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published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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Overend et al [68] designed a viscometer to measure the viscosity of slurries that have a tendency to settle. This viscometer consists of a rotating ribbed rotor surrounded by a stationary slotted sleeve; this system is then placed eccentrically within an inclined rotating bowl. It, is claimed that this overcomes most of the difficulties encountered when attempting to obtain accurate measurements for these types of mixtures. If the mixture being sheared within the annulus does not represent the true composition of the slurry being, tested then the results are expected to be inaccurate. The presence of sediment at the bottom of the rotor or the formation of large masses of particles within the flow domain will affect the accuracy of the measurements obtained. This dissertation studies the amount of flow through the slotted sleeve and the region, or regions, of low shear rate within the flow domain. Assuming that end-effects are unimportant and that the slurries can be replaced by a single-phase fluid, three two-dimensional models are proposed. These models are designed to capture the large-slot construction of the sleeve and the, approximate, non-Newtonian behaviour of the slurries. The first two models solve analytically (using a regular perturbation scheme) and numerically (using a finite volume method) the moderate-and large-Reynolds-number flow, and the third model uses a finite volume method to study the flow patterns developed by pseudoplastic fluids. The results show that the mixing of the slurry is expected to be enhanced by moving the concentric system (i.e., the rotor and the slotted sleeve) close to the rotating bowl and using low to moderate speeds for the rotor and bowl. In addition, when the cylinders rotate in the same directions, two (counter-rotating) eddies are present within the flow domain; whereas, only one eddy (rotating counter-clockwise) is ++ / present when the cylinders rotate in opposite directions. The presence of eddies in the former situation inhibits the flow through the sleeve; while, for moderate rotorspeeds, the flow through the sleeve is enhanced in the latter. When the slurry assumed pseudoplastic, we observe a region of low shear rate located near the dividing streamline present within the flow field. The distribution of shear rate within the flow field is shown to be affected by factors such as the rate of diffusion of the apparent viscosity and the value of the power law index. Therefore, this study suggests that for certain types of slurries, concentrations of particles exist within the domain and that the mixing of slurries can be impeded by the presence of eddies within the main flow field.

A direct numerical simulation study has been conducted to examine the flow that develops on long circular cylinders that are aligned, or nearly aligned, with the freestream. Results are presented for turbulent boundary layers and vortex - shedding yawed flow. Although flows of these types occur in a range of engineering applications, they remain relatively unexplored compared with flat - plate flow. The numerical scheme employed for solution of the governing Navier - Stokes equations is similar to that used in some previously published simulations, but here rather different boundary conditions are adopted. At the outer edge of the cylindrical computational domain, the imposed boundary conditions confine the vorticity field within a finite radius while allowing the continuous velocity field to converge to the free - stream velocity at large distances from the cylinder. Axial flows are considered with radius Reynolds numbers in the range 311 to 20800, ratios of boundary layer thickness to cylinder radius in the range 0.15 to 27.5, and boundary layer thicknesses of between 160 and 800 viscous units ( v / u [subscript τ] ). The mean - flow and turbulence statistics for axisymmetric boundary layers are found to differ significantly from flat - plate results when the boundary layer is strongly curved, that is when the boundary layer is thick in relation to the cylinder radius. The effects of curvature are mainly observed in the outer flow except when the cylinder radius in viscous units is small. Particular attention is given to the assessment of similarity scaling relations for the mean velocity profile, velocity fluctuation statistics and temporal wall - pressure spectra. Structural features of axisymmetric turbulence are examined by inspection of instantaneous flow fields, correlation functions and conditionally - averaged flow structures. In very thick boundary layers on thin cylinders, the simulations reveal evidence of large - scale fluid motion across the cylinder, although the mechanisms of turbulence generation do not appear to be significantly different from those in flatplate flow. Simulations of turbulence in near - axial flow over cylinders are considered with radius Reynolds numbers up to 674 and yaw angles up to 0.5 degrees. No previous flow simulations of this kind are reported in the literature. The mean - flow and turbulence statistics are found to depart rapidly from axisymmetry as the yaw angle increases. The quality of the calculated results suggests that the computational procedure is suitable for use in a more comprehensive investigation of near - axial flow over cylinders. For cylinders inclined at sufficiently large yaw angles to the free - stream, turbulent boundary layer flow gives way to oblique vortex - shedding from the cylinder. Simulated flow fields corresponding to a radius Reynolds number of 311 and a yaw angle of 3 degrees are examined to reveal the three - dimensional structure of the flow. The results suggest that the oscillating flow fields in the cylinder wake have the character of a wave travelling in the axial direction at the same speed as the axial component of the free - stream. / Thesis (Ph.D.)--School of Mechanical Engineering, 2006.

Flow-induced vibration of small elastic cylinders mounted in the shear flow of a two-dimensional jet is investigated experimentally. There has been a great deal of work concerned with different vibrating conditions and practical dynamic responses by way of mass ratios and diameters of various different cylinders. In such cases, the amplitude of the cylinder oscillation changed along with the variation of the jet velocity is due to the influence of fluid elastic instability. The experiment is based on the method of the magnetic field induction to measure the motion of the small cylinder, and it involves measurements of the varying velocity in a jet through the hot- wire anemometer. The critical velocity of the cylinder vibration in the shear flow with different diameters, mass ratios and damping factors are examined. Moreover, the oscillation traces of the cylinder by fluid elastic instability were observed when the jet velocity was increased, and then decreased for examination of hysteresis phenomena. The results show that the bifurcation of the cylinder vibration traces is remarkable especially for cylinders with high mass ratios. By the amplitude diagrams of the cylinder vibration, the critical velocity for onset of fluid elastic vibration was determined. The dependence of the critical velocity and hysteresis phenomena on the mass ratio and damping factor are discussed.

Concentric rotating cylinder and turbulent mixing devices have been frequently used in studying mixing and particle coagulation. However, these apparatus develop simple laminar flow (concentric rotating cylinders) or do not have well-defined flow (turbulent mixing devices). In this work, the eccentric rotating cylinder apparatus was investigated to find applicability for the improved study of coagulation based on the modified analytical solution of Ballal and Rivlin. Various eccentricity ratios, rotation speeds and viscosities were simulated to obtain optimum operating conditions. Inertial forces working on the fluid increased as the eccentricity ratio and rotation speed increase. As inertial forces increase, the eddy developed in wide clearance was more skewed in the direction of rotation. Both root-mean-square velocity gradient and average principal strain-rate, were increased by increasing eccentricity ratio. avaerage principal strain-rate were linearly increased as rotation speed increases, which suggested that average prinipal strain-rate can properly represent mixing intensity. Comparison of average principal strain-rate and RMS velocity gradient revealed that RMS velocity gradient overestimated mixing intensity and its error increased as eccentricity ratio increases. This study showed that the eccentric rotating cylinder apparatus has a non-uniform velocity distribution with well-defined fluid dynamics. Therefore, the eccentric rotating cylinder apparatus can be applicable as a model flocculator. However, in order to achieve reliable model predictability, the fluid Reynolds number must be below 200.

Thesis (Ph. D.)--School of Mechanical Engineering, Georgia Institute of Technology, 2004. Directed by Kenneth A. Cunefare. / Vita. Includes bibliographical references (leaves 129-136).