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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

Advanced receiver design for modernised GNSS signals

Simons, Edmund January 2015 (has links)
The Double Estimation Technique (DET) is a comparatively new GNSS tracking loop architecture that mitigates the ambiguity present in pseudoranges from Binary Offset Carrier (BOC) signals. These signals are part of ‘modernised’ GNSS development, and will see extensive use on both existing and in-development constellations. Many techniques have been proposed that either partly or wholly eliminate the problem of ambiguous BOC pseudoranges: proposal and investigation of new techniques is still an active area of research, however many of the techniques described in literature must make a trade-off between ambiguity mitigation and computational complexity. Few of the techniques have been demonstrated on hardware receivers and so knowledge of their effectiveness and practicality is limited. Comparison between the Double Estimator Technique (DET) and the commonly-used alternative Bump-Jumping (BJ) channels’ operation was obtained from a hardware receiver running customised tracking channels and from simulation channels (developed from MathCAD programs supplied by Dr. Stephen Hodgart) designed to closely match the parameters of the hardware receiver. Both DET and BJ channels were compiled for the SGR-ReSI – an FPGA-based receiver developed by SSTL as the first in the new generation of GNSS receivers. These comparison data show that an implementation of DET as described in [Blunt 2007] is not without some limitations, and this work shows how it is possible to produce in the DET a ‘false-lock’ condition that was previously not thought possible due to the two independent estimates. Several updates to the DET tracking architecture are herein proposed and tested, and with these additions, the DET can be made more robust to conditions that can severely disrupt operation of other techniques such as Bump-Jumping.
42

From tree to plank : a multidisciplinary approach to the study of wood use in boatbuilding in the Red Sea

Semaan, Lucy January 2014 (has links)
This thesis looks at the use of wood in boatbuilding in the Red Sea from classical antiquity until present times. It draws on primary sources and archaeological evidence from the Graeco-Roman period extending from the 8th century BC to the eve of Islam in the 7th century AD, and from the medieval Islamic period stretching from the 7th century AD to the 15th century AD. In doing so, it seeks to shed the light on timber trade and the timber exploitation processes; and examine how these were interlinked with the geopolitics and socio-economics of the time. It then portrays the Red Sea areas not only as wood importers from a wide array of regions such as the Mediterranean, East Africa, and South Asia, but also as beneficiaries of local wood resources for their boatbuilding needs. In that respect, it stresses that little attention has been given to the exploitation of local woods in favour of a greater reliance on Indian timber, and more specifically on teak. Thus, it suggests the need to consider other timber species than teak which were equally suitable for shipwrightry, as well as other western Indian Ocean wood providers. To illuminate and colour past interpretations of the subject, in addition to document a vanishing craftsmanship, this thesis has drawn on ethnographic research in Egypt, Sudan, Eritrea, Djibouti, Saudi Arabia and Yemen. The ethnographic enquiry focused on the types of timber used in boatbuilding in the Red Sea and their vernacular names, the timber exploitation processes, and the factors and variables that come play in the metamorphosis of a tree into a boat part. Finally, by interlinking diverse pathways of enquiry drawing on primary sources, archaeology and ethnography, this thesis aims to provide the field of maritime culture and maritime archaeology a deeper understanding of wooden boatbuilding in the Red Sea.
43

The behaviour of inclined anchors subjected to axial static and repeated loading in dry sand

El-Haggan, Sherif Mostafa January 1978 (has links)
No description available.
44

Development of the boundary conditions required for simulating a wave tank using smoothed particle hydrodynamics

Pearce, Mark Graham January 2017 (has links)
Smoothed Particle Hydrodynamics, SPH, is a Lagrangian method that has been applied to a number of fields including maritime applications. However little work has been done concerning the simulation of the motion of ships' hulls. A weakness of SPH is in the modelling of boundaries, both permeable and impermeable. This thesis investigates some of the latest SPH methods used to simulate these boundaries. Particular emphasis is placed upon the development of an approach that idealises the permeable inflow and outflow boundaries of a domain that are required to simulate the boundaries of a wave tank. In the method presented the inflow boundary has been designed such that it can also generate waves. This allows for the simulation of a ship hull subject to head waves. The inflow boundary is also capable of creating a mean flow speed along with the wave generation. The outflow boundary serves as both a plane which allows particles that have crossed it to be removed and also incorporates a sponge layer that is designed to damp out incoming waves and prevent any unphysical wave reflection. These developments, used together, allow a small section of a wave tank to be simulated, this requires a minimum of computational resources. Each new development has been tested against published data from experiments or numerical simulation. The computational models discussed in this thesis compare the performance of the new approach against experimental data and simulations using both classic SPH and Reynolds-Averaged Navier-Stokes methods. The wave generation and damping methods have been compared against the motion predicted by the Airy wave theory. The hull motion simulations have been compared against Wigley hull experimental data.
45

