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1.
Wind Conditions in Idealized Building Clusters: Macroscopic Simulations Using a Porous Turbulence Model 总被引：3，自引：0，他引：3
Simulating turbulent flows in a city of many thousands of buildings using general high-resolution microscopic simulations
requires a grid number that is beyond present computer resources. We thus regard a city as porous media and divide the whole
hybrid domain into a porous city region and a clear fluid region, which are represented by a macroscopic k–e{\varepsilon} model. Some microscopic information is neglected by the volume-averaging technique in the porous city to reduce the calculation
load. A single domain approach is used to account for the interface conditions. We investigated the turbulent airflow through
aligned cube arrays (with 7, 14 or 21 rows). The building height H, the street width W, and the building width B are the same (0.15 m), and the fraction of the volume occupied by fluid (i.e. the porosity) is 0.75; the approaching flow
is parallel to the main streets. There are both microscopic and macroscopic simulations, with microscopic simulations being
well validated by experimental data. We analysed microscopic wind conditions and the ventilation capacity in such cube arrays,
and then calculated macroscopic time-averaged properties to provide a comparison for macroscopic simulations. We found that
the macroscopic k–e{\varepsilon} turbulence model predicted the macroscopic flow reduction through porous cube clusters relatively well, but under-predicted
the macroscopic turbulent kinetic energy (TKE) near the windward edge of the porous region. For a sufficiently long porous
cube array, macroscopic flow quantities maintain constant conditions in a fully developed region. 相似文献
2.
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
3.
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
4.
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
5.
6.
Formation of horizontal convective rolls in urban areas 总被引：6，自引：0，他引：6
The formation of horizontal convective rolls (HCRs) in urban areas is investigated in this paper using observations and fine-scale numerical simulations. Cloud streets organized parallel to the mean boundary-layer wind (a manifestation of HCRs) are seen in the Fengyun-2C satellite imagery around local noon in Beijing. Observed vertical velocity and horizontal wind fields from an urban wind profiler suggest that the time scale for alternating updraft and downdraft in the boundary layer is about 30 min, and the length of the updraft/downdraft is about 9 km. Numerical simulations show that most HCRs occur in the urban areas with − zi / L < 25 (zi: the boundary-layer depth, L: the Monin–Obukhov length). Sensitivity tests reveal that HCRs are common in urban boundary layers, while rural areas are more conducive to forming cellular convection; the aspect ratio of HCRs in urban areas is smaller than the typical value over natural landscapes. 相似文献
7.
地面强风可直接造成人员伤亡和经济损失,严重影响出行安全、工农业生产等社会秩序。强风的发生与天气系统和复杂下垫面的共同作用密切相关,在城市区域尤为明显。受数值模拟技术和计算资源的限制,对实际天气条件下大范围城区的强风现象进行建筑物分辨率的大规模数值模拟研究仍是一个挑战。本研究利用中尺度气象模式嵌套流体计算动力模式的超高分辨率局地气象预报系统,对强冷空气过程造成广州市区的一次强风事件进行数值模拟,深入探讨强风的精细结构和形成机制。结果表明,伴随着强冷空气入侵,广州市区的平均风速和风场高频扰动均明显增强,且在城市冠层顶尤为明显,呈现区域不均匀的三维结构,数值模拟与地面观测相一致。较大范围的强风速和阵风主要出现在建筑物较为低矮的老城区上空,并持续影响下游河道等开阔区域。在高层建筑密集的新城区,虽然整体风速明显减弱,但能在平行风向的街道狭管和下游区域形成局地强风。特别是,超高层建筑群引起显著的垂直环流,导致强风扰动向下传播,造成最大风速达8 m s?1的地面局地强风,阵风指数接近2。上游建筑群引起的风场扰动呈现大尺度湍流结构,能沿着平均气流传播影响数公里之远的下游地区。当风场扰动经过广州塔等单体超高层建筑时,可在其两侧绕流区再次加强,形成局地强风。局地强风和阵风还出现在垂直于风向排列的沿江高层建筑群的侧边,与建筑屏风的阻挡效应和缺口溢出有关。研究结果促进认识城市强风的时空特征和物理机制,有助于提升城市气象的精细化预报水平。 相似文献
8.
