共查询到20条相似文献，搜索用时 31 毫秒
1.
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
2.
Walkability and livability in cities can be enhanced by creating comfortable environments in the streets. The profile of an urban street canyon has a substantial impact on outdoor thermal conditions at pedestrian level. This paper deals with the effect of asymmetrical street canyon profiles, common in the historical centre of Camagüey, Cuba, on outdoor thermal comfort. Temporal-spatial analyses are conducted using the Heliodon2 and the RayMan model, which enable the generation of accurate predictions about solar radiation and thermal conditions of urban spaces, respectively. On these models, urban settings are represented by asymmetrical street canyons with five different height-to-width ratios and four street axis orientations (N-S, NE-SW, E-W, SE-NW). Results are evaluated for daytime hours across the street canyon, by means of the physiologically equivalent temperature (PET index) which allows the evaluation of the bioclimatic conditions of outdoor environments. Our findings revealed that high profiles (façades) located on the east-facing side of N-S streets, on the southeast-facing side of NE-SW streets, on the south-facing side of E-W street, and on the southwest-facing side of SE-NW streets, are recommended to reduce the total number of hours under thermal stress. E-W street canyons are the most thermally stressed ones, with extreme PET values around 36 °C. Deviating from this orientation ameliorates the heat stress with reductions of up to 4 h in summer. For all analysed E-W orientations, only about one fifth of the street can be comfortable, especially for high aspect ratios (H/W > 3). Optimal subzones in the street are next to the north side of the E-W street, northwest side of the NE-SW street, and southwest side of the SE-NW street. Besides, when the highest profile is located on the east side of N-S streets, then the subzone next to the east-facing façade is recommendable for pedestrians. The proposed urban guidelines enable urban planners to create and renovate urban spaces which are more efficient in diminishing pedestrian thermal stress. 相似文献
3.
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
4.
Aurore Porson Ian N. Harman Sylvia I. Bohnenstengel Stephen E. Belcher 《Boundary-Layer Meteorology》2009,132(1):107-128
We investigate the question of how many facets are needed to represent the energy balance of an urban area by developing simplified
3-, 2- and 1-facet versions of a 4-facet energy balance model of two-dimensional streets and buildings. The 3-facet model
simplifies the 4-facet model by averaging over the canyon orientation, which results in similar net shortwave and longwave
balances for both wall facets, but maintains the asymmetry in the heat fluxes within the street canyon. For the 2-facet model,
on the assumption that the wall and road temperatures are equal, the road and wall facets can be combined mathematically into
a single street-canyon facet with effective values of the heat transfer coefficient, albedo, emissivity and thermodynamic
properties, without further approximation. The 1-facet model requires the additional assumption that the roof temperature
is also equal to the road and wall temperatures. Idealised simulations show that the geometry and material properties of the
walls and road lead to a large heat capacity of the combined street canyon, whereas the roof behaves like a flat surface with
low heat capacity. This means that the magnitude of the diurnal temperature variation of the street-canyon facets are broadly
similar and much smaller than the diurnal temperature variation of the roof facets. Consequently, the approximation that the
street-canyon facets have similar temperatures is sound, and the road and walls can be combined into a single facet. The roof
behaves very differently and a separate roof facet is required. Consequently, the 2-facet model performs similarly to the
4-facet model, while the 1-facet model does not. The models are compared with previously published observations collected
in Mexico City. Although the 3- and 2-facet models perform better than the 1-facet model, the present models are unable to
represent the phase of the sensible heat flux. This result is consistent with previous model comparisons, and we argue that
this feature of the data cannot be produced by a single column model. We conclude that a 2-facet model is necessary, and for
numerical weather prediction sufficient, to model an urban surface, and that this conclusion is robust and therefore applicable
to more general geometries. 相似文献
5.
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
6.
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
7.
Wind and temperature measurements from within and above a deep urban canyon (height/width = 2.1) were used to examine the
thermal structure of air within the canyon, exchange of heat with the overlying atmosphere, and the possible impacts of surface
heating on within-canyon air flow. Measurements were made over a range of seasons and primarily analysed for sunny days. This
allowed the study of temperature differences between opposing canyon walls and between wall and air of more than 15°C in summer.
