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191.
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
192.
A microscale three-dimensional urban energy balance model for studying surface temperatures 总被引:1,自引:2,他引:1
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
193.
陆地上空标量辐射对地表反射率和大气气溶胶散射都具有很强的敏感性,而偏振反射只对大气气溶胶敏感,对地表不敏感。根据这个原理并结合POLDER(POLarization and Directionality of Earth Reflectance)资料的特点,作者提出综合利用标量辐射和偏振反射信息来实现陆地上空大气气溶胶和地表反照率的同时反演。首先,利用多角度偏振辐射观测提取大气气溶胶光学参数,再利用标量辐射测量对偏振反演结果作进一步筛选和订正,同时获得地表反射率。数值模拟试验结果证明,仅利用偏振信息只能获取大气气溶胶信息,而且其结果误差较大,特别是对于散射作用较强的短波长通道如670 nm误差更大,但经过标量辐射订正后的结果得到明显改善,气溶胶光学厚度和地表反射率与真实值之间相关系数都达到0.99以上。为提高查找表的计算效率,提出并建立了反演方案所需要的半参数化数值表,利用内插方法寻求气溶胶光学厚度和地表反射率的数值解的反演方法。 相似文献
194.
无线GIS空间数据动态副本管理研究 总被引:2,自引:0,他引:2
针对GIS海量空间数据更新要求,研究了无线网络环境下空间数据传输和存储的副本动态复制与管理技术。通过分析空间数据移动复制服务策略,设计了基于无线网络的动态副本管理系统(DRMS)来实现空间数据的移动动态复制,并研究副本管理模型和副本更新算法,保证了无线网络传输的稳定性。 相似文献
195.
196.
We present the first diffraction-limited K-band image of the Red Rectangle with 76 mas resolution, an H-band image with 75 mas resolution, and an RG 715 filter image ( 800 nm wavelength) with 78 mas resolution (corresponding to 25 AU for a distance of 330 pc). The H and K images were reconstructed from 6 m telescope speckle data and the RG 715 image from 2.2 m telescope data using the speckle masking bispectrum method. At all wavelengths the images show a compact, highly symmetric bipolar nebula, suggesting a toroidal density distribution of the circumstellar material. No direct light from the central binary can be seen as it is obscured by a dust disk or circumbinary torus. Our first high-resolution H−K color image of the nebula shows a broad red plateau of H−K≈ 2m in the bright inner regions.The optical and near-infrared images and the available photometric continuum observations in a wide range of ultraviolet to centimeter wavelengths enabled us to model the Red Rectangle in detail using a two-dimensional radiative transfer code. Our model matches both the high-resolution images and the spectral energy distribution of this object very well, making the following picture much more certain. The central close binary system with a total luminosity of 3000 L is embedded in a very dense, compact circumbinary torus which has an average number density nH ≈5×1012 cm−3, an outer radius of the dense inner region of R≈30 AU (91 mas), and a ρ∝r−2 density distribution. The full opening angle of the bipolar outflow cavities in our model is 70°. By comparing the observed and theoretical images, we derived an inclination angle of the torus to the line of sight of 7°±1°.The radiative transfer calculations show that the dust properties in the Red Rectangle are spatially inhomogeneous. The modeling confirms that the idea of large grains in the long-lived disk around the Red Rectangle (Jura et al., 1997 [ApJ, 474, 741]) is quantitatively consistent with the observations. In our models, unusually large, approximately millimeter-sized grains dominate the emission of the compact, massive torus. Models with smaller average grain sizes can possibly be found in future studies, for instance, if it turns out that the radio spectrum is not mainly caused by continuum dust emission. Therefore, the large grains suggested by our models require further confirmation by both new observations and radiative transfer calculations. Assuming a dust-to-gas ratio ρd/ρg of 0.005, the dense torus mass is 0.25 M. The model gives a lower limit of 0.0018 M, for the mass of the large particles, which produce a gray extinction of A≈ 28m, towards the center. A much smaller mass of submicron-sized dust grains is presumably located in the polar outflow cavities, their conical surface layers, and in the outer low-density parts of the torus (where ρ∝r−4, in the region of 30 AUr 2000 AU corresponding to 0.′′09–6′′). 相似文献
197.
阿拉斯加巴罗AB—67钻孔岩芯的沉积环境分析 总被引:3,自引:0,他引:3
巴罗AB-67钻孔岩芯的48个样品被进行了25个项目的粒度分析和44个项目的地球化学分析。本文对这些数据进行了Q型因子分析,并得到了4个主因子。研究表明,这4个因子分别代表了沉积环境的四项指标,它们分别为沉积速率,降水,海平面和温度。从岩芯顶部5个样品与巴罗气象台观测记录研究了古温度和古降水的转换函数,从^210Pb测年和几个环境事件确定了沉积速率的转换函数。 相似文献
198.
André F. Lotter H. John B. Birks Wolfgang Hofmann Aldo Marchetto 《Journal of Paleolimnology》1998,19(4):443-463
Surface sediments from 68 small lakes in the Alps and 9 well-dated sediment core samples that cover a gradient of total phosphorus (TP) concentrations of 6 to 520 g TP l-1 were studied for diatom, chrysophyte cyst, cladocera, and chironomid assemblages. Inference models for mean circulation log10 TP were developed for diatoms, chironomids, and benthic cladocera using weighted-averaging partial least squares. After screening for outliers, the final transfer functions have coefficients of determination (r2, as assessed by cross-validation, of 0.79 (diatoms), 0.68 (chironomids), and 0.49 (benthic cladocera). Planktonic cladocera and chrysophytes show very weak relationships to TP and no TP inference models were developed for these biota. Diatoms showed the best relationship with TP, whereas the other biota all have large secondary gradients, suggesting that variables other than TP have a strong influence on their composition and abundance. Comparison with other diatom – TP inference models shows that our model has high predictive power and a low root mean squared error of prediction, as assessed by cross-validation. 相似文献
199.
The equations for a compositional model for simulation of a two-phase, three-component system with inter-phase mass transfer are developed. Emphasis is placed on development of inter-phase mass transfer equations for incorporation of rate-limited inter-phase mass transfer. Due to the nature of the three-component systems considered, a single-film model may be inadequate and a two-film model must be utilized. A two-film model accounts for the simultaneous transfer of components in both directions across phase interfaces. The effect of interaction between components on diffusion is considered using a general form of Fick's Law. A Hand Plot representation of ternary phase behavior is chosen since it allows for straightforward calculation of miscibility of bulk phases under conditions of local non-equilibrium. The developed set of equations form the basis for a numerical model to simulate the enhanced removal of non-aqueous phase liquids (NAPLs) from porous media using single-component alcohol floods. 相似文献
200.
近年来的一些研究表明,由于地城前孕震区地下介质的电磁结构的变化,将导致地磁短周期转换函数发生异常变化。本文通过对1991年唐山5.2、5.6级地城前后静海台地磁短周期转换函数参数的计算及其结果的分析和研究,提取出了可靠的地震短临前兆信息,确立了将其应用于唐山地区5级以上的震预报的短临异常特征及其指标。 相似文献