published_or_final_version / Mechanical Engineering / Master / Master of Philosophy

Linear oligosacchardies of 1,4 linked β-D-glucopyranose are commonly referred to as cellodextrins (CD) or cellooligosaccharides (CO). They are of interest to those working in disciplines involving cellulose chemistry because they are often used as model substrates for cellulose itself. They are of interest to food scientists and nutritionists because they are easily incorporated into foods as non-digestible oligosaccharides, a category of food ingredients that is thought to be beneficial lo human health. The intent of the research presented in this thesis was to evaluate the potential of using cellulose supports for the chromatographic separation of soluble CDs differing in their degree of polymerization (DP; a numerical value indicating the number of glucose substituents per molecule). Soluble CDs range in DP from 2 to 8. Thin layer chromatography (TLC), using cellulose-coated TLC plates, was used as a model chromatographic system. Mixed CD preparations, containing CDs ranging in DP from 2 to 8 were prepared by incomplete acid-catalyzed hydrolysis of cellulose. The DP profiles of the different CD preparations were qualitatively demonstrated by TLC using silica-coated plates, an organic solvent-based mobile phase, and a standard carbohydrate visualizing reagent (p-anisaldehye in sulfuric acid). CD-preparations were then chromatographed on cellulose-coated TLC plates. Visualization of the chromatographed CDs was accomplished using a silver nitrate-sodium hydroxide reagent system, a reducing-sugar visualizing reagent. The silver nitrate-sodium hydroxide system was found to be the most appropriate, based on detection limits, simplicity and safety, of the several visualization reagents tested. Eight different mobile phases, all aqueous-based, were tested as potential solvents for the resolution of CDs, differing in DP, on cellulose-coated tlc plates at room temperature. The optimum solvent was found to be 60% ethanol/40% water. This solvent clearly resolved CDs of DP 3, 4 and 5. CD preparations chromatographed with the same mobile phase, but with silica-coated TLC plates, were not resolved. These combined results suggest that the TLC system with the cellulose stationary phase behaves similar to an affinity system, since silica and cellulose are both relatively hydrophilic stationary phases (i.e. both systems are typically considered examples of normal phase adsorption chromatography). The results further illustrate that cellulose supports have potential for use in the preparation of CDs of defined DP. / Graduation date: 2002

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The evolution of the large scale structures and the mean field were investigated in axisymmetric and plane mixing layers. Some aspects of the linear instability of an axisymmetric jet have been demonstrated. The axisymmetric geometry admits two additional length scales with relation to the two-dimensional shear layer: the radius of the jet column and the azimuthal wavelength. The importance of these two length scales in governing the instability of an axisymmetric jet was explored. The special case of a thin axisymmetric shear layer was analyzed and the results stressing the evolution of different azimuthal modes were compared with some phase-locked data which was produced by subjecting the jet to axisymmetric and helical excitation. The importance of the initial spectral distribution in a natural jet was demonstrated when it is used as an input to the amplification curve obtained from linear stability theory to predict a measured spectral distribution at a further downstream location. The inclusion of the nonlinear terms in the stability analysis reveals two main interactions: mean flow-wave interaction and wave-wave interaction. The modification of the mean flow of an axisymmetric jet was examined by exciting two azimuthal modes simultaneously. The interaction resulted in an azimuthal modulation of the mean velocity profile having a cosine shape. Effectively, the geometry of the jet was modified without changing the geometry of the nozzle. The coupling between an excited periodic disturbance and the mean flow was analyzed and the spatial evolution of both were compared with experimental results obtained in a plane mixing layer. The behavior of the concommittant Reynolds stresses is discussed in detail. The conditions under which one disturbance will transfer energy to another were derived and demonstrated in an axisymmetric jet. The interaction between a large amplitude plane wave with a weak subharmonic component was shown to enhance the amplification rate of the subharmonic. It was further shown that the nonlinear interaction between two azimuthal modes can produce a third azimuthal mode which was not initially present in the flow. The coupling between a fundamental wave and its subharmonic in a parallel plane mixing layer was demonstrated numerically.

This thesis focuses on secondary instability mechanisms of high-speed boundary layers over cones with a circular cross section. Hypersonic transition investigations at Mach 8 are performed using Direct Numerical Simulations (DNS). At wind-tunnel conditions, these simulations allow for comparison with experimental measurements to verify fundamental stability characteristics.To better understand geometrical influences, flat-plate and cylindrical geometries are studied using after-shock conditions of the conical investigations. This allows for a direct comparison with the results of the sharp cone to evaluate the influence of the spanwise curvature and the cone opening angle. The ratio of the boundary-layer thickness to the spanwise radius is used to determine the importance of spanwise curvature effects. When advancing in the downstream direction the radius increaseslinearly while the boundary-layer thickness stays almost constant. Hence, spanwise curvature effects are strongest close to the nose and decrease in downstream direction. Their influences on the secondary instability mechanisms provide some rudimentary guidance in the design of future high-speed air vehicles.In experiments, blunting of the nose tip of the circular cone results in an increase in critical Reynolds number (c.f. Stetson et al. (1984)). However, once a certain threshold of the nose radius is exceeded, the critical Reynolds number decreases even to lower values than for the sharp cone. So far, conclusive explanations for this behavior could not be derived based on the available experimental data. Therefore, here DNS is used to study the effect of nose bluntness on secondary instability mechanisms in order to shed light on the underlying flow physics. To this end, three different nose tip radii are considered-the sharp cone, a small nose radius and a large nose radius. A small nose radius moves the transition on-set downstream, while for a large nose radius the so-called transition reversal is observed. Experimentalists hold influences of the entropy layer responsible but detailed numerical studies may lead to alternateconclusions.

Semiconductors are characterized by atomic resolution imaging and density of states measurements (DOS) obtained through the use of a scanning tunneling microscope (STM). The DOS of the conduction and valence bands can be measured separately with a STM as opposed to an optical measurement which measures only the joint DOS. Layered-structure semiconductors are characterized both in the bulk form and in the isolated cluster form. Images of three bulk layered-structure semiconductors, MoS₂, WSe₂, and SnS₂, were obtained with both positive and negative sample-to-tip bias voltages. Curves of tunneling current as a function of bias voltage were measured, from which the DOS of the valence and conduction bands can be inferred. We obtained an atomically resolved image of an isolated fragment of a semi-conductor cluster which was deposited on a graphite surface from a colloidal suspension of BiI₃. Also imaged were clusters of MoS₂ layered-structure semiconductors.

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