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An experimental and numerical study has been performed on a 4-inch globe control valve with the objective of diagnosing local flow behavior and quantifying the different flow parameters to the total valve performance. A computer-based data acquisition system, coupled with pressure transducers, were used to obtain experimental measurements of both the upstream and downstream pressures to calculate pressure drop across the valve. The acquired data was used to validate the results of numerical simulations. Four models of single stage multi-hole trims with different hole sizes were numerically studied. The pressure-drop and hence flow coefficient (Cv) of individual holes were quantified. Flow coefficient (Cv) is defined as defined as the volume of water in gallons per minute (GPM) at 60°F that will flow through a fully open valve with a pressure differential of 1 psi across the valve. The flow behavior around the trim was obtained, and these provided valuable insight into the mechanisms that determine the performance of a multi-hole trim. These were used to establish the relationship between the flow coefficient of the valve with passageways in the cage. The focus of the study was directed towards the development and optimization of a single stage multi-hole trim for a globe control valve. Relationships between parameters such as trim hole diameter, hole height (row position) and hole angular position were examined extensively. The results were used to develop a series of analytical expressions to represent the effect of each geometrical feature and predict local Cv. It is envisaged that proposed expressions will feed into current design methodologies for control valve multi-hole trims which offers the prospect of improved overall performance. The final part of the study was to optimise the control valve through a least cost method where the valve selection is based on total cost as a function of valve diameter. A case study example is discussed to show the control valve life cycle cost which has been incorporated in the optimised design.

Complex terrain covers a large fraction of the land surface of the Earth. Due to the limited uses of steeply sloped terrain a lot of this is undeveloped and often covered with a canopy of trees or shrubs. Vegetation canopies are historically modelled using a roughness length at the surface, in line with Monin-Obukhov similarity theory. While this approach has been shown to be effective for shallow canopies such as crops and grasses, for deep canopies such as mature trees this method does not replicate the within and above canopy flows accurately. New approaches were developed in the 1990s to model the canopy explicitly and with a true vertical extent. These models account for the sink of momentum and turbulent kinetic energy within the canopy. In the present work, terms representing these effects are added to these equations in the open-source Weather Research and Forecasting model so that numerical simulations can be performed. This implementation of the canopy model is then tested against a benchmark, idealised case of measurements from a wind-tunnel experiment with an artificial canopy on a ridge with a two-dimensional profile. The model is found to provide significant improvements over simulations using an identical setup but with the canopy parameterised using increased roughness at the surface. The impact of resolution on the effectiveness of the model is also explored, with higher resolutions providing a more accurate representation of the flow over the forested ridge. The model is then applied to a real-world scenario using a case study of the flow over two parallel ridges that form a valley system in the area around Perdig˜ao, Portugal. A 3-hour period during the night is simulated and in this case the canopy model still outperformed the surface roughness method. Many features of the flow were only reproduced properly by simulations using a canopy model. In particular the likelihood of recirculation in the lee of the ridges and the mean flow within the valley. Use of high resolution input data characterising properties and distribution of the canopy did not provide significantly better results than using lower resolution land use datasets with averaged canopy properties. The canopy model is shown to provide significantly better results than the surface roughness parameterisation but does require finer spatial and temporal resolution, leading to a higher computational cost. There is however scope for the implementation of the model that is used here to be improved to avoid instabilities at shorter time steps. When studying the flow over truly complex terrain covered in a canopy, it is difficult to disentangle the effect of the canopy from the effect of the terrain. Further experimental work is therefore suggested that could help to improve the understanding of canopy dynamics in complex terrain and also provide further benchmarks against which canopy models could be tested in numerical simulations. Please note that Chapters 3 and 4 are based on an article co-authored with my supervisor Charles Chemel and Chapter 4 was also co-authored by Robert Menke, who assisted with the processing of the lidar measurements. The author of this thesis carried out all simulations and analyses presented in these chapters as well as writing the text.

Alternative jet fuels are being developed for use with existing jet engines, however there are still knowledge gaps concerning how unusual compositions and properties of these fuels will affect combustion performance. Physical and chemical processes leading to problematic engine stability phenomena like flame extinction and lean blow-off (LBO) are still not well-understood for conventional spray flames, but alternative fuels provide additional challenges as they have been observed to have increased variability from expected behaviour at conditions close to LBO. Evaporation is known to be the limiting factor for combustion in spray flames, and experiments have shown both gaseous and spray flames exhibit increased amounts of local extinctions as the equivalence ratio is decreased. The flame structure and transient behaviour of spray flames behave very differently compared to gaseous flames at near-blow-off conditions and during the blowoff transient. Fuel starvation has been proposed in past experiments as a significant reason for why spray flames blow off more quickly and at richer equivalence ratio compared to gaseous flames but has been explored very little in computational studies. The prediction of fuel starvation and LBO phenomena using numerical simulations with detailed chemistry are the primary focus in this work. Large Eddy Simulations (LES) with the Conditional Moment Closure (CMC) turbulence-combustion model are used, as this methodology has shown good results in simulating extinction and blow-off in both gaseous and spray flames in a lab-scale bluff body swirl spray flame configuration. The jet fuels simulated are the Dagaut Jet-A1 surrogate and the U.S. National Jet Fuels Combustion Program (NJFCP) fuels of interest: A2, C1, and C5. A2 is a conventional Jet-A used as a reference fuel, whereas C1 and C5 are synthetic kerosenes with unusual fuel chemistry or liquid property characteristics. These NFJCP fuels are represented using the Hybrid Chemistry "HyChem" lumped pyrolysis detailed kinetic mechanisms. Simulations in non-premixed laminar counterflow flamelet configurations are conducted at pressures of 1 atm and 10 atm for stable scalar dissipation value flamelets up to extinction, and during the extinction transient. Species trends in the three HyChem fuels and the Dagaut Jet-A1 surrogate are compared in detail. In comparison with experimental blow-off trends, only C5 deviates from expected behaviour and is the most robust fuel against extinction via high scalar dissipation rate. This highlights the interplay of both chemical and physical forces contributing to a real fuel's tendency for LBO. Reignition of an extinguishing laminar flamelet using the HyChem A2 mechanism is also achieved through decrease of the scalar dissipation rate, although after a certain time the flamelet is not recoverable due to lack of chain-branching radical species. A stable condition LES-CMC simulation using the HyChem A2 (Jet-A) chemical mechanism is used as a starting point and reference for lean blow-off simulations. The computational domain is based on the Cambridge bluff body swirl burner, with a structured LES mesh and a coarse structured CMC grid. The simulation is run using an Eulerian-Lagrangian framework for multiphase flow with the Abramzon and Sirignano evaporation model. Overall flame size and shape from the LES are fairly similar to experimental OH* and OH-PLIF with Mie scattering results, however there are significant differences in location of peak heat release rate and further work is required for validation of the simulations against experiments. CH is discussed as a promising experimental marker for local extinction and location of heat release. Three fuel mass flow rates from the experimental blow-off curve for the Jet-A flame are simulated. The three simulations exhibited LBO at air flows between 5-20% greater than experimental bulk air blow-off velocities. Heat release rate decreased by at least 80% in the flame zone around the stoichiometric mixture fraction, however globally the combustor saw an increase in heat release rate due to the presence of unburnt droplets continuing to vaporise downstream. The asymmetric flame structure and duration of the blow-off transient in the simulations align very well with previous experiments with kerosene and other low-volatility fuels. The LBO transient lasted between 10-30 ms. Fuel starvation is suggested to be a driver of spray flame extinction, through decreased temperature and reduced evaporation caused by increased quantities of cold air in the system. Unburnt vaporised fuel remains in regions of temperature below 1200 K, where the fuel is no longer able to pyrolyse completely, resulting in non-flammable local mixtures. The quantity of local extinctions observed in both conditional and unconditional space is lower than expected compared to gaseous flames, and is linked to low values of the conditional scalar dissipation rate. Changing the model used to close the conditional scalar dissipation rate in the CMC equations is suggested as a potential way to improve the LBO results, as the Amplitude Mapping Closure model does not account for the very lean mixtures experienced at LBO conditions.

Atrial fibrillation (AF) is a type of supraventricular arrhythmias and one of the most common types of cardiac arrhythmia. In 2014, it was estimated that in the UK alone around 1.4 million people suffered from AF, a trend which is increasing as a result of ageing. AF can seriously impact cardiac function and subsequently it affects cardio/cerebrovascular circulation with serious complications such as heart failure, stroke, and cognitive impairments. Apart from the healthcare related issues, it imposes a massive financial burden to the patients and healthcare systems. Despite all developments in prognosis, diagnosis and treatment of AF, there are still many uncertainties regarding the underlying phenomena resulting AF and the way AF alters the haemodynamics of cardiovascular flow. In fact, AF is a complex disease and since it occurs in conjunction with other diseases, thus, its correlation with different maladies has not been fully established. Stroke is suggested as one of the frequent side effects of AF, which occurrence highly depends on flow haemodynamics. Scientists have postulated that the primary reason of stroke is clot formation in a pocket-like appendage of the left atrium (left atrial appendage) and its movement through the arterial system. However, there is some evidence suggest that the risk of thrombogenic plaques inside the aorta increases in AF patients, so, it prompts the notion of the extended effects of AF on aortic and cerebral circulations. Therefore, the present study examines effects of AF-induced poor flow circulation on aortic flow and haemodynamics throughout the vascular network. Numerical modelling and computational fluid dynamics (CFD) are prominent procedures in biofluid mechanics to explore in-isolation effects of different phenomena. The current project suggests a workflow to study AF effects on aortic circulation. Hence, 4D flow phase contrast magnetic resonance imaging (PC-MRI) along with lumped and CFD modelling techniques are employed. Then, the possible effects of AF on aortic haemodynamics, impacts of ageing, and the way AF may reduce the heart functionality are investigated. The current findings emphasise that the AF can substantially elevate thrombogenic factors in aorta, and they become more critical once the AF is accompanied by ageing. Finally, invoking the concept of exergy, the results suggest that the AF can expediate the heart ageing process.

Stellar rotation, convection, and magnetism are intricately linked in low-mass stars like the Sun. In their outer convective envelopes, the interplay of rotation and convection form a magnetic dynamo capable of sustaining both large and small scale magnetic fields. The strength of these magnetic fields are observed to grow with increasing rotation rate. The coronae of low-mass stars are heated by these magnetic fields (the exact mechanism of which remains under debate), such that the thermal pressure drives a quasi-steady outflow of plasma, referred to as a stellar wind. Due to the interaction of the large-scale magnetic field with the outflowing plasma, stellar winds are able to efficiently remove angular momentum from these stars. Therefore, the evolution of rotation for low-mass stars (on the the main sequence) is governed by their stellar winds, and by interrelation, the evolution of their magnetic activity and stellar wind output. In this thesis I attempt to better constrain the angular momentum-loss rates of the Sun and other Sun-like stars through the use of magnetohydrodynamic simulations combined with a broad range of observations. Though I do not find a concrete value for the solar case, I reduce the uncertainty in its value to within a factor of a few by locating key factors/quantities which limit our predictions, and further highlight the importance of understanding the solar angular momentum-loss rate in an astrophysical context. For the other Sun-like stars, I find the simulation results largely under-predict the angular momentum-loss rates implied by current rotation-evolution models. The reason(s) for this are uncertain, but likely involve uncertainties in both the observed magnetic field strengths and mass-loss rates of these stars, along with the under-prediction of how much of the surface magnetic field is ``opened'' by the stellar wind.

This thesis deals with the analysis and numerical simulation of anisotropic nonlinear partial differential equations (PDEs) and dynamical systems in biology. It is divided into two parts, motivated by the simulation of fingerprint patterns and the modelling of biological transport networks. The first part of this thesis deals with a class of interacting particle models with anisotropic repulsive-attractive interaction forces and their continuum counterpart. These models are motivated by the simulation of fingerprint databases, which are required in forensic science and biometric applications. In existing interacting particle models, the forces are isotropic and the continuum limits of these particle models are given by nonlocal aggregation equations with radially symmetric potentials. The central novelty in the models we consider is an anisotropy induced by an underlying tensor field. This innovation does not only lead to the ability to describe real-world phenomena more accurately, but also renders their analysis significantly harder compared to their isotropic counterparts. We discuss the role of anisotropic interaction, study the steady states and present a stability analysis of line patterns. We also show numerical results for the simulation of fingerprints, based on discrete and continuum modelling approaches. The second part of this thesis focuses on a new dynamic modeling approach on a graph for biological transportation networks which are ubiquitous in living systems such as leaf venation in plants, blood circulatory systems, and neural networks. We study the existence of solutions to this model and propose an adaptation so that a macroscopic system can be obtained as its formal continuum limit. For the spatially two-dimensional rectangular setting we prove the rigorous continuum limit of the constrained energy functional as the number of nodes of the underlying graph tends to infinity and the edge lengths shrink to zero uniformly. We also show the global existence of weak solutions of the macroscopic gradient flow. Results of numerical simulations of the discrete gradient flow illustrate the convergence to steady states, their non-uniqueness as well as their dependence on initial data and model parameters. Based on this model we propose an adapted model in the cellular context for leaf venation, investigate the model analytically and show numerically that it can produce branching vein patterns.

