Validation of microparameters in discrete element modeling of sea ice failure process

A GPU-based discrete element method (DEM) with bonded particles is investigated to simulate the mechanical properties of sea ice in uniaxial compressive and three-point bending tests. Both the uniaxial compressive strength and flexural strength of sea ice are related to the microparameters in DEM simulation including particle size, sample size, bonding strength, and interparticle friction coefficient. These parameters are analyzed to build the relationship between the material macrostrengths of sea ice and the microparameters of the numerical model in DEM simulations. Based on this relationship, the reasonable microparameters can be calculated by given macrostrengths in the applications of simulating the failure processes of sea ice. In this simulation, both uniaxial compressive strength and flexural strength of ice increase with the increasing ratio of sample size and particle size. The interparticle friction coefficient is directly related to the compressive strength but has little effect on the flexural strength. In addition, numerical simulations are compared with experimental data to show the performance of the proposed model, and a satisfactory agreement is achieved. Therefore, this microparameter validation approach based on macrostrengths can be applied to simulate the complicated failure process of sea ice interacting with offshore platform structures.     

The topographic normalization of hyperspectral data: implications for the selection of spectral end members and lithologic mapping

Compact Airborne Spectrographic Imager (CASI) hyperspectral data is used to investigate the effects of topography on the selection of spectral end members, and to assess whether the topographic correction improves the discrimination of rock units for lithologic mapping. A publicly available Digital Elevation Model (DEM), at a scale of 1:50,000, is used to model the radiance variation of the scene as a function of topography, assuming a Lambertian surface. Skylight is estimated and removed from the airborne data using a dark object correction. The CASI data is corrected on a pixel-by-pixel basis to normalize the scene to a uniform solar illumination and viewing geometry. The results show that topography has the effect of expanding end member clusters at times resulting in the overlap of clusters and that the correction process can effectively reduce the variation in detected radiance due to changes in local illumination. When topographic effects are embedded in the hyperspectral data, methods typically used for the selection of end members, such as the convex hull method, can miss end members or result in the selection of nonrepresentative pixels as end members. Thus, end members selected by some conventional methods are very likely “incomplete” or “nonrepresentative” if the topographic effect is embedded in the data. As shown in this study, the topographic correction can reveal hidden end members and achieve a better representation of end members via the statistical center of isolated clusters.     

先进星载热发射和反射辐射仪(ASTER)的特点及应用

简要介绍了ASTER的性能及技术参数,通过与其它传感器(如ETM+)进行比较分析,得出ASTER存在的几大优势,归纳出其目前的应用和研究领域,包括冰川学、火山学、地质学、水文学、农业、城镇变化、自然灾害、气候变化和DEM的生成与应用等,并讨论了其尚需改进的地方。     

大区域虚拟战场环境研究中的数据源模拟

大区域虚拟战场环境研究需用大规模的数据源作为实验数据，其中数字高程模型数据和相应的彩色影像数据是2种重要的数据源，但这些数据源售价较高，远非一般研究单位所能承受；提出了一种由小范围数据源模拟大范围数据源的无缝翻转法和一种用灰度影像数据模拟彩色影像数据的方法。前者解决了当前放大方法不能实现无缝拼接的问题，后者实现了将灰度图转换成彩色图。这2种方法可为大区域虚拟战场环境研究提供满足实验要求的大规模数字高程模型和彩色影像数据。     

模型维数对岩土离散元试验影响的宏-微观研究

对实验室常规三轴试验分别进行了二维和三维的离散元数值试验,从试样宏观力学响应和剪切带演化的角度比较模拟结果,分析模型维数对岩土力学性质模拟结果的影响。数值试验结果表明,最密实状态的、具有相同参数的二维和三维试样,在剪切过程中均会表现出软化特性,试样峰值应力均随围压的提高而增大,其峰值应力的数值相近,但三维数值试样达到峰值应力所对应的应变约为二维试样的1/4。随着围压的增大,二维和和三维离散元数值试样剪切带均趋于集中;但在相同围压作用下,不同维数数值试样宏观剪切带的形状和厚度均不同,二维试样剪切带的范围和厚度均较大。     

复杂边坡三维地质可视化和数值模型构建

复杂边坡三维地质可视化和数值模型的构建是岩土工程和水利水电工程中常常遇到的关键问题。三维地质可视化可以展示边坡结构面的空间组合,揭示边坡的宏观破坏模式;可视化模型与数值模型的结合和转换可以反映地质信息的仿真性和动态可修改性。基于AutoCAD平台,借助AutoLisp语言,采用滑动最小二乘法插值拟合,构建三维可视化模型,同时讨论可视化模型与数值模型的转入。优点在于结合了AutoCAD优越的图形编辑功能和ITASCA公司数值软件(FLAC3D和3DEC)的计算功能,并可根据地质信息的变化更新可视化模型和数值模型。该技术与三维可变形离散元法相结合,在白鹤滩水电站进水口高边坡三维可视化和应力变形分析中进行了应用。同时针对3DEC软件前后处理的不足,结合3DEC内嵌的FISH语言和AutoCAD的AutoLISP语言进行编程,探讨3DEC加锚方法、三维合位移等值线云图和塑性区可视化实现。     