The effects of a building's density on urban flows are investigated using a CFD model with the
RNG k-ε turbulence closure scheme. Twenty-seven cases with different building's density parameters
(e.g., building and street-canyon aspect ratios) are numerically simulated. As the building's density parameters
vary, different flow regimes appear. When the street canyon is relatively narrow and high, two counter-rotating
vortices in the vertical direction are generated. The wind speed along streets is mainly affected by the
building's length. However, it is very difficult to find or generalize the characteristics of the street-canyon
flows in terms of a single building's density parameter. This is because the complicated flow patterns appear
due to the variation of the vortex structure and vortex number. Volume-averaged vorticity magnitude is a very
good indicator to reflect the flow characteristics despite the strong dependency of flows on the variation of
the building's density parameters. Multi-linear regression shows that the volume-averaged vorticity magnitude
is a strong function of the building's length and the street-canyon width. The increase in the building's
length decreases the vorticity of the street-canyon flow, while, the increase in the street-canyon width
increases the vorticity. 相似文献
9.
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
10.
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
11.
In order to estimate the impacts of buildings on air pollution dispersion, numerical simulations are performed over an idealized
urban area, modelled as regular rows of large rectangular obstacles. The simulations are evaluated with the results of the
Mock Urban Setting Test (MUST), which is a near full-scale experiment conducted in Utah’s West Desert area: it consists of
releases of a neutral gas in a field of regularly spaced shipping containers. The numerical simulations are performed with
the model Mercure_Saturne, which is a three-dimensional computational fluid dynamics code adapted to atmospheric flow and dispersion simulations. It
resolves complex geometries and uses, in this study, a k–∈ closure for the turbulence model. Sensitivity studies focus on how to prescribe the inflow conditions for turbulent kinetic
energy. Furthermore, different sets of coefficients available in the literature for the k–∈ closure model are tested. Twenty MUST trials with different meteorological conditions are simulated and detailed analyses
are performed for both the dynamical variables and average concentration. Our results show overall good agreement according
to statistical comparison parameters, with a fraction of predictions for average concentration within a factor of two of observations
of 67.1%. The set of simulations offers several inflow wind directions and allows us to emphasize the impact of elongated
buildings, which create a deflection of the plume centerline relative to the upstream wind direction. 相似文献
12.
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
13.
Transport of pollution and heatout of streets into the boundary layer above is not currently understood and so fluxes cannot be quantified. Scalar concentration within the street is determined by the flux out of it and so quantifying fluxes for turbulent flow over a rough urban surface is essential. We have developed a naphthalene sublimation technique to measure transfer from a two-dimensional street canyon in a wind tunnel for the case of flow perpendicular to the street. The street was coated with naphthalene, which sublimes at room temperature, so that the vapour represented the scalar source. The transfer velocity wT relates the flux out of the canyon to the concentration within it and is shown to be linearly related to windspeed above the street. The dimensionless transfer coefficient wT/U represents the ventilation efficiency of the canyon (here, wT is a transfer velocity,U is the wind speed at the boundary-layer top). Observed values are between 1.5 and 2.7 ×10-3 and, for the case where H/W0 (ratio of buildingheight to street width), values are in the same range as estimates of transfer from a flat plate, giving confidence that the technique yields accurate values for street canyon scalar transfer. wT/U varies with aspect ratio (H/W), reaching a maximum in the wake interference regime (0.3 < H/W < 0.65). However, when upstream roughness is increased, the maximum in wT/U reduces, suggesting that street ventilation is less sensitive to H/W when the flow is in equilibrium with the urban surface. The results suggest that using naphthalene sublimation with wind-tunnel models of urban surfaces can provide a direct measure of area-averaged scalar fluxes. 相似文献
14.