By the introduction of modern power supplies capable of producing low-temperature plasma under atmospheric pressure, the interaction between plasmas and liquids have presented great potentials in many exciting applications in recent years. Cancer treatment, wound healing, nanomaterial production, water disinfection and chemical analysis are just a few examples of emerging applications of plasmas interacting with liquids. Despite the large attention received in recent years, research in this area is still in its infancy. To take the current technologies further and develop practical solutions for these applications, many fundamental questions need to be answered first. Although reactive oxygen species (ROS) and reactive nitrogen species (RNS) are known to play a dominant role in these applications. The underlying physics of plasma-liquid interaction, the chemistry involved in the liquid phase, the interfacial effects governing the mass transfer between the plasma and the liquid, mass transfer quantities and the propagation mechanisms of the transferred species throughout the liquid media are just a few sample questions that remain unanswered. A combination of computational modelling and experimental methods are used throughout this thesis to shine some light on some of these questions. New insights into the dynamics of the liquid phase chemistry as well as the physical effects of the plasma on the liquid bulk are addressed as part of this thesis. The developed computational model not only provides a better understanding of the system in general, but also predicts properties and quantities which are difficult or impossible to measure experimentally with current available apparatus and measurement techniques. The results are then employed to layout guidelines for optimized configurations of plasma-liquid systems in practical applications. Since the gas phase computational study has been explored extensively in previous works, in this thesis our main focus will be the interaction between the plasma and the plasma effluent with the liquid phase and the subsequent physicochemical reactions. The problem is broken down into three parts. In the first part, the plasma gas phase is studied independent of the liquid phase to clarify the kinetics of the plasma medium. The main chemical reaction pathways are studied as well as the effect of input power modulation on the chemical pathway variations and final gas composition. The next part focuses on the transfer of heavy reactive species into the liquid and the subsequent chemical reactions. This is relevant in remote plasma systems in which the plasma is not in electric contact with the liquid. In particular we study an epoxidation reactor that relies on a He + O2 to epoxidate alkenes in liquid phase. In the third part, the focus is on the transfer of electrons into the liquid phase. In this case, the plasma is electrically connected to the liquid and electrons are delivered to the liquid to drive liquid phase reactions. The electrochemical properties of the liquid are studied along with the effect of the surface tension gradient caused by the plasma on the liquid phase mixing patterns.

Turbulent spray combustion is characterised by a strong coupling of evaporation, mixing and chemical reaction. This leads to a wide spectrum of combustion regimes, where self-propagating premixed flames and diffusion-controlled non-premixed flames may occur simultaneously within the same flame. The physical processes involved in spray combustion and their interaction take place over a broad range of scales, which makes their modelling in numerical simulations challenging. This thesis presents the development of Doubly Conditional Moment Closure (DCMC) for the modelling of turbulent spray combustion. This modelling approach allows us to consider the effects of finite-rate chemistry and spray evaporation on the flame. Using a parametrisation of the flame structure, based on mixture fraction and reaction progress variable permits us to resolve premixed, non-premixed and intermediate combustion modes. In the first part of this thesis, the model development is presented. With its foundation as a statistical model, DCMC does not require any strong assumption in terms of the combustion mode or regime. The DCMC equation is derived in a general form, which involves only a minimum number of modelling assumptions about the physical processes involved. Closure for the DCMC equation is discussed and a complete set of models is suggested. Since little experience exists in the modelling of doubly-conditional terms, the closure models were generalised from conventional Conditional Moment Closure (CMC) or adapted from other combustion models with similar parametrisation. In the second part, the DCMC model is validated for two test flames. The DCMC model was first applied to the Cambridge spray jet flame using the Reynolds-Averaged Navier-Stokes (RANS) approach. This flame is characterised by significant pre-vaporisation and behaves as a propagating spray flame, with similarities to premixed flames, but with small-scale inhomogeneity in the gaseous mixture and the presence of liquid droplet interacting with the flame - a problem which requires the doubly-conditional description of the flame structure employed in the DCMC model. The role of the spray terms on the flame structure and mixing field were assessed using RANS and promising results were obtained. Finally, a Large-Eddy Simulation (LES) with DCMC acting as sub-grid scale combustion model was applied to the Rouen spray jet flame. LES-DCMC was found to accurately predict the spray statistics, lift-off height and flame shape. Small-scale effects of the spray on the flame could be resolved thanks to the doubly-conditional parametrisation of the flame structure. Temporal fluctuations and spatial variations of the flame structure were investigated. Spatial gradients of the doubly-conditional flame structure were small and convective transport was found to play a minor role on the flame structure compared to the effects of micro-mixing and chemical reaction in the DCMC equation. The findings of this work suggest that, besides spray combustion, DCMC shows great potential for the modelling of partially premixed flames and extinction.

Thin elastic sheets are found throughout nature and are also extremely important in industrial applications. Sheets can be described using plate and shell models which, in systems where shear is negligible, are typically fourth-order, two-dimensional, partial differential equations. Many such models exist; however, the circumstances in which a particular model is appropriate to use may not be readily apparent. Therefore a means of comparing different unshearable plate and shell models in a general setting is of interest, and is yet to be systematically addressed in the literature. The focus of this thesis is the implementation of a generic numerical framework for the discretization of two-dimensional, fourth-order boundary-value problems, using the method of boundary patches. We build upon the literature for curved triangular Hermite elements by outlining the explicit construction formulas for a known class of curved elements, compatible with Bell elements. We implement these elements within the finite element library oomph-lib . In this study we consider three plate models: the well-known moderate-rotation Foppl-von Karman model, the arbitrary-rotation Koiter-Steigmann plate model and a new moderate- to-large rotation model, which we derive herein. We then implement these plate models within the library, so that they can be solved on generic domains. Finally, we use the implemented plate models, along with analytic and finite difference approaches, to compare the models in three different contexts. The systems we study are the clamped inflation of a circular sheet, the inflation of a circular sheet subject to a rolling clamp, which undergoes a wrinkling instability, and the large cantilever-type displacement of a complicated curved domain, respectively. In all of these systems the choice of plate models is demonstrated to be important: in particular in all cases the predictions of the Foppl-von Karman model break down for moderate-thickness sheets, yielding inaccurate predictions of the sheet morphology. This serves both to demonstrate the capability of the numerical framework as a comparison tool and highlight why such comparisons are important.

Transient premixed flames are significant in areas such as spark-ignition engines and gas explosions. However, physical understanding and accurate prediction remain challenging due to the fact that the flame typically transits from early quasi-laminar to fully turbulent, and the interactions with the surrounding solid structures often lead to the continuous stretching of the flame front. This study has considered the large eddy simulation (LES) techniques for the simulations of transient turbulent premixed flames. The LES technique has evolved as a powerful computational tool for the prediction of unsteady flame propagation. The difficulty of applying LES for turbulent premixed combustion is to account for the thin flame front using appropriate methods. This thesis considers a dynamic flame surface density (DFSD) model to close the filtered reaction rate. It automatically computes the model parameter based on the characteristics of the resolved flame front. The model is first validated in a one-dimensional laminar case to ensure the correct behaviours including the filtered flame thickness and laminar burning velocity with the absence of sub-grid turbulence.

Buoyant plumes in cold fresh water are of interest because of the possibility of buoyancy reversal due to the nonlinear relation between temperature and density in water. Thus an initially rising plume may become a fountain. This project aims to mathematically model such plumes and fountains using numerical simulation by the means of a commercial software, Comsol Multiphysics. Both turbulent and lam- inar cases were investigated in different geometries, and with the assumption that density is a quadratic function of temperature. The turbulent flow cases as con- sidered here in this thesis are relevant to practical applications such as industrial discharge in cold lakes: whereas, the laminar flow case relates to laboratory experi- ments which are typically at scales too small for the flow to be turbulent. Previous investigation on warm discharge placed more attention on the biological implications of the spread along the lake bed, and not interested in analysing the dynamics of such flow, which turns out to be our focus. Furthermore, investigations on buoyant plumes that become negatively buoyant at later time (fountain flow) as considered previously, are based on the assumption that density is a linear function of tem- perature: where entrainment always reduces buoyancy. Whereas, the consideration of the temperature of maximum density is crucial and realistic in many practical situations, especially the power station warm discharge. Mixing is then bound to produce a mixture that is denser than both the discharge and the ambient water if receiving water is less than Tm: where this situation differs from plumes with linear mixing properties. Therefore, our focus is to better fathom the behaviour of warm discharge so as to give a detailed description of the flow, and also to observe buoyancy reversal whenever water that is denser than both the discharge and the receiving water is produced. The simulations were carried out for Prandtl number Pr = 7 & 11.4 and over the ranges of Froude number 0.1 ≤ Fr ≤ 5 and Reynolds numbers 50 ≤ Re ≤ 106, with source temperatures that are assumed to be higher than the temperature of maximum density Tm, and the ambient water below the Tm. Our results show some distinct behaviours from those experimental investigations by Bukreev, who also considered warm discharge where water that has temperature above the temperature Tm is initiated into a medium below Tm. The results here also showed some differences from those investigations with the linear dependence relation assumption.

In this Thesis, I explore the formation and evolution of stellar spiral structures embedded in realistic dark matter haloes with very high resolution simulations. I first study the impact of the shape of the dark matter haloes. I find that non-adiabatic changes to the dark matter halo shape, commonly found in cosmological simulations due to the assembly history of haloes, can trigger strong two-armed grand-design spiral structures extending from the inner disc to the outer region. The nature of the spiral structures is found to be consistent with kinematic density waves based on the study of their power spectra. Such grand-design spiral structures may help the formation of transient multi-armed spiral structures if the self-gravity in disc is strong enough. Evolution of spiral structures is similar when the disc and the halo are misaligned, although warps develop additionally. I further find a strong correlation between the torque strength from the halo and the strength of the corresponding spiral structures. In the second part of my Thesis I then study the influence of subhaloes by including them from realistic cosmological simulations. I identify five different massive subhaloes that hit the central region of the disc, two out of which hit the disc twice. Aside from disc heating, three distinct generations of spiral structures are found in the stellar disc, which can be related to different subhaloes. For each generation, counter-rotating single-armed spiral structures develop first. They wind up very quickly before two-armed spiral structures become prominent. These spiral structures are again identified as kinematic density waves. We find that rather than interacting with the disc through resonances, subhaloes preferentially trigger spiral structures impulsively, due to their relatively short impact time with the disc. The strength of spiral structures can be related to the integrated strength of the torque generated by subhaloes. The correlation between the torque strength exerted by a triaxial dark matter halo and by subhaloes and the spiral strength may provide constraints on the distribution of dark matter.

