CFD simulation of airflow over a regular array of cubes. Part II: analysis of spatial average properties     

Alberto Martilli  Jose Luis Santiago 《Boundary-Layer Meteorology》2007,122(3):635-654

In the first part of this study, results of a computational fluid dynamics simulation over an array of cubes have been validated against a set of wind-tunnel measurements. In Part II, such numerical results are used to investigate spatially-averaged properties of the flow and passive tracer dispersion that are of interest for high resolution urban mesoscale modelling (e.g. non resolved obstacle approaches). The results show that vertical profiles of mean horizontal wind are linear within the canopy and logarithmic above. The drag coefficient, derived from the numerical results using the classical formula for the drag force, is height dependent (it decreases with height). However, a modification of the formula is proposed (accounting for subgrid velocity scales) that makes the drag coefficient constant with height. Results also show that the dispersive fluxes are similar in magnitude to the turbulent fluxes, and that they play a very important role within the canopy. Vertical profiles of turbulent length scales (to be used in k–l closure schemes, where k is the turbulent kinetic energy and l a turbulent length scale) are also derived. Finally the distribution of the values around the mean over the reference volumes are analysed for wind and tracer concentrations.  相似文献   

Grounding the virtual: The material effects of electronic grocery shopping     

Andrew J. Murphy 《Geoforum》2007,38(5):941-953

This paper addresses the materiality of computer-mediated retailing. The paper uncovers the “hidden geography” of e-commerce, considering how the seemingly simple act of doorstep food delivery is explicated in urban form, and in transportation and communication infrastructures. The paper categorises electronic grocery shopping (EGS) enterprises into three types: “bricks and clicks”, “pure-play” and the “infomediary”, and examines how each type of operation has been materialised in the urban landscape, and the infrastructure upon which each draws. The paper then considers the strategies EGS operations have used to offset the “killer costs” of logistics for electronic commerce. The paper concludes by considering how electronic grocery shopping is embedded within broader trends in consumer behaviour, and how these trends are manifested in the materiality of urban life. The paper argues that online grocers are in the vanguard of ventures habituating customers to the remote ordering and delayed delivery of products to the home. Through controlling this “last mile”, online retailers hope to extend the product offer to higher-margin and more problematic products, which are in themselves unprofitable to distribute direct to the customer, but which can be profitably combined with a grocery shop.  相似文献   

城市地震灾害风险评价方法研究   总被引：1，自引：0，他引：1       下载免费PDF全文

金江军  潘懋  徐岳仁 《地震工程学报》2007,29(2):109-113

风险管理研究已成为防灾减灾工作从“被动救灾”到“主动预防”转化的热门课题。本文回顾了地震灾害风险评价研究进展,指出了现有评价方法的不足。提出了基于地震小区划的城市地震危险性评价方法、基于城市用地类型的城市地震易损性评价方法以及基于专家打分法的城市防震减灾能力评价方法。最后设计了城市地震灾害风险评价流程,并给出了城市地震灾害风险区划算法。  相似文献   

城市地震灾害风险评价方法研究     

金江军  潘懋  徐岳仁 《内陆地震》2007,21(2):135-141

从危险性评价、易损性评价以及防灾减灾能力评价3个方面阐述了城市地震灾害风险评价内容.回顾了地震灾害风险评价研究进展,指出了现有评价方法的不足.提出了基于地震小区划的城市地震危险性评价方法、基于城市用地类型的城市地震易损性评价方法以及基于专家打分法的城市防震减灾能力评价方法.最后设计了城市地震灾害风险评价流程,并给出了城市地震灾害风险区划算法.  相似文献   

城市下垫面反照率变化对北京市热岛过程的影响——个例分析   总被引：13，自引：1，他引：13  

江晓燕  张朝林高华　苗世光 《气象学报》2007,65(2):301-307

随着城市的不断发展,城区地表反照率等下垫面物理特征和属性会发生明显的变化,进而会对城市热岛等大气环境形成影响。文中使用中国气象科学研究院开发的新一代数值天气预报模式(GRAPES),针对2004年10月北京一次重空气污染事件中的典型城市热岛过程,分别设计了两种数值试验方案:(1)对照试验,使用模式缺省的城区下垫面反照率参数,取值0.18;(2)敏感性试验,参考同期中国科学院大气物理研究所铁塔280m高度下垫面反照率观测事实,将北京区域城市类型下垫面反照率减小至0.15。通过对比两种试验方案在1km水平分辨率下的24h模拟结果,研究了城市下垫面反照率变化对北京地区城市热岛过程的影响。结果表明:(1)GRAPES模式可成功模拟此次热岛过程中城区和郊区近地面温度的日变化趋势;(2)城市下垫面反照率的变化对城市热岛的发展非常重要,城市反照率下降0.03会使城市热岛强度增强0.8℃左右,结果也更接近实况。这说明随着城市发展引起的地表反照率减小有利于城市热岛强度增加;(3)通过分析地表的长波辐射发现,在城市区域较小反照率情形下,城区的长波辐射始终比郊区大,有利于热岛的形成;同时也有利于城区近地层的风场辐合增加,对此次污染过程的发展是有利的。  相似文献   