The wall temperature patterns follow those of incoming solar radiation loading with a secondary daytime effect from the longwave
exchange between the walls. In winter, the canyon walls receive little direct solar radiation, and temperature differences
are largely due to anthropogenic heating of the building interiors. Cool air from aloft and heated air from canyon walls is
shown to circulate within the canyon under cross-canyon flow. Roofs and some portions of walls heat up rapidly on clear days
and have a large influence on heat fluxes and the temperature field. The magnitude and direction of the measured turbulent
heat flux also depend strongly on the direction of flow relative to surface heating. However, these spatial differences are
smoothed by the shear layer at the canyon top. Buoyancy effects from the heated walls were not seen to have as large an impact
on the measured flow field as has been shown in numerical experiments. At night canyon walls are shown to be the source of
positive sensible heat fluxes. The measurements show that materials and their location, as well as geometry, play a role in
regulating the heat exchange between the urban surface and atmosphere. 相似文献
8.
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
9.
We present a scheme for parameterising scalar transfer in the urban boundary layer, which is divided into an inertial layer and a roughness layer. The latter is further divided into a shear layer and a canyon layer. In the inertial layer, scalar transfer is determined by turbulence related to canyon macroscopic features, while in the roughness layer, it is determined by shear-generated turbulence, canyon vortex and vortex-generated turbulence. We first describe a conceptual model for the canyon flow and the aerodynamic resistance network, and then estimate the resistances from the point of view of drag partition and vortex advection. The results are compared with the measurements from wind-tunnel experiments. It is found that for small canyon aspect ratio, σc, the transfer velocity increases with σc, reaching a maximum at around σc=0.5 and then decreases with σc. We also show that the scheme is not sensitive to adjustable parameters 相似文献
10.
Summary. Building walls form an important component of the total surface area in cities, but are not generally considered in most
attempts to measure urban surface temperatures. A procedure which allows sampling of the spatial and temporal variation of
wall surface temperatures in urban areas is presented. Radiometric surface temperatures are monitored using an array of infrared
thermometers mounted on a moving vehicle. It is necessary to remove observations which view sky, or mixed sky and building
scenes. Distribution truncation and distribution modelling are investigated as methods to remove the unwanted observations.
Use of the traverse method in several urban areas reveals strong temporal variations in wall temperatures due to solar loading.
Significant spatial variations in temperatures suggest caution be used in the extrapolation of data from single canyon studies
to larger scales. Times of strong surface temperature contrast between different walls are linked to canyon and solar geometry.
In the light industrial and residential observation areas, north-south streets show mid-morning and late afternoon peaks of
approximately equal magnitude in the difference between wall surface temperatures. East-west streets show a single peak in
the hour following solar noon. The downtown study area, with streets oriented northeast-southwest and northwest-southeast,
shows a double wave pattern with one peak dominant. These patterns are of interest because they can lead to large biases in
remotely-observed surface temperature with view direction.
Received May 30, 1997 Revised July 20, 1997 相似文献
11.
12.
Valéry Masson 《Boundary-Layer Meteorology》2000,94(3):357-397
An urban surface scheme for atmospheric mesoscale models ispresented. A generalization of local canyon geometry isdefined instead of the usual bare soil formulation currently usedto represent cities in atmospheric models. This allows refinement ofthe radiative budgets as well as momentum, turbulent heat and ground fluxes.The scheme is aimed to be as general as possible, in order to representany city in the world, for any time or weather condition(heat island cooling by night, urban wake, water evaporation after rainfalland snow effects).Two main parts of the scheme are validated against published data.Firstly, it is shown that the evolution of the model-predictedfluxes during a night with calm winds is satisfactory, considering both the longwave budget and the surface temperatures. Secondly, the original shortwave scheme is tested off-line and compared to the effective albedoof a canyon scale model. These two validations show that the radiative energy input to the urban surface model is realistic.Sensitivity tests of the model are performed for one-yearsimulation periods, for both oceanic and continental climates. The scheme has the ability to retrieve, without ad hoc assumptions, the diurnal hysteresis between the turbulent heat flux and ground heat flux. It reproduces the damping of the daytime turbulent heat flux by the heat storage flux observed in city centres. The latent heat flux is negligible on average,but can be large when short time scales are considered (especially afterrainfall). It also suggests that in densely built areas, domesticheating can overwhelm the net radiation, and supply a continuous turbulentheat flux towards the atmosphere. This becomes very important inwinter for continental climates. Finally, a comparison with a vegetation scheme shows that the suburban environment can be represented with a bare soil formulation for large temporal or spatial averages (typical of globalclimatic studies), but that a surface scheme dedicated to the urban surface is necessary when smaller scales are considered: town meteorological forecasts, mesoscale or local studies. 相似文献
13.
