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51.
A method for collecting suspended sediment samples has been developed that pumps a discharge-weighted volume of water from fixed depths at four to 40 locations across a river and separates the suspended sediment in the sample using a continuous-flow centrifuge. The efficacy of the method is evaluated by comparing the particle size distributions of sediment collected by the discharge-weighted pumping method with the particle size distributions of sediment collected by depth integration and separated by gravitational settling. The pumping method was found to undersample the suspended sand sized particles (> 63 μm) but to collect a representative sample of the suspended silt and clay sized particles (< 63 μm). The centrifuge separated the silt and clay sized particles (< 63 μm) into three fractions. Based on the average results of processing 17 samples from the Mississippi River and several of its large tributaries in 1990, about 10% of the silt and clay sized material was trapped in a centrifuge bowl-bottom sealing unit containing the nozzle and consisted of mostly medium and coarse silt from 16 to 63 μm. About 74% was retained on a Teflon liner in the centrifuge bowl and consisted of sizes from 0–1 to 63 μm. About 9% was discharged from the centrifuge in the effluent and was finer than 0–1 μm. About 7% was lost during the processes of removing the wet sediment fractions from the centrifuge, drying and weighing. The success of the discharge-weighted pumping method depends on how homogeneously the silt and clay sized particles (< 63 μm) are distributed in the vertical direction in the river. The degree of homogeneity depends on the composition and degree of aggregation of the suspended sediment particles. 相似文献
52.
This study uses a numerical model to investigate the groundwater flow and salt transport mechanisms below islands in the Okavango Delta. Continuous evapotranspiration on the islands results in accumulation of solutes and the formation of a saline boundary layer, which may eventually become unstable. A novel Lagrangian method is employed in this study and compared to other numerical methods. The numerical results support the geophysical observations of density fingering on Thata Island. However, the process is slow and it takes some hundreds of years until density fingering is triggered. The results are sensitive to changes of the hydraulic gradient and the evapotranspiration rate. Small changes may lead to different plume developments. Results further demonstrate that density effects may be entirely overridden by lateral flow on islands embedded in a sufficiently high regional hydraulic gradient. 相似文献
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运用Ansys有限元技术,分析了在竖向荷载作用下大直径空心桩桩顶沉降、桩端沉降与桩身压缩三者的关系,桩长L对桩身轴力的影响,桩长L、桩径D、桩体弹性模量E对桩P-s曲线的影响,桩长L、桩径D对桩P-Se(桩端沉降与桩顶沉降的比值)曲线的影响,具有工程上的实际意义。 相似文献
57.
Investigation of a breakage probability model published by Vogel and Peukert [Vogel, L. and Peukert, W., 2004. Determination of material properties relevant to grinding by practicable labscale milling tests. Int. J. Miner. Process., 74S, 329–338.] has led to a modification of their model to describe the degree of impact breakage, t10. The modified model takes a form similar to the JKMRC prior art breakage model, but with particle size and breakage properties incorporated explicitly in the model. 相似文献
58.
以位于三峡库区的龙门河森林自然保护区为研究区,综合利用线性光谱混合模型和几何光学模型,基于高光谱遥感数据提取森林结构参数是本文研究的重点。在研究区地面调查数据的基础上,通过高光谱数据和混合光谱分解法,获得反演几何光学模型所需的四分量参数,根据背景光照分量与森林植被冠层各参数间的关系,反演得到森林冠层郁闭度及平均冠幅的定量分布图,并利用37个野外实测样本进行结果验证。 相似文献
59.
Particle image velocimetry (PIV) data obtained in a wind-tunnel model of a canopy boundary layer is used to examine the characteristics
of mean flow and turbulence. The vector spacing varies between 1.7 and 2.5 times the Kolmogorov scales. Conditional sampling
based on quadrants, i.e. based on the signs of velocity fluctuations, reveals fundamental differences in flow structure, especially
between sweep and ejection events, which dominate the flow. During sweeps, the downward flow generates a narrow, highly turbulent,
shear layer containing multiple small-scale vortices just below canopy height. During ejections, the upward flow expands this
shear layer and the associated small-scale flow structures to a broad region located above the canopy. Consequently, during
sweeps the turbulent kinetic energy (TKE), Reynolds stresses, as well as production and dissipation rates, have distinct narrow
peaks just below canopy height, whereas during ejections these variables have broad maxima well above the canopy. Three methods
to estimate the dissipation rate are compared, including spectral fits, measured subgrid-scale (SGS) energy fluxes at different
scales, and direct measurements of slightly underresolved instantaneous velocity gradients. The SGS energy flux is 40–60%
of the gradient-based (direct) estimates for filter sizes inside the inertial range, while decreasing with scale, as expected,
within the dissipation range. The spectral fits are within 5–30% of the direct estimates. The spectral fits exceed the direct
estimates near canopy height, but are lower well above and below canopy height. The dissipation rate below canopy height increases
with velocity magnitude, i.e. it has the highest values during sweep and quadrant 1 events, and is significantly lower during
ejection and quadrant 3 events. Well above the canopy, ejections are the most dissipative. Turbulent transport during sweep
events acts as a source below the narrow shear layer within the canopy and as a sink above it. Transport during ejection events
is a source only well above the canopy. The residual term in the TKE transport equation, representing mostly the effect of
pressure–velocity correlations, is substantial only within the canopy, and is dominated by sweeps. 相似文献
60.
K. Bernlöhr W. Hofmann G. Leffers V. Matheis M. Panter R. Zink 《Astroparticle Physics》1998,8(4):253-264
Data taken with ten Cosmic Ray Tracking (CRT) detectors and the HEGRA air-shower array on La Palma, Canary Islands, have been analysed to investigate changes of the cosmic
ay mass composition at the ‘knee’ of the cosmic-ray flux spectrum near 1015 eV energy. The analysis is based on the angular distributions of particles in air showers. HEGRA data provided the shower size, direction, and core position and CRT data the particle track information. It is shown that the angular distribution of muons in air showers is sensitive to the composition over a wide range of shower sizes and, thus, primary cosmic-ray energies with little systematic uncertainties. Results can be easily expressed in terms of ln A of primary cosmic rays. In the lower part of the energy range covered, we have considerable overlap with direct composition measurements by the JACEE collaboration and find compatible results in the observed rise of ln A. Above about 1015 eV energy we find no or at most a slow further rise of ln A. Simple cosmic-ray composition models are presented which are fully consistent with our results as well as the JACEE flux and composition measurements and the flux measurements of the Tibet ASγ collaboration. Minimal three-parameter composition models defined by the same power-law slope of all elements below the knee and a common change in slope at a fixed rigidity are inconsistent with these data. 相似文献