Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: In the present study, two-phase natural circulation flow in a multi-parallel channel system was investigated using experimental and numerical modelling. The investigation was carried out under different power excitations and various system operations (open system, closed system and heat pipe mode). The multichannel system was equipped at the upper end with a condenser enclosed within a steam drum, while the lower portion of each channel was heated to heat the system. For the numerical modelling, transient one-dimensional conservation equations were derived from first principle for both single- and two-phase fluids and used to computer program the system’s discretised simulation model. Temperatures and mass flow rates of the fluid responses as a result of different power excitations and operations were obtained for both the experimental and numerical modelling. It was observed from the results that the fluid experiences a start-up transient before accomplishing steady-state conditions. It was further noted that the transient duration varies with power excitations and system operation modes and hence with the stability of the system. A rise in power proved not to necessarily increase the fluid mass flow rate, but invited oscillations with higher amplitudes, depending on the system’s mode of operation. Type I instability and low-quality steam oscillations were witnessed at low power and open system operation mode (system open to the atmosphere). Type II instabilities and flashing instability were observed to be associated with medium and high-power excitations for the open system mode of operation. The fluid flow became more stable and less oscillatory at all power excitations for the closed system operation mode (system not open to the atmosphere). However, a sub-cooling effect was evident at higher power, where the two-phase fluid temperatures oscillated in a sinusoidal manner. However, the mass flow rates oscillated with high amplitudes in the forward direction in some channels and assumed a unidirectional flow in other channels. In general, steady-state conditions were obtained earlier when the system was operated as a closed system. For the heat pipe mode of operation, the system transient response in all channels exhibited a geysering instability followed by flashing-induced boiling. In-phase (flow in channels exhibiting the same behaviour) and out-of-phase (flow in channels exhibiting contradictory conduct) behaviour between adjacent channels were observed at all power excitations and system operation modes. Flow reversal in heated channels of a natural circulation system were proven to exists even under equal power excitations. / AFRIKAANSE OPSOMMING: In hierdie studie is tweefasige natuurlike sirkulasievloei in ’n parallelle multikanaalstelsel ondersoek deur middel van eksperimentele en numeriese modellering. Die ondersoek is onder verskillende kragopwekkings en verskeie stelselwerkings (oop stelsel, toe stelsel en hittepypmodus). Die multikanaalstelsel is aan die bopunt met ’n kondensor binne ’n stoomdrom toegerus, terwyl die laer gedeelte van elke kanaal verhit is om die stelsel te verhit. Vir die numeriese modellering, is oorgangseendimensionele behoundsvergelykings vanaf die eerste beginsel vir beide een- en tweefasige vloeistowwe afgelei en dit is gebruik om die stelsel se gediskretiseerde simulasiemodel vir ’n rekenaar te programmeer. Temperature en massavloeitempo’s van die vloeistofrespons as gevolg van verskillende kragopwekkings en -werkings is vir beide die eksperimentele en die numeriese modellering verkry. Dit is in die resultate waargeneem dat die vloeistof ’n aansitoorgang ervaar voor dit vloeiewewigstoestande bereik. Daar is verder waargeneem dat die duur van die oorgang wissel volgens kragopwekkings en stelselwerkingsmodusse en dus op grond van die stabiliteit van die stelsel. ’n Toename in krag het nie noodwendig die vloeitempo van die vloeistofmassa verhoog nie, maar het aanleiding gegee tot ossillasies met groter amplitudes, afhangende van die stelsel se metode van werking. Tipe I-onstabiliteit en stoom-ossillasies van ’n lae intensiteit is teen lae krag en oop stelselwerkingsmodus waargeneem (stelsel oop aan die atmosfeer). Tipe II-onstabiliteit en flitsingsonstabiliteit (flashing instability) is met medium- en hoë kragopwekkings vir die oop stelsel modus van werking waargeneem. Die vloeistofvloei het meer stabiel en minder ossillerend geraak by alle kragopwekkings in die geslote stelsel van werking (stelsel nie oop na die atmosfeer nie). ’n Subverkoelingseffek was egter teen hoër krag duidelik, waar die tweefasige vloeistof se temperature sinusvormig geossilleer het. Die massavloeitempo’s het egter met hoë amplitudes in die vorentoe rigting in sommige kanale gevloei en eenrigtingvloei in ander kanale vertoon. Oor die algemeen is vloei-ewewigstoestande vroeër verkry toe die stelsel as ’n geslote stelsel bedryf is. Vir die hittepypmodus van werking het die stelsel se oorgangsweergawe in alle kanale ’n geysering onstabiliteit getoon, gevolg deur flitsinggeïnduseerde (flashing induced) kook. Gelykfasige gedrag (vloei in kanale vertoon dieselfde gedrag) en ongelykfasige gedrag (vloei in kanale vertoon teenstrydige gedrag) tussen langsliggende kanale is met al die kragopwekkings en stelselwerkingsmodusse waargeneem. Vloei-omkering in die verhitte kanale van ’n natuurlike sirkulasiestelsel is bewys om selfs onder gelyke kragopwekkings te bestaan.

