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This thesis focuses on three-dimensional thermally-driven boundary layers with three distinct types of (steady) thermal forcing (i) a streamwise-aligned heat source that is spanwise localised, (ii) a spanwise and streamwise localised heat source and (iii) a non-localised globally driven natural convection problem. What ties these problems together is that the induced three-dimensionality is on a spanwise lengthscale commensurate with the boundary-layer thickness, and so their leading-order flow is governed by the 'boundary-region' equations, attributed to Kemp (1951). A lot of work has been undertaken within this boundary-region framework in an isothermal context, for example forced by injection or roughness, but presently there has been limited work in which temperature variation has been applied. In our first problem (i), we embed a streamwise-aligned spanwise-localised heated strip, which has an O(Re^(-1/2)) spanwise scale, in a horizontal semi-infinite flat plate. At large spanwise distances from the forcing region, the flow takes the form of the two-dimensional Falkner--Skan boundary layer (1930). The three-dimensional flow response is determined numerically and a robust streak-roll structure forms downstream. We undertake an asymptotic analysis at large streamwise coordinates, and obtain the downstream transverse growth of the nonlinear response. We also determine the flow over a streamwise-aligned injection slot, and make qualitative comparisons with the thermally forced problem. A localised heat source with an O(1) streamwise scale and an O(Re^(-1/2)) spanwise scale is then considered (ii), and the corresponding nonlinear problem is solved. For weak thermal forcing, the isolated heated region results in a downstream response which has an algebraically developing velocity field. By seeking a linear bi-global eigenvalue problem in terms of the downstream algebraic spatial growth, we show that there is a new class of linear spanwise-localised algebraically developing disturbances in the Falkner--Skan boundary layer when thermal effects are included. The velocity field of the dominant mode grows (algebraically) downstream, and is driven by a decaying (weak) temperature field. Above a critical pressure gradient the dominant mode decays, and we determine the critical Prandtl number and pressure gradient parameter for downstream algebraic growth. The linear algebraically growing response ultimately gives rise to a nonlinear spanwise-localised streak-roll response downstream. We assess the stability of the streak response to time-harmonic (viscous) disturbances and inviscid Rayleigh modes. Our third problem (iii) is the natural convection flow in a heated vertical corner. This is a globally driven, non-localised problem, however in a high Grashof number regime the flow in a region near to the corner is governed by the boundary-region equations. Two self-similar solutions are determined: (A) a solution first discussed by Riley and Poots (1972), and (B) a new solution which, at large spanwise coordinates, has a coupled linearly-developing crossflow component. We show that the streamwise velocity and temperature fields determined by Riley and Poots (1972) for solution A were surprisingly accurate, given that their computational resources were limited. However, they concluded that there is an in-plane flow away from the corner which is not present in the properly resolved solution. The non-parallel stability of the self-similar profiles is tackled with the parabolised stability equations (PSE).

Protocells have been established as an important platform in the understanding of contemporary naturally occurring, cellular phenomena or in the study of the origin of life. By designing protocells from the bottom up, the cell complexity can be reduced to the bare minimum, which allows the research of very specific functionalities, which often mimic naturally occurring properties of cells or microcompartments. The previously published work by Kumar et al. from the group of Prof. Stephen Mann found inspiration in gas vesicles of Halobacterium salinarium, which allowed the organism to move vertically via the manipulation of the buoyant force the organism experiences. In a similar manner, Kumar et al. designed giant buoyant microcapsules which maintained their vertical motility through enzymatic control of an O₂-microbubble inside the capsule. The first experimental chapter of this thesis presents a novel protocell-system in protamine/DNA-microcapsules and compares it to the previously presented AMP/DNA-microcapsules from Kumar et al. The cells are demonstrated as very robust and easy to fabricate with high stability in various chemical environments and under physical stress. What makes the capsules so valuable for the further experiments of this thesis though is their capability of entrapping various functional components like enzymes, or micro- or nano particles within them. The second half of this chapter then investigates the entrapment capabilities, by assessing the capsule's membrane thickness, permeability and its structure microscopically to establish a thorough hypothesis on the localisation of the entrapped enzymes, their stability, how they are entrapped and what the capsule-membrane looks like. The following chapter then establishes the core functions of microcapsule motility. Entrapment of catalase and glucose oxidase enables the nucleation and consumption of an O₂-microbubble which in return causes the capsule to ascend or descend, facilitated by buoyancy. The first half of this chapter focuses on showcasing both the catalase mediated ascent and the glucose oxidase-mediated descent before combining both concepts into an oscillatory motion. Furthermore, the effect of the growing O₂-bubble on the capsule and its structural integrity is studied, which finally establishes protamine/DNA-microcapsules as superior to their predecessors. The second half of this chapter then focuses on the concept of microcapsule oscillations. Due to the complexity of the experimental design and since diffusion of spatially separated substrates plays a big role in them, the experimental devices will be explained and assessed both through computational simulation and practical diffusion studies with dyes. Stable oscillations and the novel concept of damped oscillations are presented in context to their experimental setups and are analysed through computer-assisted tracking of the microcapsule. Since the understanding of the growth- and depletion rate of the O₂-microbubble are so important, the last part of this chapter focuses on analysing the microbubble dynamics in relation to the substrate concentrations. Furthermore, enzyme leakage appeared as a reoccurring phenomenon throughout this thesis, which will be discussed regarding microcapsule oscillations and whether it causes any issues for the general legitimacy of the concept. Lastly, the third experimental chapter utilises the previously established tools to exploit microcapsule oscillations to perform a rudimentary uptake, transport and release of functional cargo. This work introduces polyoxometalate coacervate vesicles (PCVs) as secondary cargo containers, which can retain molecular cargo via sequestration, and which are subsequently loaded onto protamine/DNA-microcapsule through electrostatic- and other non-defined surface-surface interactions. Next, the uptake conditions for protamine/DNA-capsules and PCVs and the release of the cargo, which is conceptualised around disintegration of the PCVs in response to a rising pH to ~9 will be discussed. To initiate the PCV-disintegration internally, urease was entrapped and used as the trigger for PCV-disintegration through the reaction with urea. Finally, multiple full and successive uptake, transport and release cycles are presented and discussed with several different cargo species.

