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1.
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
2.
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
3.
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
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 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
6.
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
7.
A large-eddy simulation (LES) model, using the one-equation subgrid-scale (SGS) parametrization, was developed to study the
flow and pollutant transport in and above urban street canyons. Three identical two-dimensional (2D) street canyons of unity
aspect ratio, each consisting of a ground-level area source of constant pollutant concentration, are evenly aligned in a cross-flow
in the streamwise direction x. The flow falls into the skimming flow regime. A larger computational domain is adopted to accurately resolve the turbulence
above roof level and its influence on the flow characteristics in the street canyons. The LES calculated statistics of wind
and pollutant transports agree well with other field, laboratory and modelling results available in the literature. The maximum
wind velocity standard deviations σ
i
in the streamwise (σ
u
), spanwise (σ
v
) and vertical (σ
w
) directions are located near the roof-level windward corners. Moreover, a second σ
w
peak is found at z ≈ 1.5h (h is the building height) over the street canyons. Normalizing σ
i
by the local friction velocity u
*, it is found that σ
u
/u
* ≈ 1.8, σ
v
/u
* ≈ 1.3 and σ
w
/u
* ≈ 1.25 exhibiting rather uniform values in the urban roughness sublayer. Quadrant analysis of the vertical momentum flux
u′′w′′ shows that, while the inward and outward interactions are small, the sweeps and ejections dominate the momentum transport
over the street canyons. In the x direction, the two-point correlations of velocity R
v,x
and R
w,x
drop to zero at a separation larger than h but R
u,x
(= 0.2) persists even at a separation of half the domain size. Partitioning the convective transfer coefficient Ω
T
of pollutant into its removal and re-entry components, an increasing pollutant re-entrainment from 26.3 to 43.3% in the x direction is revealed, suggesting the impact of background pollutant on the air quality in street canyons. 相似文献
8.
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
9.
Jae-Jin Kim Eric Pardyjak Do-Yong Kim Kyoung-Soo Han Byung-Hyuk Kwon 《Asia-Pacific Journal of Atmospheric Sciences》2014,50(3):365-375
The effects of building-roof cooling on flow and air temperature in 3D urban street canyons are numerically investigated using a computational fluid dynamics (CFD) model. The aspect ratios of the building and street canyon considered are unity. For investigating the building-roof cooling effects, the building-roof temperatures are systematically changed. The traditional flow pattern including a portal vortex appears in the spanwise canyon. Compared with the case of the control run, there are minimal differences in flow pattern in the cases in which maximum building-roof cooling is considered. However, as the building roof becomes cooler, the mean kinetic energy increases and the air temperature decreases in the spanwise canyon. Building-roof cooling suppresses the upward and inward motions above the building roof, resultantly increasing the horizontal velocity near the roof level. The increase in wind velocity above the roof level intensifies the secondarily driven vortex circulation as well as the inward (outward) motion into (out of) the spanwise canyon. Finally, building-roof cooling reduces the air temperature in the spanwise canyon, supplying much relatively cool air from the streamwise canyon into the spanwise canyon. 相似文献
10.
Effect of Incoming Turbulent Structure on Pollutant Removal from Two-Dimensional Street Canyon 总被引:2,自引:2,他引:0
Large-eddy simulations are conducted to investigate the effects of the incoming turbulent structure of the flow on pollutant removal from an ideal canyon. The target canyon is a two-dimensional street canyon with an aspect ratio of 1.0 (building height to street width). Three turbulent flows upwind of the street canyon are generated by using different block configurations, and a tracer gas is released as a ground-level line source at the centre of the canyon floor. Mean velocity profiles for the three flows are similar, except near the roof. However, the root-mean-square values of the velocity fluctuations and the Reynolds shear stress increase with the friction velocity of the incoming turbulent flow. The spatially-averaged concentration within the canyon decreases with increasing friction velocity. Coherent structures of low-momentum fluid, generated above the upwind block configurations, contribute to pollutant removal, and the amount of pollutant removal is directly related to the size of the coherent structure. 相似文献
11.
Arash Nemati Hayati Rob Stoll J. J. Kim Todd Harman Matthew A. Nelson Michael J. Brown Eric R. Pardyjak 《Boundary-Layer Meteorology》2017,164(2):217-247
Three computational fluid dynamics (CFD) methods with different levels of flow-physics modelling are comprehensively evaluated against high-spatial-resolution wind-tunnel velocity data from step-down street canyons (i.e., a short building downwind of a tall building). The first method is a semi-empirical fast-response approach using the Quick Urban Industrial Complex (QUIC-URB) model. The second method solves the Reynolds-averaged Navier–Stokes (RANS) equations, and the third one utilizes a fully-coupled fluid-structure interaction large-eddy simulation (LES) model with a grid-turbulence inflow generator. Unlike typical point-by-point evaluation comparisons, here the entire two-dimensional wind-tunnel dataset is used to evaluate the dynamics of dominant flow topological features in the street canyon. Each CFD method is scrutinized for several geometric configurations by varying the downwind-to-upwind building-height ratio (\(H_\mathrm{d}/H_\mathrm{u}\)) and street canyon-width to building-width aspect ratio (S / W) for inflow winds perpendicular to the upwind building front face. Disparities between the numerical results and experimental data are quantified in terms of their ability to capture flow topological features for different geometric configurations. Overall, all three methods qualitatively predict the primary flow topological features, including a saddle point and a primary vortex. However, the secondary flow topological features, namely an in-canyon separation point and secondary vortices, are only well represented by the LES method despite its failure for taller downwind building cases. Misrepresentation of flow-regime transitions, exaggeration of the coherence of recirculation zones and wake fields, and overestimation of downwards vertical velocity into the canyon are the main defects in QUIC-URB, RANS and LES results, respectively. All three methods underestimate the updrafts and, surprisingly, QUIC-URB outperforms RANS for the streamwise velocity component, while RANS is superior to QUIC-URB for the vertical velocity component in the street canyon. 相似文献
12.