The internet has had a profound effect on communication, entertainment, buying, and selling (Webb 2002) and, in particular, as a distribution channel (Van den Poel and Leunis 1999). Increasingly companies in a variety of industries have established their own online direct sales channels instead of merely relying on conventional intermediaries (Coughlan et al 2006). Hence, multi-channel distribution strategies, combining both off- and online channels are being adopted. Frazier (1999:232) argues: “The utilization of multiple channels of distribution is now becoming the rule rather than the exception”. Companies which combine physical and web channels have been referred to as “clicks and mortar,” “bricks and clicks,” “surf and turf,” “cyber-enhanced retailing,” or “hybrid e-commerce,” (Gulati and Garino 2000; Steinfield et al 2002 a b; Agatz et al 2008; Sharma and Mehrotra 2007). Despite the apparent popularity of internet channels, adding an internet channel to the distribution mix creates potentially significant challenges for channel managers (Webb 2002). A major concern is that by going multi-channel, a firm might face ‘channel conflict’ between channel members (Sharma and Mehrotra 2007). As Rosenbloom (2007:7) claims: “Perhaps the most significant obstacle to building successful multi-channel strategies is the emergence of conflict between the different channels used for reaching customers”. The main objective in this study is to explore the impact of the internet as being a distribution channel within the multi-channel system. Three research issues which have seldom been discussed in previous literature are explored in this study. These three questions are: Q1: What are the factors encouraging manufacturers to develop the internet channel, especially manufacturers with a well established distribution channel? Q2: What are the channel design patterns of the multi-channel structure? Q3: How can channel conflict arising from the development of the internet channel be minimized? Interview was utilized as a main data collection method in this research. A total of 25 interviews were conducted and the majority of interviewees are managers in Taiwanese IT companies. The main product lines in their firms included wireless equipment, DRAM module, motherboard, and scanner. In addition, a wide range of documents were examined as a secondary data source and compared with data from the interviews. According to the statements from interviewees, six main factors were identified as encouraging the manufacturer to develop the internet channel. These are: (1) customer information management, (2) retailer management issue, (3) launching a new product, (4) to be a secondary channel, (5) management support, and (6) me-too strategy. Furthermore, it seems that when firms are at the different stages of the product life cycle, managers would design their multi-channel structure into different patterns. In addition, a further two conflict resolution styles, communication and differentiation, were found in the sample of firms studied which have seldom been mentioned in previous research and firms appear to adopt different conflict resolution styles according to stage in the product lifecycle.