This thesis comprises a series of investigations into isolated vortices that exist within the atmosphere. It consists of numerical and experimental investigations backed up by mathematical analysis. The main thrust of the work is in using laminar analogues of complex phenomena to aid the understanding of the key physical processes involved. The first portion of the research concerns the dynamics of eyes (regions of reversed flow) at the centres of vortices. We expand upon previous work investigating the process of eye formation in shallow rotating convection. Through a series of numerical simulations we observe that, as thermal forcing is increased, the system undergoes a Hopf bifurcation from a steady state to one in which the eye oscillates. Examining the nature of the oscillations we propose that this behaviour results from a trapped inertial wave, providing a range of evidence to support this theory. Following on from this we present a series of laboratory investigations designed to replicate our numerical studies. In addition to examining large scale circulations we also include some observations of rotating cellular convection. Though unsuccessful in generating a steady eye, our discussions of experiment design and implementation provide a number of insights, and we hope that future experimental work will build upon this preliminary study. The latter portion of the thesis is given over to the study of thermals. We consider the life cycle of an axisymmetric laminar thermal as it transitions through a number of distinct stages undergoing several morphological changes. A significant achievement of the study is to establish a mathematical framework that can be used throughout the life cycle, allowing us to shed light on the transitions between stages and address some previously unresolved questions. Our numerical results show the early stages of development to be key in determining the final properties of the buoyant vortex ring that is produced, with thermals displaying an independence above a critical Reynolds number. Another notable observation is that the wake left behind by the first vortex ring can itself roll up to form a second ring that follows after the first. It is hoped that this framework and our observations of laminar thermals might perhaps be useful in providing new approaches for studying atmospheric convection.

The study of magnetohydrodynamics unifies the fields of fluid mechanics and electrodynamics to describe the interactions between magnetic fields and electrically conducting fluids. Flows described by magnetohydrodynamics form a significant aspect in a wide range of engineering applications, from the liquid metal blankets designed to surround and remove heat from nuclear fusion reactors, to the delivery and guidance of nanoparticles in magnetic targeted drug delivery. The ability to optimize these, and other, processes is increasingly reliant on the accuracy and stability of the numerical models used to predict such flows. This thesis addresses this by providing a detailed assessment on the performance of two electromagnetically extended Reynolds-averaged Navier-Stokes models through computations of a number of electromagnetically influenced simple channel and Rayleigh-Bènard convective flows. The models tested were the low-Re k-ε linear eddy-viscosity model of Launder and Sharma (1974), with electromagnetic modifications as proposed by Kenjereš and Hanjalić (2000), and the low-Re stress-transport model of Hanjalić and Jakirlić (1993), with electromagnetic modifications as proposed by Kenjereš and Hanjalić (2004). First, a one-dimensional fully-developed turbulent channel flow was considered over a range of Reynolds and Hartmann numbers with a magnetic field applied in both wall-normal and streamwise directions. Results showed that contributions from the electromagnetic modifications were modest and, whilst both models inherently captured some of the reduction in mean strain that a wall-normal field imposed, results from the stress-transport model were consistently superior for both magnetic field directions. Then, three-dimensional time-dependent Rayleigh-Bènard convection was considered for two different Prandtl numbers, two different magnetic field directions and over a range of Hartmann numbers. Results revealed that, at sufficiently high magnetic field strengths, a dramatic reorganization of the flow structure is predicted to occur. The vertical magnetic field led to a larger number of thinner, more cylindrical plumes whilst the horizontal magnetic field caused a striking realignment of the roll cells' axes with the magnetic field lines. This was in agreement with both existing numerical simulations and physical intuition. The superior performance of the modified stress-transport model in both flows was attributed to both its ability to provide better representation of stress generation and other processes, and its ability to accommodate the electromagnetic modifications in a more natural, and exact, fashion. The results demonstrate the capabilities of the stress-transport approach in modelling MHD flows that are relevant to industry and offer potential for those wishing to control flow structure or levels of turbulence without recourse to mechanical means.

The use of Large Eddy Simulation (LES) for modelling air flows in buildings is a growing area of Computational Fluid Dynamics (CFD). Compared to traditional CFD techniques, LES provides a more detailed approach to modelling turbulence in air. This offers the potential for more accurate modelling of low energy natural ventilation which is notoriously difficult to model using traditional CFD. Currently, very little is known about the performance of LES for modelling natural ventilation, and its computational intensity makes its practical use on desk top computers prohibitive. The objective of this work was to apply LES to a variety of natural ventilation strategies and to compile guidelines for practitioners on its performance, including the trade-off between accuracy and cost.

An experimental study has been carried out to investigate the effects of buoyancy on smoldering of polyurethane foam. The experiments are conducted with a high void fraction flexible polyurethane foam as fuel and air as oxidizer, in a geometry that approximately produces a one dimensional smolder propagation. The potential effect of buoyancy in the process is analyzed by comparing upward and downward smolder propagation through a series of normal gravity and variable gravity experiments. Both opposed and forward mixed (free and forced) flow smolder configurations are studied. In opposed smolder the oxidizer flow opposes the direction of smolder propagation, and in forward smolder both move in the same direction. Variable gravity free flow tests are also conducted in an aircraft flying a parabolic trajectories that provides low gravity periods of up to 25 sec. Measurements are performed of the smolder reaction propagation velocity and temperature as a function of the location in the sample interior, the foam and air initial temperature, the direction of propagation and the air flow velocity. This information is used in conjunction with previously developed smolder theoretical models to determine the smolder controlling mechanisms and the effect of gravity. Three zones in the fuel sample with clearly defined smolder characteristics are identified. A zone close to the igniter where smolder is affected by the external heat, a zone at the end of the sample where smolder is affected by the environment, and a zone at the end of the sample where smolder is affected by the environment, and a zone, in the middle of the foam, that is free from external effects. This last zone is the most characteristic of one dimensional, self-supported smolder, and the one that is studied in greater detail. In mixed flow convection buoyancy induced flows together with the forced flow are the primary mechanism of oxidizer transport to the reaction zone, while diffusion has a secondary importance. In natural convection, downward smoldering is of the opposed type while upward smoldering resembles more the forward type. For opposed flow smoldering, both natural and forced, the smolder propagation velocity is found to increase with the oxidizer mass flux reaching the reaction zone. This result confirms predictions from previously developed theoretical models that the smolder velocity is proportional to the oxygen mass flow. The experimental data is correlated in terms of a non-dimensional smolder velocity derived from these models, the results show very good agreement between theory and experiments for strong smolder. To implement the models, an analysis of the gas flow field is developed where the effect of significantly different permeabilities between char and foam is been Extinction is observed for very low and for very high flow rates, which shows that smolder is controlled by a sensitive competition between oxygen supply and heat losses to and from the reaction zone. Under these conditions the models do not describe the experiments well. The forward flow smolder experiments show that forward smoldering is controlled not only by the competition between oxygen supply and heat losses to and from the reaction zone but also by the competition between pyrolysis and oxidation. For low flow velocities a regime resembling the opposed flow is observed. As the air flow velocity is increased, foam pyrolysis followed by char oxidation is the controlling smolder mechanism. For both these conditions the theoretical models describe the experiments well. Increasing the flow velocity further results in a smolder propagation velocity controlled by total fuel consumption, in downward burining. For upward burning transition to flaming is observed for very high air flow velocities. This last regime is not well predicted by the theoretical models. The results from the experiments in variable gravity environment conducted in the KC-135A and Leajet airplanes confirm the normal gravity observations that the competition between heat losses and oxidizer transport is the major mechanism controlling smolder. The absence of convective flow in low gravity results in higher temperature in the unburnt fuel and char due to smaller heat losses to the surroundings. However, the oxidizer transport to the reaction zone also decreases and as a result the temperature at the reaction zone decreases indicating a weakening of the eaction, The presence of pyrolytic reactions in foward smolder and their capability to inhibit smoldering complicates the above described smolder mechanisms.