Transfer processes in a simulated urban street canyon     

E. Solazzo  R. E. Britter 《Boundary-Layer Meteorology》2007,124(1):43-60

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   

A microscale three-dimensional urban energy balance model for studying surface temperatures   总被引：1，自引：2，他引：1  

E. Scott Krayenhoff  James A. Voogt 《Boundary-Layer Meteorology》2007,123(3):433-461

A microscale three-dimensional (3-D) urban energy balance model, Temperatures of Urban Facets in 3-D (TUF-3D), is developed to predict urban surface temperatures for a variety of surface geometries and properties, weather conditions, and solar angles. The surface is composed of plane-parallel facets: roofs, walls, and streets, which are further sub-divided into identical square patches, resulting in a 3-D raster-type model geometry. The model code is structured into radiation, conduction and convection sub-models. The radiation sub-model uses the radiosity approach and accounts for multiple reflections and shading of direct solar radiation. Conduction is solved by finite differencing of the heat conduction equation, and convection is modelled by empirically relating patch heat transfer coefficients to the momentum forcing and the building morphology. The radiation and conduction sub-models are tested individually against measurements, and the complete model is tested against full-scale urban surface temperature and energy balance observations. Modelled surface temperatures perform well at both the facet-average and the sub-facet scales given the precision of the observations and the uncertainties in the model inputs. The model has several potential applications, such as the calculation of radiative loads, and the investigation of effective thermal anisotropy (when combined with a sensor-view model).  相似文献   

城市小区规划对大气环境影响的评估研究   总被引：5，自引：3，他引：5  

苗世光  王晓云  蒋维楣  奚文  季崇萍  刘凤辉  陈鲜艳 《高原气象》2007,26(1):92-97

不同城市小区规划方案对大气环境的影响不同。本文在已建城市小区尺度气象和污染扩散模式的基础上,针对城市小区规划对气象及大气环境的影响,提出了一套以人为本的客观、科学、可操作的影响评估指标及方法,并以北京市某小区两种规划方案为例,具体评估了它们对气象及大气环境的不同影响。结果表明:由于第二种规划方案在小区中部有一条较宽的东西向道路,并且道路中绿化较多,这条道路是气流东西向流动的重要通道,道路附近风速较大、气温较低、污染较轻,并且对整个小区的气象和大气环境都有改善。由此可见,基于城市小区尺度模式,对城市小区规划对大气环境的影响进行评估是必要和可行的。本文提出的人体舒适度、行人舒适度、地面污染物浓度、最高建筑高度以下污染物浓度、建筑物表面污染物浓度及扩散能力等六项评估指标,并分别给予该6种指标的权重系数为0.2,0.1,0.1,0.1,0.1及0.4,然后再加权平均的评估方法是合理可行的。  相似文献   

Spatio-temporal GIS和GPS优化集成在城市交通管制中的应用研究     

朱洪军  郑方艳 《国土资源信息化》2007,(4):33-37

本文基于城市交通日益恶化现状和3S集成的迅速发展,表述了Spatio-temporal GIS的定义、时空路径、人类活动的四种时空关系及其坐标系统模型的建立;并根据GPS空间定位、空间导航的原理和模型,将Spatio-temporal GIS和GPS的功能优化集成,阐述了其对城市交通信息的收集、分析、显示等功能;最后论述了Spatio-temporal GIS和GPS的集成对城市交通预警、报警、协调等管制作用。  相似文献   

加拿大的城市旅游开发评述   总被引：6，自引：0，他引：6  

杨新军  崔凤军  佟玉权 《世界地理研究》1998,(2)

旅游业是加拿大发展最快的产业之一,加拿大也是国际上开展城市旅游比较成熟的国家,其城市和地方政府都把旅游业看作创造就业机会和增加税收的源泉。加拿大的城市旅游开发包括文化旅游、体育旅游、会议旅游三个主要方面。文章指出加拿大城市在城市吸引旅游者方面采取和其他商业活动类似的竞争手段,如参与市场促销、创建城市形象和建设基础设施等。30多年来城市增加了博物馆、体育运动场、会议中心和大型节事活动等项目吸引旅游者。这些投资重新塑造了城市结构,城市旅游景观也已经有了雏形。本文对此作出了评述。  相似文献