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Routing in sensor networks is a challenging issue due to inherent constraints such as power, memory, and CPU processing capabilities. In this thesis, we assume an All to All communication mode in an N × N grid sensor network. We explore routing algorithms which load balance the network without compromising the shortest paths constrain. We analyzed the Servetto method and studied two routing strategies, namely Horizontal-Vertical routing and Zigzag routing. The problem is divided into two scenarios, one being the static case (without failed nodes), and the other being the dynamic case (with failed nodes). In static network case, we derived mathematical formulae representing the maximum and minimum loads on a sensor grid, when specific routing strategies are employed. We show improvement in performance in load balancing of the grid by using Horizontal-Vertical method instead of the existing Servetto method. In the dynamic network scenario, we compare the performance of routing strategies with respect to probability of failure of nodes in the grid network. We derived the formulae for the success-ratio, in specific strategies, when nodes fail with a probability of p in a predefined source-destination pair communication. We show that the Servetto method does not perform well in both scenarios. In addition, Hybrid strategy proposed does not perform well compared to the studied strategies. We support the derived formulae and the performance of the routing strategies with extensive simulations.

The use of full wave electromagnetic modeling and simulation tools allows for accurate performance predictions of unique RF structures that exhibit multi-dimensional scales. Full wave simulation tools need to cover the broad range of frequency including RF and terahertz bands that is focused as RF technology is developed. In this dissertation, three structures with multi-dimensional scales and different operating frequency ranges are modeled and simulated. The first structure involves nanostructured solar cells. The silicon solar cell design is interesting research to cover terahertz frequency range in terms of the economic and environmental aspects. Two unique solar cell surfaces, nanowire and branched nanowire are modeled and simulated. The surface of nanowire is modeled with two full wave simulators and the results are well-matched to the reference results. This dissertation compares and contrasts the simulators and their suitability for extensive simulation studies. Nanostructured Si cells have large and small dimensional scales and the material characteristics of Si change rapidly over the solar spectrum. The second structure is a reconfigurable four element antenna array antenna operating at 60 GHz for wireless communications between computing cores in high performance computing systems. The array is reconfigurable, provides improved transmission gain between cores, and can be used to create a more failure resilient computing system. The on-chip antenna array involves modeling the design of a specially designed ground plane that acts as an artificial magnetic conductor. The work involves modeling antennas in a complex computing environment. The third structure is a unique collar integrated zig-zag antenna that operates at 154.5 MHz for use as a ground link in a GPS based location system for wildlife tracking. In this problem, an intricate antenna is modeled in the proximity of an animal. Besides placing a low frequency antenna in a constricted area (the collar), the antenna performance near the large animal body must also be considered. Each of these applications requires special modeling details to take into account the various dimensional scales of the structures and interaction with complex media. An analysis of the challenges and limits of each specific problem will be presented.

La croissance de l'Internet en tant que plateforme d'approvisionnement à grande échelled'approvisionnement de contenus multimédia a été une grande success story du 21e siécle.Toutefois, les applications multimédia, avec les charactéristiques spécifiques de leur trafic ainsique les les exigences des nouveaux services, posent un défi intéressant en termes de découverte,de mobilité et de gestion. En outre, le récent élan de l'Internet des objets a rendu très nécessairela revitalisation de la recherche pour intégrer des sources hétérogènes d'information à travers desréseaux divers. Dans cet objectif, les contributions de cette thèse essayent de trouver un équilibreentre l'hétérogénéité et l'interopérabilité, pour découvrir et intégrer les sources hétérogènesd'information dans le contexte de l'Internet du Futur.La découverte de sources d'information sur différents réseaux requiert une compréhensionapprofondie de la façon dont l'information est structurée et quelles méthodes spécifiques sontutilisés pour communiquer. Ce processus a été régulé à l'aide de protocoles de découverte.Cependant, les protocoles s'appuient sur différentes techniques et sont conçues en prenant encompte l'infrastructure réseau sous-jacente, limitant ainsi leur capacité à franchir la limite d'unréseau donné. Pour résoudre ce problème, le première contribution dans cette thèse tente detrouver une solution équilibrée permettant aux protocoles de découverte d'interagir les uns avecles autres, tout en fournissant les moyens nécessaires pour franchir les frontières entre réseaux.Dans cet objectif, nous proposons ZigZag, un middleware pour réutiliser et étendre les protocolesde découverte courants, conçus pour des réseaux locaux, afin de découvrir des servicesdisponibles dans le large. Notre approche est basée sur la conversion de protocole permettant ladécouverte de service indépendamment de leur protocole de découverte sous-jacent. Toutefois,dans les réaux de grande échelle orientée consommateur, la quantité des messages de découvertepourrait rendre le réseau inutilisable. Pour parer à cette éventualité, ZigZag utilise le conceptd'agrégation au cours du processus de découverte. Grâce à l'agrégation, ZigZag est capabled'intégrer plusieurs réponses de différentes sources supportant différents protocoles de découverte.En outre, la personnalisation du processus d'agrégation afin de s'aligner sur ses besoins,requiert une compréhension approfondie des fondamentaux de ZigZag. À cette fin, nous proposonsune seconde contribution: un langage flexible pour aider à définir les politiques d'unemanière propre et efficace.