Marcin Idczak Patrice Mestayer Jean-Michel Rosant Jean-Francois Sini Michel Violleau 《Boundary-Layer Meteorology》2007,124(1):25-41
In order to investigate the microclimatic conditions in a street canyon, a physical model was used to conduct the Joint ATREUS-PICADA
Experiment (JAPEX) in situ experimental campaign. Four lines of buildings simulated by steel containers were installed to
form three parallel street canyons at 1:5 scale, with width/height aspect ratio approximately 0.40. The reference wind and
atmospheric conditions were measured, as well as the flow velocity and direction in the street. Preliminary results concern
street canyon ventilation and thermal effects on in-canyon airflow, and show that vortical motions appear for reference wind
directions perpendicular to the street axis. The presence of adjacent rows of buildings did not appear to significantly influence
the flow character within the canyon for the case of a low aspect ratio corresponding to a skimming flow regime. The flow
structure was not significantly affected by the thermal effects although some slight interference occurred in the lower part
of the canyon. An analysis of horizontal temperature gradients indicated that a thin boundary layer develops near the heated
facade. These facts imply that the thermal effects are considerable only very close to the wall. 相似文献
14.
An urban canopy model is developed for use in mesoscale meteorological and environmental modelling. The urban geometry is
composed of simple homogeneous buildings characterized by the canyon aspect ratio (h/w) as well as the canyon vegetation characterized by the leaf aspect ratio (σ
l
) and leaf area density profile. Five energy exchanging surfaces (roof, wall, road, leaf, soil) are considered in the model,
and energy conservation relations are applied to each component. In addition, the temperature and specific humidity of canopy
air are predicted without the assumption of thermal equilibrium. For radiative transfer within the canyon, multiple reflections
for shortwave radiation and one reflection for longwave radiation are considered, while the shadowing and absorption of radiation
due to the canyon vegetation are computed by using the transmissivity and the leaf area density profile function. The model
is evaluated using field measurements in Vancouver, British Columbia and Marseille, France. Results show that the model quite
well simulates the observations of surface temperatures, canopy air temperature and specific humidity, momentum flux, net
radiation, and energy partitioning into turbulent fluxes and storage heat flux. Sensitivity tests show that the canyon vegetation
has a large influence not only on surface temperatures but also on the partitioning of sensible and latent heat fluxes. In
addition, the surface energy balance can be affected by soil moisture content and leaf area index as well as the fraction
of vegetation. These results suggest that a proper parameterization of the canyon vegetation is prerequisite for urban modelling. 相似文献
15.
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
16.
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
17.
An Urban Surface Exchange Parameterisation for Mesoscale Models 总被引：9，自引：11，他引：9
A scheme to represent the impact of urban buildings on airflow in mesoscale atmospheric models is presented. In the scheme, the buildings are not explicitly resolved, but their effects on the grid-averaged variables are parameterised. An urban quarter is characterised by a horizontal building size, a street canyon width and a building density as a function of height. The module computes the impact of the horizontal (roof and canyon floor) and vertical (walls) surfaces on the wind speed, temperature and turbulent kinetic energy. The computation of the shortwave and longwave radiation, needed to compute the temperature of the urban surfaces, takes into account the shadowing and radiation trapping effects induced by the urban canyons. The computation of the turbulent length scales in the TKE equation is also modified to take into account the presence of the buildings.The parameterisation is introduced into a mesoscale model and tested in a bidimensional case of a city over flat terrain. The new parameterisation is shown to be able to reproduce the most important features observed in urban areas better than the traditional approach which is based only on the modification of the roughness length, thereby retaining the Monin–Obukhov similarity theory. The new surface exchange parameterisation is furthermore shown to have a strong impact on the dispersion characteristics of air pollutants in urban areas. 相似文献
18.
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
19.
Radiative Exchange in an Urban Street Canyon 总被引：1，自引：4，他引：1
The influence of building geometry on the radiation terms ofthe surface energy balance is a principal reason for surfacetemperature differences between rural and urban areas.Methods exist to calculate the radiation balance in an urban area,but their validity across the range of urban geometries andmaterials has not been carefully considered.Here the exchange of diffuse radiation in an urban street canyon isinvestigated using a method incorporating all reflections of radiation.This exact solution is compared to two commonly used approximationsthat retain either no reflections, or just one reflection of radiation.The area-averaged net radiative flux density from the facets of the canyondecreases in magnitude monotonically as the canyon aspect ratio increases.The two approximate solutions possess unphysical differences from thismonotonic decrease for high canyon aspect ratios or low materialemissivities/high material albedos.The errors of the two approximate solutions are small for near blackbodymaterials and small canyon aspect ratios but can be an order ofmagnitude for intermediate material properties and deep street canyons.Urban street canyon models need to consider at least one reflectionof radiation and multiple reflections are desirable for full applicability. 相似文献
20.
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