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1.
We investigate the dispersion of a passive scalar within an idealised urban district made up of a building-like obstacle array.
We focus on a street network in which the lateral dimension of the buildings exceeds the street width, a geometry representative
of many European cities. To investigate the effect of different geometries and wind directions upon the pollutant dispersion
process, we have performed a series of wind-tunnel experiments. Concentration measurements of a passive tracer have enabled
us to infer the main features characterising its dispersion within the street network. We describe this by focusing on the
roles of different transfer processes. These are the channelling of the tracer along the street axes, the mixing at street
intersections, and the mass exchange between the streets and the overlying atmospheric flow. Our experiments provide evidence
of the dependence of these processes on the geometrical properties of the array and the direction of the overlying atmospheric
flow. 相似文献
2.
Evaluation of a micro-scale wind model’s performance over realistic building clusters using wind tunnel experiments 总被引：2，自引：0，他引：2
The simulation performance over complex building clusters of a wind simulation model(Wind Information Field Fast Analysis model, WIFFA) in a micro-scale air pollutant dispersion model system(Urban Microscale Air Pollution dispersion Simulation model, UMAPS) is evaluated using various wind tunnel experimental data including the CEDVAL(Compilation of Experimental Data for Validation of Micro-Scale Dispersion Models) wind tunnel experiment data and the NJU-FZ experiment data(Nanjing University-Fang Zhuang neighborhood wind tunnel experiment data). The results show that the wind model can reproduce the vortexes triggered by urban buildings well, and the flow patterns in urban street canyons and building clusters can also be represented. Due to the complex shapes of buildings and their distributions, the simulation deviations/discrepancies from the measurements are usually caused by the simplification of the building shapes and the determination of the key zone sizes. The computational efficiencies of different cases are also discussed in this paper. The model has a high computational efficiency compared to traditional numerical models that solve the Navier–Stokes equations, and can produce very high-resolution(1–5 m) wind fields of a complex neighborhood scale urban building canopy(～ 1 km ×1km) in less than 3 min when run on a personal computer. 相似文献
3.
A microscale model for air pollutant dispersion simulation in urban areas: Presentation of the model and performance over a single building 总被引：1，自引：0，他引：1
A microscale air pollutant dispersion model system is developed for emergency response purposes. The model includes a diagnostic wind field model to simulate the wind field and a random-walk air pollutant dispersion model to simulate the pollutant concentration through consideration of the influence of urban buildings. Numerical experiments are designed to evaluate the model's performance, using CEDVAL(Compilation of Experimental Data for Validation of Microscale Dispersion Models) wind tunnel experiment data, including wind fields and air pollutant dispersion around a single building. The results show that the wind model can reproduce the vortexes triggered by urban buildings and the dispersion model simulates the pollutant concentration around buildings well. Typically, the simulation errors come from the determination of the key zones around a building or building cluster. This model has the potential for multiple applications; for example, the prediction of air pollutant dispersion and the evaluation of environmental impacts in emergency situations; urban planning scenarios;and the assessment of microscale air quality in urban areas. 相似文献
4.
Pollutant Concentrations in Street Canyons of Different Aspect Ratio with Avenues of Trees for Various Wind Directions 总被引：2，自引：0，他引：2
This study summarizes the effects of avenues of trees in urban street canyons on traffic pollutant dispersion. We describe various wind-tunnel experiments with different tree-avenue models in combination with variations in street-canyon aspect ratio W/H (with W the street-canyon width and H the building height) and approaching wind direction. Compared to tree-free street canyons, in general, higher pollutant concentrations are found. Avenues of trees do not suppress canyon vortices, although the air ventilation in canyons is hindered significantly. For a perpendicular wind direction, increases in wall-average and wall-maximum concentrations at the leeward canyon wall and decreases in wall-average concentrations at the windward wall are found. For oblique and perpendicular wind directions, increases at both canyon walls are obtained. The strongest effects of avenues of trees on traffic pollutant dispersion are observed for oblique wind directions for which also the largest concentrations at the canyon walls are found. Thus, the prevailing assumption that attributes the most harmful dispersion conditions to a perpendicular wind direction does not hold for street canyons with avenues of trees. Furthermore, following dimensional analysis, an estimate of the normalized wall-maximum traffic pollutant concentration in street canyons with avenues of trees is derived. 相似文献
5.
