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1.
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
2.
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
3.
An Improved Three-Dimensional Simulation of the Diurnally Varying Street-Canyon Flow 总被引：3，自引：3，他引：0
The impact of diurnal variations of the heat fluxes from building and ground surfaces on the fluid flow and air temperature distribution in street canyons is numerically investigated using the PArallelized Large-eddy Simulation Model (PALM). Simulations are performed for a 3 by 5 array of buildings with canyon aspect ratio of one for two clear summer days that differ in atmospheric instability. A detailed building energy model with a three-dimensional raster-type geometry—Temperature of Urban Facets Indoor-Outdoor Building Energy Simulator (TUF-IOBES)—provides urban surface heat fluxes as thermal boundary conditions for PALM. In vertical cross-sections at the centre of the spanwise canyon the mechanical forcing and the horizontal streamwise thermal forcing at roof level outweigh the thermal forces from the heated surfaces inside the canyon in defining the general flow pattern throughout the day. This results in a dominant canyon vortex with a persistent speed, centered at a constant height. Compared to neutral simulations, non-uniform heating of the urban canyon surfaces significantly modifies the pressure field and turbulence statistics in street canyons. Strong horizontal pressure gradients were detected in streamwise and spanwise canyons throughout the day, and which motivate larger turbulent velocity fluctuations in the horizontal directions rather than in the vertical direction. Canyon-averaged turbulent kinetic energy in all non-neutral simulations exhibits a diurnal cycle following the insolation on the ground in both spanwise and streamwise canyons, and it is larger when the canopy bottom surface is paved with darker materials and the ground surface temperature is higher as a result. Compared to uniformly distributed thermal forcing on urban surfaces, the present analysis shows that realistic non-uniform thermal forcing can result in complex local airflow patterns, as evident, for example, from the location of the vortices in horizontal planes in the spanwise canyon. This study shows the importance of three-dimensional simulations with detailed thermal boundary conditions to explore the heat and mass transport in an urban area. 相似文献
4.
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
5.
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
6.
The effects of street bottom heating and inflow turbulence on urban street-canyon flow are experimentally investigated using a circulating water channel. Three experiments are carried out for a street canyon with a street aspect ratio of 1. Results from each experiment with bottom heating or inflow turbulence are compared with those without bottom heating and appreciable inflow turbulence. It is demonstrated that street bottom heating or inflow turbulence increases the intensity of the canyon vortex. A possible explanation on how street bottom heating or inflow turbulence intensifies the canyon vortex is given from a fluid dynamical viewpoint. 相似文献
7.
The transfer processes within and above a simulated urban street canyon were investigated in a generic manner. Computational
fluid dynamics (CFD) was used to aid understanding and to produce some simple operational parameterisations. In this study
we addressed specifically the commonly met situation where buoyancy effects arising from elevated surface temperatures are
not important, i.e. when mechanical forces outweigh buoyancy forces. In a geophysical context this requires that some suitably
defined Richardson number is small. From an engineering perspective this is interpreted as the important case when heat transfer
within and above urban street canyons is by forced convection. Surprisingly, this particular scenario (for which the heat
transfer coefficient between buildings and the flow is largest), has been less well studied than the situation where buoyancy
effects are important. The CFD technique was compared against wind-tunnel experiments to provide model evaluation. The height-to-width
ratio of the canyon was varied through the range 0.5–5 and the flow was normal to the canyon axis. By setting the canyon’s
facets to have the same or different temperatures or to have a partial temperature distribution, simulations were carried
out to investigate: (a) the influence of geometry on the flow and mixing within the canyon and (b) the exchange processes
within the canyon and across the canyon top interface. Results showed that the vortex-type circulation and turbulence developed
within the canyon produced a temperature distribution that was, essentially, spatially uniform (apart from a relatively thin
near-wall thermal boundary layer) This allowed the temperatures within the street canyon to be specified by just one value
T
can
, the canyon temperature. The variation of T
can
with wind speed, surface temperatures and geometry was extensively studied. Finally, the exchange velocity u
E
across the interface between the canyon and the flow above was calculated based on a heat flux balance within the canyon
and between the canyon and the flow above. Results showed that u
E
was approximately 1% of a characteristic wind velocity above the street canyon. The problem of radiative exchange is not
addressed but it can, of course, be introduced analytically, or computationally, when necessary. 相似文献
8.
Numerical and experimental studies on flow and pollutant dispersion in urban street canyons 总被引：1，自引：0，他引：1
In this study numerical simulations and water tank experiments were used to investigate the flow and pollutant dispersion in an urban street canyon. Two types of canyon geometry were tested. The studies indicate that in a step-up notch canyon （higher buildings on the downstream side of the canyon）, the height and shape of the upstream lower buildings plays an important role in flow pattern and pollutant dispersion, while in a step-down notch canyon （lower buildings on the downstream side）, the downstream lower buildings have little influence. The studies also show that the substitution of tall towers for parailelepiped buildings on one side of the canyon may enhance the street ventilation and decrease the pollutant concentration emitted by motor vehicles. 相似文献
9.
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
10.
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
11.
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
12.
Boundary-Layer Meteorology - Coherent flow structures and pollutant dispersion in a spanwise-long street canyon are investigated using a parallelized large-eddy-simulation model. Low- and... 相似文献
13.