Application of 3D-CFD modelling for dynamic behaviour of ship in waves

Lakshmynarayanana, P. Arun January 2017 (has links)
Modern seakeeping computations are carried out using a variety of techniques ranging from two-dimensional (2-D) strip theory employing potential flow to three-dimensional (3-D) computations using fully nonlinear unsteady RANS (Reynolds-averaged Navier-Stokes equations) codes. The ever increasing size of ships and off-shore platforms has resulted in ‘softer’ hull which require hydroelastic effects to be taken into account when predicting fluid-structure interactions. Majority of such investigations are carried out numerically using potential flow solvers. Although nonlinear potential flow methods are also used, RANS/CFD can fully take into ac-count the nonlinearities and viscous effects. It is important, therefore, to verify and validate the predictions from such numerical predictions. This thesis aims to investigate the symmetric motions and responses of flexible floating bodies by coupling RANS/CFD and Finite Element software. The two-way interaction between a fluid solver, Star-CCM+, and a structural solver, Abaqus, is applied by exchanging pressures and nodal displacements more than once every time step, namely implicit scheme. A combination of overset and mesh morphing approaches and finite volume solution to allow for the motions of a body at the free surface is used. The air-water interface is captured using a VOF method and improved using HRIC scheme. The proposed method is applied for a validation case for a flexible barge and a containership in regular head waves for a range of wave frequencies and wave heights. The computational results are compared with experimental measurements and 2-D linear hydroelastic predictions. It is shown that the numerical predictions of flexible bodies can be carried out using the present two-way coupling method. The nonlinearities in wave loads arising from severe flare impact and green water (containership) are well predicted, which also provides some initial insights into the nature of 2-node flexible component.
46

The use of flexible biomimetic fins in propulsion

Riggs, Paul R. January 2010 (has links)
This thesis documents a series of investigations exploring the role of stiffness profile in propulsion using pitching flexible fins. Stiffness profile is defined as the variation in local bending stiffness along the chord of a fin, from leading to trailing edge. An unmanned robotic submarine was created, using simple pitching flexible fins for propulsion. Its design and performance prompted a review of literature covering many aspects of oscillating fin propulsion, paying special attention to the studies of pitching flexible fins, of the type used in the submarine. In the body of previous work, fin stiffness profile was a consequence of the external shape profile of a fin; fins had not thus far been designed with stiffness profile specifically in mind. A hypothesis was proposed: “Use of a biomimetic fin stiffness profile can improve the effectiveness of a flexible oscillating fin, over that of a standard NACA designated fin shape.” Rectangular planform flexible fins of standard NACA 0012 design and 1:1 aspect ratio were tested alongside similar fins with a stiffness profile mimicking that of a pumpkinseed sunfish (Lepomis gibbosus). The fins were oscillated with a pitching-only sinusoidal motion over a range of frequencies and amplitudes, while torque, lateral force and static thrust were measured. Over the range of oscillation parameters tested, it was shown that the fin with a biomimetic stiffness profile offered a significant improvement in static thrust over a fin of similar dimensions with a standard NACA 0012 aerofoil shape, and produced thrust more consistently over each oscillation cycle. A comparison of different moulding materials showed that the improvement was due to the stiffness profile itself, and was not simply an effect of altering the overall stiffness of the fin, or changing its natural frequency. Within the range of stiffnesses and oscillation conditions tested, fins of the same stiffness profile were found to follow similar thrust-power curves, independently of their moulding material. Biomimetic fins were shown to produce between 10% and 25% more thrust per watt of mechanical input power.
47

