Instabilities in multiphase and icing flows

The problem of the stability of water-coated ice layer is investigated for a free surface flow down an inclined plane for the cases of normal and thin (boundary- layer scale) water films. For the case of boundary-layer scale water film a Froude- based double-deck theory is developed which is then used to investigate linear two-dimensional (2D), three-dimensional (3D) and nonlinear 2D stability of the problem. The new mode of upstream-propagating instability arising because of the ice layer is found and its properties investigated. In the linear double-deck, analytic expressions for the dispersion relation and neutral curves are obtained for the case of Pr = 1. For the general case, linear stability problem is solved numerically using new 4th order finite-difference scheme developed for Orr-Sommerfeld equations. Non-linear double-deck equations are solved with a new 2nd order in space global- marching type scheme and the effects of nonlinearity are analysed. An explanation of the physical mechanism leading to the upstream propagation is derived. The effect of the intersection and branch exchange between the interfacial mode and a shear mode is discovered in a 2-fluid plane Poiseuille flow and investigated in detail using linear stability theory and the numerical approach developed for the free-surface flows. The interaction between three instability modes present in the problem is analysed. It is shown that the question of mode identity becomes complicated because of the discovered intersection and the methods of establishing mode identity are discussed. Finally, the longwave asymptotic analysis of the ice layer under a water/air plane Poiseuille flow is performed. The effect of ice on the modes present in the problem is discussed

Microfluidic systems for in situ formation of nylon 6,6 membranes.

A microfluidics based, localised formation of nylon 6,6 membranes has been undertaken. The study demonstrates the feasibility of maintaining stable aqueous/organic interfaces for xylene within simple linear flow channels. Glass fabricated structures were used with adipoyl chloride and hexamethylenediamine in the organic and aqueous phases, respectively, in order to achieve nylon 6,6 interfacial polymerisation. Localised membrane formation was investigated in flow channels of different geometries over a wide range of flow rates (500–4000 μl/min), with Reynolds numbers ranging from 8.4 to 67.2. The results demonstrate that interfacial polymerisation occurs consistently over a wide range of flow rates and of flow entry angles for dual aqueous/organic solvent input. However, creation of uniform planar film structures required careful optimisation, and these were best achieved at 2000 μl/min with a flow entry angle of 45°. The resulting membranes had thicknesses in the range between 100 and 300 μm. Computational modelling of the aqueous/organic flow was performed in order to characterise flow stability and wall shear-stress patterns. The flow arrangement establishes a principle for the fabrication of micromembrane structures designed for low sample volume separation, where the forming reaction is a facile and rapid interfacial process

Computational modelling of cavity arrays with heat transfer using implicit large eddy simulations

This PhD programme was sponsored by the United Kingdom Atomic Energy Authority (UKAEA). The aim of this study is to conduct advanced computational modelling of a cooling device used in the fusion process which recycles waste energy. The development of efficient, water cooled tiles, that can sustain heat loads of approximately 20 MW (in quasi-steady state conditions) is the motivation of the current work. The information presented here will contribute to thermal-mechanical analysis, to be conducted at the Joint European Torus (JET) in future years. The devices known as “Hypervapotrons” have been used successfully at JET to provide a ion dump that dissipates residual energy from the fusion process. A capability to model the flow structure and heat transfer, across a large number of geometric and material options is provided within. Differences in geometry, result in changes to the flow structure and heat transfer rates. The desire to optimise such designs relies upon the fundamental understanding of the flow field within the main section, where the geometry may be defined as a cavity array. The benchmark case of a lid driven cavity flow was used for the validation of the flow field solution. Solutions using high resolution methods in the formulation provided a good comparison with established experimental data. Therefore, validation of incompressible, Implicit Large Eddy Simulations (ILES) for a wall bounded, three dimensional, turbulent flow is provided within. The sensitivity of the high order reconstruction in conjunction with the characteristics based scheme (Drikakis ;Rider, 2005), to resolve turbulent flow structure is provided here. The solution response to grid resolution and a regularised velocity profile at the upper lid surface is also detailed. The investigation provided insight and confidence in the turbulence modelling approach which is relatively recent. It was also demonstrated through the lid driven case (and later in the Hypervapotron cases) that high order reconstruction was a simulation prerequisite, based on grid resolutions used within. Additional validation was also provided against numerical and analytical solutions for the Conjugate Heat Transfer (CHT) and scalar temperature field. Where appropriate both unsteady and steady problems based on a composite, three layer medium are detailed to provide preliminary validation for the implementation of the temperature scalar and conjugate boundary conditions. Unfortunately, it was not feasible to solve the coupled problem with an explicit solver as used in this study. However, it is suggested that the initial stages of thermal boundary layer development may be observed leading to the locations of incipient boiling. Two different Reynolds numbers were considered for the Hypervapotron ”Standard” geometry, Re=12000 and Re=18000. The different flow structures show that the cavity aspect ratio of the Standard design promotes lower flow speeds at the cavity base, since two or three counter rotating vortices coexist inside the cavities depending on Reynolds number. A detailed analysis on the impact of the number of repeating units within the computational domain is also provided. Results are presented of ensemble averaged quantities based on the Reynolds decomposition. The temperature distribution present in the solid, fluid and its interface for the thermally developing case is achieved. In addition the total and decomposed heat fluxes are presented for the Hypervapotron (Standard design) which provides similar comparison with recent Reynolds Averaged Navier-Stokes (RANS) simulations.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Advanced Ignition Modelling for Pre-chamber Combustion in Lean Burn Gas Engines