Advancements in remotely piloted aircraft systems (RPAS) introduced a new way to observe the atmospheric boundary layer (ABL). Adequate sampling of the lower atmosphere is key to improving numerical weather models and understanding fine-scale processes. The ABL’s sensitivity to changes in surface fluxes leads to rapid changes in thermodynamic variables. This study proposes using low-level buoyancy to characterize ABL transitions. Previously, buoyancy has been used as a bulk parameter to quantify stability. Higher-resolution data from RPASs highlight buoyancy fluctuations. RPAS profiles from two field campaigns are used to assess the evolution of buoyancy in convective and stable boundary layers. Data from these campaigns included challenging events to forecast accurately, such as convective initiation, a low-level jet, and katabatic flows. Results show that the ABL depth (ABLD) determined by the minimum in vertical buoyancy gradient agrees well with proven ABLD metrics, such as potential temperature gradient maxima. Moreover, in the cases presented, low-level buoyancy rapidly increases prior to convective initiation and rapidly decreases prior to the onset of a low-level jet. This study expounds on the utility of buoyancy in the ABL and contextualizes its use in comparison to Richardson number profiles. Additionally, RPAS profiles are reviewed as an operational way to aid the forecasting of aviation weather. The concept of operations is described. Based on conversations with partners in the aviation community, a visualization of the RPAS data was designed to deliver the most desirable information. Restrictions on field campaigns caused by the COVID19 pandemic impeded the goal of regular profiling at a nearby airport. Nevertheless, a handful of scenarios are assessed from the perspective of a pilot. Finally, a discussion is provided on the complications of flying an RPAS at an active airport.

The rise of soft robots poses a promising revolution across a variety of fields, such as invasive surgical procedures or aquatic animal monitoring and sampling, by providing a softer solution to delicate problems. However, with their youth comes a need for growth, particularly in regard to increasing mobility in aquatic environments seeing as motion is often slow and belabored. Additionally, exit strategies in breaking the air-water interface are not thoroughly explored to date. To address these challenges, this study looks to bioinspiration for the answer in the form of Emperor Penguins. By utilizing microbubbles in their plumage to decrease drag forces on their bodies, Emperor Penguins are able to propel themselves out of the water to heights not theoretically achievable through buoyancy alone. Not only is the strategy highly effective, it lends well to the soft robotic field as pneumatic actuation is a commonly used mechanism of locomotion. To explore this behavior and simulate its effects, this study tests a hollow silicone ellipsoid with hole punctures applied to its surface for microbubble release. Bubble characteristics such as separation point, bubble diameter, and downstream bubble expansion were monitored when subjected to a fluid flow to determine ideal air pressure through the ellipsoid body. Drag reduction is tested by measuring the robot’s leap height out of the water.

Artificial micro/nanomotors (MNMs), inspired by mobile biomolecular entities, have demonstrated great potential as miniaturized robots performing diverse tasks from environmental remediation to biological treatment owing to their great mobility and versatility. The reported MNMs can be propelled using various power sources, including magnetic field, electric field, ultrasound, light, and chemical reaction. MNMs that operate on chemical reactions are usually equipped with higher velocity due to the superior energy conversion efficiency, which dominantly present as bubble propelled systems. However, the majority of the bubble propelled MNMs utilize bubble ejection and detachment force, which result in swarming and linear motion for Janus and tubular motors, respectively. It is still challenging for chemical propelled MNMs to have absolute control in direction without external field. A crafty design to circumvent this limitation is to develop biocatalytic MNMs with bubble buoyancy propulsion. This thesis focuses on the design, fabrication, and applications of submarine-like buoyancy-propelled MNMs that move in the vertical direction. I fabricated buoyancy propelled nanomotors with one-pot synthesis and provided the first work characterizing detailed motion behavior with electrochemistry. With coupled biocatalytic cascade reaction converting glucose as the fuel to oxygen bubbles, the nanomotor was propelled by buoyancy, which dominated the initiative collision at the electrode surface. Four representative electrical impact signals were observed and corresponded to four types of motion patterns. The corresponding relationship was confirmed with a numerical simulation. The integration of MNMs and electrochemistry provided a new dimension to characterize and understand the complex dynamics of the self-propelled nanoparticles. I further investigated the buoyancy propelled MNMs in biomedical applications. The pH-sensitive polymer incorporated micromotor exhibited regulated vertical motion via hydrophilic/hydrophobic phase shifting in different pH environments, and the system was proved to be applicable for anti-cancer drug delivery in a proof-of-concept three-dimensional cell culture. The proposed micromotor opens up new avenues in autonomous robotic fabrication for in vivo drug delivery in complex media. I also investigated the buoyancy propelled MNMs in water remediation. The buoyancy propelled nanomotor exhibited reversible vertical motion in low concentrations of H2O2, which induced the convection of the micro-environment and increased the pollutants to get in contact with the absorbents. The proposed nanomotor showed efficient removal of both inorganic heavy metal ions and organic per- and poly-fluoroalkyl substances (PFAS) in complex environments. At last, the buoyancy propelled MNMs were studied for the vertically spatial separation of targeted cancer cells in mixed samples. With the aid of antibody surface modification, the buoyancy-propelled nanomotors can autonomously attach to the targeted cells and endow the cancer cells with vertical motion. With a customized glass tube, the floated cells can be easily separated. The proposed nanomotor exhibited great isolating efficiency with facile operations, which broadened the development of cell separation methods towards biocompatible nanostructures. The findings presented in this thesis open up new avenues for the development of buoyancy propelled MNMs in diverse applications.