Despite their importance for pollutant dispersion in urban areas, the special features of dispersion at street intersections
are rarely taken into account by operational air quality models. Several previous studies have demonstrated the complex flow
patterns that occur at street intersections, even with simple geometry. This study presents results from wind-tunnel experiments
on a reduced scale model of a complex but realistic urban intersection, located in central London. Tracer concentration measurements
were used to derive three-dimensional maps of the concentration field within the intersection. In combination with a previous
study (Carpentieri et al., Boundary-Layer Meteorol 133:277–296, 2009) where the velocity field was measured in the same model,
a methodology for the calculation of the mean tracer flux balance at the intersection was developed and applied. The calculation
highlighted several limitations of current state-of-the-art canyon dispersion models, arising mainly from the complex geometry
of the intersection. Despite its limitations, the proposed methodology could be further developed in order to derive, assess
and implement street intersection dispersion models for complex urban areas. 相似文献
6.
The transport of a passive scalar from a continuous point-source release in an urban street network is studied using direct numerical simulation (DNS). Dispersion through the network is characterized by evaluating horizontal fluxes of scalar within and above the urban canopy and vertical exchange fluxes through the canopy top. The relative magnitude and balance of these fluxes are used to distinguish three different regions relative to the source location: a near-field region, a transition region and a far-field region. The partitioning of each of these fluxes into mean and turbulent parts is computed. It is shown that within the canopy the horizontal turbulent flux in the street network is small, whereas above the canopy it comprises a significant fraction of the total flux. Vertical fluxes through the canopy top are predominantly turbulent. The mean and turbulent fluxes are respectively parametrized in terms of an advection velocity and a detrainment velocity and the parametrization incorporated into a simple box-network model. The model treats the coupled dispersion problem within and above the street network in a unified way and predictions of mean concentrations compare well with the DNS data. This demonstrates the usefulness of the box-network approach for process studies and interpretation of results from more detailed numerical simulations. 相似文献
7.
城市湍流边界层内汽车尾气扩散规律数值模拟研究 总被引：2，自引：1，他引：1
以纳维斯托克斯方程组、大气平流扩散方程、湍流动能及湍流动能耗散率方程组为基础．采用伪不定常方法,建立了一个数值模式．利用该模式列城市湍流边界层内流场结构及汽车排放污染物扩散规律进行了研究。结果表明：街谷内会形成一个涡旋型流场．汽车排放污染物浓度在地面及建筑物背风面产生堆积,且其沿高度方向的梯度变化在背风面大．迎风而小。随着街谷两侧建筑物屋顶风速的增大,峡谷内形成的涡旋流场的强度增大,污染物扩散速率增大：当屋顶来流与街道之间的夹角逐渐增大时．涡旋中心位置由街道中心偏向于背风面及更高层且污染物扩散速度加快。 相似文献
8.
9.