Lean burn combustion systems present a viable route to emissions reductions. However, sustainable and conrolled combustion of lean mixtures can be challenging. Scavenged pre-chamber ignition systems aim to address this challenge by creating favourable ignition conditions close to stoichiometry in the spark region. The main lean charge ignition is then delivered by flame jets propagating through the nozzles connecting the pre-chamber to the cylinder. Accurate and fast CFD modelling of the mixture formation and early flame kernel development in the pre-chamber are essential for the design of such systems. The initial stages of ignition in spark-ignited engines typically occur at time scales, temperatures and length scales falling outside of the remit of conventional CFD techniques prompting development of specialised ignition models. A review of the models currently available highlighted a gap in the technology currently available in commercial CFD codes. Simplistic models based on direct energy transfer or fixed temperature kernel development are readily available but these models to not provide an accurate representation of the spark growth. More complex models are also available which are based on complex chemistry/turbulence interaction. While these models can provide accurate solutions they require standalone chemistry solvers or spatial temperature distribution calculations. These can be time consuming to solve, making the models less than ideal for simulations in a production environment. A novel spark model has been developed by Ricardo and implimented into the CFD software VECTIS, allowing for an accurate specification of the spark-ignition process. The model covers all stages of spark discharge from breakdown and the formation of the initial kernel and includes a predictive model for the initial flame kernel size. The flame kernel evolution is computed via a 1D variable temperature model incorporating plasma physics with two-way coupling with 3D CFD. The detailed chemistry effects are included through flame speed and mixture properties tabulation. This paper illustrates the principles and applications of the developed model. The model is then applied to the analysis of a novel pre-chamber ignition system and the results are compared with measurement data. A study is performed to investigate the sensitivity of the results to the input parameters within the spark ignition model

Mesoscale modelling and simulation of macromolecule transport in microfludic channels

This thesis concerns the numerical simulation of dilute macromolecular solutions. Present work details the development of a novel mesoscale simulation method. The developed modeling approach is capable to describe both the macroscopic flow field of the carrier liquid and the micromechanical behaviour of the transported large molecules. In this modeling method, the concept of micromechanical structures is introduced in order to represent macromolecules. The motion of the considered mechanical structures is governed by forces arising from the motion of the bulk fluid phase and microscopic forces arising from stochastic Brownian motion of the solvent molecules. This document presents the motivation, the objectives and systematic steps of the model development. The work presents detailed discussion, verification and validation of the developed modeling method.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Implicit large eddy simulation of environmental urban flows

Many environmental flows are turbulent flows. Depending on the physical aspects of the wind and the urban topology, turbulence might result into unfavourable or even dangerous conditions for the pedestrians. Turbulence can also play a very important role in the transport of toxic pollutants from accidental or intentional releases. Thus, it is vital to understand its complex characteristics so that its features are accurately predicted when computational methods are used. Real urban environment involving separation and reattachment regions provides an excellent testcase for investigating such complex flows. This thesis is focused on analysing the physics involved in flows around building models pertinent to environmental flows in urban areas and to evaluate the applicability of Implicit Large-Eddy Simulation in simulating the specific type of flows. For this purpose, a number of high resolution schemes in the context of Implicit Large-Eddy Simulation (each representing di erent degrees of spatial discretisation accuracy) was assessed. The evaluation of the schemes involved direct validation against experimental data as well as comparisons with DNS and LES data regarding flows within roughness element arrays in staggered arrangements. Initially, the flow within an uniform height cubical matrix was simulated. Four numerical schemes were tested in three di erent grid resolutions. The results were found in very good agreement with the Laser Doppler Anemometry data and they even exhibit DNS-like characteristics in specific locations of comparisons. Thus, it was concluded that high order spatial discretisation schemes allow the accurate representation of reality even in relatively coarse computational meshes. The second case under investigation involved flows within a more realistic representation of urban topology. Results obtained within an array of sixteen elements with five di erent heights reveal that although the roughness of the area is increased, the wind’s velocity profile above the obstacles shares almost the same slope as in the case of the array of the four cubical element. It is believed that this thesis has expanded the range of applications in the context of Implicit Large Eddy Simulation using high resolution schemes and contributed in persuading the scientific community for its potentials.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Implicit large eddy simulation of turbulent duct flows