Ammonoid cephalopods have chambered shells that regulated buoyancy. The morphology of their shells strongly influenced the physical properties acting on these animals during life. Heteromorph ammonoids, which undergo changes in coiling throughout ontogeny, are the focus of this dissertation. The biomechanics of these cephalopods are investigated in a framework involving functional morphology, paleoecology, and possible modes of life.Constructional constraints were investigated for the marginally-corrugated septal walls within the chambered ammonoid shell. These constraints governed the positive relationship between septal complexity and terminal size. Furthermore, increased septal complexity facilitated liquid retention via surface tension. More complex septa would have increased liquid retention at larger scales, which could have been used as liquid ballasts, reserves for buoyancy adjustment, or to prevent disruptive sloshing of cameral liquid.New methods for the virtual reconstruction of cephalopod shells are described. The shell constrains the volume and shape of each material of unique density that influenced organismal mass and its distribution. Therefore, these virtual models can be used to compute the conditions for neutral buoyancy, hydrostatic stability, syn vivo orientation, and the directional efficiency of movement. The rigid shell also constrains how the living ammonoid would have interacted with fluids in a dynamic setting. A new method for the construction of neutrally-buoyant, physical models is described, which can be used to compute hydrodynamic properties such as drag and swimming velocity.The biomechanics of three heteromorph ammonoid morphotypes from the North American Western Interior Seaway are discussed. These morphotypes represent the families Baculitidae, Nostoceratidae, and Scaphitidae. These investigations provide a better understanding of the hydrostatic and hydrodynamic properties that constrained the modes of life for the majority of prominent ammonoid taxa from the Late Cretaceous of the U.S. Western Interior. Novel modeling techniques provide data that suggests heteromorph ammonoids had selective pressures imposed on them for specific biological functions or life habits, rather than pointless morphological experimentation or liberation from such evolutionary pressures. These syn vivo physical properties also influenced the biogeographic dispersal and paleoecology for these enigmatic creatures that were once vital components of marine ecosystems.

Active control of combustion has always been important given the potential practical applications to improve efficiency and stability, and to reduce unwanted emissions. Body force effects, such as applied electric fields and buoyancy, have been objects of attention given their capability to change small flame behavior. However, distinguishing between different body force effects and then controlling those body forces to affect combustion performance in predictable ways is a challenge that has not yet been solved. In addition to the inherent difficulties defining the flame chemistry in thermally driven buoyant flows, there is the further complication of capturing the relationship between flame charged species chemistry and the ultimate physical influences produced by electromagnetic forces on them. This work presents the development of numerical simulation tools to provide more insights into the coupling between flame behavior, flame chemistry, and different body force effects. Both buoyancy and electric field effects are explored. Also, the comparison of the resultant flame behavior when these body forces are applied to the flame is analyzed to provide possible equivalences in body force control, as well as to examine what might occur during combustion in alternative gravitational environments. There are four major parts in this work. The first one relates to the chemical kinetic model for the flame chemistry with charged and excited species; the second looks into the effects that two different jet burner geometries have on a flame that is exposed to different gravity environments; the third section explores the implementation of applied electric fields in the simulations and how this affects a non-premixed flame at 1g; and the last section carries out a comparison between the effects on flame behavior when body forces of different nature, buoyancy and electric field force, are present. This last section also provides insight into the possible similarities and regimes when both forces could be equivalent. This work starts with the development of a comprehensive chemical kinetic model including excited and charged species. Excited species are rarely included in combustion chemical kinetic models due to their low concentrations and low impact on major chemical pathways in the flame, yet they are crucial for the visual characterization of the flame. Chemiluminescent species CH* and OH* are added to the model since they are well-known markers for flame location and heat release. Additionally, naturally produced charged species in the flame are also included in the chemical kinetic model developed in this work since they will be interacting with the electric field when this is applied to the flame. Considered as the first reaction that naturally produces charged species in the flame, the chemi-ionization reaction -- as well as the subsequent reactions for charged species -- is also included in the chemistry model. Then, the model is validated against detailed chemistry and experimental literature results employing one- and two-dimensional burner geometries. The final reduced model, named Model 1, contains a total of 45 species and 216 reactions. The second part of this work reproduces how buoyancy forces affect the flame. For that purpose, simulations under gravities that vary from 0g to 2g are performed employing two-dimensional CFD calculations. The results are then contrasted with previous experimental and numerical literature. A comparison between both different gravities and different burners is performed, as well as a non-dimensional analysis of the behavior of the flame. Next, an implementation of the electric field solver to the 1g two-dimensional flame geometry is explored. The results are compared with experimental literature, showing similarities for the major characteristics, such as for the I-V curves and the downstream ion current spatial distribution. However, a detailed investigation of the electric field characteristics reveals that a non-physical behavior is obtained for the physical distribution of the charged species when employing the solving method proposed for the subsaturation regime, which is also discussed. Finally, an analogy between body forces and the possible similarities among them is analyzed. This comparison corroborates previous literature mentioning that, even though electric and buoyancy forces are from a different nature, they might be considered equivalent when applying an electric field in a 1g flame to achieve an equivalent supergravity flame. It is concluded that the 2g flame resembles the 1g flame with 0.5kV/cm applied. However, a large regime of supergravity conditions where both forces are equivalent is yet to be explored.