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
14.
As urbanization progresses, more realistic methods are required to analyze the urban microclimate. However, given the complexity and computational cost of numerical models, the effects of realistic representations should be evaluated to identify the level of detail required for an accurate analysis. We consider the realistic representation of surface heating in an idealized three-dimensional urban configuration, and evaluate the spatial variability of flow statistics (mean flow and turbulent fluxes) in urban streets. Large-eddy simulations coupled with an urban energy balance model are employed, and the heating distribution of urban surfaces is parametrized using sets of horizontal and vertical Richardson numbers, characterizing thermal stratification and heating orientation with respect to the wind direction. For all studied conditions, the thermal field is strongly affected by the orientation of heating with respect to the airflow. The modification of airflow by the horizontal heating is also pronounced for strongly unstable conditions. The formation of the canyon vortices is affected by the three-dimensional heating distribution in both spanwise and streamwise street canyons, such that the secondary vortex is seen adjacent to the windward wall. For the dispersion field, however, the overall heating of urban surfaces, and more importantly, the vertical temperature gradient, dominate the distribution of concentration and the removal of pollutants from the building canyon. Accordingly, the spatial variability of concentration is not significantly affected by the detailed heating distribution. The analysis is extended to assess the effects of three-dimensional surface heating on turbulent transfer. Quadrant analysis reveals that the differential heating also affects the dominance of ejection and sweep events and the efficiency of turbulent transfer (exuberance) within the street canyon and at the roof level, while the vertical variation of these parameters is less dependent on the detailed heating of urban facets. 相似文献
15.
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
16.
Negin Nazarian Alberto Martilli Leslie Norford Jan Kleissl 《Boundary-Layer Meteorology》2018,168(2):321-341
Urban morphology and inter-building shadowing result in a non-uniform distribution of surface heating in urban areas, which can significantly modify the urban flow and thermal field. In Part I, we found that in an idealized three-dimensional urban array, the spatial distribution of the thermal field is correlated with the orientation of surface heating with respect to the wind direction (i.e. leeward or windward heating), while the dispersion field changes more strongly with the vertical temperature gradient in the street canyon. Here, we evaluate these results more closely and translate them into metrics of “city breathability,” with large-eddy simulations coupled with an urban energy-balance model employed for this purpose. First, we quantify breathability by, (i) calculating the pollutant concentration at the pedestrian level (horizontal plane at \(z\approx 1.5\)–2 m) and averaged over the canopy, and (ii) examining the air exchange rate at the horizontal and vertical ventilating faces of the canyon, such that the in-canopy pollutant advection is distinguished from the vertical removal of pollution. Next, we quantify the change in breathability metrics as a function of previously defined buoyancy parameters, horizontal and vertical Richardson numbers (\(Ri_\text {h}\) and \(Ri_\text {v}\), respectively), which characterize realistic surface heating. We find that, unlike the analysis of airflow and thermal fields, consideration of the realistic heating distribution is not crucial in the analysis of city breathability, as the pollutant concentration is mainly correlated with the vertical temperature gradient (\(Ri_\text {v}\)) as opposed to the horizontal (\(Ri_\text {h}\)) or bulk (\(Ri_\text {b}\)) thermal forcing. Additionally, we observe that, due to the formation of the primary vortex, the air exchange rate at the roof level (the horizontal ventilating faces of the building canyon) is dominated by the mean flow. Lastly, since \(Ri_\text {h}\) and \(Ri_\text {v}\) depend on the meteorological factors (ambient air temperature, wind speed, and wind direction) as well as urban design parameters (such as surface albedo), we propose a methodology for mapping overall outdoor ventilation and city breathability using this characterization method. This methodology helps identify the effects of design on urban microclimate, and ultimately informs urban designers and architects of the impact of their design on air quality, human health, and comfort. 相似文献
17.
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
18.
19.
Kyung-Hwan Kwak Jong-Jin Baik Sang-Hyun Lee Young-Hee Ryu 《Boundary-Layer Meteorology》2011,141(1):77-92
Urban surface and radiation processes are incorporated into a computational fluid dynamics (CFD) model to investigate the
diurnal variation of flow in a street canyon with an aspect ratio of 1. The developed CFD model predicts surface and substrate
temperatures of the roof, walls, and road. One-day simulations are performed with various ambient wind speeds of 2, 3, 4,
5, and 6 ms−1, with the ambient wind perpendicular to the north–south oriented canyon. During the day, the largest maximum surface temperature
for all surfaces is found at the road surface for an ambient wind speed of 3 ms−1 (56.0°C). Two flow regimes are identified by the vortex configuration in the street canyon. Flow regime I is characterized
by a primary vortex. Flow regime II is characterized by two counter-rotating vortices, which appears in the presence of strong
downwind building-wall heating. Air temperature is relatively low near the downwind building wall in flow regime I and inside
the upper vortex in flow regime II. In flow regime II, the upper vortex expands with increasing ambient wind speed, thus enlarging
the extent of cool air within the canyon. The canyon wind speed in flow regime II is proportional to the ambient wind speed,
but that in flow regime I is not. For weak ambient winds, the dependency of surface sensible heat flux on the ambient wind
speed is found to play an essential role in determining the relationship between canyon wind speed and ambient wind speed. 相似文献