Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

The formation process and characteristics of sprays from annular liquid jet breakup in moving gas streams have been investigated. In the first part of the thesis, a linear instability analysis is carried out for the instability and breakup of annular liquid jets. A dispersion relation has been derived and solved numerically by using Muller's method. Temporal instability analysis shows that two independent unstable modes, para-sinuous and para-varicose, exist for the annular jet instability. The para-sinuous mode outgrows the para-varicose one at relatively low gas-liquid density ratios and large Weber numbers as typically encountered in the twin-fluid atomization. The curvature of the annular jet promotes the jet instability and may not be neglected for the breakup processes of annular liquid jets. Not only the velocity difference across each interface but also the absolute velocity of each fluid is important for the jet instability. Co-flowing gas at high velocities is found to significantly improve atomization performance. A mesh-searching method has been developed to determine absolute mode of instability. The numerical results indicate that both absolute and convective instability exist for para-sinuous and para-varicose modes under certain flow conditions. Para-sinuous unstable waves outgrow para-varicose ones, and hence dominate the jet instability according to both absolute and convective instability analysis. The liquid viscosity has a simple stabilizing effect on the jet instability while the gas inertial force shows fairly complex influence on the absolute instability of the jet. The convective growth rates for various inner gas velocities indicate that not only the velocity difference between, but also the absolute velocity of the liquid and gas, determine the jet breakup process. In the second part of this thesis, experimental investigations have been conducted for the breakup process of annular water jets exposed to an inner air stream by photographic technique, and the characteristics of the resultant sprays by Phase Doppler Particle Analyzer. Two annular nozzles of the same structure but different dimensions are designed and constructed especially to provide smooth contraction for the liquid flow. The test apparatus is constructed to produce the annular liquid sheets or sprays of good quality. Flow visualization reveals that there exist three regimes. i.e., bubble formation, annular jet formation and atomization regime for the jet breakup process. Within the bubble formation regime, the jet breakup characteristics measured from the photographs taken under various liquid and gas velocities show that uniform bubbles are observed for various air-to-water velocity ratios. The jet breakup and wave lengths decrease with the air-to-water velocity ratio. The measurements are compared with the predictions by the linear instability analysis, and fair agreement is obtained. Spray characteristics measured by a Phase Doppler Particle Analyzer indicate that using atomizing air enhances the jet breakup process and improves the atomization performance by producing fine sizes of droplets and increasing the uniformity of drop sizes. The drop axial velocity has a jet-type distribution in the radial direction, and decreases monotonically along the spray axis. Increase in the water and air velocities results in higher drop axial velocity. The droplet size described by its Sauter mean diameter (SMD) reaches a minimum value at the central region of the spray and increases towards the spray edge. The SMD has a complex variation along the spray axis. / Graduate

We investigate the dynamics of spontaneous jumps of water drops from electrically charged superhydrophobic dielectric substrates during a sudden step reduction in gravity level. In the brief free-fall environment of a drop tower, with a non-homogeneous external electric field arising due to dielectric surface charges (with surface potentials 0.4-1.8 kV), body forces acting on the jumped drops are primarily supplied by polarization stress and Coulombic attraction instead of gravity. This electric body force leads to a drop bouncing behavior similar to well-known phenomena in 1-g0, though occurring for much larger drops (~0.5 mL). We show a simple model for the phenomenon, its scaling, and asymptotic estimates for drop time of flight in two regimes: at short-times close to the substrate when drop inertia balances Coulombic force due to net free charge and image charges in the dielectric substrate and at long-times far from the substrate when drop inertia balances free charge Coulombic force and drag. The drop trajectories are controlled primarily by the dimensionless electrostatic Euler number Eu, which is a ratio of inertial to electrostatic forces. To experimentally determine values of Eu we conduct a series of drop tower experiments where we observe the effects of drop volume, net free charge, and static surface potential of the superhydrophobic substrate on drop trajectories. We use a direct search optimization to obtain a Maximum Likelihood Estimate for drop net charge, as we do not measure it directly in experiment. For φEu/8π > 1 drops escape the electric field, where φ is a drop to substrate aspect ratio. However, we do not observe any escapes in our dataset. With an eye towards engineering applications we consider the results in light of the so-called low-gravity phase separation problem with a worked example.

Relationships between structure and dynamics in fluids have a wide variety of applications. Because theories for fluid structure are now well developed, such relationships can be used to “predict” dynamic properties. Also, recasting dynamic properties in terms of structure may provide new insights. In this thesis, we explore whether some of the relationships between structure and dynamics that have proven useful for understanding simple atomic liquids can also be applied to complex fluid systems. In particular, we focus on model fluid systems with particles that interact with attractive forces that are shortranged (relative to the particle diameter), and display properties that are anomalous when compared to those of simple liquids. Examples of fluids with short-range attractive (SRA) interactions include colloidal suspensions and solutions of micelles or proteins. We show via simulations that common assumptions regarding free volume and dynamics do not apply for SRA fluids, and propose a revision to the traditional free volume perspective of dynamics. We also develop a model which can predict the free volume behavior for hard-sphere and SRA fluids. Next, we demonstrate that the dynamic properties of SRA fluids can be related to structural order. In terms of structural order, the properties of SRA fluids can be related to those of another anomalous fluid, liquid water. In both fluids, anomalous dynamics are closely related to anomalous structure, which can be traced to changes in second and higher coordination shells. We also find that a similar relationship between structural order and dynamics approximately holds for fluids under shear. Motivated by previous work, we explore via simulation how tuning the particle-wall interactions to flatten or enhance the particle layering in a confined fluid impacts its self-diffusivity, viscosity, and entropy. We find that the excess entropy explains the observed trends. Finally, we present preliminary simulation data regarding the relationship between heterogeneous dynamics and structure. We show that the mobility of particles is related in a simple way to the structure of the particles surrounding them. In particular, our results suggest that a critical amount of local disorder allows a particle to be mobile on intermediate time scales. / text

No description available.

