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151.
Prashant Kumar Paul S. Fennell Allan N. Hayhurst Rex E. Britter 《Boundary-Layer Meteorology》2009,131(1):3-18
Dispersion of particles, as evidenced by changes in their number distributions (PNDs) and concentrations (PNCs), in urban
street canyons, is still not well understood. This study compares measurements by a fast-response particle spectrometer (DMS500)
of the PNDs and the PNCs (5–1000 nm, sampled at 1 Hz) at street and rooftop levels in a Cambridge UK street canyon, and examines
mixing, physical and chemical conversion processes, and the competing influences of traffic volume and rooftop wind speed
on the PNDs and the PNCs in various size ranges. PNCs at street level were ≈6.5 times higher than at rooftop. Street-level
PNCs followed the traffic volume and decreased with increasing wind speed, showing a larger influence of wind speed on 30–300 nm
particles than on 5–30 nm particles. Conversely, rooftop PNCs in the 5–30 nm size range increased with wind speed, whereas
those for particles between 30 and 300 nm did not vary with wind speed. 相似文献
152.
Although studies of sediment transport in steep and coarse-grained channels have been more numerous in recent years, the dynamics of sediment transport in step–pool river systems remain poorly understood. This paper investigates displacements of individual clasts through Spruce Creek (Québec, Canada), a classic step–pool channel, and the effects of the channel morphology on the path length of the clasts. Passive integrated transponder tags (PIT) were used to track the displacement of 196 individual particles over a range of discharges including the bankfull stage. Clasts were tracked after five sequences of flood events. The results showed that the distance distributions match a two-parameter Gamma model. Equal mobility transport occurs for the particle size investigated during each sequence of flood events. Mean travel distance of the clasts can be estimated from excess stream power, and the mobility of the clasts is more than an order of magnitude less than the model reported in riffle–pool channels. The dominant morphological length scale of the bed also controls the path length of the clasts. These results confirm some preliminary observations on sediment transport in step–pool channels. 相似文献
153.
154.
Particle size distribution (PSD) and carbonate content are two of the most frequently used parameters for describing lake sediments. This paper presents a method for rapidly measuring both these parameters using a laser particle-sizer, as well as an original way in which to separately interpret the PSD of the carbonate and non-carbonate fractions. Differential particle size analyses are produced by measuring the PSD of each sample twice. The first PSD is measured using the bulk sample. The second PSD is measured after the carbonate fraction has been completely dissolved by adding hydrochloric acid to the sample. As well as particle size analyses, laser particle-sizers automatically give obscuration values for the samples being analysed. By comparing the obscuration value for the bulk sample with the value for the de-carbonated fraction, the carbonate content of the sediment can be determined. In order to test the accuracy of the method, the results were compared with chemical analyses of carbonate content. This comparison showed that when applied to diatom-rich samples the method underestimates the carbonate content. This is due to the fact that the differential particle size method measures the volume concentrations of the carbonate and non-carbonate fractions, whereas chemical analysis measures the weight concentrations. As diatoms are hollow and have relatively low-densities, differential particle size analyses of diatom-rich samples overestimate the non-carbonate fraction. Despite this limitation, and bearing in mind that laser particle-sizer distributions are expressed in terms of volume percent for each size class and not weight percent, PSDs can provide useful data for interpreting sedimentary processes. One of the main advantages of this method is that it can be used to separate the PSDs of the carbonate (autochthonous) fraction and the non-carbonate (allochthonous) fraction from the PSD of the bulk sample. The value of such an approach was demonstrated by the analysis and interpretation of a 9 m-long core from Lake Bourget in France. An analysis of the bulk sample PSDs plotted as a function of depth showed the consistency of sedimentary processes over the last 7200 years. Amongst these processes, it was possible to identify two modes of clastic material deposition: river-borne fine sedimentation during major floods and gravity reworking of sandy sediment from the lakeshore. 相似文献
155.
156.
利用NASA/CERES发布的2001~2015年云参数资料,选取高层云、雨层云、层积云的云水含量和云粒子有效半径,统计分析了西南地区云参数的时空分布特征和变化趋势。结果表明:从年均空间分布来看,西南地区液水和冰水含量均东部高于西部,海拔低的地区高于海拔高的地区;高层云和雨层云液相和冰相云粒子有效半径在川西高原最大。从数值大小来看,雨层云液水和冰水含量最多,分别介于90~230 g/m2和100~300 g/m2,层积云最少,分别介于0~80 g/m2和0~60 g/m2;冰相云粒子有效半径高于液相2~6 μm。从季节分布来看,雨层云液水和冰水含量秋季和冬季偏高,夏季和春季偏少,高层云和层积云季节差异较小;液相云粒子有效半径均夏季最大。从变化趋势来看,西南地区各地液水和冰水含量均呈减少趋势,液相和冰相云粒子有效半径有呈减少或增加趋势。 相似文献
157.