Magnetic flux emergence is a fundamental process in the Sun, during which magnetic fields emerge from the solar interior to the surface, to build up active regions and give onset to spectacular dynamic phenomena, such as eruptions and jets. In this thesis, we performed 3D, resistive MHD simulations to study the emergence and the associated magnetic activity of a quadrupolar region in the Sun. Our aim behind the setup of this initial condition (i.e. a quadrupolar region) was to study a magnetic field configuration, which has not been studied in detail before, although it has been repeatedly observed in the Sun and it has been shown that it can host intense magnetic activity (e.g. in the form of jets, flares and eruptions). The results of our experiments showed that the internal dynamics of such regions leads to the onset of eruptions in the form of twisted magnetic flux tubes (flux ropes). These eruptions are recurrent but they cannot escape the outermost field of the emerging flux (envelope field). They remain confined within the envelope field, as the downward tension of the outermost field lines overwhelms the upward Lorentz force of the erupting field. When we add an ambient magnetic field in the solar atmosphere, external reconnection between the emerging and the ambient field triggers the emission of (standard) reconnection jets. The external reconnection also releases the tension of the ambient field lines and, thus, the eruptions move in an ejective way towards the outer space. Namely, the confined eruptions become ejective eruptions, which escape from the numerical domain. These ejective eruptions drive a newly observed class of jets, the so called "blowout" jets. Our experiments reproduce some of the main observed characteristics of the "blowout" jets. We showed that "blowout" jets emit hot and cool plasma into the outer solar atmosphere simultaneously, and they undergo untwisting motion due to the relaxation of twist during their ejection. We found that the untwisting motion of the "blowout" jets is associated with the propagation of torsional Alfvén waves. Finally, we performed a parametric study to explore the effect of the ambient field strength on the onset and dynamics of the eruptive events. We found that one of the main effects is that the stronger ambient field suppresses the vertical expansion of the magnetic envelope of the quadrupolar region due to the higher magnetic pressure above it. This result has an effect on the emission of jets, which are emitted due to reconnection between the two fields. When the ambient field is relatively weak, it is pushed away from the strong emerging field and reconnection between them is not so persistent. On the other hand, when the ambient field is relatively strong, we find that more jets are ejected due to more efficient and more frequent reconnection between the two flux systems. As a consequence, we find that more mass and flux is being transferred into the solar corona by the reconnection jets. Also, we find that there are more eruptions when the ambient field is stronger. The study of the total energy flux carried by the jets showed that it is sufficient to provide the energy required to accelerate the high speed solar wind. This indicates that the "blowout" jets may play an important role in driving the solar wind.

Water transport properties in cement are important for the cement industry. At the nanoscale, a nondestructive experimental method, 1 H nuclear magnetic resonance [NMR] relaxometry, can be used to quantify these properties. However, recent results have proven difficult to reconcile with current understanding of cement. The purpose of this work is to use Molecular Dynamics [MD] simulations to try and better understand water in cement and hence better interpret some of the NMR data. In particular, MD simulations are used to investigate water dynamics in two sizes of nanopores in analogues of calcium-silicate-hydrate [C-S-H], which is the active phase of cement paste. These pores are gel pores (3-5 nm) and interlayer spaces (1 nm). First, a bulk water system is studied and the water diffusion coefficient and NMR relax ation times are calculated. The results are compared to literature values and used to validate the methods. Then, different C-S-H analogues based on SiO 2] α -quartz crystal, tobermorite 11 ̊ A and modified tobermorite 14 ̊ A are presented. Two different sets of interatomic poten tials are used for these model simulations: CLAY FF+SPC/E and Freeman+TIP4P. These simulations are then compared. A model called MD4 which is based on modified tobermorite 14 ̊ A and using CLAY FF+SPC/E potentials is selected for further work. The density profile of water oxygen in MD4 is used to identify four water layers with different properties in the gel pore (L1, L2, TL and B) and one water layer in the interlayer pore (IL). Diffusivity and desorption analyses are performed on water populations related to these layers. The importance of the calcium ions close to the surface is highlighted. The NMR dipolar correlation function is generated for water using data from the MD4. This function underpins relaxation analysis. These outputs are compared to Korb’s single water layer model of surface NMR relaxation. Korb’s model is not supported by the new data. However, a new relaxation model of surface relaxation that takes into account water in two layers is supported by the data. Exchange is possible between these layers and is important for diffusivity as well as relaxation. Simulations are carried out as a function of temperature and used to calculate water trans- port activation energies in bulk and in MD4. Finally, the analysis of water exchange between the interlayer and gel pores is performed. It is shown that the exchange time in simulations is ≈69000 times smaller than measured experimentally. Some possible failings in the model that would account for this are discussed.

Thermal improvement of traditional and historic buildings is going to play a crucial role in the achievement of established carbon emission targets. The suitable retrofit options for traditional buildings are, however, very limited and their long term performance is still uncertain. Evaluation of risks, prior to any alteration of building physics, is critical to avoid future damage to the fabric or occupants’ health. Moisture dynamics in building envelopes are affected by the enclosure’s geometry, materials properties and external and internal boundary conditions. Since the internal boundary is heavily influenced by users, understanding their behaviour is essential to predict the outcome of energy retrofit measures more accurately. The effect of user behaviour on energy demand has been extensively investigated; however, its impact on the hygrothermal performance of the envelopes has barely been explored. This research approached the connection between users and buildings from a new angle looking at the effect that user behaviour has on moisture dynamics of buildings’ envelopes after the retrofit. Qualitative and quantitative research methods were used to develop a holistic evaluation of the question. Firstly, factors influencing the adoption of energy efficiency measures in traditional buildings were explored by means of semi-structured interviews with private owners and project managers. Subsequently, a multi-case study including interviews with occupants and monitoring of environmental conditions was conducted. Data collected at this stage was used to explore users’ daily practices of comfort and to characterise the internal climate of traditional dwellings. Lastly, users’ impact was quantified using Heat, Air and Moisture (HAM) numerical simulation. This allowed for the evaluation of the hygrothermal performance of walls under different internal climate scenarios. Combined results of interviews, environmental monitoring and simulation showed that internal climate can compromise envelope performance after the retrofit and highlighted the need to consider users in the decision making process. Ultimately, the results of this research will help to increase awareness about the potential impact of user behaviour and provide recommendations to decision makers involved in the energy retrofit of traditional structures.

Downslope flows and regions of enhanced cooling have important impacts on society and the environment. Parameterisation of these often subgrid-scale phenomena in numerical models requires a sound understanding of the underlying physical processes, which has been the overarching aim of this work. A numerical model has been used to characterise the development of a region of enhanced cooling in an idealised alpine valley with width and depth of order 10 and 1 km, respectively, under stable, decoupled, poorly-drained conditions. A focus of this work has been to remove the uncertainty surrounding the forcing mechanisms behind the development of regions of enhanced cooling. The average valley-atmosphere cooling has been found to be almost equally partitioned between radiative and dynamics effects. Complex interactions between the downslope flows and the region of enhanced cooling have been quantified for the first time. For example, relatively large variations in the downslope flows are generally restricted to the region of enhanced cooling and cannot solely be attributed to the analytical model of [McNider, 1982a]. These flow variations generally coincide with return flows above the downslope flows, where a thin region of unstable air occurs, as well as coinciding with elongated downslope flow structures. The impact of these interactions on the dispersion of passive pollutants has been investigated. For example, pollutants are generally trapped within the region of enhanced cooling. The concentration of pollutants within the region of enhanced cooling, emitted over the lower half of the slopes, increase as the emission source moves away from the ground-based inversion that expands from the bottom of the valley. The concentration of pollutants within the region of enhanced cooling is very similar when varying the location of the emission source over the top half of the valley slopes. This work includes a test of the effects of varying the horizontal numerical grid resolution on average valley-atmosphere temperature changes.

Laser welding of metals and alloys is extensively used in industry due to its advantages of controlled heating, narrow weld bead, low heat affected zone (HAZ) and its ability to weld a wide range of metals and dissimilar metals. Laser welding of dissimilar metals such as carbon steels and stainless steel is still a challenging task, particularly due to the formation of brittle phases in the weld, martensitic formation in the HAZ and solidification cracking in the fusion zone. These issues can significantly deteriorate the strength of the welded joint. The aim of this work is to investigate the fundamental phenomena that occur inside the dissimilar weld zone and their effect on weld quality. In order to establish the key process variables, an initial study concentrated on the effect of different laser process parameters on dissimilar weld quality. In the second part of the work, a comprehensive study was performed to understand and subsequently control the alloying composition in laser dissimilar welding of austenitic stainless steel and low carbon steel. A dissimilar weld that is predominantly austenitic and homogeneous was obtained by controlling the melt pool dynamics through specific point energy and beam alignment. The significance of dilution and alloying elements on joint strength was established. A coupled CFD and FEM numerical model was developed to assist in understanding the melt pool dynamics and transportation processes of alloying elements. The model has been validated by a series of laser welding experiments using various levels of specific point energy. The laser welding characteristics in terms of geometric dimensions, surface morphology, alloying concentration, and dilution, were compared, and it is concluded that the specific point energy and laser beam position are the key parameters that can be controlled to obtain a weld bead with characteristics most suitable for industrial applications. In the third part of the work, a comparative study was performed to understand the significance of cooling rate, and alloying composition on the microstructure and phase structure of the dissimilar weld zone. Results show that the HAZ within the high carbon steel has significantly higher hardness than the weld area, which severely undermines the weld quality. A new heat treatment strategy was proposed based on the results of the numerical simulation, and it is shown to control the brittle phase formation in HAZ of high carbon steel. A series of experiments was performed to verify the developed thermo-metallurgical FEA model and a good qualitative agreement of the predicted martensitic phase distribution is shown to exist. Although this work is presented in the context of dissimilar laser welding of mild steel to stainless steel, the concept is applicable to any dissimilar fusion welding process.

Concrete-Filled Steel Tubular (CFST) columns are increasingly being used because of their many advantages, including high strength, high ductility, and higher fire resistance than conventional steel or concrete columns of the same size. In order to maximise the advantages of CFST column, composite action of the column should be ensured. In realistic structures, the load is not directly applied to the entire CFST column section and is introduced from the beam-column connection. Simple shear connections, which are usually preferred in constructions, are only connected to the external face of the steel tube and there is an issue about how this load is introduced to the concrete core, through the bond at the steel/concrete interface. There are fundamental errors in the load introduction mechanism assumed in various current design methods. Furthermore, based on this erroneous load introduction mechanism, construction methods, such as placing shear connectors inside the steel tube or using through-column plates, are recommended to ensure complete load introduction. However, these methods are either impractical or uneconomical. The aim of this project, therefore, is to develop a thorough understanding of the load introduction mechanism and to use the new insights to assess design implications, for both ambient temperature and fire safety design. The research has been conducted through physical testing, extensive numerical modelling and detailed analytical derivations. A series of new load introduction tests, in which square CFST columns are loaded through simple fin plate connections, are carried out. These tests are designed to investigate the effects of changing column lengths below and above the connection, the effectiveness of using shear connectors inside the steel tube below the connection (according to Eurocode 4) and using a cap plate on the column top for load introduction into the concrete core. The test results indicate that the connection load is introduced to the concrete core through the column length above and within the connection or the cap plate on top of the column. This is different from the currently assumed mechanism of load introduction which assumes that load introduction occurs from underneath the connection. Below the connection, there is transfer of forces from the steel tube to the concrete core, but the total force in the column remains unchanged. Consequently, using shear connectors below the connection is ineffective in increasing CFST column strength, as has been demonstrated by the tests. The physical tests are supplemented by an extensive numerical parametric study to check whether the conclusions are applicable to different design conditions and to provide data for development of a new design method. The parameters include: section geometry (square, circular, and rectangular), position of load application to CFST column, dimensions of the square column cross-section, steel tube thickness, connection length, column length above the connection, column length below the connection, and maximum bond stress at the steel-concrete interface. The numerical simulation results confirm the experimental observations. Furthermore, the numerical simulation results indicate that the entire column length and the entire perimeter of the steel-concrete interface above and within the connection are engaged in load introduction. Based on the experimental and numerical simulation results, a simple calculation method has been proposed to calculate the column cross-section resistance under compression. According to this equation, the concrete compression resistance to the composite column is the minimum of the plastic resistance or the bond strength within and above the connection. This gives rise to a “concrete strength reduction factor” to account for incomplete load introduction, being the ratio of the load introduced to the concrete core through the interface bond to the concrete plastic resistance. Based on the new load introduction calculation method and using representative values of column dimensions and concrete cylinder strength, it has been demonstrated that complete load introduction can be achieved in almost all practical arrangements of concrete-filled tubular construction. For slender CFST column design, this concrete strength reduction factor should also be used to calculate the CFST column cross-section flexural stiffness. For a CFST column under combined axial compression and bending, the concrete strength reduction factor should be used when calculating the compression force, but should be ignored when calculating the bending resistance because composite action is not necessary for bending of the CFST column. The new load introduction mechanism induces additional compression in the concrete core and possible tension in the steel tube above the connection. Therefore, the concrete core of the column above the connection in multi-storey construction should be designed to resist the additional compression force. For the steel tube, in ambient temperature design, the steel contribution ratio (steel section resistance/plastic resistance of composite cross-section) of the top floor column should be at least 0.25. For fire resistance design, the steel contribution ratio of the top floor columns, those on the floor below the top floor, and those two floors below the top floor, should not be less than 0.5, 0.33, and 0.25 respectively.

