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1.
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
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A box model to simulate mass transfer inside deep street canyons and with atmospheric flow above is introduced and discussed.
Two ideal deep street canyons with aspect ratios of 3 and 5 (the aspect ratio being the ratio between building height and
street width H/W) are considered. This range of aspect ratios, found in many densely populated historical centres in Mediterranean cities
as well as in other cities around the world, potentially creates high air pollutant concentration levels. Our model is based
on a combination of analytical solutions and computation fluid dynamics (CFD) simulations using carbon monoxide (CO) as a
tracer pollutant. The analytical part of the model is based on mass transfer velocity concepts while CFD simulations are used
both for a preliminary validation of the physical hypothesis underlying the model (steady-state simulations) and to evaluate
the concentration pattern with time (transient or wash-out simulations). Wash-out simulation curves were fitted by model curves,
and mass transfer velocities were evaluated through a best-fitting procedure. Upon introducing into the model the contribution
of traffic-produced turbulence, the modelled CO concentration levels became comparable with those obtained in real-world monitoring
campaigns. The mass transfer rate between the canyon and the above atmosphere was then expressed in terms of an overall mass
transfer velocity, which directly allows the evaluation of the mass transfer rate between the bottom volume of the canyon
(pedestrian level) with the above atmosphere. Overall mass transfer velocities are reported as a function of the operating
conditions studied (H/W = 3–5 and wind speeds = 2–8 ms−1). Finally, a simple expression is reported for determining pollutant concentrations at the pedestrian level based on the
overall mass transfer velocity defined. 相似文献
4.
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
5.
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
6.
为研究机动车辆排出的污染物在大气中的扩散规律，在北京做了小风条件下的街谷示踪试验。当楼顶风速u接近或大于1米/秒时，街谷内可形成一稳定的原生涡；u＜0.6米/秒时原生涡将消失。对于楼之间空间较小的街谷，背风面和迎风面的示踪剂浓度平均比值可达8。浓度值沿楼层高度无明显变化；由于快车路旁松墙的阻挡和抬升作用，可能造成沿高度方向楼层中段的浓度偏高。在街谷外，除下风方路面上有一按下风距离的负幂指数衰减的浓度分布外，上风方路面上也有一按较大负幂指数衰减的分布。根据上述试验，给出了用以预测街谷中机动车辆排出的惰性气体污染物的扩散模式；模式中，对原生涡和小尺度湍流，做了分别处理。 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
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Flow over urban surfaces depends on surface morphology and interaction with the boundary layer above. However, the effect of the flow on scalar fluxes is hard to quantify. The naphthalene sublimation technique was used to quantify scalar vertical fluxes out of a street canyon under neutral conditions. For an array of eight canyons with aspect ratio H/W=0.75 (here, H is building height and W is the street width), increased flux was observed in the first two or three canyons for moderate and low roughness upstream. This is consistent with predictions of the length scale for initial adjustment of flow to an urban canopy. The flux was constant after the initial adjustment region and thus dependent only on local geometry. For a street canyon in the equilibrium part of the array, each facet of the street canyon was coated with naphthalene to simulate scalar release from street, walls and roof, to evaluate the effect of street canyon geometry on fluxes for H/W=0.25, 0.6, 1 and 2. Fluxes from the roof and downstream wall were considerably larger than fluxes from the street and upstream wall, and only the flux from the downstream wall exhibited a simple decrease with H/W. For each H/W there was a monotonic decrease between downstream wall, street and upstream wall transfer. This suggests that flow decelerates around the recirculation region in the lee of the upstream building, i.e. a recirculating jet rather than a symmetrical vortex. The addition of a second source within the street canyon resulted in reduced fluxes from each facet for H/W>0.25, due to increased concentration of naphthalene in the canyon air. 相似文献
10.
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 between 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 1,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. 相似文献
11.
Summary The paper presents an overview of the influence of street architecture on the wind and turbulence patterns in street canyons and discusses the effects on local air quality. The findings of recent experimental and numerical studies are summarized and wind-tunnel data sets are presented that illustrate the flow-field variability. It is shown that small-scale features of the street architecture play an important role. The formation of a vortex inside the street canyon is affected by the roof configuration. In shorter street canyons, the flow component along the street becomes important for pollutant transport. These results are of importance for urban air quality modeling in particular when dealing with pollution problems caused by road traffic. Furthermore, the findings should be taken into account in fast response models that are used to assess critical areas in the case of accidental or non-accidental releases of hazardous material in urban areas. 相似文献
12.
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
13.
Marcin Idczak Patrice Mestayer Jean-Michel Rosant Jean-Francois Sini Michel Violleau 《Boundary-Layer Meteorology》2007,124(1):25-41
In order to investigate the microclimatic conditions in a street canyon, a physical model was used to conduct the Joint ATREUS-PICADA
Experiment (JAPEX) in situ experimental campaign. Four lines of buildings simulated by steel containers were installed to
form three parallel street canyons at 1:5 scale, with width/height aspect ratio approximately 0.40. The reference wind and
atmospheric conditions were measured, as well as the flow velocity and direction in the street. Preliminary results concern
street canyon ventilation and thermal effects on in-canyon airflow, and show that vortical motions appear for reference wind
directions perpendicular to the street axis. The presence of adjacent rows of buildings did not appear to significantly influence
the flow character within the canyon for the case of a low aspect ratio corresponding to a skimming flow regime. The flow
structure was not significantly affected by the thermal effects although some slight interference occurred in the lower part
of the canyon. An analysis of horizontal temperature gradients indicated that a thin boundary layer develops near the heated
facade. These facts imply that the thermal effects are considerable only very close to the wall. 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.
