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The design of high Q oscillators, using Crystals at lower frequencies, (and dielectric resonators at much higher frequencies), has long been considered a black art. This may be due to the fact that a systematic approach with optimized design guideline for crystal oscillators could not be found after extensive literature search.
In this dissertation, after analyzing the first crystal oscillator by W.G. Cady (1921), other high performance crystal oscillators will be discussed, analyzed and calculated.
A single transistor crystal oscillator design as used by HP (Hewlett Packard) in one of their designs, the HP10811A is considered in this thesis for mathematical analysis and CAD (Computer aided Design) simulation. This was also measured on state-of-the-art signal source analyzer. After validation, this design is scaled to 100MHz, the frequency of interest for this dissertation.
Though most designers use a single transistor based oscillator circuit, it is not an optimized design because of limited control over key design parameters such as loop gain, dc current etc.
This dissertation is an attempt to overcome the limitation due to the single transistor circuit and to give a step by step procedure, explaining the significance of a two transistor design with thorough analysis and design simulation results. This two stage transistor circuit is also not yet a best solution in terms of phase noise performance and output power, and some add-on circuitry will be needed for an optimized performance.
An important contribution of this work is to show that since the voltage gain is the ratio of the collector resistor and the emitter resistor, the performance is practically independent of the VHF transistor and gives better control over various parameters of the oscillator, in order to optimize the design.
A grounded-base amplifier is then introduced and added for improving the isolation and the output power. Unlike most oscillators, that take the output from the collector, a novel concept introduced by Rohde [14], is incorporated here, where the crystal is used as a filter that is then connected to the grounded base amplifier, a technique which many companies have been using. This dissertation will show that this technique increases the output power without significantly affecting the phase noise. Such a validation is needed for better understanding and as per my knowledge, has not been done so far.
For the oscillator, the tuning diode sensitivity and flicker noise contribution are also taken into consideration, by calibrating the mathematics and its validation is shown.
Crystal resonators of the type AT and stress-compensated (SC) cut devices will be considered as they give the best performance. The one port Colpitts type oscillator is considered first and the two port two transistor design later. Both will need a post amplifier/buffer stage.
A complete step by step design procedure for an optimized 100MHz crystal oscillator is then presented.
For completeness, CAD Simulation and Experimental results are provided for 10 MHz, 128 MHz and 155 MHz VCO circuits.
An empirical and simulation-based assessment of tree growth in temperate alley-cropping systems
(2020)
The potential of biomass generated from dedicated energy crops, used in short-rotation coppices (SRC), progressively grows recognition as a flexible primary source for the generation of energy, heat, fuel, and bio-based materials and chemicals. The alley-cropping systems (ACSs), which can integrate tree strips managed as SRC into agriculturally managed fields, are often regarded as an adaptable multi-crop land-use strategy that can provide ecological and economic benefits. The research aim of the present dissertation has focused on investigating the prospective implications of different site-specific conditions and scenarios on tree growth in ACSs with SRC, thus incorporating several experimental and simulation-based studies. For this, the ability of a process-oriented, eco-physiological tree growth model was investigated in order to (i) impute missing empirical data, thus securing a reliable repository of tree growth characteristics, (ii) simulate the tree growth in terms of woody biomass production in strong relation to the interactions with adjacent crops and their respective resource capture, (iii) predict and evaluate the tree growth sensitivity to prospective climate changes, thus performing risk assessments for the near and distant future, and (iv) derive and assess the land equivalent ratio (LER) and gross energy yield for different climatic, soil, and management scenarios. The findings have corroborated the potential tree growth vulnerability to prospective climatic changes, particularly to changes in water availability, and have underlined the importance of coping management strategies in SRC for forthcoming risk assessments and adaptation scenarios. Both LER and gross energy yields had resulted in a convex curve where the maximum values were achieved when either the tree or crop component was dominant (>75% of the land area) and minimum when these components shared similar proportions of land area. Collectively, the implications of different site-specific conditions and scenarios on tree growth in ACSs with SRC have been investigated in order to improve the decision-making, optimization, and adaptation of such systems. Last but not least, this dissertation has emphasized the considerable potential of modelling approaches in ACSs, as they can impute missing data from scarce available data and simulate tree and crop yields for specific site-conditions in a non-intrusive, inexpensive, and prompt way while supporting early site-setup planning.