No description available.

All modern High Temperature Reactors (HTR) thermal cycles have one thing in common: the use of some form of heat exchanger. This heat exchanger is used to pre-heat or cool the primary loop gas, from where the secondary power generation cycle is driven. The Compact Heat Exchanger (CHE) type offers high heat loads in smaller volumes. Various studies have been done to improve the heat transfer in the flow channels of these CHEs but little focus has been placed on the thermal design of surrounding material in such a heat exchanger. The focus of this study is on the effective material usage in a CHE. Three test cases were investigated (trapezoidal, serpentine and zigzag layouts with semi-circular cross-sections) all under the same boundary conditions. Computational Fluid Dynamics (CFD) was used to simulate these test cases and the results were evaluated according to four factors, the volume ratio, heat spots, temperature difference and the combined enhancement factor. From the results it was concluded that the zigzag layout performs best when evaluated according to the volume ratio and the temperature difference and gave the best overall enhancement factor. The serpentine layout performed the worst when evaluated according to the enhancement factor. / Thesis (MIng (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2013

All modern High Temperature Reactors (HTR) thermal cycles have one thing in common: the use of some form of heat exchanger. This heat exchanger is used to pre-heat or cool the primary loop gas, from where the secondary power generation cycle is driven. The Compact Heat Exchanger (CHE) type offers high heat loads in smaller volumes. Various studies have been done to improve the heat transfer in the flow channels of these CHEs but little focus has been placed on the thermal design of surrounding material in such a heat exchanger. The focus of this study is on the effective material usage in a CHE. Three test cases were investigated (trapezoidal, serpentine and zigzag layouts with semi-circular cross-sections) all under the same boundary conditions. Computational Fluid Dynamics (CFD) was used to simulate these test cases and the results were evaluated according to four factors, the volume ratio, heat spots, temperature difference and the combined enhancement factor. From the results it was concluded that the zigzag layout performs best when evaluated according to the volume ratio and the temperature difference and gave the best overall enhancement factor. The serpentine layout performed the worst when evaluated according to the enhancement factor. / Thesis (MIng (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2013

Measurement of all the mechanical properties of carbon nanotubes is extremely difficult because of its small size. In the present work, all the five transverse isotropic properties of single wall carbon nanotubes (SWCNTs) and double wall carbon nanotubes are estimated through molecular structural mechanics for different chirality, length and assumed thickness. Armchair, zigzag & chiral SWCNTs and polychiral DWCNTs are considered for the analysis. Longitudinal and lateral Young’s modulus; longitudinal and lateral Poisson’s ratio and shear modulus are estimated for 1080 SWCNTs and 1170 polychiral DWCNTs. Effect of temperature on all the properties of SWCNT are investigated. Modal characterization of SWCNT is carried out in base fixed condition and different mode shapes viz. axial, torsion and bending mode shapes are identified based on the effective mass. Once the transverse isotropic properties of SWCNTs are estimated, these are used to estimate the transverse isotropic properties of nanocomposites embedded with SWCNT agglomerates. During the manufacturing of nanocomposite, SWCNT agglomerates are formed due to sticking of number of SWCNTs. Parametric studies are carried out to see the effect of SWCNT length on the properties of nanocomposite. Empirical formulae for all the transverse isotropic properties of SWCNT at room temperature and elevated temperature; frequency of SWCNT are derived. Empirical formulae for polychiral DWCNT transverse isotropic properties are estimated. Input for these empirical formulae are the length, chirality and assumed thickness. Empirical formulae were also derived for nanocomposite embedded with different number of SWCNTs having different chirality. The derived empirical formulae were validated with available analytical and experimental results for some sample cases.