Understanding magnetohydrodynamic (MHD) phenomena associated with complex duct geometries and buoyancy effects is required to effectively design liquid metal (LM) blankets for fusion reactors. These topics are investigated in the present work by numerically simulating 3D LM MHD flow using HIMAG (HyPerComp Incompressible MHD solver for Arbitrary Geometry), a code developed by HyPerComp with support from UCLA. In Part I of this dissertation, the simulated geometry is a manifold consisting of a rectangular feeding duct which abruptly expands along the applied magnetic field direction to distribute LM into several parallel channels. As a first step in qualifying the flow, a magnitude of the curl of the induced Lorentz force is used to distinguish between inviscid, irrotational core flows and boundary and internal shear layers where inertia and/or viscous forces are important. Scaling laws are obtained which characterize the 3D MHD pressure drop and flow balancing as a function of the flow parameters and the manifold geometry. Associated Hartmann (Ha) and Reynolds (Re) numbers in the computations are ~10^3 and ~10^1-10^3 respectively while the expansion ratio is varied from 4 to 12. An accurate model for the pressure drop is developed for the first time for inertial-electromagnetic and viscous-electromagnetic regimes based on 96 computed cases. Analysis shows that increasing the distance between the manifold inlet and the entrances of the parallel channels can improve flow balance by utilizing the effect of flow transitioning to a quasi-two-dimensional state in the expansion region of the manifold. Lastly, a Resistor Network Model is developed to describe the effect of the length of the poloidal channels on flow balancing in LM manifold. As the poloidal channels lengthen, the flow balance improves.The simulated geometry in Part II consists of a straight, vertical duct which runs perpendicular to a strong, fringing applied magnetic field. There is also a region of applied heating as the primary goal of Part II is to explore buoyancy effects in MHD duct flows. The unsteady 3D MHD equations are solved using HIMAG. Results are presented for both upwards and downwards flows in electrically conducting (wall conductance ratio cw=0.12) and nonconducting ducts for a range of Ha~10^2, Re~10^3-10^4, and Grashof (Gr) numbers~10^7-10^8. While increasing Gr or decreasing Re increases buoyancy effects, increasing Ha was shown to increase maximum temperature by enhancing flow stability. The extent to which the flow is quasi-2D is analyzed and buoyant effects, in competition with Joule dissipation, are shown to bring about 3D flow features and newly discovered MHD mixed convection phenomena. Steeply diminishing volumetric heating, which approximates nuclear heating, is applied to the vertical MHD flows for comparison to flows with surface heating only. Surface heating generates stronger buoyancy effects than volumetric heating of the same total power; however, many of the same phenomena occur. Therefore, surface heating, the only option for lab experiments, can be useful in exploring the effects of volumetric heating in MHD flows. Lastly, the results of a surface heating case are presented for the purpose of comparison with other codes and experiments, especially the MaPLE-U experiment that is currently is underway at UCLA.

In light of growing concerns over rising atmospheric concentrations of greenhouse gases like carbon dioxide (CO₂), carbon capture and storage (CCS) has been suggested as a means to reduce the rate of net addition of CO₂ to the atmosphere. One potential CCS method involves injecting CO₂ into deep saline aquifers, where they are designed to reside for long periods of time. High-pressure and high-temperature CO₂/brine flow through porous media is the subject of active research, but faithfully recreating the conditions and forces found deep in the subsurface remains a challenge. In particular, the role of buoyant forces in transporting CO₂ must be studied further, since the long-term migration of CO₂ is dominated by buoyancy. This study consists of two parts. Chapter 1 discusses buoyancy as relevant to the context of CO₂ sequestration and prior methods used to study buoyancy-dominated flow. Four methods to experimentally recreate buoyancy-driven flow in high-pressure corefloods are presented: “inject low and let rise,” progressive pressure increase, simplified Darcy’s Law, and the Buckley-Leverett approach. Chapter 2 investigates the potential of using surface-coated silica nanoparticles to improve the conformance of CO₂ during flow through aquifers. The Buckley-Leverett approach is used to determine a single buoyancy-driven flow rate, and a vertical coreflood is conducted using this flow rate. Core-average saturation and pressure drop measurements across the core are measured, and the in-situ CO₂ distribution is visualized by taking axial X-ray CT scans of the core during the experiment. The effect of the nanoparticles is studied by conducting the experiment with three different nanoparticle concentrations: 0 wt% (as a control), 0.5 wt%, and 5 wt%. The addition of 0.5 wt% of nanoparticles (NP) does not markedly improve the conformance of CO₂ when compared to the control. However, at concentrations of 5 wt% NP, steady-state and residual CO₂ saturation increases, sweep efficiency increases, and CO₂ mobility decreases significantly when compared to the control. The lack of effectiveness of the 0.5 wt% formulation may be due to the influence of perpendicular-to-flow bedding layers that are present in the cross-bedded sandstone core used in the experiments. There are mixed indications regarding the suitability of the Buckley-Leverett approach to predicting the buoyancy-driven flow regime.

Moist convective clouds play a key role in the earth's energy and water cycles, but their turbulent and localized nature makes it difficult to study how they interact with the large-scale atmospheric circulation. In this dissertation, we study the interaction between convection and the large-scale tropical atmosphere in two ways. First, we develop simple analytical models for how convection interacts with large-scale circulations in the tropical atmosphere. These models are primarily based on a traditional-but-crude approximation of gravity wave dynamics, in which the tropopause is assumed to be a rigid lid. We test these models in cloud-resolving numerical simulations of a small patch of tropical atmosphere and determine that some of these methods pass a simple test of self-consistency, but are far from perfect.Alternately, we take a more sophisticated analytical approach-- instead of using the traditional rigid lid model of the tropical atmosphere, we derive novel analytical solutions for pulses of buoyancy (a crude representation of the effects of convective clouds) in an atmosphere with both a troposphere and a stratosphere. This construction allows wave energy to realistically radiate out of the troposphere. We find that allowing wave energy to radiate out of the troposphere causes buoyancy anomalies in the troposphere to decay on timescales of hours to days, in stark contrast to the rigid lid model, which predicts that buoyancy anomalies persist forever in the absence of dissipation.