Due to the use of fossil fuel resources, many environmental problems have been increasingly growing. Thus, the recent research focuses on the use of environment friendly materials from sustainable feedstocks for future fuels, chemicals, fibers and polymers. Lignocellulosic biomass has become the raw material of choice for these new materials. Recently, the research has focused on using lignin as a substitute material in many industrial applications. The antiradical and antimicrobial activity of lignin and lignin-based films are both of great interest for applications such as food packaging additives. DPPH assay was used to determine the antioxidant activity of Kraft lignin compared to Organosolv lignins from different biomasses. The purification procedure of Kraft lignin showed that double-fold selective extraction is the most efficient confirmed by UV-Vis, FTIR, HSQC, 31PNMR, SEC, and XRD. The antioxidant capacity was discussed regarding the biomass source, pulping process, and degree of purification. Lignin obtained from industrial black liquor are compared with beech wood samples: Biomass source influences the DPPH inhibition (softwood > grass) and the TPC (softwood < grass). DPPH inhibition affected by the polarity of the extraction solvent. Following the trend: ethanol > diethylether > acetone. Reduced polydispersity has positive influence on the DPPH inhibition. Storage decreased the DPPH inhibition but increased the TPC values. The DPPH assay was also used to discuss the antiradical activity of HPMC/lignin and HPMC/lignin/chitosan films. In both binary (HPMC/lignin) and ternary (HPMC/lignin/chitosan) systems the 5% addition showed the highest activity and the highest addition had the lowest. Both scavenging activity and antimicrobial activity are dependent on the biomass source; Organosolv of softwood > Kraft of softwood > Organosolv of grass. Lignins and lignin-containing films showed high antimicrobial activities against Gram-positive and Gram-negative bacteria at 35 °C and at low temperatures (0-7 °C). Purification of Kraft lignin has a negative effect on the antimicrobial activity while storage has positive effect. The lignin leaching in the produced films affected the activity positively and the chitosan addition enhances the activity for both Gram-positive and Gram-negative bacteria. Testing the films against food spoilage bacteria that grow at low temperatures revealed the activity of the 30% addition on HPMC/L1 film against both B. thermosphacta and P. fluorescens while L5 was active only against B. thermosphacta. In HPMC/lignin/chitosan films, the 5% addition exhibited activity against both food spoilage bacteria.
This thesis is an attempt to assess some of the effects that electroquasistatic body forces exert on turbulent internally forced convective flows. In order to do that, a stochastic turbulence model is employed, namely, the One-Dimensional Turbulence (ODT) model.
The reduced dimensionality of ODT demands a reduction of the Navier-Stokes equations (and in this case, also the Maxwell equations), into a 1-D system. This is done by performing an asymptotic analysis in terms of the nondimensional numbers of the flow. Also, a validation step due to the relative novel character of the cylindrical ODT formulation is done for an incompressible and constant properties flow regime and a variable density flow regime. The validation is presented for both the temporal (T-ODT) and a novel spatial (S-ODT) formulation in both planar and cylindrical geometries. Results in the constant property case show that wall normal (and radial) profiles, in both the T-ODT and S-ODT formulations, show good agreement with each other and to the data of Direct Numerical Simulations (DNSs). For the evaluated variable density heated pipe flow, gradients at the wall can be better reproduced with S-ODT.
After validating the model, ODT is applied first into a planar configuration which emulates the flow in a wire-plate Electrostatic Precipitator (ESP). For this flow, the additional input energy due to the electroquasistatic body force has an effect on the modification of the bulk velocity, and subsequently, the skin friction coefficient. Some qualitative DNS trends are confirmed with ODT, such as the localized increase of the Reynolds stress, as a consequence of increased eddy activity close to the discharge electrodes. Next, the results of ODT simulations in a cylindrical wire-tube ESP are presented. Here, ODT results are compared to experimental results. ODT results for global integral quantities such as the streamwise pressure gradient and the Nusselt number enhancement ratio are able to match in a reasonable way the experimental results. The competing relevance between the EHD contribution to turbulence by momentum, and by affecting the temperature and density due to the Joule heating effect is also analyzed, showing the leading order relevance of the former one. Specifically for the Nusselt number results, the sensitivity of the EHD flow to transition effects is shown to be very significant.
