban street canyons with a viaduct

In this paper,the influences of the ambient wind speed and the height and width of a viaduct in a 2-dimensional street canyon on the diffusion of reactive pollutants emitted by motor vehicles were investigated using computational fluid dynamics(CFD)method.Pollutants were treated as reactive by including a NO-NO_(2)-O_(3) photochemical reaction mechanism in the simulation.The Reynolds-averaged Navier-Stokes(RANS)k-ε turbulence model and the discrete phase model were used to simulate the airflow movement and the concentration distribution,respectively,of the reactive pollutants in the street canyon.Three indices,i.e.,the chemical reaction contribution of NO(CRC_(NO)),the chemical reaction contribution of NO_(2)(CRC_(NO_(2))),and the O_(3) depletion rate,were used to evaluate the relative importance of the photochemical reactions.It was found that the presence of a viaduct changed the flow field structure in the street canyon.The CRC_(NO) and CRC_(NO_(2))decreased from the windward side to the leeward side of the canyon.The maximum values of the CRC_(NO) and CRC_(NO_(2)) were observed at the pollution source(x=245 m)due to the influence of a clockwise vortex in the street canyon.As the height and width of the viaduct increased,concentration of the ground pollutants and the O_(3) depletion rate increased.The O_(3) depletion rate was much higher on the leeward side(90%)than on the windward side.The pollutant concentrations after the reaction were twice as high with the viaduct as without the viaduct when the viaduct height was the same as the building height.The viaduct had a significantly larger influence on the concentration of the reactive pollutants than the chemical reactions.The O_(3) depletion rate in the canyon and the pollutant concentrations decreased as the ambient wind speed increased,whereas the CRC_(NO_(2)) increased.