The purpose of this study was to examine the conceptual understandings of 55 elementary preservice teachers for the concept of buoyancy. This study used Ausubel’s Assimilation Theory (Ausubel, 1963) as a framework for a 15-week intervention that used pre/post concept maps (Cmaps), pre/post face-to-face semi-structured interviews, and drawings as evidences for change of formation of cognitive structures. Using a convergent parallel design and mixed methods approach, preservice teachers’ conceptions were analyzed using these evidences. Results of the study show that preservice teachers held both scientific conceptions and misconceptions about buoyancy as a force before and after an instructional intervention. Of importance were the existence of robust misconceptions about buoyancy that included inaccurate scientific knowledge about the foundational concepts of gravity, weight, mass, and density. The largest gains in scientific knowledge included the concepts of gravity, surface area, opposing forces, and the buoyant force. These concepts were consistently supported with evidence from post-concept maps, post, semi-structured interviews, and drawings. However, high frequencies of misconceptions were associated with these same aforementioned concepts as well as additional misconceptions about buoyancy-related concepts (i.e., weight, density, displacement, and sinking/floating). A paired t test showed a statistically significant difference (t = -3.504, p = .001) in the total number of scientifically correct concepts for the pre-concept maps (M = 0.51, SD = .879) and post-concept maps (M = 1.25, SD = 1.542). The Cohen’s d effect size was small, .47. Even through gains for the pre/post concept maps were noted, a qualitative analysis of the results indicated that not only were there serious gaps in the participant’s scientific understanding of buoyancy, after the instructional intervention an increased number of misconceptions were presented alongside the newly learned concepts. A paired t test examining misconceptions showed that there was a statistically significant difference (t = -3.160, p = .003) in the total number of misconceptions for the pre-concept maps (M = 2.709, SD = 1.449) and post-concept maps (M = 3.363, SD = 2.094) after the intervention. The Cohen’s d effect size was small, .43. Taken together, these results revealed that, in general, the preservice teachers had understandings of buoyancy that align with children in preschool and elementary school (Biddulph and Osborne, 1983; Grimellini-Tomasini et al., 1990; Halford, Brown & Thompson, 1986; Hsin and Wu, 2011; Kohn, 1993; Rappolt-Schlichtmann et al., 2007; Yin et al., 2008). Based on these findings, implications for this study suggest that elementary preservice teacher candidates should be carefully screened to ensure they have mastered foundational scientific knowledge that they are expected to teach to children. As such knowledge is a prerequisite to the development of pedagogical content knowledge, it is unlikely that large numbers of robust misconceptions will be significantly reduced or eliminated during a science methods course that is designed to focus on pedagogical content knowledge.

‘Cold pools’ are pools of air that have been cooled by rain evaporation, and which subsequently slump down and spread out across the Earth’s surface due to their negative buoyancy. Such cold pools, which typically arise from rain produced by convection, also feed back upon convection by kicking up new convection at their edges.This thesis studies the interaction of cold pools and convection at two levels of detail: on one end, we study the dynamics and thermodynamics of a single, idealized cold pool, and on the other, we study the interplay between a steady-state ensemble of convection and the many cold pools that accompany it. A recurring notion is that of ‘effective buoyancy’, which is the net acceleration experienced by a density anomaly such as a cold pool, including the back-reaction of the environment (i.e. the ‘virtual mass effect’) which reduces the net acceleration from its Archimedean value. We derive analytical formulae for the effective buoyancy of cold pools and other roughly cylindrical density anomalies, and use the same framework to understand the forces at play when cold pools trigger new convection. We also analyze the sizes and lifetimes of cold pools, and examine the impact of cold pools on the organization (i.e. clustering) of convection.

The primary objectives of this research were to understand the potential for gravity-stable surfactant floods for enhanced oil recovery without the need for mobility control agents and to optimize the performance of such floods. Surfactants are added to injected water to mobilize the residual oil and increase the oil production. Surfactants reduce the interfacial tension (IFT) between oil and water. This reduction in IFT reduces the capillary pressure and thus the residual oil saturation, which then results in an increase in the water relative permeability. The mobility of the surfactant solution is then greater than the mobility of the oil bank it is displacing. This unfavorable mobility ratio can lead to hydrodynamic instabilities (fingering). The presence of these instabilities results in low reservoir sweep efficiency. Fingering can be prevented by increasing the viscosity of the surfactant solution or by using gravity to stabilize the displacement below a critical velocity. The former can be accomplished by using mobility control agents such as polymer or foam. The latter is called gravity-stable surfactant flooding, which is the subject of this study. Gravity-stable surfactant flooding is an attractive alternative to surfactant polymer flooding under certain favorable reservoir conditions. However, a gravity-stable flood requires a low velocity less than the critical velocity. Classical stability theory predicts the critical velocity needed to stabilize a miscible flood by gravity forces. This theory was tested for surfactant floods with ultralow interfacial tension and found to over-estimate the critical velocity compared to both laboratory displacement experiments and fine-grid simulations. Predictions using classical stability theory for miscible floods were not accurate because this theory did not take into account the specific physics of surfactant flooding. Stability criteria for gravity-stable surfactant flooding were developed and validated by comparison with both experiments and fine-grid numerical simulations. The effects of vertical permeability, oil viscosity and heterogeneity were investigated. Reasonable values of critical velocity require a high vertical permeability without any continuous barriers to vertical flow in the reservoir. This capability to predict when and under what reservoir conditions a gravity-stable surfactant flood can be performed at a reasonable velocity is highly significant. Numerical simulations were also used to show how gravity-stable surfactant flooding can be optimized to increase critical velocity, which shortens the project life and improves the economics of the process. The critical velocity for a stable surfactant flood is a function of the microemulsion viscosity and it turns out there is an optimum value that can be used to significantly increase the velocity and maintain stability. For example, the salinity gradient can be optimized to gradually decrease the microemulsion viscosity. Another alternative is to inject a polymer drive following the surfactant solution, but using polymer complicates the process and adds to its cost without significant benefit in most gravity-stable surfactant floods. A systematic approach was introduced to make decisions on using polymer in applications based on stability criteria and cost. Also, the effect of an aquifer on gravity-stable surfactant floods was investigated as part of a field-scale study and strategies were developed to minimize its effect on the process. This study has provided new insights into the design of an optimized gravity-stable surfactant flood. The results of the numerical simulations show the potential for high oil recovery from gravity-stable surfactant floods using horizontal wells. Application of gravity-stable surfactant floods reduces the cost and complexity of the process. The widespread use of horizontal wells has greatly increased the attractiveness and potential for conducting surfactant floods in a gravity-stable mode. This research has provided the necessary criteria and tools needed to determine when gravity-stable surfactant flooding is an attractive alternative to conventional surfactant-polymer flooding.