This thesis may open the door to a vast new field of phenomena which can not only serve for the further validation of the ODT model against DNSs or experiments, but also for the real use of ODT in applications which are so far inaccessible for traditional DNSs.
In the last 30 years, communication became one of the most important pillars of our civilization. Every day terabytes of information are moved wired and wireless between computers. In order to transport this amount of data, researchers and industry increase the data rates of the underlying communication networks with impressive speed. However, such ultra-high data rates are unavailable at the communication endpoints.
One reason why ultra-high data rates are still not available for the communication endpoints is their inability to handle the protocol processing at this data rate. In order to enable communication endpoints to process high-volume data streams, the protocol processing has to be parallelized and optimized on all processing levels. However, parallelization and optimization are cumbersome tasks, which are further complicated as the protocol processing is traditionally carried out by the operating system.
This thesis aims at circumventing these problems by moving the protocol processing into external processing hardware and interpreting communication protocols as stream processing problems. In order to achieve ultra-high data rates at the communication endpoints, a protocol stream processing design approach was developed and evaluated. The design process is separated into implementation, soft real-time analysis, parallelization, and mapping steps, which allow a scalable protocol implementation without paradigm changes. Furthermore, a data link protocol for 100 Gbit/s wireless was developed and implemented with the new stream processing design concept, in order to show its feasibility. The data link protocol is configurable for different communication conditions and easy to parallelize by providing different granularities of packets. The proposed design-process has shown to be suitable for uncovering bottlenecks and helping with debugging the individual stages of the protocol.
Die vorliegende Arbeit befasst sich mit der Analyse der Dauerfestigkeit von Schweißverbindungen nach erfolgter Reparatur des vorhandenen Ermüdungsrisses. In den aktuellen Regelwerken (wie Eurocode bzw. International Institute of Welding) liegen bis heute keine normativen Bemessungskonzepte für sanierte Schweißverbindungen vor. Diesbezüglich werden im Rahmen der vorliegenden Arbeit die herkömmlichen Ermüdungsfestigkeitsnachweise (nach Nenn-, Struktur- und Kerbspannungskonzept) um die ermittelten FAT-Klassen für sanierte bzw. nachbehandelte Schweißverbindungen ergänzt.
Die Analyse betrachtet ein typisches kleinmaßstäbliches Bauteilelement des Stahlbaus, d.h. das Konstruktionsdetail der Quersteife. Dabei handelt es sich um nicht „belastungstragende“ Kehlnähte, die eine bevorzugte Ausgangsstelle für Ermüdungsrisse an Nahtübergängen darstellen und daher stark ermüdungsfestigkeitsmindernd wirken.
Die experimentellen Ermüdungsuntersuchungen in Form von klassischen Wöhlerversuchen verfügen über drei Gruppen von Versuchsprüfkörpern. Die Gruppe I bezieht sich auf fehlerfreie Schweißnähte, d.h. im Schweißzustand belassene Prüfkörper. Die Gruppe II betrifft die fehlerhaften Schweißnähte, die durch Ausschleifen der potenziellen Risse und Wiederschweißen saniert wurden. Bei der Gruppe III werden die Prüfkörper zusätzlich durch eine Schweißnahtnachbehandlungsmethode, d.h. mit höherfrequentem Hämmern bearbeitet. Neben den klassischen Ermüdungsversuchen werden die Kerbwirkungen in ermüdungskritischen Stellen mit Hilfe der Finite-Elemente-Methode analysiert. Somit können die FAT-Klassen nach Nenn-, Struktur- und Kerbspannungskonzept für Reparaturschweißungen ermittelt werden. Um die relevante Vergleichbarkeit der entsprechenden Schweißnahtzustände bzw. Effektivität solcher Reparaturmaßnahmen sicherzustellen wird zusätzlich eine ganze Serie von sog. Begleituntersuchungen durchgeführt. Dabei handelt es sich um Temperatur-, Verzugs- und Härtemessungen, röntgenografische Eigenspannungsbestimmungen sowie metallographische Untersuchungen zur Bestimmung der Schmelzbadgeometrie bzw. des Nahtprofils. Vor allem wird im Rahmen dieser Arbeit ein Fokus auf die Eigenspannungsanalyse gelegt, um die tatsächliche Eigenspannungsverteilung in den entsprechenden Reparaturzuständen zu ermitteln und somit eine Basis für verbesserte Bemessungsmodelle von sanierten und nachbehandelten Schweißnähten zu erarbeiten. Bei den Gruppen I und II der Versuchsprüfkörper betrifft die Eigenspannungsanalyse ebenso die Schweißsimulation.