Effects of Street-Bottom and Building-Roof Heating on Flow in Three-Dimensional Street Canyons 总被引：4，自引：0，他引：4
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. 相似文献
16.
A model for the calculation of the turbulence flow field and air pollutant concentrations in urban canyons is developed. A two-dimensional set of hydrodynamical equations and a threedimensional diffusion equation are solved numerically with a personal computer. Different boundary conditions were investigated. Three flow regimes were found: without vortex, with one vortex, and with two vortexes, within an urban canyon. The influence of building density and wind speed components along the street was also investigated. 相似文献
17.
An analytical model has been developed for the flow along a street canyon (of height H and width W), generated by an external wind blowing at any angle relative to the axis of the street. Initially, we consider the special
case of a wind blowing parallel to the street. The interior of the street is decomposed into three regions, and the flow within
each region is assumed to depend only on the external wind and the distance to the closest solid boundary. This decomposition
leads to two different flow regimes: one for narrow streets (H/W > 1/2) and one for wide streets (H/W < 1/2). The theoretical model agrees well with results obtained from numerical simulations using a Reynolds-Averaged Navier–Stokes
model. We then generalize the model to the case of arbitrary wind direction. Numerical solutions show that the streamlines
of the mean flow in the street have a spiral form, and for most angles of incidence, the mass flux along the street scales
on the component of the external wind resolved parallel to the street. We use this result to generalize the model derived
for wind blowing parallel to the street, and the results from this model agree well with the numerical simulations. The model
that has been developed can be evaluated rapidly using only very modest computing power, so it is suitable for use as an operational
tool. 相似文献
18.
Large-eddy simulations were conducted to investigate the mechanism of pollutant removal from a three-dimensional street canyon. Five block configurations with aspect ratios (building height to length) of 1, 2, 4, 8 and $\infty $ were used to create an urban-like array. A pollutant was released from a ground-level line source at the centre of the target canyon floor. For smaller aspect ratios, the relative contribution of the turbulent mass flux to net mass flux at the roof level, which was spatially averaged along the roof-level ventilation area, was closer to unity, indicating that turbulent motions mainly affected pollutant removal from the top of the canyon. As aspect ratio increased, the relative contribution became smaller, owing to strong upwind motions. However, the relative contribution again reached near unity for the infinite aspect ratio (i.e. a two-dimensional street canyon) because of lowered lateral flow convergence. At least 75 % of total emissions from the three-dimensional street canyon were attributable to turbulent motions. Pollutant removal by turbulent motions was related to the coherent structures of low-momentum fluid above the canyons. Though the coherent structure size of the low-momentum fluid differed, the positions of low-momentum fluid largely corresponded to instantaneous high concentrations of pollutant above the target canyon, irrespective of canyon geometry. 相似文献
19.
This study constitutes a further step in the analysis of the performances of a street network model to simulate atmospheric pollutant dispersion in urban areas. The model, named SIRANE, is based on the decomposition of the urban atmosphere into two sub-domains: the urban boundary layer, whose dynamics is assumed to be well established, and the urban canopy, represented as a series of interconnected boxes. Parametric laws govern the mass exchanges between the boxes under the assumption that the pollutant dispersion within the canopy can be fully simulated by modelling three main bulk transfer phenomena: channelling along street axes, transfers at street intersections, and vertical exchange between street canyons and the overlying atmosphere. Here, we aim to evaluate the reliability of the parametrizations adopted to simulate these phenomena, by focusing on their possible dependence on the external wind direction. To this end, we test the model against concentration measurements within an idealized urban district whose geometrical layout closely matches the street network represented in SIRANE. The analysis is performed for an urban array with a fixed geometry and a varying wind incidence angle. The results show that the model provides generally good results with the reference parametrizations adopted in SIRANE and that its performances are quite robust for a wide range of the model parameters. This proves the reliability of the street network approach in simulating pollutant dispersion in densely built city districts. The results also show that the model performances may be improved by considering a dependence of the wind fluctuations at street intersections and of the vertical exchange velocity on the direction of the incident wind. This opens the way for further investigations to clarify the dependence of these parameters on wind direction and street aspect ratios. 相似文献
20.
Prashant Kumar Paul S. Fennell Allan N. Hayhurst Rex E. Britter 《Boundary-Layer Meteorology》2009,131(1):3-18
Dispersion of particles, as evidenced by changes in their number distributions (PNDs) and concentrations (PNCs), in urban
street canyons, is still not well understood. This study compares measurements by a fast-response particle spectrometer (DMS500)
of the PNDs and the PNCs (5–1000 nm, sampled at 1 Hz) at street and rooftop levels in a Cambridge UK street canyon, and examines
mixing, physical and chemical conversion processes, and the competing influences of traffic volume and rooftop wind speed
on the PNDs and the PNCs in various size ranges. PNCs at street level were ≈6.5 times higher than at rooftop. Street-level
PNCs followed the traffic volume and decreased with increasing wind speed, showing a larger influence of wind speed on 30–300 nm
particles than on 5–30 nm particles. Conversely, rooftop PNCs in the 5–30 nm size range increased with wind speed, whereas
those for particles between 30 and 300 nm did not vary with wind speed. 相似文献