This study provides independent analysis of Residual Oil Zones (ROZs) in the Permian Basin from a regional perspective, focusing on the formation mechanism and present ROZ locations. Results demonstrate widespread potential for ROZs, defined here as thick volumes of reservoir rock containing near-residual saturations of predominantly immobile oil formed by natural imbibition and displacement of oil by dynamic buoyant or hydrodynamic forces. Previous work suggests hydrodynamic forces generated by regional tectonic uplift drove widespread oil remobilization and ROZ creation. To test the hypothesis, uplift and tilting are quantified and the resulting peak regional potentiometric gradient used as a physical constraint to compute and compare predicted ROZ thicknesses from hydrodynamics for several ROZ-bearing San Andres fields with known ROZ thicknesses. Late-Albian Edwards Group geologic contacts, which are interpreted to have been deposited near sea level prior to uplift, are used as a regional datum. Approximate elevations determined for the present datum show ~1800 m of differential uplift since Edwards deposition, with an average regional slope of ~0.128˚. This post-Edwards tilting increased the pre-existing regional structural gradient of the San Andres Formation to ~0.289˚. Using the calculated post-Edwards gradient results in to prediction of ROZ thicknesses from hydrodynamics that is consistent with measured ROZ thicknesses at several fields. When compared with countervailing buoyancy forces, hydrodynamics is calculated to be the more dominant driving force of oil movement for reservoirs with structural dips less than 1.5˚, which is the common dip for San Andres Formation platform deposits where ROZs have been identified. To predict the location of ROZs, ROZ-related oil field properties were identified and analyzed for over 2,800 Permian Basin reservoirs. A strong basin-wide correlation between API and crude sulfur content is consistent with the expected outcome of oil degradation driven by oil-water interaction, and supports the use of API and sulfur content as proxies for ROZ potential in the Permian Basin. Spatial analysis of sulfur data shows that the highest probability for ROZ existence exists in Leonardian through Guadalupian-age reservoirs, distributed primarily in shelf and platform areas of Permian structures. Combined, these results support the widespread potential for ROZs across the Permian Basin generated primarily by regional scale tilting and resultant hydrodynamic forces.

ABSTRACT This Master s by Research thesis is an Industry Oriented Project, and the Researcher is a full-time employee in an Australian Polyurethane Company as a Senior Chemical Engineer. The thesis examines the application viability of the Methyl formate blown rigid foams. In this regard the real trial applications, e.g. pipe filling, rigid foam spray and moulding have been conducted, which are commonly adopted in making most of the end products. In this thesis the physical properties that are most crucial from application point of view have been focused like dimensional stability, Compressive Strength, close cell contents, thermal conductivity and buoyancy. Comparison of few of these properties is also done, where needed, between the rigid foams blown with Methyl formate Vs 141b - otherwise only Methyl formate blown foam is focused. The effects of different parameters on thermal conductivity of the foam have also been examined which is the most important characteristic of the foam used for insulation purpose. All these application trials have been found quite satisfactory and also achieved satisfactory results in respect to all those required characteristics of the foam e.g. flowability in pipe filling trial, thermal conductivity in insulation spray trial, dimensional stability and close cell contents, compressive strength in Buoyancy loss test. It is obvious and proved that the lower thermal conductivity value of 141b compared to that of Methyl formate and Carbon dioxide gives lower thermal conductivity values both in spray foams and the hand pour. But the Methyl formate blown foams also show quite reasonable values to make it a choice as insulation foam. The Methyl formate blown spray insulation foams are successfully being produced and sold in the market by FSI USA and Australian Urethane Systems in Australia. Their trade name for Methyl formate is ECOMATE. In buoyancy test the rigid PU foam with moulded density range 40-42 kg/m3 gave close cell content greater than 95% and compressive strength of the range 0.230 MPa to 0.330 MPa and dimensional stability well within the spec as required for floatation vessels. The % buoyancy loss of the foam immersed for 30 days in pure water was found -3.9551 % and in 5% trisodium phosphate water solution was -4.2423%. This promising buoyancy results make the Methyl formate blown rigid foam the good choice for floating vessels body cavity filling as well. In a nutshell Methyl formate blown rigid foams are the good choice of the time.

Welding is used extensively in aerospace, automotive, chemical, manufacturing, electronic and power-generation industries. Thermally-induced residual stresses due to welding can significantly impair the performance and reliability of welded structures. Numerical simulation of weld pool dynamics is important as experimental measurements of velocities and temperature profiles are difficult due to the small size of the weld pool and the presence of the arc. From a structural integrity perspective of welded structures, it is necessary to have an accurate spatial and temporal thermal distribution in the welded structure before stress analysis is performed. Existing research on weld pool dynamics simulation has ignored the effect of fluid flow in the weld pool on the temperature field of the welded joint. Previous research has established that the weld pool depth/width (D/W) ratio and Heat Affected Zone (HAZ) are significantly altered by the weld pool dynamics. Hence, for a more accurate estimation of the thermally-induced stresses it is desired to incorporate the weld pool dynamics into the analysis. Moreover, the effects of microstructure evolution in the HAZ on the mechanical behavior of the structure need to be included in the analysis for better mechanical response prediction. In this study, a three-dimensional model for the thermo-mechanical analysis of Gas Tungsten Arc (GTA) welding of thin stainless steel butt-joint plates has been developed. The model incorporates the effects of thermal energy redistribution through weld pool dynamics into the structural behavior calculations. Through material modeling the effects of microstructure change/phase transformation are indirectly included in the model. The developed weld pool dynamics model includes the effects of current, arc length, and electrode angle on the heat flux and current density distributions. All the major weld pool driving forces are included, namely surface tension gradient, plasma drag force, electromagnetic force, and buoyancy. The weld D/W predictions are validated with experimental results. They agree well. The effects of welding parameters (like welding speed, current, arc length, etc.) on the weld D/W ratio are documented. The workpiece deformation and stress distributions are also highlighted. The transverse and longitudinal residual stress distribution plots across the weld bead and their variations with welding speed and current are also provided. The mathematical framework developed here serves as a robust tool for better prediction of weld D/W ratio and thermally-induced stress evolution and distribution in a welded structure by coupling the different fields in a welding process.

It is incumbent upon teachers to reach out to students through methods that capitalize on the students' own motivations. Because of the diversity of self-referential personal styles of learning, reaching every student with a cookie-cutter approach to teaching is nearly impossible. This report explores the application of a type of problem-based learning known as "Legacy Cycles" that apply web technology to answer challenges presented as scenarios. The scenarios give students a similar experience to scientists pursuing investigation and research. Students then search for answers to questions, learn more about the processes being taught with hands-on activities, and prepare a product to demonstrate mastery of the content. In this example of the Legacy Cycle, three challenges are used to teach concepts of density, ocean currents and plastic pollution.