Ashok K. Luhar 《Boundary-Layer Meteorology》2002,102(1):1-38
The mean concentration distributionwithin a plume released from a point source in the atmosphericboundary layer can be greatly influenced by the systematic turningof wind with height (i.e. vertical wind direction shear). Such aninfluence includes a deflection of the plume centroid, with anassociated shearing of the vertical plume cross-section, and anenhancement of dispersion, in the horizontal plane. Wind directionshear is normally not accounted for in coastal fumigation models,although dispersion observations with shear acting as acontrolling parameter are not uncommon. A three-dimensionalLagrangian stochastic model is used to investigate the influenceof uniform wind direction shear on the diffusion of a point-sourceplume within the horizontally homogeneous convective boundarylayer, with the source located at the top of the boundary layer.Parameterisations are developed for the plume deflection andenhanced dispersion due to shear within the framework of aprobability density function (PDF) approach, and compared with theLagrangian model results. These parameterisations are thenincorporated into two applied coastal fumigation models: a PDFmodel, and a commonly used model that assumes uniform andinstantaneous mixing in the vertical direction. The PDF modelrepresents the vertical mixing process more realistically. A moreefficient version of the PDF model, which assumes a well-mixedconcentration distribution in the vertical at large times, isapplied to simulate sulfur dioxide data from the Kwinana CoastalFumigation Study. A comparison between the model results and thedata show that the model performs much better when the wind-sheareffects are included. 相似文献
158.
159.
超前区域注浆是煤层底板灰岩水害防治的有效手段,而注浆工程中的劈裂注浆过程是决定注浆效果的关键环节,但由于对受注灰岩地层裂隙起裂机制认识不清,致使劈裂注浆过程中注浆压力、浆液水灰比等参数缺少有效控制,难以保证注浆效果。针对上述问题,利用颗粒元(Particle Flow Code,PFC)数值模拟软件,考虑浆液水灰比、地应力水平、弱面和裂隙的倾角和宽度等因素,开展奥陶系灰岩顶部劈裂注浆数值模拟计算。结果表明:在弱面和裂隙条件下起裂压力均随浆液水灰比(1∶1、2∶1、3∶1)的增大而减小,随最大主应力与最小主应力差值(9、12、15 MPa)的减小而增大,随弱面或裂隙宽度(3、8、15 mm)及其与最大主应力夹角(30°、60°、90°)的增大而减小;起裂裂隙沿平行于最大主应力方向延展;起裂压力值随弱面与基质强度比(0.30、0.03)的减小而减小,当弱面强度较高时,起裂压力大于裂隙条件下的起裂压力,而且沿着基质起裂;当注浆孔置于弱面两端或裂隙中间时,起裂裂隙沿弱面或裂隙的两端起裂,当注浆孔置于弱面中间位置时,起裂裂隙沿中间位置的基质起裂。研究结果有助于底板水害超前区域改造劈裂注浆的有效控制,指导注浆参数的选取,为解放深部煤炭资源提供技术支撑。
相似文献160.
In this paper an adaptive algorithm for Smoothed Particle Hydrodynamics (SPH) for the Shallow Water Equations (SWEs) is presented. The area of a particle is inversely proportional to depth giving poor resolution in small depths without particle refinement. This is a particular limitation for flooding problems of interest here. Higher resolution is created by splitting the particles, while particle coalescing (or merging) improves efficiency by reducing the number of the particles when acceptable. The new particle coalescing procedure merges two particles together if their area becomes less than a predefined threshold value. Both particle splitting and coalescing procedures conserve mass and momentum and the smoothing length of new particles is calculated by minimizing the density error of the SPH summation. The new dynamic particle refinement procedure is assessed by testing the numerical scheme against analytical, experimental and benchmark test cases. The analytical cases show that with particle splitting and coalescing typical convergence rates remain faster than linear. For the practical test case, in comparison to using particle splitting alone, the particle coalescing procedure leads to a significant reduction of computational time, by a factor of 15. This makes the computational time of the same order as mesh-based methods with the advantage of not having to specify a mesh over a flood domain of unknown extent a priori. 相似文献