The acquisition of representative subsea fluid sampling from offshore field development asset is crucial for the correct evaluation of oil reserves and for the design of subsea production facilities. Due to rising operational expenditures, operators and manufacturers have been working hard to provide systems to enable cost effective subsea fluid sampling solutions. To achieve this, any system has to collect sufficient sample volumes to ensure statistically valid characterisation of the sampled fluids. In executing the research project, various subsea sampling methods used in the offshore industry were examined and ranked using multi criteria decision making; a solution using a remote operated vehicle was selected as the preferred method, to compliment the subsea multiphase flowmeter capability, used to provide well diagnostics to measure individual phases – oil, gas, and water. A mechanistic (compositional fluid tracking) model is employed, using the fluid properties that are equivalent to the production flow stream being measured, to predict reliable reservoir fluid characteristics on the subsea production system. This is applicable even under conditions where significant variations in the reservoir fluid composition occur in transient production operations. The model also adds value in the decision to employ subsea processing in managing water breakthrough as the field matures. This can be achieved through efficient processing of the fluid with separation and boosting delivered to the topside facilities or for water re-injection to the reservoir. The combination of multiphase flowmeter, remote operated vehicle deployed fluid sampling and the mechanistic model provides a balanced approach to reservoir performance monitoring. Therefore, regular and systematic field tailored application of subsea fluid sampling should provide detailed understanding on formation fluid, a basis for accurate prediction of reservoir fluid characteristic, to maximize well production in offshore field development.

Tooling design is crucial for the production of cost-effective and durable composite products. As part of the current search for cost reduction (by reducing capital investment, energy use and cycle time), integrally-heated tooling is one of the technologies available for ‘out-of-autoclave’ processing of advanced thermoset polymer composites. Despite their advantages, integrally-heated tools can suffer from uneven distribution of temperature, variability in heat flow rate and inconsistency in heating/cooling time. This research, therefore, investigates a number of design variables such as shape and layout of heating channels in order to improve the heating performance of an integrally-heated tool. Design of Experiments (DoE) has been carried out using Taguchi’s Orthogonal Array (OA) method to set several combinations of design parameters. Each of these design combinations has been evaluated through numerical simulation to investigate heating time and mould surface temperature variation. The simulation results suggest that the layout of the channels and their separation play a vital role in the heating performance. Signal-to-Noise (S/N) ratio and analysis of variance (ANOVA) have been applied to the results obtained to identify the optimal design combination of the integrally-heated tool. Statistical analysis reveals that the heating performance of an integrally-heated tool can be significantly improved when the channels’ layout is parallel. The shape of the channels has negligible effect and the distance between the channels should be determined based on the production requirement. According to the predicted optimal design, a developed integrally water-heated tool is manufactured. The actual thermal properties of the constituent materials of the produced tool are also measured. Then a numerical model of the experimental tool model is simulated in ANSYS software, with setting the actual material properties and boundary condition to define the temperature uniformity and heating rate of the experimental tool. Comparison of the experimental and numerical results of the experimental tool confirmed the well assigning of the boundary conditions and material properties during simulation the heated tool. The experimental results also confirmed the predicted optimal design of the integrally heated tool. Finally, in order to define its thermomechanical behaviour under the effective (in service) thermal loads, a tool model is simulated. Numerical results presented that the produced extremes of thermal deformation, elastic strain, normal and plane shear stresses, under the effective thermal loading, are within the allowable elastic limits of the participated materials.

In vitro tissue engineering is a method for developing living and functional tissues external to the body, often within a device called a bioreactor to control the chemical and mechanical environment. However, the quality of bone tissue engineered products is currently inadequate for clinical use as the implant cannot bear weight. In an effort to improve the quality of the construct, hydrostatic pressure, the pressure in a fluid at equilibrium that is required to balance the force exerted by the weight of the fluid above, has been investigated as a mechanical stimulus for promoting extracellular matrix deposition and mineralisation within bone tissue. Thus far, little research has been performed into understanding the response of bone tissue cells to mechanical stimulation. In this thesis we investigate an in vitro bone tissue engineering experimental setup, whereby human mesenchymal stem cells are seeded within a collagen gel and cultured in a hydrostatic pressure bioreactor. In collaboration with experimentalists a suite of mathematical models of increasing complexity is developed and appropriate numerical methods are used to simulate these models. Each of the models investigates different aspects of the experimental setup, from focusing on global quantities of interest through to investigating their detailed local spatial distribution. The aim of this work is to increase understanding of the underlying physical processes which drive the growth and development of the construct, and identify which factors contribute to the highly heterogeneous spatial distribution of the mineralised extracellular matrix seen experimentally. The first model considered is a purely temporal model, where the evolution of cells, solid substrate, which accounts for the initial collagen scaffold and deposited extracellular matrix along with attendant mineralisation, and fluid in response to the applied pressure are examined. We demonstrate that including the history of the mechanical loading of cells is important in determining the quantity of deposited substrate. The second and third models extend this non-spatial model, and examine biochemically and biomechanically-induced spatial patterning separately. The first of these spatial models demonstrates that nutrient diffusion along with nutrient-dependent mass transfer terms qualitatively reproduces the heterogeneous spatial effects seen experimentally. The second multiphase model is used to investigate whether the magnitude of the shear stresses generated by fluid flow, can qualitatively explain the heterogeneous mineralisation seen in the experiments. Numerical simulations reveal that the spatial distribution of the fluid shear stress magnitude is highly heterogeneous, which could be related to the spatial heterogeneity in the mineralisation seen experimentally.

Compressive residual stresses (CRS) are beneficial for enhancing the fatigue life of metal components. Shot Peening (SP) is an industrial cold working process that is applied to induce a field of CRS and modify the mechanical properties of the metal component. The SP process involves impacting a surface with tiny shots with forces sufficient to create plastic deformation. The process is governed by a number of important parameters such as the shot size, angle of attack, initial velocity, mass flow rate and the distance from the shot nozzle to the surface being peened. The relationship between the optimal peening outcome, particularly the residual stress distribution of the treated surface, and the peening parameters is still unknown and needs to be investigated further. Manufacturers are interested in producing a uniform peening process for complex geometries which optimises the SP parameters. Modelling the process is complex as it involves the interaction of a metallic surface with a large number of shots of very small diameter. Conventionally, such problems are solved using finite element software to predict stresses and strains of a single shot impact then applying superposition. At the moment there are no Finite Element Method (FEM) modelling solutions involving more than tens of shots. The number of shots and elements required for such a modelling process made the approach unfeasible prior to the work described herein. The objective of this work is to develop an appropriate numerical modelling approach that can better simulate the real SP process. The model will be provided by combining Discrete Element Method (DEM) with FEM. The DEM is employed to get a distribution of impact velocities over space and time which are then implemented into a FEM analysis. A discrete element model with randomly distributed steel shots bombarding a steel component at various velocities has been developed as benchmark example. With this model the SP shot - shot interaction, the shot - target interaction, the surface coverage, angle of impingement, shot size, impact velocity and the overall shot flow can be parametrically studied in details and with little computational effort. The novel approach also proposes a new method to dynamically change the coefficient of restitution for repeated impacts during the simulation and predicts the CRS more effectively. The effects of SP on different materials of relevance to gas turbine engine components will be investigated in order to improve the understanding of the interaction between the shots and the targeted material. Initially, an uncoupled analysis was peforned, in order to assess the capabilities of the two modelling systems, DEM and FEM, to delivery an improved solutuion when combining two commercially available codes. This parametric analysis is performed using the state-of-the-art Discrete Element (DE) application EDEM. In the subsequent part of this work, a dynamic Finite Element (FE) application Abaqus will be used to investigate single shot impacts and to obtain the residual stress distribution. This gives us a prescribed residual stress distribution and peening coverage. A Combined DEM/FEM tool (DEST) is proposed that eliminates any manual pre-processing required for linking/coupling, eliminating the use of two different applications and provide an integrated solution for the simulation of the Shot Peening process. In the subsequent chapter, the implementation of essential tools for the enchanced modelling of Shot Peening process functionalities, such as the nozzle, bounding box, coverage and intensity is described. A number of computational improvements are also implemented to reduce the computation time. The existing binary search is enhanced to self-balancing search tree and further improved to allow insertion and deletion of elements. A bounding box feature which removes shots that move out of the domain during the course of the simulation is also implemented. Experiments featuring single shot impacts are performed to gain better understanding the deformation process in the target material subjected to impact conditions to those occurring in the production peening. The single shot impacts are experimentally examined using SEM and EBSD. During final chapter, case studies are performed to compare the results of the simulations with large-scale experimental work. The coverage of peening of single and multiple nozzles with different angle of impingements are assessed. Finally, possible directions for further research concerning the accurate quantification of material responses to SP are identified in the report.

Two-phase flow in porous media is an important subsurface process that has significant impacts on the global economy and environments. To study two-phase system in porous media, capillary pressure (Pc ), relative permeability (Kr), bulk electrical conductivity (σb) and bulk relative permittivity (εb) are often employed as characterization parameters. Interestingly, all of these parameters are functions of water saturation (S). However, the non-uniqueness in the Pc -S, Kr-S,σb-S and εb-S relationships pose considerable challenges in employing them for effective monitoring and control of the two-phase flow processes. In this work, laboratory scale experiments and numerical simulations were conducted to investigate the factors and conditions contributing to the non-uniqueness in the above relationships for silicone oil-water and supercritical CO2-water flow in porous media, with a special emphasis on geological carbon sequestration. Specifically, the dynamic capillary pressure effect, which indicates the dependence of the Pc - S relationship on the rate of change of saturation (αS/αt) during two-phase flow in porous media was investigated. Using a silicone oil-water system, the dynamic capillary pressure effect was quantified in term of the parameter named the dynamic coefficient, τ , and it was found to be dependent on the domain scale and the viscosity ratio of the two fluids. It was found that τ increases with the domain scale and the viscosity ratio. It is inversely affected by αS αt , which is related to the degree of resistance to the fluid motion, namely, viscosity. In almost all cases, τ was found to decrease monotonically with an increase in water saturation, S. An order increase in magnitude of τ was observed as the domain scale increases from 4cm scale to 8cm in height. A similar order of increase in τ was observed in the 12cm high domain scale. There is an order increase in the value of τ for the silicone oilwater system as the viscosity ratio increases from 200 to 500. For the supercritical CO2 (scCO2) and water system in porous media, the experiments and numerical simulations showed that τ increases with rising system temperature and decreasing porous media permeability. Dimensionless analysis of the silicone oil-water experimental results showed that by constructing non-dimensional groups of quantities expressing a relationship among different variables on which τ depends, it is possible to summarise the experimental results and determine their functional relationship. A generalised scaling relationship for τ was derived from the dimensionless analysis which was then validated against independent literature data. The exercise showed that the τ-S relationship obtained from the literature and the ii scaling relationship match reasonably well. This work also demonstrated the applicability of an artificial neural network (ANN) as an alternative computational platform for the prediction of the domain scale dependence of τ . The dependence of the Kr-S relationship on αS/αt was also investigated. The results showed that the Kr-S curve under dynamic flow condition is different from that under the quasi-static condition. Kr for water (Krw) increases with increasing water saturation and decreases with the increase in viscosity ratio while Kr for silicone oil (Krnw) increases with decreasing water saturation as well as with the increase in viscosity ratio. Also, Krw decreases while Krnw increases with the increasing boundary pressure. However, the εb-S and σb-S relationships were found to be independent of αS/αt for the scCO2-water system in carbonate and silicate porous media. Nevertheless, the εb and σb values decrease as the water saturation decreases in the two porous media samples. While εb decreases with increase in temperature in silica sand, the trend in the limestone showed a slight increase with temperature, especially at high water saturation. Also, the εb-S relationship is shown to be affected by pressure in silica sand increasing with the pressure of the domain. On the contrary, the σb-S relationship increases as the temperature increases with more significance at higher water saturation in the silica sand sample. This work further demonstrated the application of a membrane in the monitoring of the CO2 in geological sites used for carbon sequestration. Commercial silicone rubber coupled with a pressure transducer showed potential in the detection of CO2 leakage from geological sites. The response of the device in terms of the mass of permeated gas, permeability and gas flux were investigated for both CO2 and N2. In addition, the monitoring of potable water contamination in a shallow aquifer by the migrating or leaking of CO2 is demonstrated with the combination of the pH analysis, geoelectrical measurement techniques and the membrane-sensor system. Overall, the work in this PhD research demonstrated robust applications of two-phase systems'characterization parameters under different scenarios in the porous media. Implications of the findings in this work to the monitoring and control of two-phase systems in porous media are expatiated.