Thesis (MSc)--University of Stellenbosch, 2007. / Bibliography. Also available via the Internet.

The concept of moving surface boundary-layer control, as applied to the Joukowsky and NACA airfoils, is investigated through a planned experimental program complemented by theoretical and flow visualization studies. The moving surface was provided by one or two rotating cylinders located at the leading edge, the trailing edge, or the top surface of the airfoil. Three carefully designed two-dimensional models, which provided a wide range of single and twin cylinder configurations, were tested at a subcritical Reynolds number (Re = 4.62 x 10⁴ or Re — 2.31 x 10⁵) in a laminar-flow tunnel over a range of angles of attack and cylinder rotational speeds. The test results suggest that the concept is indeed quite promising and can provide a substantial increase in lift and a delay in stall. The leading-edge rotating cylinder effectively extends the lift curve without substantially affecting its slope. When used in conjunction with a second cylinder on the upper surface, further improvements in the maximum lift and stall angle are possible. The maximum coefficient of lift realized was around 2.22, approximately 2.6 times that of the base airfoil. The maximum delay in stall was to around 45°. In general, the performance improves with an increase in the ratio of cylinder surface speed (Uc) to the free stream speed (U). However, the additional benefit derived progressively diminishes with an increase in Uc/U and becomes virtually negligible for Uc/U > 5. There appears to be an optimum location for the leading-edge-cylinder. Tests with the cylinder at the upper side of the leading edge gave quite promising results. Although the CLmax obtained was a little lower than the two-cylinder configuration (1.95 against 2.22), it offers a major advantage in terms of mechanical simplicity. Performance of the leading-edge-cylinder also depends on its geometry. A scooped configuration appears to improve performance at lower values of Uc/U (Uc/U ≤ 1). However, at higher rates of rotation the free stream is insensitive to the cylinder geometry and there is no particular advantage in using the scooped geometry. A rotating trailing-edge-cylinder affects the airfoil characteristics in a fundamentally different manner. In contrast to the leading-edge-cylinder, it acts as a flap by shifting the CL vs. α plots to the left thus increasing the lift coefficient at smaller angles of attack before stall. For example, at α = 4°, it changed the lift coefficient from 0.35 to 1.5, an increase of 330%. Thus in conjunction with the leading-edge- cylinder, it can provide significant improvements in lift over the entire range of small to moderately high angles of incidence (α ≤ 18°). On the theoretical side, to start with, the simple conformal transformation approach is used to obtain a closed form potential-flow solution for the leading-edge-cylinder configuration. Though highly approximate, the solution does predict correct trends and can be used at a relatively small angle of attack. This is followed by an extensive numerical study of the problem using: • the surface singularity approach including wall confinement and separated flow effects; • a finite-difference boundary-layer scheme to account for viscous corrections; and • an iteration procedure to construct an equivalent airfoil, in accordance with the local displacement thickness of the boundary layer, and to arrive at an estimate for the pressure distribution. Effect of the cylinder is considered either through the concept of slip velocity or a pair of counter-rotating vortices located below the leading edge. This significantly improves the correlation. However, discrepancies between experimental and numerical results do remain. Although the numerical model generally predicts CLmax with a reasonable accuracy, the stall estimate is often off because of an error in the slope of the lift curve. This is partly attributed to the spanwise flow at the model during the wind tunnel tests due to gaps in the tunnel floor and ceiling required for the connections to the externally located model support and cylinder drive motor. However, the main reason is the complex character of the unsteady flow with separation and reattachment, resulting in a bubble, which the present numerical procedure does not model adequately. It is expected that better modelling of the cylinder rotation with the slip velocity depending on a dissipation function, rotation, and angle of attack should considerably improve the situation. Finally, a flow visualization study substantiates, rather spectacularly, effectiveness of the moving surface boundary-layer control and qualitatively confirms complex character of the flow as predicted by the experimental data. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

No description available.

published_or_final_version / Civil Engineering / Master / Master of Philosophy

This thesis is concerned with issues relating to the development of an active sealing system for hydraulic actuators where the sealing elements can be radially extended and retracted to vary the friction and leakage characteristics. In order to determine the feasibility of the active sealing concept it is necessary to establish that varying the seal geometry may achieve useful improvements in the friction-leakage trade-off and that a practical method of achieving this seal extension can be realised. Experimental and simulation approaches for seal friction prediction have been developed and active seal prototypes produced to demonstrate the concept. <br /> Experiments were carried out to measure the constant velocity friction for single-lip and double-lip seals over a range of sliding speeds and sealed pressures with special consideration applied to the instroke-outstroke direction dependence. Additional experiments were performed with sinusoid motion to provide an indication of the transient friction characteristics. Friction was shown to increase towards the end of the outstroke cycle and decrease once the instroke motion began. <br /> Tribology simulations were produced based on the results of a FEA simulation of the rod-seal contact pressure. Empirical friction-load relationships and novel contact mechanics approaches for high loads were considered. Simulations based on the Reynolds equation including standard inverse EHL theory and the GW-average Reynolds lubrication are also presented. Experimental agreement could be improved if loading is assumed to transfer to the fluid to maintain a fluid film. A hysteresis friction model was also developed in attempt to improve the prediction of speed dependent friction. <br /> Two active seal prototypes were produced, each with an adjustable external pressure supplied to the outer circumference of the sealing element. Constant velocity friction measurements for different external pressures and the transient response following step changes in this pressure are presented.