200 research outputs found

    Asymptotic theory for a Leidenfrost drop on a liquid pool

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    Droplets can be levitated by their own vapour when placed onto a superheated plate (the Leidenfrost effect). It is less known that the Leidenfrost effect can likewise be observed over a liquid pool (superheated with respect to the drop), which is the study case here. Emphasis is placed on an asymptotic analysis in the limit of small evaporation numbers, which proves to be a realistic one indeed for not so small drops. The global shapes are found to resemble "superhydrophobic drops" that follow from the equilibrium between capillarity and gravity. However, the morphology of the thin vapour layer between the drop and the pool is very different from that of classical Leidenfrost drops over a flat rigid substrate, and exhibits different scaling laws. We determine analytical expressions for the vapour thickness as a function of temperature and material properties, which are confirmed by numerical solutions. Surprisingly, we show that deformability of the pool suppresses the chimney instability of Leidenfrost drops

    Digital Microfluidics as a Reconfiguration Mechanism for Antennas

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    This dissertation work concentrates on novel reconfiguration technologies, including design, microfabrication, and characterization aspects with an emphasis on their applications to multifunctional reconfigurable antennas. In the literature, reconfigurable antennas have made use of various reconfiguration techniques. The most common techniques utilized revolved around switching mechanisms. Other techniques such as the incorporation of variable capacitors, varactors, and physical structure manipulation surfaced recently to overcome many problems faced in using switches and their biasing. Usage of fluids (micro-fluidic or otherwise) in antennas provides a conceptually easy reconfiguration mechanism in the aspect of physical alteration. However, a requirement of pumps, valves, etc. for liquid transportation makes the antenna implementations rather impractical for the real-life scenarios. This work reports on design and experiments conducted to evaluate the electrowetting on dielectric (EWOD) driven digital microfluidics as a reconfiguration mechanism for antennas

    Design of robust slow-speed ships for sustainable operation

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    Phd ThesisMulti-objective optimisation that considers the energy efficiency and economic success is an important aspect of ship design and operation. Both the hydrodynamic and economic performance characteristics need to be addressed in the early stages of the design, and secured during the life span of a ship. Because of the conflicting nature of these two objectives, there are various trade-offs at stake in the goal for making ships more efficient and greener to comply with IMO regulations while reducing the building and operating costs and increasing the profitability at the same time for all stakeholders especially owners and operators. In attempt to reduce the amount of greenhouse gas emissions from ships, and hence to achieve a lower EEDI value, this research approaches the problem of improving the energy efficiency of ships. That is achieved by optimising the hull design over a speed range through parametric modification to reduce resistance and required power, and also through adopting slow steaming concept. Moreover, the research aims to determine the best practice to reduce the annual cost of running a ship and to increase the annual revenue as well as to make the ship a more profitable investment over her life span. The profit per tonne.mile and the net present value NPV are estimated in the economic analysis to be used as indicators to compare alternative designs for different routes and market conditions scenarios. To achieve this aim, the main operational and economic aspects such as the fluctuations in the fright rates and fuel prices in the shipping market are covered in the economic analysis. In addition, the acquiring price and salvage value are included in order to obtain solid comparisons. An optimisation framework using a VBA macro code has been developed based on the concept of Pareto optimality to assess decision making, and to determine robust designs as well as operational profiles based on results from the hydrodynamic model, environmental impact model, and the economic model. The optimisation process is carried out for a Panamax tanker case study using 5 parameters and a set of constraints for the hull parameters and speed. The outcome from the optimisation framework is a set of Pareto optimal solutions where weight factors are appointed to give the flexibility when addressing the importance of each individual function. The solutions are presented graphically to form what is known as Pareto front which determines the design space and the trade-offs between the different competing objective ii functions. This optimisation framework could assist decision making where it is possible to choose a robust design or designs that offer a near-optimum performance regardless any fluctuations in the market and or the operation profile, and eliminate any significant sub-optimal design

    The broaching of ships in following seas

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    The two aims of this work were: (1) to develop a theoretical technique for determining the conditions where a broach would occur, and (2) to identify the principal factors affecting the liability of a ship to broach. The first step was to develop a mathematical model based on the conventional manoeuvring equations with coefficients which were functions of the ship's longitudinal position in the wave, but independent of encounter frequency. Next, a theoretical method for calculating the values of some of the coefficients as functions of wave position was developed using a strip theory approach and the results compared with those obtained experimentally. The experimental technique involved using a planar motion mechanism to oscillate a constrained model balanced on a wave created by a wave dozer in a circulating water channel. Although the agreement was poor and experimental scatter high for some of the coefficients, the more important ones were predicted quite well using the theory. Constrained model experiments were also --carried out in calm water in order to determine the approximate value of the roll coupling terms and it was found that, since they were small, the roll equation could be ignored as a first approximation. It was then possible to study the stability of the lateral and longitudinal motions separately for various wavelengths and to determine that the principal factor causing a broach was the large wave induced yaw moment combined with the small restoring moment available from the rudder operating with reduced effectiveness. The lateral and longitudinal equations were then combined using a digital/analogue hybrid simulation permitting the conditions which caused a broach to be determined. When the results from the simulation-were compared with results which had already been carried out by the Admiralty Marine Technology Establishment at Haslar there was fairly good agreement, implying that this method could be used to determine whether a proposed design would meet an acceptable standard. Finally, possible improvements to the simulation were suggested and guidelines for reducing the liability to broach were given both for the operator and the designer

    Numerical modelling of a sloped wave energy device

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    Dynamics and distribution of immunoglobolin E receptors : a dialog between experiment and theory

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    This dissertation explores the dynamics and distribution of immunoglobulin E receptors (FceRI) on mast cells by drawing on the techniques of experimental and theoretical physics. The motivation for these investigations is provided by a considerable interest in the transmembrane signaling mechanisms of immunoreceptors, especially when triggered with membrane-bound ligands. Experimental investigations quantify the spatiotemporal dynamics of the redistribution of FceRI due to membrane-bound monovalent ligands, using total internal reflection fluorescence microscopy and single-particle tracking. When mast cells contact such substrates, receptor clusters form at cell-substrate contact points. The initial rate of accumulation of receptors into these contact points or cell protrusions is consistent with diffusion-limited trapping. Over longer timescales (\u3e10 s), individual clusters move with both diffusive and directed motion components and eventually coalesce to form a large central receptor patch surrounded by a receptor cluster depletion zone. Detailed analysis of single-particle trajectories show that receptors maintain their diffusivity when confined within receptor clusters, and increase their diffusivity (above that of monomeric unliganded FceRI) in central patches. To study the kinetics of central patch formation, a new coalescence theory described by a melding process, which is not instantaneous, was developed. In these theoretical investigations, the difficult problem of moving boundaries is encountered. To handle the complexity, which stems from boundary growth due to particle melding, the study is divided into three parts. The first is about stationary trapping problems investigated by the standard defect technique, and the second is about a validity study of an adiabatic approximation for moving boundaries. In the last part of this dissertation, a new coalescence theory is developed, which is based on a completely self-consistent approach. Here, the time dependence of the moving boundary is not prescribed but obtained through feedback. Comparison of experiment and theory shows that observed biological cluster coalescence is delayed at early times and occurs at a faster rate at later times than predicted by a simple theory. The incompatibility at early times is addressed by a generalization of the theory to incorporate a time-dependent melding process by a memory concept, which quantitatively explains the observed delay
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