Numerical simulation is becoming increasingly used to support theoretical effort into understanding the turbulence problem. We develop theoretical ideas related to the transfer and dissipation of energy, which clarify long-standing issues with the energy balance in isotropic turbulence. These ideas are supported by results from large scale numerical simulations. Due to the large number of degrees of freedom required to capture all the interacting scales of motion, the increase in computational power available has only recently allowed flows of interest to be realised. A parallel pseudo-spectral code for the direct numerical simulation (DNS) of isotropic turbulence has been developed. Some discussion is given on the challenges and choices involved. The DNS code has been extensively benchmarked by reproducing well established results from literature. The DNS code has been used to conduct a series of runs for freely-decaying turbulence. Decay was performed from a Gaussian random field as well as an evolved velocity field obtained from forced simulation. Since the initial condition does not describe developed turbulence, we are required to determine when the field can be considered to be evolved and measurements are characteristic of decaying turbulence. We explore the use of power-law decay of the total energy and compare with the use of dynamic quantities such as the peak dissipation rate, maximum transport power and velocity derivative skewness. We then show how this choice of evolved time affects the measurement of statistics. In doing so, it is found that the Taylor dissipation surrogate, u^3 / L, is a better surrogate for the maximum inertial flux than dissipation. Stationary turbulence has also been investigated, where we ensure that the energy input rate remains constant for all runs and variation is only introduced by modifying the fluid viscosity (and lattice size). We present results for Reynolds numbers up to Rλ = 335 on a 1024^3 lattice. Using different methods of vortex identification, the persistence of intermittent structure in an ensemble average is considered and shown to be reduced as the ensemble size increases. The longitudinal structure functions are computed for smaller lattices directly from an ensemble of realisations of the real-space velocity field. From these, we consider the generalised structure functions and investigate their scaling exponents using direct analysis and extended self-similarity (ESS), finding results consistent with the literature. An exploitation of the pseudo-spectral technique is used to calculate second- and third-order structure functions from the energy and transfer spectra, with a comparison presented to the real-space calculation. An alternative to ESS is discussed, with the second-order exponent found to approach 2/3. The dissipation anomaly is then considered for both forced and free-decay. Using different choices of the evolved time for a decaying simulation, we show how the behaviour of the dimensionless dissipation coefficient is affected. The Karman-Howarth equation (KHE) is studied and a derivation of a work term presented using a transformation of the Lin equation. The balance of energy represented by the KHE is then investigated using the pseudo-spectral method mentioned above. The consequences of this new input term for the structure functions are discussed. Based on the KHE, we develop a model for the behaviour of the dimensionless dissipation coefficient that predicts Cɛ= Cɛ(∞)+CL/RL. DNS data is used to fit the model. We find Cɛ(∞) = 0.47 and CL = 19.1 for forced turbulence, with excellent agreement to the data. Theoretical methods based on the renormalization group and statistical closures are still being developed to study turbulence. The dynamic RG procedure used by Forster, Nelson and Stephen (FNS) is considered in some detail and a disagreement in the literature over the method and results is resolved here. An additional constraint on the loop momentum is shown to cause a correction to the viscosity increment such that all methods of evaluation lead to the original result found by FNS. The application of statistical closure and renormalized perturbation theory is discussed and a new two-time model probability density functional presented. This has been shown to be self-consistent to second order and to reproduce the two-time covariance equation of the local energy transfer (LET) theory. Future direction of this work is discussed.

Micrometer injection moulding is a type of moulding in which moulds have geometrical design features on a micrometer scale that must be transferred to the geometry of the produced part. The difficulties encountered due to very high shear and rapid heat transfer of these systems has motivated this investigation into the fundamental mathematics behind polymer heat transfer and associated processes. The aim is to derive models for polymer dynamics, especially heat dynamics, that are considerably less approximate than the ones used at present, and to translate this into simulation and optimisation algorithms and strategies, Thereby allowing for greater control of the various polymer processing methods at micrometer scales.

The stringing regulation on greenhouse gases emissions coupled with the rising fuel price and the growth in aviation transportation have imposed increasing demands on the aircraft industry to develop revolutionary technologies to meet such challenges. Methods of delaying flow separation on aircraft high lift systems have been sought which can lead to an increase in the aircraft performance and ultimately a reduction in aircraft operational costs and its impact on the environment. Synthetic jet actuators are a promising method of delivering flow control for aircraft applications due to their ability to inject momentum to an external flow without net mass flux and their potential in being integrated in MEMS through micro-fabrication with relative ease. It has been demonstrated in many laboratory experiments that synthetic jets are capable of delaying flow separation on aerodynamic bodies of various shapes. However, currently the operating conditions of synthetic jets are mostly chosen by trial-and-error, and thus the flow control effectiveness varies from one experiment to another. In order to deliver an effective flow separation control which achieves a desired control effect at minimum energy expenditure, a better understanding of the fluid mechanics of the behaviour of synthetic jets and the interaction between synthetic jets and a boundary layer are required. The aims of the present research were to achieve such a goal through a series of purposely designed numerical simulations. Firstly, synthetic jets issued from a circular orifice into quiescent air were studied to understand the effect of dimensionless parameters on the formation and the extent of roll-up of vortex rings. The computational results confirmed that the Stokes number determines the strength of vortex roll-up of a synthetic jet. Based on the computational results, a parameter map was produced in which three different operational regimes of synthetic jets were indentified and a criterion for vortex roll-up was also established. A circular synthetic jet issued into a zero-pressure-gradient laminar boundary layer was then investigated. The capability of FLUENT in modelling the key characteristics of synthetic jets was validated using experimental data. The formation and evolution of coherent structures produced by the interaction between synthetic jets and a boundary layer, as well as their near-wall effect in terms of the wall shear stress, were examined. A parameter map illustrating how the appearance of the vortical structures and their corresponding shear stress patterns vary as the synthetic jet operating condition changes was established. In addition, the increase in the wall shear stress relative to the jet-off case was calculated to evaluate their potential separation control effect.Finally, the study moved one step forward to investigate the flow separation control effect of an array of three circular synthetic jets issued into a laminar boundary layer which separates downstream on an inclined plate. The impact of synthetic jets on the boundary layer prior to separation and the extent of flow separation delay on the flap, at a range of synthetic jet operating conditions, were examined and the correlation between them was investigated. Furthermore, the optimal operating conditions for this synthetic jet array in the current study were identified by considering both the flow control effect and the actuator power consumption. The characteristics of the corresponding vortical structures were also examined.The findings from this work have produced some further insights of the behaviour and the interaction between synthetic jets and a boundary layer, which will be useful for ensuring an effective application of synthetic jets in practical settings.

In this thesis a numerical study has been undertaken to investigate turbulent flow and heat transfer in a number of flow problems, representing the gas-cooled reactor core flows. The first part of the research consisted of a meticulous assessment of various advanced RANS models of fluid turbulence against experimental and numerical data for buoyancy-modified mixed convection flows, such flows being representative of low-flow-rate flows in the cores of nuclear reactors, both presently-operating Advanced Gas-cooled Reactors (AGRs) and proposed ‘Generation IV’ designs. For this part of the project, an in-house code (‘CONVERT’), a commercial CFD package (‘STAR-CD’) and an industrial code (‘Code_Saturne’) were used to generate results. Wide variations in turbulence model performance were identified. Comparison with the DNS data showed that the Launder-Sharma model best captures the phenomenon of heat transfer impairment that occurs in the ascending flow case; v^2-f formulations also performed well. The k-omega-SST model was found to be in the poorest agreement with the data. Cross-code comparison was also carried out and satisfactory agreement was found between the results.The research described above concerned flow in smooth passages; a second distinct contribution made in this thesis concerned the thermal-hydraulic performance of rib-roughened surfaces, these being representative of the fuel elements employed in the UK fleet of AGRs. All computations in this part of the study were undertaken using STAR-CD. This part of the research took four continuous and four discrete design factors into consideration including the effects of rib profile, rib height-to-channel height ratio, rib width-to-height ratio, rib pitch-to-height ratio, and Reynolds number. For each design factor, the optimum configuration was identified using the ‘efficiency index’. Through comparison with experimental data, the performance of different RANS turbulence models was also assessed. Of the four models, the v^2-f was found to be in the best agreement with the experimental data as, to a somewhat lesser degree were the results of the k-omega-SST model. The k-epsilon and Suga models, however, performed poorly. Structured and unstructured meshes were also compared, where some discrepancies were found, especially in the heat transfer results. The final stage of the study involved a simulation of a simplified 3-dimensional representation of an AGR fuel element using a 30 degree sector configuration. The v^2-f model was employed and comparison was made against the results of a 2D rib-roughened channel in order to assess the validity and relevance of the precursor 2D simulations of rib-roughened channels. It was shown that although a 2D approach is extremely useful and economical for ‘parametric studies’, it does not provide an accurate representation of a 3D fuel element configuration, especially for the velocity and pressure coefficient distributions, where large discrepancies were found between the results of the 2D channel and azimuthal planes of the 3D configuration.

The rapid depletion of hydrocarbon fields around the world has led the industry to search for these resources in ever increasing water depths. In this context, the large diameter (D > 100mm) vertical riser has become a subject of great interest. In this research work, a major investigation was undertaken to determine the two phase flow hydrodynamics in a 254mm vertical riser. Two types of experiments were performed for range of air-water superficial velocities. The first experimental campaign addresses the issue of the two gas injector’s performances (conventional vs. novel design gas injector) in the large diameter vertical riser. The experimental results show that the novel design gas injector should be the preferential choice. The second set of the experimental work investigates the two phase flow hydrodynamics in the vertical riser in detail. The two phase flow patterns and their transitions were identified by combination of visual observations and statistical features. Based on the results, the experimental flow regime map was developed and compared with the existing vertical upflow regime maps/models. None of the flow regime transition models adequately predicted the flow regimes transitions in large diameter vertical risers as a whole. In this regard, the Taitel et al. (1980) bubble to slug flow transition model has been modified for large diameter vertical upflow conditions, based on the physical mechanism observed. The general trends of modified criteria agreed well with the current and other large diameter experimental results. The effect of upstream conditions on the vertical riser flow behaviour was also investigated in detail by two different inlet configurations (i) near riser base injection and (ii) upstream flowline injection. It was found that no significant differences exist in flow behaviour at low air-water superficial velocities for both the inlet configuration, at high air-water superficial velocities, the intermittent flow behavior in flowline influences the riser flow pattern characteristics and thereby controls the riser dynamics. It is found that liquid slugs from the flowline naturally dissipate to some extent in the riser as a consequence of compression of succeeding bubble that rapidly expands and break through the liquid slug preceding it when it enters the riser. The experimental work corroborates the general consensus that slug flow does not exist in large diameter vertical upflow condition. Experimental data has been further compared to increase the confidence on the existing two phase flow knowledge on large diameter vertical riser: (a) by comparing with other experimental studies on large diameter vertical upflow in which generally, a good agreement was found, (b) by assessing the predictive capability of void fraction correlations/pressure gradient methods. The important implication of this assessment is that the mechanistic approach based on specific flow regime in determining the void fraction and pressure gradient is more successful than conventional empirical based approaches. The assessment also proposes a proposed set a of flow regime specific correlations that recommends void fraction correlations based on their performances in the individual flow regimes. Finally, a numerical model to study the hydrodynamic behaviour in the large diameter horizontal flowline-vertical riser system is developed using multiphase flow simulator OLGA. The simulated results show satisfactory agreement for the stable flows while discrepancies were noted for highly intermittent flows. The real time boundary application was partially successful in qualitatively reproducing the trends. The discrepancies between the predicted results and experimental data are likely to be related to the incorrect closure relations used based on incorrect flow regimes predictions. The existence of the multiple roots in the OLGA code is also reported for the first time.