K. Fortuniak 《Theoretical and Applied Climatology》2008,91(1-4):245-258
Summary The paper focuses on the absorption of shortwave radiation in an urban street canyon. To test the effective albedo of the
canyon an analytic solution of the multiple reflection problem is applied. The infinitesimally long canyon is divided into
slices and a matrix of view factors for the slices is defined. Incoming shortwave radiation includes direct and diffuse parts
and shadowing effects are included in the analysis. The model is validated against Aida’s (1982) scale model data and measurements
in a real canyon. The results demonstrate a rapid decrease of the effective albedo as the canyon aspect ratio (its height
to width, H/W) are increased. It is also shown that diurnal changes of the effective albedo can be very complex depending on the particular
combination of H/W ratio, surface reflectivity and canyon orientation.
Author’s address: Krzysztof Fortuniak, Department of Meteorology and Climatology, University of Łódź, Narutowicza 88, 09-139
Łódź, Poland. 相似文献
13.
14.
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
15.
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
16.
Air flow inside an array of cubes is simulated. Cubes (edge length 0.15 m) are arranged in a regular array, separated by 0.15 m
in the streamwise and spanwise directions. Numerical simulations are performed based on Reynolds-averaged Navier–Stokes equations
(RANS), solved in a computational fluid dynamics model (CFD), with standard k–ε turbulent closure (two prognostic equations are solved for the turbulent kinetic energy k and its dissipation ε, respectively). Simulations are validated against wind-tunnel data using a technique based on hit-rate
calculations, and calculated statistical parameters. The results show that the horizontal velocity is very well modelled,
and despite some discrepancies, the model that fulfils the hit-rate test criteria gives useful results that are used to investigate
three-dimensional (3-D) flow structures. The 3-D analysis of the flow shows interesting patterns: the centre of the canyon
vortex is at 3/4 of the canyon height, and stronger downward than upward motions are present within the canyon. Such behaviour
is explained by the presence of a compensation flow through the side of the canyon, which enters the canyon from the upper
part and exits from the lower part. This complex 3-D structure affects the tracer dispersion, and is responsible for pollutant
transport and diffusion. 相似文献
17.
Ken-ichi Narita 《Boundary-Layer Meteorology》2007,122(2):293-320
A major problem in urban climate modelling is determining how the heat fluxes from various canyon surfaces are affected by
canyon flow. To address this problem, we developed a water evaporation method involving filter paper to study the distribution
of the convective transfer velocity in urban street canyons. In this method, filter paper is pasted onto a building model
and the evaporation rate from the paper is measured with an electric balance. The method was tested on 2D (two-dimensional)
street canyon models and 3D model arrangements. Moreover, in this technique, it is easy to restrict the flux within an arbitrary
surface in question. That is, the evaporation distribution on a surface can be studied by using several small pieces of filter
paper. In the 2D case, the wall transfer velocity was strongly dependent on the canyon aspect ratio for perpendicular wind
directions and it varied widely with height within both windward and leeward wall surfaces. For 3D cubic arrays, the relation
to canyon aspect ratio was largely different from that of the 2D canyon. And, as a case study, the variation of wind direction
was investigated for a city-like setting. The area-averaged transfer velocity was insensitive to wind direction but its local
deviation was significant. Finally, we measured the transfer velocity for a clustered block array surrounded by relatively
wide streets. The effect of spatial heterogeneity on the transfer velocity was significant. Moreover, for a fixed total building
volume, the transfer velocity was considerably larger when the building height varied than when it was uniform. Therefore,
the water evaporation method with filter paper is expected to be useful for studying the transfer velocity and ventilation
rates in urban areas with various canyon shapes. 相似文献
18.
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
19.
Numerical studies on flow fields around buildings in an Urban street canyon and cross-road 总被引:8,自引:2,他引:6
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. 相似文献
20.
Large-Eddy Simulation for the Mechanism of Pollutant Removal from a Two-Dimensional Street Canyon 总被引:3,自引:3,他引:0
Takenobu Michioka Ayumu Sato Hiroshi Takimoto Manabu Kanda 《Boundary-Layer Meteorology》2011,138(2):195-213
Large-eddy simulation (LES) is conducted to investigate the mechanism of pollutant removal from a two-dimensional street canyon
with a building-height to street-width (aspect) ratio of 1. A pollutant is released as a ground-level line source at the centre
of the canyon floor. The mean velocities, turbulent fluctuations, and mean pollutant concentration estimated by LES are in
good agreement with those obtained by wind-tunnel experiments. Pollutant removal from the canyon is mainly determined by turbulent
motions, except in the adjacent area to the windward wall. The turbulent motions are composed of small vortices and small-scale
coherent structures of low-momentum fluid generated close to the plane of the roof. Although both small vortices and small-scale
coherent structures affect pollutant removal, the pollutant is largely emitted from the canyon by ejection of low-momentum
fluid when the small-scale coherent structures appear just above the canyon where the pollutant is retained. Large-scale coherent
structures also develop above the canyon, but they do not always affect pollutant removal. 相似文献