Gegenstand der vorliegenden Arbeit sind Steuerungsprobleme sowohl mit endlichem, als auch mit unendlichem Zeithorizont. Auf beide Probleme werden Transformationen des Zustandes, der Steuerung und der Zeit angewendet. Dadurch wird für Aufgaben mit unendlichem Zeithorizont die Klasse der Dynamiken erweitert, für welche notwendige Bedingungen existieren. Bemerkenswert ist, dass diese stets eine explizite Transversalitätsbedingung enthalten.
An einem Beispiel wird demonstriert, warum die Verwendung des Riemannschen Integralbegriffes und der Regel der partiellen Integration im Riemannschen Sinne ungeeignet erscheint, wenn man Steuerungsprobleme mit unendlichem Zeithorizont oder Singularitäten im Zielfunktional betrachtet. Stattdessen wird diese Regel für Lebesgue-Integrale formuliert. Hierbei ergeben sich in natürlicher Weise als Voraussetzung an die eingehenden Funktionen, dass sie gewichteten Sobolevräumen angehören. Das erklärt, warum die theoretische Grundlage der Arbeit ein Existenzsatz und notwendige Bedingungen in Form eines Pontrjaginschen Maximumprinzips für Steuerungsprobleme mit unendlichem Zeithorizont bilden: Beide Sätze wurden unter Verwendung von Hilbertraum-Techniken bewiesen und nutzen, dass die zugrunde gelegten Funktionenräume Hilberträume sind. Als Hilfsmittel werden neben der Substitutionsregel für Lebesgue-Integrale, dem Gronwall-Lemma und der Hölderschen Ungleichung lediglich Grundlagen der gewöhnlichen Differentialrechnung benötigt, um mittels des Beweises der Äquivalenz zweier Aufgabenklassen neue Resultate zu erzielen. So ist es gelungen, neue notwendige Bedingungen und einen neuen Existenzsatz für ein Steuerungsproblem mit fester Zeit zu formulieren. Die besondere Erkenntnis ist, dass es bei erfüllten Voraussetzungen keine Rolle spielt, ob das betrachtete Problem Singularitäten im Zielfunktional, in der Dynamik oder auch in den Zustandsfunktionen hat oder nicht. Als geeigneter Funktionenraum für Steuerungsprobleme mit Singularitäten ergibt sich wiederum ein gewichteter Sobolevraum, der stets aus den Daten einer Aufgabe passend bestimmt werden kann. Anhand dieses Raumes lässt sich explizit nachprüfen, welche analytischen Eigenschaften für die Lösung zu erwarten sind.
Die Konstruktionsplanung von neuen Transitrouten oder Energieleitungen auf einem topografischen Gelände wird von Ingenieuren in der Regel manuell vorgenommen, ohne dass eine Optimalität garantiert werden kann. Wir stellen einen neuen Ansatz zur Berechnung von Trajektorien für die Entwicklung neuer optimaler Transitrouten und Energieleitungen zwischen zwei Standorten auf einer Untermannigfaltigkeit U von IR³ vor. Diese Untermannigfaltigkeit repräsentiert die Topographie eines Geländes. U wird näherungsweise durch ein spezielles gewichtetes Gitternetz modelliert. Auf diesem Gitternetz werden die kürzesten Wege für den Bau neuer Routen bestimmt, wobei wir drei Optimierungskriterien betrachten werden: Routen mit minimaler Länge, Routen mit geringsten Baukosten und Routen mit minimalen absoluten Höhenvariationen oder minimalen absoluten Steigungen. Anschließend wird eine Kombination dieser Kriterien gebildet, um dieses Problem zu einem multikriteriellen Optimierungsproblem zu erweitern. Ein Algorithmus für den kürzesten Weg, wie der Dijkstra-Algorithmus, wird verwendet, um optimale Kompromisse für die Konstruktion neuer Routen zu berechnen.