This thesis explores the links between different ways of modelling the physical world. Finite difference numerical simulations may be used to encode the behaviour of physical systems, allowing us to gain insight into their workings and even to predict their behaviour. Similarly, one can investigate the properties of gravitational black holes through the use of analogue black holes, physical systems which share at least some part of the physics of the astronomical objects. Concentrating on black hole analogues using Bose-Einstein condensates, I show how simulations of these systems may be greatly assisted through the use of a proper absorbing boundary condition, the Perfectly Matched Layer. Such a boundary condition allows the effcient truncation of the computational domain, both saving computational time and increasing accuracy. I then apply this technique to the simulation of the supersonic flow of a Bose-Einstein condensate through a Laval nozzle, a black hole analogue, showing that such a flow should be stable and observable in the laboratory. Moving to a related system, I investigate the optical analogue of the Iordanskii force - the friction resulting from interaction between excitations in a superfluid's normal component and a superfluid vortex - through the simulation of such a vortex in a Bose-Einstein condensate illuminated by slow light, which is light whose group velocity is on the order of metres per second. The interaction of the slow light with the vortex should produce a momentum transfer due to the optical Aharonov-Bohm effect, exerting a force on the vortex. The coupled system of equations describing the condensate-slow light system is simulated, giving some surprising results.

A series of local and bench scale laboratory experiments and bench and field scale numerical simulations were conducted to develop a better understanding of the interrelationship between nonwetting phase (NWP) source zones and downgradient aqueous phase concentrations in saturated porous media contaminated by immiscible organic liquids. Specific emphasis was placed on the factors governing the rate of NWP source zone evolution and the factors governing the rate of mass transfer from the NWP to the aqueous phase. Hysteretic NWP relative permeability-saturation (krN-SW) relationships were measured at the local scale for six sands to examine the relationship between krN-SW functions and porous media type. Parameterization of the measured constitutive relationships revealed a strong correlation between mean grain diameter and the maximum value of NWP relative permeability. The measured krN-SW relationships, were validated through a bench scale experiment involving the infiltration, redistribution, and immobilisation of NWP in an initially water saturated heterogeneous porous medium. This match of simulation to experiment represents the first validation of a multiphase flow model for transient, fixed volume NWP releases. Multiphase flow simulations of the bench scale experiment were only able to reproduce the experimental observations, in both time and space, when the measured krN-SW relationships were employed. Two-dimensional field scale simulations of a fixed volume NWP release into a heterogeneous aquifer demonstrate the influence of spatially variable krN-S relationships correlated to porous media type. Both the volume of the NWP invaded porous media, and the length of time during which NWP is migrating, will be under predicted if variable (correlated) kr,N is not accounted for in the numerical model iv formulation. This under prediction is exacerbated as the mean intrinsic permeability of the release location decreases. A new, thermodynamically-based interfacial area (IFA) model was developed for use in the single-boundary layer expression of mass transfer as an alternative to existing empirical correlation expressions. The IFA model considers consistency and continuity of constitutive relationships, energy losses, effective specific interfacial area for mass transfer, and dissolution of residual NWP. A bench scale experiment involving the release and dissolution of a transient NWP source zone in heterogeneous porous media was conducted to evaluate the appropriateness of the developed IFA model when utilised to predict NWP dissolution rates. Comparison of measured downgradient dissolved phase concentrations and source zone NWP saturations in time and space with those from numerical simulations of the experiment reveal that the proposed IFA model is superior to both a local equilibrium assumption and existing empirical correlation expressions. This represents the first mass transfer model validated for the dissolution of a complex NWP source zone. Twodimensional simulations at the field scale of multiphase flow and dissolution suggest that employing existing mass transfer expressions instead of the IFA model lead to incorrect predictions of the life spans of NWP source zones, downgradient dissolved phase concentrations, and the rate of mass flux through a downgradient boundary. The practical implication of this research is that accurate numerical predictions of the evolution of a transient NWP source in porous media require consideration of krN-S relationships and NWP / aqueous phase IFA, as these factors dictate the rates of the key subsurface contaminant processes of migration and dissolution, respectively.

The liquid-metal fusion blanket constitutes a pivotal element in the infrastructure of magnetic field confined fusion nuclear power plants, undertaking a multifaceted role crucial to their operation. Its responsibilities span from breeding tritium, essential fuel for fusion reactions, to converting the energy from high-energy neutrons and plasma into electricity, while also shielding structural components from the impact of high-energy species produced during fusion processes. Specifically engineered for this purpose, a liquid metal fusion blanket employs materials such as lithium or lithium alloys, serving as coolants and breeding materials simultaneously. Within the domain of liquid-metal (LM) blankets, mixed-convection presents a significant challenge, emerging as the dominant flow phenomenon in most fusion LM blanket designs. The magnetohydrodynamics (MHD) flows of liquid breeders, like PbLi, within blanket conduits experience notable buoyancy forces due to heightened temperature gradients resulting from intense heat loads. The intricate interplay of MHD effects and buoyant forces gives rise to strongly coupled phenomena. Understanding and predicting the complex flow behaviors arising from the interaction of these multiple effects necessitate both experimental data and numerical investigations of three-dimensional mixed-convection MHD flows for advancing LM blanket designs.Chapter two of this thesis outlines the establishment of the MaPLE-U facility, dedicated to high-temperature liquid metal experiments under intense magnetic fields and various flow orientations with respect to gravity. Subsequently, the first experimental dataset is presented, focusing on PbLi, a prominent blanket breeding candidate, to elucidate mixed-convection MHD flow behaviors and heat transfer phenomena. The findings challenge the assumption of complete flow laminarization under strong MHD effects, widely adopted in LM MHD R&D strategy, emphasizing the need for simultaneous consideration of multiple effects. Chapter three delves into numerical investigations employing COMSOL Multiphysics, where flow predictions are validated against analytical solutions, benchmarked experimental data, and results from other MHD codes. A novel MHD-heat transfer flow model is developed to address the lack of numerical simulation tools for wall-bounded fully developed flows concurrently coupling MHD flow and heat transfer equations under harsh nuclear fusion reactor conditions (Ha ~ 10^4,Gr ~ 10^11). Building upon this groundwork, chapter four provides further insights into mixed-convection MHD flows within complex LM blanket geometries under fusion-relevant conditions, showcasing the versatility and computational accuracy of the COMSOL Multiphysics platform, particularly in scenarios surpassing existing experimental and numerical studies.

A finite element code has been developed at the University of Sheffield to simulate the structural response of steel and composite framed buildings subjected to fire. The steel skeleton is represented using two-noded line elements, the steel-to-steel connections using spring elements and the flooring system by isotropic flat shell elements. Structures are therefore considered as a complete entity, allowing a more realistic prediction of structural behaviour at elevated temperature. A series of numerical simulations of fire tests carried out on the full-scale, eight-storey composite frame at the BRE laboratory at Cardington in 1995 and 1996 have been conducted. These tests have been subject to a number of significant parametric studies including slab thickness and secondary beam connection strength and stiffness. The concrete floor slab element has also been extended to a layered flat shell element allowing the inclusion of material non-linearities, thermal bowing, thermal degradation, anisotropic properties and a more advanced cracking model. Using the new concrete floor slab element the Cardington fire tests have been simulated in detail, to further understanding of the structural reaction in fire. Another series of parametric studies have been conducted considering again the thickness of the floor slab, the effect of the slab temperature gradient, the compressive strength, tensile strength and load ratios. These have all been compared to results from the Cardington fire tests. Current design methods based on isolated element design are considered by comparing the results of analyses in which the concrete floor is either included as a continuous slab in an extensive subframe, or is treated simply as forming the flanges of composite beams in a three-dimensional skeleton. These examples show clearly the effects of membrane and bridging actions of the continuous floor slab. The implications for future design developments are discussed with particular reference to the parametric studies conducted.

Abstract: As internal tides propagate in the ocean, they carry and dissipate energy over hundreds and even thousands of kilometers. We perform fully nonlinear simulations to examine the evolution of horizontally propagating, vertical mode-1 internal tides in non-uniformly stratified fluids, as it depends on wave amplitude, ocean depth, Coriolis forces, and the spanwise extent of the waves. The background stratification is set up according to ocean measurements southwest of Hawaii and in the South China Sea. Two-dimensional (2D) simulations on the beta-plane are based on the internal tides originating near the Hawaiian Ridge and propagating southwest towards the equator. The results are compared to the ocean measurements from the EXperiment on Internal Tidal Scattering (EXITS). Another series of 2D simulations on the f-plane is set up based on the internal tides propagating westward in the South China Sea and are compared to the observations. The simulations in both research domains align qualitatively and somewhat quantitatively with the observations. A three-dimensional (3D) model simulating spanwise-localized waves is utilized to characterize the evolution of internal tides in the streamwise and spanwise direction. The spanwise evolution of the 3D waves is examined in terms of the lateral spreading, radius of curvature, and sea surface signature. The evolution of sea surface signature is compared favourably to a satellite image in the South China Sea. The 3D model can thus be used to reversely deduce the initial conditions of internal tides.

Abstract: Lithium-ion batteries are the leading contender for high density energy storage for applications such as electric vehicles and personal electronic devices. While they promise well over 4 V of potential per cell, actually realizing such high voltages is quite difficult, given the many modes of energy loss during discharge. One dominant loss, especially at high currents (e.g., fast-charging) is losses due to ionic transport in the electrolyte. Mathematical models and their computational implementation have been used to simulate Lithium-ion battery discharge to better understand its complex physics and to optimize its design. Because of the complex and dynamic nature of Lithium-ion batteries, these models are transient and highly non-linear. A difficulty arises in that the scales on which key physics occurs varies over many orders of magnitude, meaning simulations often decouple scales to some degree to conserve computational resources. In this thesis, a transient and non-linear numerical model is developed for the lithium-ion battery macro-scale, that is, the scale on which heterogeneities due to microscopic components can be ignored. Additionally, the electrolyte is studied separately by isolating the separator contribution by modelling a symmetric Li-foil cell. The numerical model is supported by a rigorous set of mathematical derivations for the governing equations. This process utilizes concentrated solution theory and the finite element method. First the symmetric cell system is verified by reproducing the voltage response to a constant current step reported from experiments in the literature. A sensitivity analysis is performed for the electrolyte characteristics and discussed in terms of their influence on cell performance. After concluding that the electrolyte model accurately reproduces the real-world system, the model is applied to a novel electrolyte for which no previous numerical modelling has been performed. It is concluded that the experimental results do not match what is expected from the model, due to the non-reproducibility of the experimental data. The full macro-scale battery model is analyzed in terms of its voltage response and solution variable profiles during a constant current discharge. Following a study of the cell’s hysteresis, the battery’s capacity and efficiency are calculated. Another sensitivity analysis provides insight into the importance of the active material’s characteristics on cell performance. Finally, running the battery at different current densities confirms that increasing the rate of cell charge/discharge will negatively impact the efficiency of the system.

Abstract: A series of three studies investigates theoretically and numerically the evolution, stability, and pseudomomentum transport of fully localized three-dimensional internal gravity wave packets, as they self-interact nonlinearly with their induced mean flow. The first study considers a rotating, uniformly stratified Boussinesq fluid that is stationary in the absence of waves. We derive through perturbation theory an integral expression for the mean ``Bretherton flow'' induced by fully localized wave packets influenced by the Coriolis force. We perform numerical simulations of fully localized wave packets with the predicted Bretherton flow superimposed, for a range of initial amplitudes, wave packet aspect ratios, and relative vertical wavenumbers spanning the hydrostatic and nonhydrostatic regimes. Results are compared with predictions based on linear theory of wave breaking due to overturning, convection, self-acceleration, and shear instability. We find that nonhydrostatic wavepackets tend to destabilize due to self-acceleration, eventually overturning although the initial amplitude is well below the overturning amplitude predicted by linear theory. Strongly hydrostatic waves are found not to attain amplitudes sufficient to become shear unstable, overturning instead due to localized steepening of isopycnals. Results are discussed in the broader context of previous studies of one- and two-dimensional wave packet overturning, and recent observations of oceanic internal waves. The second study considers the transmission and reflection of finite amplitude internal gravity wave packets across a reflection level in a nonrotating Boussinesq fluid with a nonuniform retrograde shear flow. We derive the critical amplitude for wave packets to transmit partially above the reflection level predicted by linear theory. We find that transmitted and reflected wave packets corresponding to strongly nonhydrostatic primary waves can interact resonantly to generate quadratically nonlinear secondary wave packets. We propose a novel weakly nonlinear mechanism to explain the generation of secondary wave packets by nonbreaking moderately nonhydrostatic primary waves, and predict the critical amplitude for its onset. Simulations are performed for a range of nonhydrostatic wave packets with small to moderately large initial amplitudes with their predicted Bretherton flow superimposed. Transmission is quantified using the pseudomomentum corresponding to upward-propagating waves above the reflection level. In most cases transmission transiently grows and decays as wave packets first cross and then reflect from the reflection level. For all but the most strongly nonhydrostatic wave packets, larger-amplitude waves exhibit smaller peak transmission, relative to the total pseudomomentum. Strongly nonhydrostatic wave packets exhibit continuous transmission well above the reflection level. When we consider the time interval for transmission to decrease to half its peak value, we find this becomes longer with larger initial amplitude. These behaviours result from the combined effects of modulational instability, and the generation and evolution of secondary wave packets. Results are discussed in the context of previous studies of one- and two-dimensional wave packet transmission and reflection. The third study considers the transmission and reflection of three-dimensional internal gravity wave packets in an anelastic gas in which the background flow models the Quasi-Biennial Oscillation (QBO). We derive an integral expression for the anelastic Bretherton flow, and the conditions for wave packets to tunnel partially through the QBO winds. Simulations are performed for a range of moderately nonhydrostatic wave packets with their predicted Bretherton flow superimposed, incident upon two model QBO profiles. Transmission is quantified using the pseudomomentum of waves above the QBO. Transmission decreases as wave packets are initialized to be progressively more nonhydrostatic. Varying initial wave amplitude is found to have no quantitative effect on transmission (relative to the initial pseudomomentum) for physically relevant initial amplitudes because nonlinear interactions with the Bretherton flow occur on a significantly slower time scale than that of transmission. Transmitted wave packets tend to grow exponentially in amplitude due to the exponentially decreasing atmospheric background mass density, ultimately inducing a local mean flow that acts to drive the waves to overturn and break turbulently. Results are discussed in the context of previous studies of one- and two-dimensional wave packet transmission and reflection, and of the theorized role of internal gravity waves in driving QBO dynamics.