Ducts can be found in ventilation systems, cooling ducts and blade passages of turbines, centrifugal pumps and many other engineering installations. The properties of the flow in ducts can significantly affect the performance and efficiency of these installation areas. The majority of the flows in ducts and engineering applications are turbulent. The work presented in this thesis focuses on the analysis of turbulent flows inside square sectioned ducts and ducts with bends. The accuracy of three different high resolution high order schemes in the context of Implicit Large Eddy Simulation (ILES) is analysed. The influence of a low Mach limiting technique, Low Mach Number Treatment (LMNT) is also studied. The schemes employed are Monotonic Upwind Scheme for Scalar Conservation Laws (MUSCL) with a 2nd order Monotonized Central (MC) and 5th order limiter, and a 9th order Weighted Essential Non-Oscillatory (WENO) limiter. The first case studied is a duct of square cross section . In the absence of experimental data for the duct case, the data from a plain channel flow is used to shed light on the results. The flow analysis points out the generation of secondary motions created by the existence of surrounding walls. All schemes employed lead to a similarly developed turbulent flow that is used to provide the turbulent boundary profile for the following case. LMNT proves to significantly assist MUSCL 2nd and 5th, that use it, in providing a turbulent profile similar to that of WENO 9th that did not employ the technique but is inherently less dissipative. The second case under study is that of a square sectioned duct with a 90o bend. The simulation output is in good agreement both qualitatively and quantitatively with the experimental data available in the literature. The generation of secondary flows inside the bend is observed without flow separation. Although the turbulent flow entering the domain is almost the same for all cases, differences between the schemes are noticed especially after the middle of the bend. LMNT leads to an overprediction of turbulence after that area for both schemes employing it while WENO 9th without LMNT provides the most accurate results compared to those provided by the experiment. The results demonstrate applicability of ILES to strongly confined flows with secondary motions and shed light on cognitive properties of a wide range of state of the art schemes.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

High resolution and high order methods for RANS modelling and aerodynamic optimization

With the optimisation of fixed aerodynamic shapes reaching its limits, the active flow control concept increasingly attracts attention of both academia and industry. Adaptive wing technology, and shape morphing airfoils in particular, represents a promising way forward. The aerodynamic performance of the morphing profiles is an important issue affecting the overall aerodynamic performance of an adaptive wing. A new concept of active flow, the Active Camber concept has been investigated. The actuator is integrated into the aerofoil and aerofoil morphing is realized via camber deformation. In order to identify the most aerodynamically efficient designs, an optimisation study has been performed using high resolution methods in conjunction with a two equation eddy viscosity model. Several different types of previously proposed compressible filters, including monotone upstream-centered schemes for conservation laws (MUSCL) and weighted essential non-oscillatory (WENO) filters, are incorporated and investigated in the present research. The newly developed CFD solver is validated and the effect that high resolution methods have on turbulent flow simulations is highlighted. The outermost goal is the development of a robust high resolution CFD method that will efficiently and accurately simulate various phenomena, such as shock/boundary layer interaction, flow separation and turbulence and thus provide the numerical framework for analysis and aerodynamic aerofoil design. With respect to the latter a multi-objective integrated design system (MOBID) has been developed that incorporates the CFD solver and a state-of-the-art heuristic optimisation algorithm, along with an efficient parametrization technique and a fast and robust method of propagating geometric displacements. The methodologies in the MOBID system resulted in the identification of the design vectors that revealed aerodynamic performance gains over the datum aerofoil design. The Pareto front provided a clear picture of the achievable trade-offs between the competing objectives. Furthermore, the implementation of different numerical schemes led to significant differences in the optimised airfoil shape, thus highlighting the need for high-resolution methods in aerodynamic optimisation.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements