This thesis presents two problems in the field of fluid mechanics. Both problems concern the flow of a Newtonian viscous fluid in the laminar and early-transitional regimes. Geometrically, they also share the following features: a square corner; a wall boundary layer; and a semi-infinite physical domain. Part 1 of this thesis, comprising Chapters 2–5, considers the laminar flow parallel to a streamwise corner. In Chapter 2 we present an in-depth study of the laminar flow internal to a square corner. The hydrodynamic stability of this flow is the subject of Chapter 3. For the special case of zero pressure gradient, our analysis suggests a critical Reynolds number of Re[subscript]c ≈ 44 000 (based on streamwise distance from the leading edge), indicating that this flow is significantly less stable than the well-known Blasius boundary layer on a semi-infinite flat plate. In Chapter 4 we derive the laminar flow external to a square corner. Finally, in Chapter 5 we summarize our findings and offer some recommendations for future research on laminar and transitional corner flows. Part 2, comprising Chapters 6–10, considers the sudden blockage of steady laminar flow within a circular pipe. Even though the blockage occurs almost instantaneously, the fluid takes an appreciable time to come to rest. Accordingly, Chapter 6 presents a detailed analysis of the laminar-decay process at an arbitrary location upstream of the blockage point. The hydrodynamic stability of this unsteady upstream flow is the subject of Chapters 7 and 8. Chapter 7 uses traditional linear eigenmode theory, originally developed for steady laminar flow, to estimate that the laminar flow is absolutely stable in the event that the pre-blockage Reynolds number does not exceed Re[subscript]c ≈ 450. The linear pseudomode analysis of Chapter 8 yields the substantially lower estimate Re[subscript]c ≈ 115, above which there exists the theoretical possibility of transient growth initiating a ‘bypass’ transition to turbulence. However, after accounting for the transient nature of the underlying flow itself, we hypothesize a significantly higher threshold Re[subscript]c ≈ 1000 for full breakdown of the laminar structure. Chapter 9 rounds off the present work by extending the laminar-flow analysis of Chapter 6 to the immediate vicinity of the blockage point. We present a direct numerical simulation of the complete laminar-decay process within this end-region, highlighting the early-phase development of an unsteady corner boundary layer and the subsequent development of vortices in the interior of the pipe. The thesis concludes in Chapter 10 by summarizing the findings from Part 2 and suggesting some fruitful directions for future research on unsteady pipe flows. / Thesis (Ph.D.) -- University of Adelaide, School of Mathematical Sciences, 2009

Thesis (Ph. D.)--University of Washington. / Vita. Bibliography: L. 150-156.

Thesis (M.S.)--University of Wisconsin--Madison, 1963. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 89-92).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1973. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (M.S.)--University of Wisconsin--Madison, 1963. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 70-71).

abstract: Over the past three decades, particle image velocimetry (PIV) has been continuously growing to become an informative and robust experimental tool for fluid mechanics research. Compared to the early stage of PIV development, the dynamic range of PIV has been improved by about an order of magnitude (Adrian, 2005; Westerweel et al., 2013). Further improvement requires a breakthrough innovation, which constitutes the main motivation of this dissertation. N-pulse particle image velocimetry-accelerometry (N-pulse PIVA, where N>=3) is a promising technique to this regard. It employs bursts of N pulses to gain advantages in both spatial and temporal resolution. The performance improvement by N-pulse PIVA is studied using particle tracking (i.e. N-pulse PTVA), and it is shown that an enhancement of at least another order of magnitude is achievable. Furthermore, the capability of N-pulse PIVA to measure unsteady acceleration and force is demonstrated in the context of an oscillating cylinder interacting with surrounding fluid. The cylinder motion, the fluid velocity and acceleration, and the fluid force exerted on the cylinder are successfully measured. On the other hand, a key issue of multi-camera registration for the implementation of N-pulse PIVA is addressed with an accuracy of 0.001 pixel. Subsequently, two applications of N-pulse PTVA to complex flows and turbulence are presented. A novel 8-pulse PTVA analysis was developed and validated to accurately resolve particle unsteady drag in post-shock flows. It is found that the particle drag is substantially elevated from the standard drag due to flow unsteadiness, and a new drag correlation incorporating particle Reynolds number and unsteadiness is desired upon removal of the uncertainty arising from non-uniform particle size. Next, the estimation of turbulence statistics utilizes the ensemble average of 4-pulse PTV data within a small domain of an optimally determined size. The estimation of mean velocity, mean velocity gradient and isotropic dissipation rate are presented and discussed by means of synthetic turbulence, as well as a tomographic measurement of turbulent boundary layer. The results indicate the superior capability of the N-pulse PTV based method to extract high-spatial-resolution high-accuracy turbulence statistics. / Dissertation/Thesis / Animation of N-pulse PIVA measurement of flow-structure interaction / Doctoral Dissertation Mechanical Engineering 2018

Nanofluids are a novel class of heat transfer fluids in which nanoparticles are dispersed in traditional heat transfer fluids. They offer enhanced thermophysical, rheological and radiative properties. These enhancements have resulted in recent research being centred on the application of nanofluids to various systems. An example of such systems is the solar volumetric flow receiver in which great efficiency improvements have been reported. To explain this efficiency increase, researchers have evaluated the impact of enhanced radiative properties of nanofluids while largely neglecting that of enhanced thermophysical properties. This study looks at the impact of enhanced thermophysical properties on the performance of nanofluid-based solar volumetric receivers. Particular focus is drawn to the impact of temperature dependent conductivity and volumetric specific heat capacity. Copper oxide - water nanofluid is employed as its temperature dependent properties have been characterised. [Please note: this thesis file has been deferred until June 2016]

No description available.