Abstract: The global increase in energy usage and greenhouse gas (GHG) emissions is largely due to the ascending trend of energy consumption in buildings. To address the negative impacts of this trend, designing energy-efficient buildings is crucial. As a potential solution, thermal energy storage (TES) systems, specifically using active TES in buildings’ mass have been proposed. This thesis focuses on reducing thermal loads (i.e., space cooling and heating loads) in cold-climate buildings by investigating the implementation of two methods of active TES in walls: the use of domestic cold water (DCW) for space cooling and ventilated concrete block wall (VBW) with supply air to zone (SAZ). DCW can be circulated through thermally massive walls before regular household consumption (e.g., shower) (herein “DCW-wall”) to provide free cooling without wasting DCW. The study evaluated the cooling potentials of DCW-wall system through 3D transient thermal simulations and revealed that the system is effective in providing cooling energy to the zone. With low inlet DCW temperatures, the system was able to deliver a significant amount of cooling energy per day, which could contribute to a substantial portion of the annual energy demand for space cooling in cities with cold climates like Toronto. In VBW system air is circulated between a zone and the voided cores of a VBW, where the air exchanges heat with the wall before returning to the zone. To evaluate the system's performance, typical-day and annual energy analyses were conducted under various boundary conditions and air circulation speeds. The study found that a VBW with a 2 m/s air circulation speed throughout the day can lead to 67% more thermal energy storage when compared to having no air circulation. The annual analysis compared the energy performance between a VBW and a traditional wood-frame wall in different cold climates. In addition, an annual energy analysis showed that substituting a traditional wood-frame wall with a VBW can yield a total assisting heating and cooling of 35 kWh/m2 (wall area) for Edmonton, Canada throughout the year. Overall, this thesis presents two methods that can potentially reduce space thermal loads in cold-climate buildings through active TES solutions in wall system. The results of this research can provide valuable insights for building design and energy management in order to create more energy-efficient buildings.

The development of nonlinear optics has benefited not only the fundamental sciences but also many applications such as laser frequency conversion, optical switching, and biomedical imaging. Optical fiber, as a type of waveguide that is cost-effective, optical-power-scalable, robust, and compact, would be an intriguing platform for nonlinear optics. For a long time, people have focused on nonlinear optical dynamics in single-mode fiber. Although the spatial evolutions are limited, the single-mode system is still complicated enough and there are many interesting nonlinear phenomena being revealed such as optical soliton propagation, stimulated Raman scattering, and stimulated Brillouin scattering. Multimode optical fibers support multiple transverse modes in the waveguide. Nonlinear propagation of short light pulses results in coupling of spatial, spectral and temporal properties of the field. It is only in the last decade that people have developed efficient conceptual and simulation tools to handle such complicated evolutions. In return, exploration of the nonlinear spatiotemporal optics in multimode fiber offers a flexible and cost-efficient platform for exploring scientifically interesting nonlinear spatiotemporal phenomena. Multimode fibers also offer many possibilities for applications, such as scaling the power of short-pulse lasers and amplifiers, and optical communication with space-division multiplexing. This thesis focuses on the study of nonlinear spatiotemporal propagation in the anomalous-dispersion regime of multimode fibers. The fundamental knowledge for multimode nonlinear study is shown in Chapter 1. Kerr beam self-cleaning (KBSC) is a non-dissipative process in which optical power flows from higher-order modes to the lowest mode in nonlinear propagation. In Chapter 2, We focus on the KBSC of femtosecond pulsed beams in the anomalous dispersion regime in multimode graded-index fiber. Our results show modest beam-cleaning and strong pulse compression by a factor of ~10. We believe the results will further the understanding of KBSC and offer the potential application in high-energy pulse compression. Lasers based on soliton-like pulse-shaping dominate ultrashort pulse generation. Spatiotemporal mode-locking with anomalous dispersion can be regarded as the multimode analog of soliton lasers based on single-mode fiber, but with richer nonlinear dynamics owing to intermodal interactions. In Chapter 3, we present numerical and experimental observations of spatiotemporal mode-locking with anomalous dispersion. The results add new understanding of spatiotemporal mode-locking and illustrate the issues that must be addressed to create multimode soliton lasers. In multimode fibers, anomalous-dispersion pulse propagation involves both spatial and temporal degrees of freedom, leading to complex and intriguing soliton features that are not well understood. In Chapter 4, We report theoretical and experimental studies of highly-multimode solitons in step-index fiber. These are the first solitons to exhibit speckled intensity profiles. Numerical simulations agree reasonably with the experimental results. The results will help provide a framework for a variety of multimode nonlinear phenomena and may be relevant to applications such as space-division multiplexing communication, and imaging. Finally, in Chapter 5, some future directions are discussed.

Bayesian machine learning methods are capable of making predictions with well-quantified uncertainty, and tend to be inherently more robust to noisy data. This makes such methods particularly interesting in engineering and scientific problems that require the use of interpretable machine learning algorithms, but where the available data is sparse and noisy. In this thesis, we explore and demonstrate the usefulness of Bayesian machine learning algorithms in several categories of computational engineering problems. First, we present two Gaussian process-based algorithms for failure prediction of structural components. Second, we show how Bayesian optimization can be applied to efficiently optimize engineering designs that require to be validated by time consuming forward simulations, such as fluid-structure interaction simulations. Third, we demonstrate how Bayesian neural networks can be used for scientific discovery, and present a method to discover unknown partial differential equations from sparse data. Finally, we present a Gaussian process-based reduced-order-model capable of efficiently collecting training data, with application to fluid dynamics simulations.

The effects of velocity shear on the resistive drift wave instability in the non-adiabatic limit and the toroidal ion temperature gradient instability are investigated for a plasma of inhomogeneous density/ion temperature, respectively. For the resistive drift wave, we find that the instability growth rate decreases monotonically with increasing magnitude of shear, but we find that complete stabilization is impossible. For the ion temperature gradient instability, we find that the standard WKB approximation is insufficient to describe the full behavior of the instability, and that an analysis of the localized eigenmode problem reveals two separate unstable solutions which the WKB approximation does not predict. The impact of flow shear on these two new unstable solutions is discussed. In both resistive drift wave and ion temperature gradient instabilities, the sheared flow causes a shifting, tilting, and sharpening of the electrostatic potential eddies.

Sheet piles are geotechnical-structural elements with interlocking edges together that are driven into the ground to deliver soil retention and excavation support. Steel sheet pile walls are widely used for retaining walls, riverbank protection, seawalls, cofferdams, etc. In Nebraska, sheet pile walls prevent scouring and protect backfill for bridge abutment systems. While sheet piling is not designed and applied to resist vertical load, several recent studies attempted to investigate whether sheet piles could be employed for axial load bearing. Many projects from Europe and some in the U.S. have utilized the axial load-bearing capacity of sheet piles in bridge abutment constructions and high buildings for years. This thesis investigates the feasibility of these sheet piles' function as axial load-bearing foundation elements and how sheet piles can cooperate with axial and lateral loads for applying a water-crossing bridge in Nebraska. The research conducted large-scale direct tests to evaluate the interface parameters between soil and sheet pile to estimate the side resistance acting on sheet piles under axial load. The test data was utilized for main input parameters for the numerical simulation of sheet pile abutment to estimate bearing capacity from the analytical method. The data obtained from the analytics and simulations were compared. Furthermore, static pile load tests were conducted to evaluate the bearing capacity of a down-sized model sheet pile in a controlled test pit. A numerical simulation of sheet pile abutment was undertaken to understand sheet pile behavior under lateral and axial loads. This research evaluated how axial load can influence the failure criteria of sheet piles like horizontal deflection, shear force, and moment and how lateral load can affect the vertical settlement. The prerequisites to utilize sheet pile abutments, like effective span lengths of the bridge, excavation levels, anchors systems, and soil conditions, are provided in the parametric study. Advisors: Seunghee Kim and Jongwan Eun

CdTe thin-film solar cells have become popular due to low manufacturing cost, but this benefitcomes at the expense of cell performance. While performance is improving, CdTe is still plagued byopen circuit voltage (VOC ) losses. These losses are attributed to interface and bulk recombination,but with current methods, evaluation of these properties is convoluted. This thesis reports onTRPL simulations on a Cd(Se,Te) double heterostructures (DHs), a semiconductor material thatacts as the absorber layer in commercially relevant thin-film solar cells. TRPL was simulated onAlumina/Cd(Se,Te)/Alumina DHs, where alumina acts as an excellent surface passivation agentfor Cd(Se,Te). Simulations were conducted with COMSOL Multiphysics®, where time-dependentcharge transport equations and Poisson’s equation were numerically solved using the finite elementmethod. Models were created to investigate the effects of bulk, surface recombination, mobility, andmaterial thickness. These models used single photon excitation (1PE) and two-photon excitation(2PE) to examine the surface and bulk of the material. Results were analyzed to develop a combinedexperimental and numerical simulation procedure to distinguish and quantify bulk and interfacerecombination mechanisms in Cd(Se,Te) photovoltaic devices. This general approach can be appliedto other thin film solar cells to help determine where significant VOC losses are occurring.

Home to a disproportionate population relative to their areas, river deltas are critically important landscapes. Their locations at the interface of the land and sea make them particularly susceptible to sea level rise, while their vast extents limit characterization based on in situ observations alone. Consequently, remote sensing and numerical modeling methods and studies are needed to better understand the current processes occurring within these systems, as well as to estimate their future evolution. In this dissertation, a suite of remote sensing and numerical modeling methods are developed and applied to better understand current processes within deltas, and to project future changes. The first study is an assessment of the accuracy of discharge partitioning estimation from remotely sensed imagery of river delta networks. This analysis aggregates data from 15 site-specific studies to find that errors associated with graph-theoretic estimates of discharge partitioning are consistent across a diverse set of delta landscapes. In the second study, reduced-complexity modeling simulates the evolution and anthropogenic modification of idealized river deltas. Simulation of different hydrodynamic scenarios, and routing of passive particles through the landscape, enables characterizations of the impact that natural morphological differences, anthropogenic modifications, and different flow conditions have on material transport. The results suggest that material type exerts a first-order control over particle behavior, and human modifications to the landscape reduce hydrological connectivity. The third and fourth studies present reduced complexity modeling of deltaic evolution over hundreds of years to investigate the relationship between surface processes and subsurface form within deltaic environments in the context of their future evolution. In the third study, testing of different input sediment compositions and steady rates of sea level rise suggests that both variables influence surface morphology and subsurface connectivity. The fourth study considers the impact of sea level rise acceleration, and finds that the dynamic response of surface channels to an unsteady rate of sea level rise changes based on its magnitude and trajectory. These changes on the surface are mirrored to an extent in the subsurface, which can only be estimated from surface information if the sea level change is relatively steady for some period of time. The results of these studies provide guidance for both policy makers and managers of deltas, as it is clear that humans are significantly impacting the natural processes of these landscapes. Taken together, research conducted as part of this dissertation provides information about current processes and potential future evolution of delta landscapes.

