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1.
Water tank experiments are carried out to investigate the convection flow induced by bottom heating and the effects of the ambient wind on the flow in non-symmetrical urban street canyons based on the PIV (Particle Image Visualization) technique. Fluid experiments show that with calm ambient wind,the flows in the street canyon are completely driven by thermal force, and the convection can reach the upper atmosphere of the street canyon. Horizontal and vertical motions also appear above the roofs of the buildings. These are the conditions which favor the exchange of momentum and air mass between the street canyon and its environment. More than two vortices are induced by the convection, and the complex circulation pattern will vary with time in a wider street canyon. However， in a narrow street canyon， just one vortex appears. With a light ambient wind, the bottom heating and the associated convection result in just one main vortex. As the ambient wind speed increases, the vortex becomes more organized and its center shifts closer to the leeward building. 相似文献
2.
二维街谷地面加热引起的流场特征的水槽实验研究 总被引：5，自引：0，他引：5
利用拖曳式水槽,采用激光粒子成像速度场测量系统(PIV),模拟了街谷存在地面加热时流场特征;讨论了环境风场对其的影响。我们发现在静风条件下,街谷中环流完全由热力驱动,对流活动可伸展至街谷上方;在建筑物层顶以上,也可发现水平和垂直方向的运动。这些对流活动有助于基本风场为零时,街谷内外动量和物质的交换。当街谷较宽时,对流形成的涡旋可能为两个以上,形态较为复杂并随时间变化,当街谷变窄时,涡旋蜕化成只有一个。当有弱环境风场存在时,街谷中的对流呈现为一个主涡旋,随着风速增加,涡旋形状更加规则,其中心并向下风向移动。 相似文献
3.
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
4.
Scalar Fluxes from Urban Street Canyons Part II: Model 总被引：1，自引：1，他引：0
A practical model is developed for the vertical flux of a scalar, such as heat, from an urban street canyon that accounts for variations of the flow and turbulence with canyon geometry. The model gives the magnitude and geometric dependence of the flux from each facet of the urban street canyon, and is shown to agree well with wind-tunnel measurements described in Part I. The geometric dependence of the flux from an urban street canyon is shown to be determined by two physical processes. Firstly, as the height-to-width ratio of the street canyon increases, so does the roughness length and displacement height of the surface. This increase leads to a reduction in the wind speed in the inertial sublayer above the street canyons. Since the speed of the circulations in the street are proportional to this inertial sublayer wind speed, the flux then reduces with the inertial sublayer wind speed. This process is dominant at low height-to-width ratios. Secondly, the character of the circulations within the street canyon also varies as the height-to-width ratio increases. The flow in the street is partitioned into a recirculation region and a ventilated region. When the street canyon has high height-to-width ratios the recirculation region occupies the whole street canyon and the wind speeds within the street are low. This tendency decreases the flux at high height-to-width ratios. These processes tend to reduce the flux density from the individual facets of the street canyon, when compared to the flux density from a horizontal surface of the same material. But the street canyon has an increased total surface area, which means that the total flux from the street canyon is larger than from a horizontal surface. The variations in scalar flux from an urban street canyon with geometry is over a factor of two, which means that the physical mechanisms responsible should be incorporated into energy balance models for urban areas. 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.
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
8.
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
9.
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
10.
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
11.
A two-dimensional, time-dependent flow model coupled with a radiative transfer module has been applied to examine the characteristics of nocturnal flow in a steep canyon in the Rocky Mountains in Colorado. The effect of nighttime surface cooling on drainage flow is examined and compared with observations. In a complementary study, tracer data have been analyzed to estimate the mass flux from a tributary canyon and to examine processes of transport and diffusion. Simulations indicate that the strength and structure of the drainage wind are controlled mainly by terrain features, ambient wind conditions, and effective radiative cooling rates. The transport of tracer from a lower secondary vortex to an upper primary vortex is largely controlled by diffusional processes; removal of tracer from the canyon is controlled by the primary vortex and its interaction with the ambient wind. Differences between mass fluxes from model simulations and those calculated from experiments involve uncertainties in both the structure of the model and the analysis of data. 相似文献
12.
