published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Master / Master of Science in Engineering

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

This master’s report seeks to increase knowledge of complex numbers and how to identify complex roots graphically. The reader will obtain a greater understanding of the history of complex numbers, the definition of a complex number and a few of the field properties of complex numbers. Readers will also be enlightened on how to visibly detect complex roots of polynomials of the second, third and fourth degree. / text

Very accurate methods, based on boundary integral techniques, are developed for the study of multiple, interacting fluid interfaces in an Eulerian fluid. These methods are applied to the evolution of a thin, periodic layer of constant vorticity embedded in irrotational fluid. Numerical regularity experiments are conducted and suggest that the interfaces of the layer develop a curvature singularity in infinite time. This is to be contrasted with the more singular vorticity distribution of a vortex sheet developing such a singularity in a finite time.

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The work presented in this thesis involves the application of room temperature ionic liquids (RTILs) as solvents for use in electrochemical experiments. Initially, the fundamentals of electrochemistry is presented, followed by a comprehensive overview of RTILs in terms of their properties, applications and their behaviour as electrochemical solvents compared to conventional aprotic solvents. The results of 8 original studies are then presented as follows: X-Ray photoelectron spectroscopy is used to quantify the concentration of bromide ions in an ionic liquid, and is independently confirmed by potential-step chronoamperometry. The reaction mechanisms and kinetics for the electrochemical reduction of some aromatic nitro compounds (namely nitrobenzene and 4-nitrophenol) are determined. The electrochemistry of phosphorus trichloride and phosphorus oxychloride is studied in detail for the first time, due to the unusual stability of these highly reactive compounds in RTILs. The reductions and oxidations of sodium and potassium nitrate are studied, giving rise to 'melt'-like behaviour. The electrodeposition of sodium oxide on platinum is also demonstrated. The electrochemical oxidation of nitrite and the oxidation and reduction of the toxic gas, nitrogen dioxide, is presented. The oxidation of hydrogen gas is studied in ten RTILs with a range of different cations and anions, and contrasting interactions with the RTIL anions are seen. The electrochemical oxidation of ammonia gas is studied in five RTILs with different anions and a general reaction mechanism is suggested. The reduction of benzoic acid is studied in six RTILs, and the kinetics of the dissociation step are found to be very fast. The first five studies are all carried out in one particular ionic liquid, and the reactions and mechanisms are compared to that observed in conventional aprotic solvents. The last three studies employ several RTILs with different cations and anions to look at the contrasting interaction of protons with the RTIL cation/anion and ultimately help to understand the pH properties of the solvent. The overall findings from the work in this thesis are that some reactions and mechanisms (e.g bromide, nitro derivatives and ammonia) are generally the same in RTILs as in conventional aprotic solvents, but other species (e.g. nitrates, phosphorus derivatives) show remarkably different behaviour. It has also been demonstrated that RTILs are suitable media for the detection of nitrogen dioxide, hydrogen and ammonia gases. This suggests that RTILs could potentially offer many advantages when employed as solvents in electrochemical reactions and in amperometric gas sensors.

In this thesis, a wavelet-based prediction method is developed for concealing packet-loss effects in wireless channels. The proposed method utilizes a wavelet decomposition algorithm in order to process the data and then applies the well known linear prediction technique to estimate one or more approximation coefficients as necessary at the lowest resolution level. The predicted sample stream is produced by using the predicted approximation coefficients and by exploiting certain sample value patterns in the detail coefficients. In order to test the effectiveness of the proposed scheme, a wireless channel based on a three-state Markov model is developed and simulated. Simulation results for transmission of image and speech packet streams over a wireless channel are reported for both the wavelet-based prediction and direct linear prediction. In all the simulations run in this work, the wavelet-based method outperformed the direct linear prediction method. / Hellenic Navy author.

Submitted in fulfillment of the academic requirements for the Masters of Applied Science (Chemistry), Durban University of Technology, Durban, South Africa, 2016. / In this work, the thermodynamic properties for the binary mixtures containing the ionic liquid (IL): 1-ethyl-3-methylimidazolium ethyl sulphate ([EMIM] [EtSO4]) were calculated. The binary systems studied were {pyridine (Py) or ethyl acetoacetate (EAA) or thiophene (TS) + [EMIM] [EtSO4]}. The results were interpreted in terms of the intermolecular interactions between the (pyridine + IL), (ethyl acetoacetate + IL), and (thiophene + IL) molecules. The physical properties: density, speed of sound, and refractive index were measured for the binary mixtures over the complete mole fraction range using an Anton Paar DSA 5000 M vibrating U- tube densimeter and an Anton Paar RXA 156 refractometer, respectively. The measurements were done at T = (298.15, 303.15, 308.15, 313.15, and 318.15) K and at p = 0.1 MPa. The experimental data was used to calculate the derived properties for the binary mixtures namely:- excess molar volume (V E ), isentropic compressibility (ks), molar refractions (R) and deviation in refractive index (Δn). For the binary mixtures, (Py or EAA or TS + IL), V E was negative throughout the whole composition range which indicates the existence of attractive intermolecular interaction between (pyridine + IL) and (ethyl acetoacetate + IL) for (thiophene + IL), V E was negative at low mole fraction of thiophene and became positive at high mole fraction of thiophene. For the binary mixtures (pyridine + IL), (ethyl acetoacetate + IL), ks was positive indicating that the binary mixtures were more compressible than the ideal mixture. For the binary mixture (thiophene + IL) ks was negative at low thiophene composition and positive at high composition indicating that the binary mixture was less compressible than the ideal mixture at low thiophene composition and more compressible at high composition of thiophene. The molar refraction, R, is positive for the (Py or EAA or TS + IL) binary systems at T = (298.15 – 318.15) K, molar refraction decreases as the organic solvent composition increases. For the binary mixture (pyridine + [EMIM] [EtSO4]), Δn is negative at mole fractions < 0.75 of pyridine and positive at mole fractions >0.75 at all temperatures and decreases with an increase in temperature. For the binary system (ethyl acetoacetate + [EMIM] [EtSO4]), Δn values are positive over the entire composition range and at all temperatures and increases with an increase in temperature. Δn values for the (thiophene + IL) system are negative for mole fractions of thiophene < 0.62 and becomes positive for mole fractions of thiophene > 0.62 and Δn increases with an increase in temperature. The Redlich-Kister smoothing equation was used successfully for the correlation of V E and Δn data. The Lorentz- Lorenz equation gave a poor prediction of V E , but a good prediction of density or refractive index. / M

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