Leakage of stored bulk phase CO₂ is of particular risk to sequestration in deep saline aquifers due to the fact that when injected into typical saline aquifers, the CO₂ rich gas phase has lesser density than the aqueous phase resulting in buoyancy driven flow of the fluids. As the CO₂ migrates upward, the security of its storage depends upon the trapping mechanisms that counteract the migration. While there are a variety of trapping mechanisms the mechanism serving as motivation for this research is local capillary trapping. Local capillary trapping occurs during buoyancy-driven migration of bulk phase CO₂ within a saline aquifer (Saadatpoor, 2009). When the rising CO₂ plume encounters a region where capillary entry pressure is locally larger than average, CO₂ accumulates beneath the region. While research is continued by means of numerical simulation, research at the bench scale is needed to validate the conclusions made from simulation work. Presented is the development of a bench scale experiment whose objective is to assess local capillary trapping. The initial step in accomplishing this objective is to understand the fluid dynamics of CO₂ and brine in a saline aquifer which is categorized as two phase immiscible buoyancy driven displacement. Parameters influencing this displacement include density, viscosity, wettability and heterogeneity. A bench scale environment created to be analogous to CO₂ and brine in a saline aquifer is created in a quasi-two dimensional experimental apparatus, which allows for observation of plume migration at ambient conditions. A fluid pair analogous to supercritical CO₂ and brine is developed to mimic the density and viscosity relationship found at pressure and temperature typical of storage aquifers. The influences of viscosity ratio, density differences, porous medium wettability and heterogeneity are observed in series of experimental sequences. Three different fluid pairs with different viscosity ratios and density differences are used to assess density and viscosity influences. Porous media of varying grain size and wettability are used to assess the influence of heterogeneity and wettability. Results are qualitatively consistent with theoretical results and those from previous works.

Hurricanes rapidly destroy large expanses land in coastal Louisiana marsh. Research shows that freshwater marsh with organic soils experience increased destruction during hurricanes compared to other marsh. A relevant question surfaces, do some restoration projects create marsh similar to marshes that are more susceptible to hurricane damage. This study analyzes soil, bulk density, plant composition, and buoyancy of restoration projects and sites adjacent to those that experienced land loss during Hurricanes Katrina and Rita. Results indicate that high organic matter percentages in marsh soil increases hurricane susceptibility attributed to decreased bulk density and increased buoyancy. Buoyancy is episodic and is highest during late summer months when soil temperature and decomposition are highest. Late summer is typically when most intense hurricanes occur. If marsh is less dense, decomposing, and buoyant when strongest hurricanes hit, then potential for destruction during a hurricane increases. Samples were collected from August 2009 to October 2009.

Experimental measurements of the lift-off velocity and lift-off height, and numerical simulations were conducted on the liftoff and stabilization phenomena of laminar jet diffusion flames of inert-diluted C3H8 and CH4 fuels. Both non-reacting and reacting jets were investigated, including effects of multi-component diffusivities and heat release (buoyancy and gas expansion). The role of Schmidt number for non-reacting jets was investigated, with no conclusive Schmidt number criterion for liftoff previously known in similarity solutions. The cold-flow simulation for He-diluted CH4 fuel does not predict flame liftoff; however, adding heat release reaction leads to the prediction of liftoff, which is consistent with experimental observations. Including reaction was also found to improve liftoff height prediction for C3H8 flames, with the flame base location differing from that in the similarity solution - the intersection of the stoichiometric and iso-velocity contours is not necessary for flame stabilization (and thus lift-off). Possible mechanisms other than that proposed for similarity solution may better help to explain the stabilization and liftoff phenomena. The stretch rate at a wide range of isotherms near the base of the lifted tribrachial flame were also quantitatively plotted and analyzed.

The evolving requirements for ballast water treatment to reduce the probability of the introduction of additional nonindigenous aquatic species into U.S. coastal waters and the Great Lakes will result in increased capital and operating costs for trans-oceanic vessels. As an alternative to ballast treatment systems, a new ship concept that will essentially eliminate the trans-oceanic transport of ballast water is investigated in this dissertation. The Ballast-Free Ship concept involves a new paradigm that approaches ballast operation as the reduction of buoyancy rather than the addition of weight to get the vessel to its required ballast drafts. The traditional ship ballast tanks are replaced by longitudinal structural ballast trunks that surround the cargo hold below the ballast draft. These trunks, which are connected to an intake plenum near the bow and a discharge plenum near the stern, are flooded in the ballast condition to decrease the ship's buoyancy. The pressure differential between the bow and the stern is utilized to drive a slow flow through the ballast trunks to ensure that the trunks always contain local seawater. At the end of the ballast voyage, the trunks are isolated and pumped dry using conventional ballast pumps. This dissertation describes the development and investigation related to this new ship concept. The available pressure differential as well as the development of the ballast water flow in the trunks is established through a number of simulations utilizing Computational Fluid Dynamics (CFD). The impact on ship resistance and propulsion is investigated through towing tank experiments as well as CFD simulations. The required structural re-arrangements to incorporate the Ballast-Free Ship concept are thoroughly examined. The effect of the structural re-design on damage survivability, intact stability and seakeeping performance of the vessel is evaluated. The economic impact of the proposed concept is compared with alternative ballast water treatment methods. The results show that the Ballast-Free Ship concept could provide a viable, more cost effective alternative to ballast treatment systems. Furthermore, it could provide superior protection from the introduction of nonindigenous aquatic species through ballast water.

A numerical study has been carried out on the natural convection in a vertical slot with a narrow upper section. The fluid in the slot is initially at a uniform temperature and motionless. The left walls are subjected to a step change in temperature while the right walls are kept at the initial temperature. The top and bottom are insulated. The resulting flow pattern and heat transfer are studied for different Rayleigh and Prandtl numbers, Ra and Pr, respectively. A program is developed for the simulation of flow and temperature fields by using the projection or fractional-step formulation which incorporates a Godunov-type discretization of convective terms. The unsteady Navier-Stokes equations under the Boussinesq approximation are solved with primitive variables. Different flow regimes are obtained depending on Ra and Pr. The flow regimes observed change from conduction-dominated to convection-dominated with increasing Ra. The steady flow for water (Pr = 6), alcohol (Pr = 20) and viscous oil (Pr = 100) at Ra = 10$\sp4$ is found to be independent of Pr although the time evolution of the flow is Pr-dependent. In the limit of very small Prandtl numbers, a series of runs have been performed to predict the onset of oscillations and the transitions to unsteady natural convection in a fixed geometry. Transitions from two to one, from one to three, and from three to two cell structure in the flow are observed depending on Grashof number. The effect of narrow upper section is examined as a function of the ratio of its height to the total height. It is found that the strength of the circulation in the narrow section increases for air and water as the height ratio increases whereas for limit case of small Pr, it is weak over the entire range of the height ratio considered. Comparison of the results between rectangular and present configuration with the height ratio of 1.3/7 reveals that the liquid metals are most significantly affected by the narrow section: as opposed to two cells observed in a rectangular cavity, three cells are found in the wide section.