The movement of fluids has a significant impact on the biological world, from the transport of critical medications, to the shaping of cellular life. The presence of a fluid-fluid interface gives rise to regions where a fluid---and its contents---can be selectively transported or trapped, and where stresses from the rearranging interface can lead to damage or even death of nearby microorganisms. First, we examine the role of local displacement on network level transport. Multiphase fluid flow through small length-scale networks---such as porous rock or tumor vasculature---can be described by examining local interactions of two adjacent channels (pores) using a pore doublet model. However, the traditional pore doublet model does not take into account the region at the interface of the two fluids, and thus the applicability of this model for low aspect ratio pores is unknown. Here we show using computational fluid dynamics (CFD) that traditional pore doublet models begin to break down for lower channel aspect ratios due to increased energy dissipation in the fluid interface region. We also show that our pore doublet model is able to extend previous models, elucidating network level behavior from a local response. Second, we focus on the generation of highly uniform droplets. When air is blown in a straw near an air-liquid interface, typically one of two behaviors is observed: a dimple in the liquid's surface, or a frenzy of sputtering bubbles, waves, and spray. Here we report and characterize an intermediate oscillatory regime that can create monodisperse aerosols from periodic angled jets. The underlying mechanisms responsible for this highly periodic regime are not well understood. We present experimentally validated scaling arguments to rationalize the fundamental frequencies driving this system, as well as the conditions that bound the periodic regime. This mechanism has the potential to aerosolize microorganisms in the bulk fluid. Third, we look at the role of fluid stresses on nearby biological life. In the biotechnology industry mammalian cells are grown in aerated tanks where locally elevated stresses---created by bubbles rapidly changing shape---can be high enough to kill nearby cells; however the effect of elevated stresses on cells at the timescales of these bubble events is unclear. Here we investigate the effect on cell viability from fluid stresses created by a bubble undergoing topological change, using a combination of CFD, numerical particle tracking, and experimental microfluidics. Using this approach we elicit an overall bubble-induced effect on a cell population's viability. Finally, we examine the role stresses can have on bacterial aerosolization. A key component of the airborne infection pathway is the survival of the pathogen during aerosolization pinch-off processes. Due to a rapidly rearranging interface, pinch-off processes have the potential to generate an enormous amount of hydrodynamic stress in the surrounding fluid. However, the magnitude and duration of the hydrodynamic stresses in these droplets is unknown. We show using numerical simulations the spatial and temporal hydrodynamic stress history of microscale aerosol droplets produced by the central jet of collapsing bubbles. This stress history can then be linked to the stress tolerance of various bacteria allowing for the creation of a new stress-based metric for bacterial survival during aerosolization. / 2021-01-28T00:00:00Z

Air bubbles in liquids have complex interactions with their surroundings. A rising bubble not only mixes the surrounding fluid but also collects suspended particles such as bacteria or microplastics on its interface, transporting them to the liquid surface. When a bubble bursts, it releases droplets that carry sea salt, microorganisms, and chemicals into the air, affecting both human health and the climate. Through experiments and theory, this dissertation studies the underlying mechanisms behind bubble-mediated biofouling prevention, air-sea particle transport, and sea spray formation. Our first study examines the relationship between the flow fields created by rising bubbles and biofouling prevention on a submerged surface. Bubble aeration is a method for preventing biofouling organisms, such as barnacles, from growing on a surface without using environmentally harmful chemicals. We identify the critical flow characteristics of periodically rising bubbles that correlate with the prevention of multi-species biofouling over a seven-week period, offering a potential framework for studying and comparing flow fields that successfully inhibit biofouling. Our next study investigates how small bubbles concentrate particles adhered to the bubble interface, such as plastics or microorganisms, into highly-contaminated droplets during the bursting process. We reexamine the assumption that only particles small enough to fit within a thin microlayer around the bubble can be transported into the influential top jet drop, and demonstrate that larger particles can also be transported and exhibit higher enrichment levels than predicted. We combine experiments and theory to develop an analytical model estimating the expected enrichment based on the bubble size, particle size, and particle position on the bubble. We proceed by focusing on plastic particle transfer into the atmosphere via bursting bubbles from breaking ocean waves. Existing estimates of micro- and nanoplastic transport through this pathway have large uncertainties due to limited detection techniques and studies. We develop a modeling framework that examines the size-dependent transport of hydrophilic and hydrophobic plastic particles, revealing the dominance of jet drops over film drops and the potential for nanometer-sized plastics to become highly concentrated in the smallest drops. Finally, we explore the role of salinity on the bursting bubble production of submicron drops, which are critical to cloud formation and other atmospheric processes. It is well-known that bubbles bursting in saltier water will produce more submicron drops. However, previous studies have attributed this trend to the suppression of bubble coalescence with higher salinity, leading to more numerous bubbles and consequently more drops. We demonstrate that submicron drop production increases with salinity, even when using a salt that does not affect bubble coalescence behavior. This finding implies that salinity has a systematic effect on the physics of submicron drop formation, even at the scale of a single bubble.