Inter-tablet coating variability: Tablet residence time variability

This paper investigates inter-tablet coating variability, specifically, the variability of tablet residence times within the spray zone of a horizontal coating pan. Results from experiments, discrete element method (DEM) computer simulations, and an analytical model developed to describe the coating mass distribution are presented.The simulations indicate that the coefficient of variation of tablet residence times, and subsequently, of coating mass, decreases with time following a power law relation. The theoretical model demonstrates that the coefficient of variation of residence time for a randomly mixed tablet bed is inversely proportional to the square root of the number of coating “trials”. DEM simulations show that during each pan revolution, tablets in the spray zone remain in a quasi-segregated state from tablets located outside the spray zone for some time period termed Δt_seg. Increasing the pan's Froude number, the spray zone aspect ratio, and the tablet-tablet and tablet-pan friction coefficient all act to decrease Δt_seg, leading to more uniform residence times and less inter-tablet coating variability for a given operating time. The relationship between Δt_seg and tablet load is more complex due changes in bed dynamics. In addition to the variability studies, a model is developed that relates coating fraction, effective mass flow rate, Δt_seg, and the desired coating mass to the allowable fraction of tablets with a coating mass lying outside of a specified range of coating masses.     

Comparison research on the neighbor list algorithms: Verlet table and linked-cell

A Discrete Element Method (DEM) simulation program of gravity driven granular collapse is developed to compare the performance of Verlet table and linked-cell algorithms. Focusing on the simulation efficiency, the two neighbor list algorithms' parameters are discussed such as search radius Rs, cell edge Lc, updating interval time step (UPTime) and the number of particles in simulation system N. The results show that appropriate parameters will improve the simulation efficiency, while the inappropriate parameters will debase the simulation efficiency, even leading to the simulation system crash. The results and discussions presented in this paper will be useful not only for parameters adjustment of DEM but also for Molecular Dynamics (MD) simulations. It will be beneficial to improving the simulation efficiency, especially for the researchers who intend to carry out DEM or MD simulation.     

SPOT5-HRS digital elevation models and the monitoring of glacier elevation changes in North-West Canada and South-East Alaska

Monitoring the response of land ice to climate change requires accurate and repeatable topographic surveys. The SPOT5-HRS (High Resolution Stereoscopic) instrument covers up to 120 km by 600 km in a single pass and has the potential to accurately map the poorly known topography of most glaciers and ice caps. The acquisition of a large HRS archive over ice-covered regions is planned by the French Space Agency (CNES) and Spotimage, France during the 2007–2008 International Polar Year (IPY). Here, we report on the accuracy and value of HRS digital elevation model (DEM) over ice and snow surfaces.

A DEM is generated by combining tools available from CNES with the PCI Orthoengine^SE software, using HRS images acquired in May 2004 over South-East Alaska (USA) and northern British Columbia (Canada). The DEM is evaluated through comparison with shuttle radar topographic mission (SRTM) DEM and ICESAT data, on and around the glaciers. A horizontal shift of 50 m is found between the HRS and SRTM DEMs and is attributed to errors in the SRTM DEM. Over ice-free areas, HRS elevations are 7 m higher than those of SRTM, with a standard deviation of ± 25 m for the difference between the two DEMs. The 7-m difference is partly attributed to the differential penetration of the electromagnetic waves (visible for HRS; microwave for SRTM) in snow and vegetation.

We also report on the application of sequential DEMs (SRTM DEM in February 2000 and HRS DEM in May 2004) for the monitoring of glacier elevation changes. We map the topographic changes induced by a surge of one tributary of Ferris Glacier. Maximum surface lowering of 42 (± 10) m and rising of 77 (± 10) m are observed in the 4 years time interval. Thinning rates up to 10 (± 2.5) m/yr are observed at low altitudes and confirm the ongoing wastage of glaciers in South-East Alaska.     

Characterization and Evaluation of Elevation Data Uncertainty in Water Resources Modeling with GIS

Grid based digital elevation models (DEM) are commonly used in water resources modeling. The quality of readily available DEM, however, varies from source to source in terms of horizontal resolution and vertical accuracy which are the two important aspects of elevation uncertainty in the modeling with raster GIS. This paper addresses the issue of elevation data uncertainty in GIS supported hydrologic simulations. The essential role of elevation data in the modeling is revealed by presenting DEM processing processes in distributed and semi-distributed hydrologic analyses. It is very difficult to examine the elevation uncertainties analytically due to complexities of the hydrologic models. An ideal approach is to assess the effect of the DEM uncertainty by applying varying resolutions or accuracies of elevation data in the modeling. Different grid sizes of DEM are used in observing DEM resolution dependence and resulting model outputs are compared to obtain a profile of its effect. Impact of DEM vertical accuracy is explored by Monte Carlo simulation with a large number of DEM realizations generated based on different levels of specified error. The approach is implemented in a case study with a topography based hydrologic model on an experimental watershed to analyze both aspects of the uncertainty. The results show that both DEM grid size and vertical accuracy could have profound effect on hydrologic modeling performance. The impact can be compensated by model calibrations due to interactions between model parameters and spatial factors. The study indicates that the DEM uncertainty can be effectively evaluated using the applied method. The work is to provide some insight into the characterization of elevation data quality and the association between topography and water resources models.