The Gaussian model of plume dispersion is commonly used for pollutant concentration estimates. However, its major parameters, dispersion coefficients, barely account for terrain configuration and surface roughness. Large-scale roughness elements (e.g. buildings in urban areas) can substantially modify the ground features together with the pollutant transport in the atmospheric boundary layer over urban roughness (also known as the urban boundary layer, UBL). This study is thus conceived to investigate how urban roughness affects the flow structure and vertical dispersion coefficient in the UBL. Large-eddy simulation (LES) is carried out to examine the plume dispersion from a ground-level pollutant (area) source over idealized street canyons for cross flows in neutral stratification. A range of building-height-to-street-width (aspect) ratios, covering the regimes of skimming flow, wake interference, and isolated roughness, is employed to control the surface roughness. Apart from the widely used aerodynamic resistance or roughness function, the friction factor is another suitable parameter that measures the drag imposed by urban roughness quantitatively. Previous results from laboratory experiments and mathematical modelling also support the aforementioned approach for both two- and three-dimensional roughness elements. Comparing the UBL plume behaviour, the LES results show that the pollutant dispersion strongly depends on the friction factor. Empirical studies reveal that the vertical dispersion coefficient increases with increasing friction factor in the skimming flow regime (lower resistance) but is more uniform in the regimes of wake interference and isolated roughness (higher resistance). Hence, it is proposed that the friction factor and flow regimes could be adopted concurrently for pollutant concentration estimate in the UBL over urban street canyons of different roughness. 相似文献
10.
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 coecient 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. 相似文献
11.
How to Parametrize Urban-Canopy Drag to Reproduce Wind-Direction Effects Within the Canopy 总被引：1，自引：1，他引：0
The mean wind direction within an urban canopy changes with height when the incoming flow is not orthogonal to obstacle faces. This wind-turning effect is induced by complex processes and its modelling in urban-canopy (UC) parametrizations is difficult. Here we focus on the analysis of the spatially-averaged flow properties over an aligned array of cubes and their variation with incoming wind direction. For this purpose, Reynolds-averaged Navier–Stokes simulations previously compared, for a reduced number of incident wind directions, against direct numerical simulation results are used. The drag formulation of a UC parametrization is modified and different drag coefficients are tested in order to reproduce the wind-turning effect within the canopy for oblique wind directions. The simulations carried out for a UC parametrization in one-dimensional mode indicate that a height-dependent drag coefficient is needed to capture this effect. 相似文献
12.
13.
Sang-Hyun Lee 《Boundary-Layer Meteorology》2011,140(2):315-342
The vegetated urban canopy model (VUCM), which includes parametrizations of urban physical processes for artificial surfaces
and vegetated areas in an integrated system, has been further developed by including physical processes associated with grass-covered
surfaces in urban pervious surfaces and the photosynthesis effects of urban vegetation. Using measurements made from three
urban/suburban sites during the BUBBLE field campaign in 2002, the model’s performance in modelling surface fluxes (momentum
flux, net radiation, sensible and latent heat fluxes and storage heat flux) and canopy air conditions (canopy air temperature
and specific humidity) was critically evaluated for the non-precipitation and the precipitation days. The observed surface
fluxes at the urban/suburban sites were significantly altered by precipitation as well as urban vegetation. Especially, the
storage heat at urban surfaces and underlying substrates varied drastically depending on weather conditions while having an
important role in the formation of a nocturnal urban surface layer. Unlike the nighttime canopy air temperature that was largely
affected by the storage-heat release, the daytime canopy air conditions were highly influenced by the vertical turbulent exchange
with the overlying atmosphere. The VUCM well reproduced these observed features in surface fluxes and canopy air conditions
at all sites while performing well for both the non-precipitation and the precipitation days. The newly implemented parametrizations
clearly improved the model’s performance in the simulation of sensible and latent heat fluxes at the sites, more noticeably
at the suburban site where the vegetated area fraction is the largest among the sites. Sensitivity analyses for model input
parameters in VUCM elucidated the relative importance of the morphological, aerodynamic, hydrological and radiative/thermal
properties in modelling urban surface fluxes and canopy air conditions for daytime and nighttime periods. These results suggest
that the VUCM has great potential for urban atmospheric numerical modelling for a range of cities and weather conditions in
addition to having a better physical basis in the representation of urban vegetated areas and associated physical processes. 相似文献
14.