The Manassas oil field was discovered in 1986 in Lincoln County, Colorado (Township 14S, Range 56W) by Petro Lewis Corporation and it completed production from the Pennsylvanian Atoka Sandstone within 24 years. It is located on the theoretical NW extension of the Haswell-Salt Lake Morrow sandstone producing trend and Morrow sandstones have found in some of the available wells in the area. The productive Morrow sandstones are fluvial channel deposits that are encased by marine shales. The distribution of the Morrow channel system is difficult to define from either data obtained from sparsely located wells alone, or directly from seismic data due to low acoustic contrast between fluvial sandstones and marine shale. Therefore, this study has used a correlation of well log data to understand regional stratigraphy of the selected study area and seismic attributes were used to develop a workflow to define the Morrow sandstone channel system in the Manassas prospect. Well logs from forty wells were used to define the early Pennsylvanian strata (Marmaton, Cherokee, upper and lower Atoka, upper and lower Morrow) and underlying Mississippian unit (St. Louis). The formation data were used to visualize the regional stratigraphy using isopach maps and stratigraphic cross sections. The acoustic and density logs from the Lockwood 27-22 well located in the Manassas field, were used to generate the synthetic seismogram for the purpose of seismic horizons interpretation. The upper Morrow isochron thick was defined using upper and lower Morrow time structure maps. The channel infill system was first detected using the Chaos attribute that identifies reflectors associated with channel infills that are normally chaotic signals with low consistency. The observation was confirmed using, Variance attribute that analyzes signal coherency, and estimates trace to trace variance. The amplitude attributes (RMS Amplitude – iterative attribute, and Envelope) highlight both channel infills and bright spots. The Genetic Inversion identifies relative variations of rock properties. The Generalized Spectral Decomposition attribute was used to visualize the detailed channel morphology through generating a RGB blending model as the final step. This study shows that the Morrow channel sandstones are present across the Manassas prospect and are interpreted as a composite meandered and anastomosed channel system. The observed high amplitude variations are interpreted as tuning effects of thin beds and relative lithological changes caused by variations of rock properties such as density and porosity.

The term of Cognitive Radio (CR) used to indicate that spectrum radio could be accessed dynamically and opportunistically by unlicensed users. In CR Networks, Interference between nodes, hidden terminal problem, and spectrum sensing errors are big issues to be widely discussed in the research field nowadays. To improve the performance of such kind of networks, this thesis proposes Cognitive Medium Access Control (MAC) protocols for Mobile Ad-Hoc Networks (MANETs). From the concept of CR, this thesis has been able to develop a cognitive MAC framework in which a cognitive process consisting of cognitive elements is considered, which can make efficient decisions to optimise the CR network. In this context, three different scenarios to maximize the secondary user's throughput have been proposed. We found that the throughput improvement depends on the transition probabilities. However, considering the past information state of the spectrum can dramatically increases the secondary user's throughput by up to 40%. Moreover, by increasing the number of channels, the throughput of the network can be improved about 25%. Furthermore, to study the impact of Physical (PHY) Layer errors on cognitive MAC layer in MANETs, in this thesis, a Sensing Error-Aware MAC protocols for MANETs has been proposed. The developed model has been able to improve the MAC layer performance under the challenge of sensing errors. In this context, the proposed model examined two sensing error probabilities: the false alarm probability and the missed detection probability. The simulation results have shown that both probabilities could be adapted to maintain the false alarm probability at certain values to achieve good results. Finally, in this thesis, a cooperative sensing scheme with interference mitigation for Cognitive Wireless Mesh Networks (CogMesh) has been proposed. Moreover, a prioritybased traffic scenario to analyze the problem of packet delay and a novel technique for dynamic channel allocation in CogMesh is presented. Considering each channel in the system as a sub-server, the average delay of the users' packets is reduced and the cooperative sensing scenario dramatically increases the network throughput 50% more as the number of arrival rate is increased.