In this work, we demonstrate the bottom-up on-surface synthesis of poly(para-dibenzo[bc,kl]-coronenylene) (PPDBC), a zigzag edge-encased analog of poly(para-phenylene) (PPP), and its lateral fusion into zigzag edge-extended graphene nanoribbons (zeeGNRs). Toward this end, we designed a dihalogenated di(meta-xylyl)anthracene monomer displaying strategic methyl groups at the substituted phenyl ring and investigated its applicability as precursor in the thermally induced surface-assisted polymerization and cyclodehydrogenation. The structure of the resulting zigzag edge-rich (70%) polymer PPDBC was unambiguously confirmed by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). Remarkably, by further thermal treatment at 450 °C two and three aligned PPDBC chains can be laterally fused into expanded zeeGNRs, with a ribbon width of nine (N = 9) up to 17 (N = 17) carbon atoms. Moreover, the resulting zeeGNRs exhibit a high ratio of zigzag (67%) vs armchair (25%) edge segments and feature electronic band gaps as low as 0.9 eV according to gaps quasiparticle calculations.

<p>Harnessing the power of data has been a driving force for computing in recently years. However, the non-vectorized or even non-Euclidean nature of certain data with complex structures also poses new challenges to the data science community. Topological data analysis (TDA) has proven effective in several scenarios for alleviating the challenges, by providing techniques that can reveal hidden structures and high-order connectivity for data. A central technique in TDA is called persistent homology, which provides intervals tracking the birth and death of topological features in a growing sequence of topological spaces. In this dissertation, we study the representative problem for persistent homology, motivated by the observation that persistent homology does not pinpoint a specific homology class or cycle born and dying with the persistence intervals. Furthermore, studying the representatives also leads us to new findings for related problems such as persistence computation.<br> </p> <p>First, we look into the representative problem for (standard) persistence homology and term the representatives as persistent cycles. We define persistent cycles as cycles born and dying with given persistence intervals and connect the definition to interval decomposition for persistence modules. We also look into the computation of optimal (minimum) persistent cycles which have guaranteed quality. We prove that it is NP-hard to compute minimum persistent p-cycles for the two types of intervals in persistent homology in general dimensions (p>1). In view of the NP-hardness results, we then identify a special but important class of inputs called weak (p+1)-pseudomanifolds whose minimum persistent p-cycles can be computed in polynomial time. The algorithms are based on a reduction to minimum (s,t)-cuts on dual graphs.<br> </p> <p>Second, we propose alternative persistent cycles capturing the dynamic changes of homological features born and dying with persistence intervals, which the previous persistent cycles do not reveal. We focus on persistent homology generated by piecewise linear (PL) functions and base our definition on an extension of persistence called the levelset zigzag persistence. We define a sequence of cycles called levelset persistent cycles containing a cycle between each consecutive critical points within the persistence interval. Due to the NP-harness results proven previously, we propose polynomial-time algorithms computing optimal sequences of levelset persistent p-cycles for weak (p+1)-pseudomanifolds. Our algorithms draw upon the idea of relating optimal cycles to min-cuts in a graph that we exploited earlier for standard persistent cycles. Note that levelset zigzag poses non-trivial challenges for the approach because a sequence of optimal cycles instead of a single one needs to be computed in this case.<br> </p> <p>Third, we investigate the computation of zigzag persistence on graph inputs, motivated by the fact that graphs model real-world circumstances in many applications where they may constantly change to capture dynamic behaviors of phenomena. Zigzag persistence, an extension of the standard persistence incorporating both insertions and deletions of simplices, is one appropriate instrument for analyzing such changing graph data. However, unlike standard persistence which admits nearly linear-time algorithms for graphs, such results for the zigzag version improving the general $O(m^\omega)$ time complexity are not known, where $\omega< 2.37286$ is the matrix multiplication exponent. We propose algorithms for zigzag persistence on graphs which run in near-linear time. Specifically, given a filtration of length m on a graph of size n, the algorithm for 0-dimension runs in $O(m\log^2 n+m\log m)$ time and the algorithm for 1-dimension runs in $O(m\log^4 n)$ time. The algorithm for 0-dimension draws upon another algorithm designed originally for pairing critical points of Morse functions on 2-manifolds. The correctness proof of the algorithm, which is a major contribution, is achieved with the help of representatives. The algorithm for 1-dimension pairs a negative edge with the earliest positive edge so that a representative 1-cycle containing both edges resides in all intermediate graphs.</p>