A large-eddy simulation (LES) with a one-equation subgrid-scale (SGS) model was developed to investigate the flow field and
pollutant dispersion inside street canyons of high aspect ratio (AR). A 1/7th power-law wall model was implemented near rigid
walls to mitigate the demanding near-wall resolution requirements in LES. This LES model had been extensively validated against
experimental results for street canyons of AR = 1 and 2 before it was applied to the cases of AR = 3 and 5. A ground-level
passive pollutant line source, located in the middle of the street, was used to simulate vehicular emissions. Three and five
vertically aligned primary recirculations were developed in the street canyons of AR 3 and 5, respectively. The ground-level
mean wind speed was less than 0.5% of the free stream value, which makes it difficult for the pollutant to be transported
upward for removal. High pollutant concentration and variance were found near the buildings where the air flow is upwards.
It was found that the velocity fluctuation, pollutant concentration and variance were all closely related to the interactions
between the primary recirculations and/or the free surface layer. Several quantities, which are non-linear functions of AR,
were introduced to quantify the air quality in street canyons of different configurations. 相似文献
15.
Flow and Pollutant Transport in Urban Street Canyons of Different Aspect Ratios with Ground Heating: Large-Eddy Simulation 总被引：2，自引：2，他引：0
Xian-Xiang Li Rex E. Britter Leslie K. Norford Tieh-Yong Koh Dara Entekhabi 《Boundary-Layer Meteorology》2012,142(2):289-304
A validated large-eddy simulation model was employed to study the effect of the aspect ratio and ground heating on the flow
and pollutant dispersion in urban street canyons. Three ground-heating intensities (neutral, weak and strong) were imposed
in street canyons of aspect ratio 1, 2, and 0.5. The detailed patterns of flow, turbulence, temperature and pollutant transport
were analyzed and compared. Significant changes of flow and scalar patterns were caused by ground heating in the street canyon
of aspect ratio 2 and 0.5, while only the street canyon of aspect ratio 0.5 showed a change in flow regime (from wake interference
flow to skimming flow). The street canyon of aspect ratio 1 does not show any significant change in the flow field. Ground
heating generated strong mixing of heat and pollutant; the normalized temperature inside street canyons was approximately
spatially uniform and somewhat insensitive to the aspect ratio and heating intensity. This study helps elucidate the combined
effects of urban geometry and thermal stratification on the urban canyon flow and pollutant dispersion. 相似文献
16.
On the Impact of Trees on Dispersion Processes of Traffic Emissions in Street Canyons 总被引：4，自引：0，他引：4
Wind-tunnel studies of dispersion processes of traffic exhaust in urban street canyons with tree planting were performed and
tracer gas concentrations using electron capture detection (ECD) and flow fields using laser Doppler velocimetry (LDV) were
measured. It was found that tree planting reduces the air exchange between street canyons and the ambience. In comparison
to treeless street canyons, higher overall pollutant concentrations and lower flow velocities were measured. In particular,
for perpendicular approaching wind, markedly higher concentrations at the leeward canyon wall and slightly lower concentrations
at the windward canyon wall were observed. Furthermore, a new approach is suggested to model porous vegetative structures
such as tree crowns for small-scale wind-tunnel applications. The approach is based on creating different model tree crown
porosities by incorporating a certain amount of wadding material into a specified volume. A significant influence of the crown
porosity on pollutant concentrations was found for high degrees of porosity, however, when it falls below a certain threshold,
no further changes in pollutant concentrations were observed. 相似文献
17.
Built terrains, with their complexity in morphology, high heterogeneity, and anthropogenic impact, impose substantial challenges in Earth-system modelling. In particular, estimation of the source areas and footprints of atmospheric measurements in cities requires realistic representation of the landscape characteristics and flow physics in urban areas, but has hitherto been heavily reliant on large-eddy simulations. In this study, we developed physical parametrization schemes for estimating urban footprints based on the backward-Lagrangian-stochastic algorithm, with the built environment represented by street canyons. The vertical profile of mean streamwise velocity is parametrized for the urban canopy and boundary layer. Flux footprints estimated by the proposed model show reasonable agreement with analytical predictions over flat surfaces without roughness elements, and with experimental observations over sparse plant canopies. Furthermore, comparisons of canyon flow and turbulence profiles and the subsequent footprints were made between the proposed model and large-eddy simulation data. The results suggest that the parametrized canyon wind and turbulence statistics, based on the simple similarity theory used, need to be further improved to yield more realistic urban footprint modelling. 相似文献
18.
Large-Eddy Simulation of Flow and Pollutant Transport in Urban Street Canyons with Ground Heating 总被引：5，自引：4，他引：1
Xian-Xiang Li Rex E. Britter Tieh Yong Koh Leslie K. Norford Chun-Ho Liu Dara Entekhabi Dennis Y. C. Leung 《Boundary-Layer Meteorology》2010,137(2):187-204
Our study employed large-eddy simulation (LES) based on a one-equation subgrid-scale model to investigate the flow field and
pollutant dispersion characteristics inside urban street canyons. Unstable thermal stratification was produced by heating
the ground of the street canyon. Using the Boussinesq approximation, thermal buoyancy forces were taken into account in both
the Navier–Stokes equations and the transport equation for subgrid-scale turbulent kinetic energy (TKE). The LESs were validated
against experimental data obtained in wind-tunnel studies before the model was applied to study the detailed turbulence, temperature,
and pollutant dispersion characteristics in the street canyon of aspect ratio 1. The effects of different Richardson numbers
(Ri) were investigated. The ground heating significantly enhanced mean flow, turbulence, and pollutant flux inside the street
canyon, but weakened the shear at the roof level. The mean flow was observed to be no longer isolated from the free stream
and fresh air could be entrained into the street canyon at the roof-level leeward corner. Weighed against higher temperature,
the ground heating facilitated pollutant removal from the street canyon. 相似文献
19.
An analytical model of atmospheric dispersion in urban areas in both daytime and nighttime conditions is presented. The model
is based on a Gaussian formulation where the horizontal and vertical diffusion coefficients are determined according to analytical
theories. The model is validated with dispersion measurements from field experiments conducted in Oklahoma City, Salt Lake
City, St. Louis and London, U.K. The theory is in good agreement with the data for both daytime and nighttime conditions.
The data support the conclusion that the magnitude of the nighttime stratification in the urban atmosphere is weak; however,
its effects on dispersion are not negligible. The predicted existence of two distinct dispersion regimes, in the near and
in the far field, is also confirmed by the data. The good collapse of the data suggests that urban dispersion is governed
by the characteristic length scales of atmospheric boundary-layer turbulence, rather than urban canopy length scales that
are more likely to affect dispersion only in the vicinity of the source. 相似文献
20.
Ashok K. Luhar 《Boundary-Layer Meteorology》2002,102(1):1-38
The mean concentration distributionwithin a plume released from a point source in the atmosphericboundary layer can be greatly influenced by the systematic turningof wind with height (i.e. vertical wind direction shear). Such aninfluence includes a deflection of the plume centroid, with anassociated shearing of the vertical plume cross-section, and anenhancement of dispersion, in the horizontal plane. Wind directionshear is normally not accounted for in coastal fumigation models,although dispersion observations with shear acting as acontrolling parameter are not uncommon. A three-dimensionalLagrangian stochastic model is used to investigate the influenceof uniform wind direction shear on the diffusion of a point-sourceplume within the horizontally homogeneous convective boundarylayer, with the source located at the top of the boundary layer.Parameterisations are developed for the plume deflection andenhanced dispersion due to shear within the framework of aprobability density function (PDF) approach, and compared with theLagrangian model results. These parameterisations are thenincorporated into two applied coastal fumigation models: a PDFmodel, and a commonly used model that assumes uniform andinstantaneous mixing in the vertical direction. The PDF modelrepresents the vertical mixing process more realistically. A moreefficient version of the PDF model, which assumes a well-mixedconcentration distribution in the vertical at large times, isapplied to simulate sulfur dioxide data from the Kwinana CoastalFumigation Study. A comparison between the model results and thedata show that the model performs much better when the wind-sheareffects are included. 相似文献