R. M. Banta L. D. Olivier W. D. Neff D. H. Levinson D. Ruffieux 《Theoretical and Applied Climatology》1995,52(1-2):27-42
Summary The study investigates two effects that a valley or canyon opening onto a plain can have on flow and contaminant dispersion over the downwind plain. The first effect is the channeling of strong ambient flow by the canyon when the wind is nearly aligned with the canyon axis. Two cases showed that these conditions produced a region of focused flow downwind of the canyon mouth. The second effect is the formation of canyon exit jets on nights with weaker ambient flow. In two case studies under these conditions strong exit jets formed that were several hundred meters deep. The jets remained narrow and strong at least 10 km onto the plains, and in one of the cases the jet extended more than 20 km over the plains. These deep jets only lasted 2–3 h, and they had a small but significant effect on surface-released tracer transport as indicated by surface sampling. We hypothesize that the near-surface advection of tracer was accomplished by a thin katabatic layer of flow, and that an elevated release or elevated sampling would have indicated a greater effect of the exit jet on tracer transport.With 18 Figures 相似文献
13.
Dry-bulb temperature, dew-point, wind speed, and wind direction were measured in and around an isolated vegetative canopy in Davis CA from 12 to 25 October 1986. These meteorological variables were measured 1.5 m above ground along a transect of 7 weather stations set up across the canopy and the upwind/downwind open fields. These variables were averaged every 15 minutes for a period of two weeks so we could analyze their diurnal cycles as well as their spatial variability. The results indicate significant nocturnal heat islands and daytime oases within the vegetation stand, especially in clear weather. Inside the canopy within 5 m of its upwind edge, daytime temperature fell by as much as 4.5 °C, whereas the nighttime temperature rose by 1 °C. Deeper into the canopy and downwind, the daytime drop in temperature reached 6 °C, and the nighttime increase reached 2 °C. Wind speed was reduced by ~ 2 ms–1 in mild conditions and by as much as 6.7 ms–1 during cyclonic weather when open-field wind speed was in the neighborhood of 8 ms–1. Data from this project were used to construct correlations between temperature and wind speed within the canopy and their corresponding ambient, open-field values.With 10 Figures 相似文献
14.
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
15.
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
16.
应用雷诺应力湍流模型,模拟了不同高度比的城市街道峡谷的气流场。结果表明:峡谷的对称性对其内部气流场有显著影响。前高后低型峡谷下部为逆时针旋涡,上部为顺时针旋涡,峡谷越深,流场发展的越充分;峡谷内部墙面存在明显的驻点。前低后高型峡谷只存在一个大的顺时针旋涡,随着峡谷的加深,内部气流速率有减小的趋势;峡谷达到一定深度后出现驻点。对称型峡谷内部形成了顺时针旋涡,强度不大;随着峡谷的加深,内部流场转为一顺一反2个旋涡的二元结构;仅当峡谷很深时才出现明显驻点。前低后高型峡谷的气流场形式更有利于污染物的迁移、扩散,在城市规划中应尽量结合主导风向设计这类建筑布局。 相似文献
17.
18.
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
19.
Summary The development of a cyclonic vortex over a polynya is investigated with the primitive equation mesoscale model METRAS. The
impact of different atmospheric processes on vortex development is determined by calculating the terms of the vorticity tendency
equation. Sensitivity studies are performed for different large-scale situations (geostrophic winds 1 ms−1, 3 ms−1, 20 ms−1, initial ice-water temperature difference of 35 K or 17.5 K) and for different polynya sizes and shapes. In general, the
vortex develops within a few hours. It is intensified by buoyancy, mainly resulting from latent heat release. Advective and
diffusive processes hinder the vortex development. The intensification depends on the actual situation and is faster over
small polynyas and heterogeneous ice cover. These situations result in intensification periods of only 12 to 18 hours for
the vortex, but create very strong vortices. Halved horizontal temperature gradients also about halve the vortex intensity.
The lifetime and intensification of a vortex increases with the time the air mass spends over the water. Thus, weak winds
show a slower development of the vortex but the vortex intensifies for more than 24 hours. Over big polynyas several vortices
develop, a long polynya results in a longer and narrower vortex which intensifies over a longer period. 相似文献
20.
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