Immersed boundary (IB) methods are attractive due to their ability to simulate flow over complex geometries on a simple Cartesian mesh. Unlike conformal grid formulation, the mesh does not need to conform to the shape and orientation of the boundary. This eliminates the need for complex mesh and/or re-meshing in simulations with moving/morphing boundaries, which can be cumbersome and computationally expensive. However, the imposition of boundary conditions in IB methods is not straightforward and numerous modifications and refinements have been proposed and a number of variants of this approach now exist. In a nutshell, IB methods in the literature often suffer from numerical oscillations, implementation complexity, time-step restriction, burred interface, and lack of generality. This limits their ability to mimic conformal grid results and enforce Neumann boundary conditions. In addition, there is no generic IB capable of solving flow with multiple potentials, closely/loosely packed structures as well as IBs of infinitesimal thickness. This dissertation describes a novel 2$ ^{\text{nd}} $ order direct forcing immersed boundary method designed for simulation of two- and three-dimensional incompressible flow problems with complex immersed boundaries. In this formulation, each cell cut by the IB is reshaped to conform to the shape of the IB. IBs are modeled as a series of 2D planes in 3D space that connect seamlessly at the edges of the cut cells, in a way that mimics conformal grid. IBs are represented in a continuous and consistent fashion from one cell to another, thus eliminating spatial pressure oscillations originating from inconsistent description of the IB as well as the traditional stair-step problem, leading to a more accurate resolution of the boundary layer. Boundary conditions are enforced at the exact location of the IB devoid of interpolation, which guarantees sound simulations even on grids with high aspect ratio, and enables simulations of flow packed with multiple IBs in close proximity. Boundary conditions for each phase across the IB are enforced independently, yielding a unique capability to solve flows with zero-thickness IBs. Simulations of a large number of 2D and 3D test cases confirm the prowess of the devised immersed boundary method in solving flows over multiple loosely/closely-packed IBs; stationary, moving and highly morphing IBs; as well as IBs with zero-thickness. Extension of the proposed scheme to solve flow with multiple potentials is demonstrated by simulating transfer and transport of a passive scalar from an array of side-by-side and tandem cylinders in cross-flow. Aquatic vegetation represented by a colony of circular cylinders with low to high solid fraction is simulated to showcase the prowess of the current numerical technique in solving flow with closely packed structures. Aquatic vegetation studies are extended to a colony of flat plates with different orientations to show the capability of the developed method in modeling zero-thickness structures.

In this thesis a four-species, four-step combustion model is coupled to an adaptive mesh refinement (AMR)-enabled compressible flow solver to simulate detonation attenuation and subsequent re-initiation following interaction with a half-cylinder obstacle in a stoichiometric methane-oxygen mixture. Six distinct categories of detonation behavior are identified: detonation quenching, critical ignition without detonation re-initiation, critical detonation re-initiation (CDR), CDR without transverse detonation, critical transmission, and unattenuated detonation transmission. It has been concluded from these simulations that the transverse detonation wave plays an important role in the re-initiation events of a quenched detonation. Only rare cases, both experimentally and numerically, show re-initiation without transverse detonation. Some cases exhibit a pocket of unburned gas, but it is not a necessary feature for re-initiation to occur. This study demonstrates the abilities of a 4-step combustion model with its ability to capture the transverse detonation wave, which has not been captured by previous simple chemistry models.

With the recent progress in technologies such as hydraulic fracturing and horizontal drilling, unconventional resource development has exploded in recent years. Nevertheless, significant challenges remain for shale reservoirs because of the extensive number of uncertainties. Without proper characterization processes, extracting economic values from these projects will be difficult, and optimizations for future plans will also be challenging. Therefore, efficient models in production mechanism, management and optimization have gradually become hot topics among the petroleum industry. This study aims to address various crucial challenges during the production process of shale reservoirs, including unconventional well gas oil ratio (GOR) characterization, choke management, and well spacing optimization. Due to the ultra-low permeability and porosity, the fluid phase behavior in shale reservoirs significantly differs from the conventional fluid behavior and increases the production forecasting complexity. A substantial effort to better understand the mechanism is to characterize the unconventional well GOR, which always plays as a critical indicator to help predict long-term oil/gas production trends and develop appropriate production strategies. In this research, GOR behavior was first evaluated by a set of comprehensive sensitivity studies in a tight oil well model, which helped to investigate the key drivers that can impact the GOR response in unconventional resources. Then, a parent-child well-set case in Eagle Ford was presented. Through detailed characterization of the producing GOR, an improved understanding of the parent-child well behavior and the fracture hit impact can be obtained. In order to improve the efficiency of field operation, seeking for proper operation plan has been the focusing topic among the oil and gas industry. Choke management strategy selection is one of the essential measures to regulate fluid flow and control downstream system pressure, which could significantly impact the well production rate and estimated ultimate recovery (EUR). In this study, we utilized the non-intrusive embedded discrete fracture model (EDFM) method to handle complicated fracture designs and predicted the long-term EUR from conservative to aggressive choke strategy. Meanwhile, a series of sensitivity studies were presented to evaluate the impacts of various factors on shale gas production, including fracture permeability modulus, fracture closure level, and natural fractures network. The model becomes a valuable stencil to design fracture closure and complex fracture networks, which is a significant improvement for a more reliable choke management model in unconventional area. Another crucial part for well performance improvement is well spacing optimization. Consistent estimates of well spacing help reduce the impact of complex uncertainties from unconventional reservoirs, thereby improving the EUR and enhancing economic growth. We demonstrated a case study on well spacing optimization in a shale gas reservoir located in the Sichuan Basin in China. By using the advanced EDFM technology, complex natural fractures can be effectively captured and simulated. In this study, five different well spacing scenarios ranging from 300 m to 500 m were simulated individually to find the optimum well spacing that maximizes the economic revenue. As the practicability and the convenience showed in this workflow, it becomes feasible to be utilized in any other shale gas well.

Numerical simulations are performed to support a combustion experiment campaign in partial gravity in a centrifuge designed for use in conjunction with the NASA Glenn Research Center’s Zero Gravity Research Facility (a 5.2 second drop tower). The centrifuge is a circular dome chamber of volume ~ 0.3 m3 with 81.3 cm diameter. The artificial gravitational field is controlled by the rotation rate of the chamber. This is complicated by gravitational gradients as a function of radius and by Coriolis force as a function of flow velocity. The model is constructed with Ansys FLUENT utilizing a rotating non-inertial reference frame and simulates the entire chamber volume containing a heptane candle with a wick length of 10 mm × 3.18 mm diameter located at 32 cm from the centrifuge center. Simulation results locally near the candle are compared to a series of experiment images where the flame tip bends in the Coriolis force direction. The study investigates the recirculation effects and the relation between the buoyancy and the Coriolis force in how they affect the flame. The model simulates the experiments well and suggestions are made to avoid recirculation effects in future centrifuge experiments.

A thorough literature review identified lack of precision control over quality of droplets generated by the currently available industrial sprayers and a growing need for higher quality droplets in the coating industry. Particularly, lack of knowledge and understanding in continuous inkjets (CIJ) and drop-on-demand (DOD) technologies is identified as significant. Motivated by these needs, this dissertation is dedicated to computational fluid dynamics (CFD) and scaling studies to improve existing inkjet technologies and develop new designs of efficient coating with single and/or multiple piezoelectric sensors to produce on-demand droplets. This dissertation study aims at developing a new DOD type coating technology, but it required understanding the effects of paint viscosity on droplet generation mechanism, an effective droplet delivery method to the coating surface, painted surface quality and control system of the DOD among others. Waterborne (WB) paints are chosen as the working liquid to identify three different DOD designs capable of creating a stream of mono-dispersed droplets. Volume-of-fluids (VOF) multiphase model explored the droplet creation process and effects of various parameters on the droplets’ quality. The law approach scaling analysis identified scaling laws to scale up these numerical results conduced for the laboratory-scale DOD to the large industrial scale inkjet nozzles.

Viscosity is an essential property of each fluid we encounter in everyday life. It can exhibit direction-dependent features, for example induced by the external orientation of suspended anisotropic particles in a homogeneous magnetic field. Experimentally and numerically, we study the effect of the anisotropic viscosity tensor resulting from magnetic alignment of ferrimagnetic rod-shaped particles on the statistical description of turbulence. As the particles are much smaller than the smallest scales of the fully turbulent flow, this allows investigating the impact of a molecular quantity on the various macroscopic scales. The experimental setup consists of a von Kármán swirling flow. In the first part of this thesis, it is shown to exhibit a large-scale flow structure with a four-quadrant in- and outflow mode in the transverse centre plane. This mean-flow instability degrades homogeneity and isotropy throughout the cell and in particular also in the centre, which is commonly assumed to be transversely isotropic. Applying a pseudo-random disc reversal suppresses this feature and yields a prototypical axisymmetric poloidal pumping flow. Variations of the forcing scheme are studied for a range of Reynolds numbers, characteristic time scales and reversal patterns. In the particle-laden fluid, the two baseline flow states with or without flow structure are then exposed to a magnetic fleld. A fixation of the azimuthal orientation of the flow structure results, leaving a signature on all scales of the flow. In the randomly forced flow, however, we do not observe a response of the small scales which is decoupled from the large-scale modification at the given measurement sensitivity and resolution. For a perfect alignment of the particles, the impact of the particle-induced anisotropic stress tensor is studied numerically in a box of homogeneous turbulence. Three volume fractions are compared at moderate Reynolds numbers. The influence of an increasing particle concentration is characterized by a proportionally enhanced dissipation and larger Taylor microscales in the field-parallel direction. The effects are strongly scale-dependent and largest at the highest wavenumbers, at which the vortices orient in the anisotropy direction and the averaged Reynolds stresses approach a one-component turbulence state.

Silicon solar cells are the leading force in the photovoltaics market due to their low mass-production costs and wide range of application scenarios. Enhancement of optical generation and reduction of recombination loss are two important aspects of high-efficiency solar cell development. Black silicon (b-Si) texturing, one of the most effective light-trapping techniques, has received considerable attention for solar cell applications. However, the development of high-efficiency b-Si solar cells is significantly hindered by a lack of in-depth understanding of the electrical properties of b-Si textures. It is widely observed that the inferior electrical performance of a b-Si emitter outweighs the gain in optical performance, resulting in lower efficiencies. However, it is also found that the surface recombination loss of a passivated b-Si texture can be unexpectedly low, which could contribute to high efficiency for some solar cell architectures. This thesis aims to determine (1) how b-Si surface morphology should be optimized to achieve high performance solar cells and (2) whether b-Si textures are indeed better than conventional textures. This thesis first provides a literature review of solar cell surface texturing techniques with an emphasis on b-Si texturing. The primary research approaches of this thesis are then shown. A systematic investigation of b-Si field-effect passivation enhancement is presented, exploring the root cause of the low surface recombination loss for undiffused b-Si surfaces and determining the optimal combinations of surface passivation schemes and b-Si morphologies. Significant field-effect passivation enhancement is found when the surface charge density is moderate, and the enhancement strength increases as the distance between the opposite surfaces decreases. Next, a fundamental study of POCl3-diffused b-Si emitters with various textures is presented, covering dopant distribution characteristics, emitter lateral conductance behaviour, and recombination loss mechanisms. Optimization strategies for b-Si emitters are also proposed based on the findings. It is shown that, in general, b-Si emitters with shallow nanofeatures can achieve higher electrical performance than those with deep nanofeatures. Finally, the weighted average reflectance (WAR) is proven as an effective surface morphology metric for a wide range of surface textures that can forecast the efficiency at the early stage of b-Si solar cell fabrication. By correlating solar cell performance reported in the literature to WAR, it is shown that multi-crystalline silicon solar cell efficiency can be improved with b-Si texturing, and this is predominately attributed to an increase in short-circuit current density via the blue response improvement. It is also found that some b-Si textures can improve the performance of mono-crystalline silicon solar cells. Device simulations show that the electrical performance of hierarchical (combination of microtexture and nanotexture) and inverted-pyramidal b-Si textures can be comparable to or even better than random pyramids. As such, these textures show great potential for mono-crystalline silicon solar cells.