We study convexities designed to characterise some of the most fundamental classes of graphs. To this end, we present some known results on this topic in a slightly different form, so as to give a homogeneous representation of a very disparate field. Furthermore, we present some new results on the Caratheodory number of interval graphs and also give a more or less exhaustive account of everything that is known in this context on AT-free graphs, including new results on characterising linear vertex orders and the structure of the intervals of this class.
We introduce the new class of bilateral AT-free graphs which is motivated by the linear order characterisation and the convexity used to describe AT-free graphs. We discuss their relation to other known classes and consider the complexity of recognition. Furthermore, as a consequence of notions from abstract convexity we present algorithmic results with regards to some natural subclasses of these.
As an application of notion of an extreme vertex of a convex geometry, we discuss structural aspects of avoidable vertices in graphs, which form a generalisation of simplicial vertices. This includes a characterisation of avoidable vertices as simplicial vertices in some minimal triangulation of the graph and a new proof of the existence result. Furthermore, we discuss the algorithmic issues regarding the problem of efficient computation of avoidable vertices in a given graph. This is complemented by an algorithmic application of the concept of avoidable vertices to the maximum weight clique problem, by identifying a rather general class of graphs in which every avoidable vertex is bisimplicial. This leads to a polynomial-time algorithm for the maximum weight clique problem in this class of graphs. Implications of this approach for digraphs are also discussed. All of these results lead to a conjecture concerning the generalisation of avoidable vertices to avoidable paths and we prove this conjecture for paths of length less or equal to two.
Finally, we analyse the properties of many different and widely used forms of graph search. Here, we discuss the problem of recognising whether a given vertex can be the last vertex visited by some fixed graph search. Moreover, we present some new aspects of the problem of deciding whether a given spanning tree of a graph is a graph search tree of a particular type of search. We generalise the concept of such trees to many well-known searches and give a broad analysis of the computational complexity of this problem. Both of these discussions are motivated by the use of graph searches in the context of computing properties of convexity.
The increasing demand for competitive, whilst also environment-friendly airplane travel, compels the design of highly efficient engines in the aeronautical field. A potential for improvement of traditional polycrystalline Ni-based superalloys, aiming higher creep resistance, was investigated. The approach adopted the concept of metal matrix composites (MMCs) to incorporate a rigid discontinuous phase, in the form of particles, to a γ’-strengthened Ni-based superalloy. In order to make the concept feasible, different microstructures resulting from diverse manufacturing techniques were investigated. By using distinct mixing and sintering methods, powders of Inconel X-750 and TiC were combined to form composites containing 15 vol.% of reinforcing particles. Powders were prepared with low and high energy milling processes, and formed by uniaxial pressure sintering and spark plasma sintering methods. Non-reinforced variants and composites had microstructures thoroughly examined at their initial state and after long isothermal aging treatments. Selected variants were further submitted to tensile and compression creep tests at temperatures between 700 and 800 °C, in the stress range of 200 to 500 MPa.
A comprehensive analysis was conducted using techniques such as EBSD, XRD Rietveld refinement, EDS and TEM to evaluate the development of γ’, η and TiC phases, determining the achievable microstructures with each fabrication method and establishing their evolution after aging treatments over times up to 1000 h. Likewise, creep properties were analyzed by obtaining parameters such as creep exponents, threshold stresses and activation energies. A creep life estimation was conducted with the use of a Monkman-Grant relationship and a Larson-Miller parametrization. Lastly, the potential for a reduction in creep strain rates in a working turbine blade, considering the density of investigated materials as a parameter, was evaluated.
All produced composites presented power law creep, with dislocations surpassing γ’ particles by climb. The variant produced by high energy ball milling and spark plasma sintering exhibited the highest creep rates, resulting from intense diffusion through grain boundaries. It also presented η phase after long isothermal aging, which affects negatively the creep resistance. Contrastingly, in the variant produced by low mixing combined with pressure sintering the lowest creep rates were observed. It was proposed that reinforcing TiC particles effectively acted as a load bearing phase, counterbalancing the adverse effects of the intergranular diffusion in the refined microstructure. Furthermore, a higher microstructural stability was observed in this variant, resulting from limited interaction between TiC particles and the matrix during fabrication.