A method using balloons as tracers for measuring mean wind velocity in street canyons or mountain valleys has been developed. Tests of the method with numerical experiments showed that the method reproduced an assumed wind field quite well provided that the buoyancy component of the balloon velocity was larger than the downward velocity component of the wind. Tests of the method with measurements of wind velocity in a street canyon of downtown Chicago showed that the method yielded flow patterns quite similar to photographic results of flow visualization of phisical simulations by other investigators. However, no direct measurements of wind velocity were available for quantitative comparison. 相似文献
15.
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
16.
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
17.
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
18.
Modelling Street-Scale Flow and Dispersion in Realistic Winds—Towards Coupling with Mesoscale Meteorological Models 总被引：4，自引：3，他引：1
Zheng-Tong Xie 《Boundary-Layer Meteorology》2011,141(1):53-75
Further to our previous work—simulations of flow and dispersion in an oblique wind over the DAPPLE site (Xie and Castro, Atmos
Environ 43:2174–2185, 2009)—large-eddy simulations of flows and dispersion over the same site in a wind perpendicular to Marylebone Road and the windward
surfaces of most of the buildings were performed. The DAPPLE site is located at the intersection of Marylebone Road and Gloucester
Place in central London. In order to investigate the effects of wind direction on flows and dispersion, the velocity and scalar
fields in the perpendicular wind were compared with those in the oblique wind. Furthermore, realistic wind conditions measured
on the BT Tower at 190 m above street level were processed and used to drive the numerical simulations of flows and dispersion
at the DAPPLE site. This leads to significant predictive improvements of the dispersion compared with field measurements,
which provides validation and confidence for coupling mesoscale meteorological models, e.g. the UK Met Office’s Unified Model
and the NCAR’s Weather Research & Forecasting Model, with the street-scale large-eddy simulation of urban environments. 相似文献
19.
A higher order closure model is applied to simulate the dynamics in an area with a deep valley characterized by complex terrain in the southwestern US. The simulation results show generally good agreement with measured profiles at two locations within the valley. Both the measurements and the simulations indicate that the flow dynamics in the area are highly influenced by the topography and meandering of the valley, and can be resolved only by the full three-dimensional model code. The wind veering simulated over the range of the topographic elevations is often larger than 100 deg and in some cases as large as 180 deg, as a consequence of topographic forcing. In the case of an infinitely long valley, as is assumed in two-dimensional test simulations, a strong low-level jet occurs within the valley during stable conditions. The jet is mainly a consequence of the Coriolis effect. However, the jet development is significantly reduced due to asymmetric effects of the actual topography treated in the three-dimensional simulations. Tests with the two-dimensional nonhydrostatic version of the model show significant wave responses for a stable stratified flow over the valley. The structure resembles nonlinear mesoscale lee waves, which are intrinsically nonhydrostatic. However, considering the three-dimensional nature of this valley system, a better understanding and verification of the nonhydrostatic effects requires both a three-dimensional nonhydrostatic numerical model and an observational data set which is fully representative in all three dimensions.List of symbols (unless otherwise defined in the text)
B
1
closure constant
-
f
Coriolis parameter
-
g
acceleration of gravity
-
K
M
,K
H
,K
R
turbulent exchange coefficients for momentum, heat and moisture
-
k
von Karman constant
-
L
Monin-Obukhov length
-
q
2
twice the turbulent kinetic energy
-
R
specific humidity
-
s
height of the model top
-
T
g
ground surface temperature
-
t
time
-
U, V
horizontal components of wind
-
U
g
,V
g
geostrophic wind components
-
u, w
perturbation components ofU andW wind components
-
u
*
friction velocity
-
W
vertical wind component in the terrain-following coordinates
-
x, y
horizontal coordinates
-
Z
actual height above sea level
-
z
actual height above ground
-
z
0
roughness length
-
z
g
terrain height
-
z
i
depth of the convective boundary layer
-
1
closure constant
-
coefficient of thermal expansion
-
height in the terrain-following coordinate
-
master length scale in the turbulent parameterization
-
scaled pressure (Exner function)
-
potential temperature
-
m
normalized vertical wind shear 相似文献
20.