Cette thèse cherche à résoudre en pratique le problème de couverture dans les réseaux de Petri et les systèmes de canaux à pertes (LCS). Ces systèmes sont intéressants à étudier car ils permettent de modéliser facilement les systèmes concurrents et les systèmes distribués. Le problème de couverture dans un système de transitions consiste à savoir si on peut, à partir d’un état initial arriver à un état plus grand qu’un état cible. La résolution de ce problème dans les systèmes de transitions bien structurés (WSTS) sera le sujet d’études de la première partie. Les réseaux de Petri et les LCS sont des WSTS. On donnera dans la première partie une méthode générale pour le résoudre rapidement en pratique. Cette méthode utilise des invariants de couverture, qui sont des sur-approximations de l’ensemble des états couvrables. La seconde partie sera consacrée aux réseaux de Petri. Elle présentera diverses comparaisons théoriques et pratiques de différents invariants de couverture. Nous nous intéresserons notamment à la combinaison de l’invariant classique de l’inéquation d’état avec une analyse de signe simple. Les LCS seront le sujet d’études de la troisième partie. On présentera une variante de l’inéquation d’état adaptée aux LCS ainsi que deux invariants qui retiennent des propriétés sur l’ordre dans lequel les messages sont envoyés. La thèse a mené à la création de deux outils, ICover et BML, pour résoudre le problème de couverture respectivement dans les réseaux de Petri et dans les LCS. / The goal of this thesis is to solve in practice the coverability problem in Petri nets and lossy channel systems (LCS). These systems are interesting to study because they can be used to model concurrent and distributed systems. The coverability problem in a transition system is to decide whether it is possible, from an initial state, to reach a greater state than a target state. In the first part, we discuss how to solve this problem for well-structured transition systems (WSTS). Petri nets and LCS are WSTS. In the first part, we present a general method to solve this problem quickly in practice. This method uses coverability invariants, which are over-approximations of the set of coverable states. The second part studies Petri nets.We present comparisons of coverability invariants, both in theory and in practice. A particular attention will be paid on the combination of the classical state inequation and a simple sign analysis. LCS are the focus of the third part. We present a variant of the state inequation for LCS and two invariants that compute properties for the order in which messages are sent. Two tools, ICover and BML, were developed to solve the coverability problem in Petri nets and LCS respectively.

In automatic speech recognition systems (ASRs), training is a critical phase to the system?s success. Communication media, either analog (such as analog landline phones) or digital (VoIP) distort the speaker?s speech signal often in very complex ways: linear distortion occurs in all channels, either in the magnitude or phase spectrum. Non-linear but time-invariant distortion will always appear in all real systems. In digital systems we also have network effects which will produce packet losses and delays and repeated packets. Finally, one cannot really assert what path a signal will take, and so having error or distortion in between is almost a certainty. The channel introduces an acoustical mismatch between the speaker's signal and the trained data in the ASR, which results in poor recognition performance. The approach so far, has been to try to undo the havoc produced by the channels, i.e. compensate for the channel's behavior. In this thesis, we try to characterize the effects of different transmission media and use that as an inexpensive and repeatable way to train ASR systems.

Traditionally, source coding (data compression) and channel coding (error protection) are performed separately and sequentially, resulting in what we call a tandem (separate) coding system. In practical implementations, however, tandem coding might involve a large delay and a high coding/decoding complexity, since one needs to remove the redundancy in the source coding part and then insert certain redundancy in the channel coding part. On the other hand, joint source-channel coding (JSCC), which coordinates source and channel coding or combines them into a single step, may offer substantial improvements over the tandem coding approach. This thesis deals with the fundamental Shannon-theoretic limits for a variety of communication systems via JSCC. More specifically, we investigate the reliability function (which is the largest rate at which the coding probability of error vanishes exponentially with increasing blocklength) for JSCC for the following discrete-time communication systems: (i) discrete memoryless systems; (ii) discrete memoryless systems with perfect channel feedback; (iii) discrete memoryless systems with source side information; (iv) discrete systems with Markovian memory; (v) continuous-valued (particularly Gaussian) memoryless systems; (vi) discrete asymmetric 2-user source-channel systems. For the above systems, we establish upper and lower bounds for the JSCC reliability function and we analytically compute these bounds. The conditions for which the upper and lower bounds coincide are also provided. We show that the conditions are satisfied for a large class of source-channel systems, and hence exactly determine the reliability function. We next provide a systematic comparison between the JSCC reliability function and the tandem coding reliability function (the reliability function resulting from separate source and channel coding). We show that the JSCC reliability function is substantially larger than the tandem coding reliability function for most cases. In particular, the JSCC reliability function is close to twice as large as the tandem coding reliability function for many source-channel pairs. This exponent gain provides a theoretical underpinning and justification for JSCC design as opposed to the widely used tandem coding method, since JSCC will yield a faster exponential rate of decay for the system error probability and thus provides substantial reductions in complexity and coding/decoding delay for real-world communication systems. / Thesis (Ph.D, Mathematics & Statistics) -- Queen's University, 2008-05-13 22:31:56.425