Topology management techniques for tolerating node failures in wireless sensor networks: A survey

In wireless sensor networks (WSNs) nodes often operate unattended in a collaborative manner to perform some tasks. In many applications, the network is deployed in harsh environments such as battlefield where the nodes are susceptible to damage. In addition, nodes may fail due to energy depletion and breakdown in the onboard electronics. The failure of nodes may leave some areas uncovered and degrade the fidelity of the collected data. However, the most serious consequence is when the network gets partitioned into disjoint segments. Losing network connectivity has a very negative effect on the applications since it prevents data exchange and hinders coordination among some nodes. Therefore, restoring the overall network connectivity is very crucial. Given the resource-constrained setup, the recovery should impose the least overhead and performance impact. This paper focuses on network topology management techniques for tolerating/handling node failures in WSNs. Two broad categories based on reactive and proactive methods have been identified for classifying the existing techniques. Considering these categories, a thorough analysis and comparison of all the recent works have been provided. Finally, the paper is concluded by outlining open issues that warrant additional research.     

Reconstructing the microstructure of polyimide–silicalite mixed‐matrix membranes and their particle connectivity using FIB‐SEM tomography

Properties of a composite material made of a continuous matrix and particles often depend on microscopic details, such as contacts between particles. Focusing on processing raw focused‐ion beam scanning electron microscope (FIB‐SEM) tomography data, we reconstructed three mixed‐matrix membrane samples made of 6FDA‐ODA polyimide and silicalite‐1 particles. In the first step of image processing, backscattered electron (BSE) and secondary electron (SE) signals were mixed in a ratio that was expected to obtain a segmented 3D image with a realistic volume fraction of silicalite‐1. Second, after spatial alignment of the stacked FIB‐SEM data, the 3D image was smoothed using adaptive median and anisotropic nonlinear diffusion filters. Third, the image was segmented using the power watershed method coupled with a seeding algorithm based on geodesic reconstruction from the markers. If the resulting volume fraction did not match the target value quantified by chemical analysis of the sample, the BSE and SE signals were mixed in another ratio and the procedure was repeated until the target volume fraction was achieved. Otherwise, the segmented 3D image (replica) was accepted and its microstructure was thoroughly characterized with special attention paid to connectivity of the silicalite phase. In terms of the phase connectivity, Monte Carlo simulations based on the pure‐phase permeability values enabled us to calculate the effective permeability tensor, the main diagonal elements of which were compared with the experimental permeability. In line with the hypothesis proposed in our recent paper (?apek, P. et al. (2014) Comput. Mater. Sci. 89 , 142–156), the results confirmed that the existence of particle clusters was a key microstructural feature determining effective permeability.     

采用多点地质统计法重构页岩的数字岩心

由于实验设备获得的页岩岩样大多是微米或毫米尺度,如果要获得更大尺度和范围内的孔隙结构,必须通过对其特征的三维重构才能实现。为此,提出了基于纳米精度的体数据与多点地质统计方法的页岩三维数字岩心重构方法。以页岩纳米CT数据为训练图像,使用数据模板提取其中的结构特征,通过多点地质统计方法将结构特征重构到数字岩心中;形成了以变差函数曲线、孔隙直径分布和孔隙连通性来判别重构岩心与真实页岩结构相似度的判别方法 ;基于格子玻尔兹曼方法验证了相似度评价方法。结果表明:(1)变差函数可以检验重构岩心与真实岩心的空间相似性,孔隙直径分布和孔隙连通性可以检验流动特征的相似性;(2)当变差函数曲线、孔隙直径分布和孔隙连通性相似时,重构岩心与真实岩心的渗透率相近,表明所提出的重构方法与结构相似度评价方法是有效的;(3)由于强非均质性,不同位置的重构页岩岩心渗透率差异可超过100倍。该研究成果对构建三维大尺寸强非均质性页岩数字岩心、认识页岩孔隙结构、分析流动规律具有指导作用。     

渭北油田长3、长6、长7储层孔隙结构特征

储层孔隙结构是影响储层的渗流能力和油气产能的主要因素.本文通过结合应用铸体薄片图像分析资料,铸体薄片照片,毛管压力曲线等资料对渭北油田长3、长6、长7储层孔隙结构特征进行研究,明确了储层微观结构特征,有利于进一步开展储层评价,对油田后期挖潜有重要的指导作用。     

Graph Theory and Open-space Network Design

A decision support system designed to aid the planning and creation of green-space or open-space networks is described. The system relies on data describing current land cover and existing conservation lands, and may be extended to include any number of other ancillary data sources. The system incorporates algorithms developed in the fields of graph theory and operations research but also describes two new algorithms. The system demonstrates the application of existing graph theory algorithms in reserve design problems by illustrating the similarity and differences between classic graph theory problems and those faced by planning officials. Notions of connectivity and parsimony important to researchers in both fields are explicitly addressed.     

Physics inspired algorithms for (co)homology computations of three-dimensional combinatorial manifolds with boundary

The issue of computing (co)homology generators of a cell complex is gaining a pivotal role in various branches of science. While this issue may be rigorously solved in polynomial time, it is still overly demanding for large scale problems. Drawing inspiration from low-frequency electrodynamics, this paper presents a physics inspired algorithm for first cohomology group computations on three-dimensional complexes. The algorithm is general and exhibits orders of magnitude speed up with respect to competing ones, allowing to handle problems not addressable before. In particular, when generators are employed in the physical modeling of magneto-quasistatic problems, this algorithm solves one of the most long-lasting problems in low-frequency computational electromagnetics. In this case, the effectiveness of the algorithm and its ease of implementation may be even improved by introducing the novel concept of lazy cohomology generators.     

Isolated nonnegative solutions of infinite-dimensional algebraic Riccati equations

For an infinite-dimensional continuous (or discrete)-time linear system, based on the study of the representation of nonnegative solutions of the algebraic Riccati equation (ARE), we get some sufficient and necessary conditions for a nonnegative solution of (ARE) to be isolated in the set of all nonnegative solution of (ARE) with respect to the norm topology, the strong operator topology and weak operator topology, respectively.     

The super connectivity of exchanged hypercubes

The exchanged hypercube EH(s,t) where s?1 and t?1 are two positive integers, proposed by Loh et al. [The exchanged hypercube, IEEE Transactions on Parallel and Distributed Systems 16 (9) (2005) 866-874], is obtained by systematically removing links from a binary hypercube Qn. This paper determines that the super connectivity and the super edge-connectivity of EH(s,t) are 2s where s?t. That is, for s?t, at least 2s vertices (resp. 2s edges) of EH(s,t) are removed to get a disconnected graph that contains no isolated vertex.