Probabilistic modelling of the process of evacuation for ship crises management

Hifi, Yasmine January 2017 (has links)
Evacuation analysis of passenger ships is mandatory since June 2016 for all passenger ships as prescribed by the International Maritime Organisation (IMO) in the revised guidelines for evacuation analysis (MSC.Circ.1533). The advanced evacuation analysis defined in the IMO guidelines, relies on the usage of computer models to simulate the movement of people and their interactions with the environment and the other evacuees. These models are powerful tools which allow for a detailed representation of theEnvironment and the people specific characteristics and behaviour. Setting up and running simulations is a very time consuming process and the bigger and more complex the ship or building is, the longer it takes. In situations when time is critical such as during emergencies and when a fast assessment of theevacuation time is needed, then relying on evacuation simulation tools might not be an option. Having a simplified model that can capture the different factors influencing the evacuation process and predict the total evacuation time would be a real advantage. In the research presented here, an attempt to develop such a simplified model has been made. The work undertaken during this research focused on investigating the possibility to derive a parametric model that could be simple enough to produce fast estimates of evacuation times but also capture the different elements of the evacuation process to satisfactory accuracy. Different parametric models were investigated. The nature of the problem led to investigate arrival processes, which were a good candidate for the underlying model explaining the evacuation of passengers. A close analysis showed that a Batch Non-Homogeneous Poisson Process (Batch NHPP) was needed to model the problem at hand. The batch arrivals and the NHPP are independent of each other so they were modelled and studied separately. The data used in this research came from a number of simulations (8 000 individual runs) performed with the Evacuation simulation software Evi as well as from the validation dataset produced by the EU-funded project SAFEGUARD and its associated Evi simulation runs (about 200 runs). The work was split between the fitting of the batch sizes of the arrivals and the NHPP. A complete analysis of the data was performed. The fitting of the batch sizes as well as the arrivals with the selected models produced very good results. Then, using the fitted models, new data was generated and analysed. The results were compared to the original data for both the batch sizes and the NHPP.
48

Numerical prediction of ship manoeuvring in regular waves by a 2.5D approach

He, Shi January 2017 (has links)
This thesis intends to study the manoeuverability of a single ship advancing in regular waves which is believed to be important for ship navigation safety as the situation is much more common experienced by a seagoing ship in real seaways instead of the calm water environment for traditional manoeuverability analysis. For this purpose, a so called two time scales model is applied to study the problem, according to the difference of the motion frequencies between the two involved sub-problems. That is to say, the total ship motions are consist of two parts, namely the wave induced motions analysed in a rapidly varying time scale system and the manoeuvring motions associated with a slowly varying time scale system. The two systems exchange data with each other at specific time intervals to reflect the interaction between two sub-motions. By this means, the analysis for an advancing ship executing a maneuver in waves can be achieved. To make concrete analysis of the sub-problems, a boundary element method (BEM) based on the 2.5 dimensional (2.5D) potential flow theory is adopted to solve the 5 degrees of freedom (DOF) rapidly varying wave induced motions, i.e., the seakeeping problem of slender ships advancing at speeds from moderate to relatively high is determined by interactively solving the discrete boundary integrate equation, kinematic and dynamic boundary conditions on the free surface in cross sections from bow to stern in time domain. Besides, this method is also used for the estimation of the manoeuvring derivatives required by the manoeuvring analysis. A numerical scheme called Multi-Transmitting Formula (MTF) is imposed on an artificial boundary to satisfy the radiation condition. The lift force, regarded as a consequence of the 3 dimensional (3D) flow effect which is important to manoeuvring motions but normally be neglected in the 2.5D theory, is taken into account for the evaluation of the total hydrodynamic forces acting on the ship during lateral motions. Furthermore, Non-Uniform Rational B-Spline (NURBS) is used for modelling the body plans of the ship and expressing the unknown quantities on the boundary elements to give more accurate and smoother solutions for the boundary value problems (BVP). To validate the established numerical tool, computations are carried out on a WigleyIII hull, a Series 60 hull and a container ship S175 hull respectively. The results are compared with the available experimental data. Regarding the manoeuvring motion simulations, the model is established based on the modular concept proposed by the Japanese Mathematical Modelling Group (MMG). The forces and moments induced by the propulsion system, the rudder system and the nonlinear viscous effect are estimated separately by empirical formulae or directly obtained from experimental measurements. The mean second order wave drift force is determined by direct pressure integration depending on the solved linear velocity potential from the seakeeping module. Simulations of the standard free running maneuvers, namely turning circle motion and Zig-zag motion, are carried out on the S175 ship in calm water and different regular waves successively. The results are compared with experimental measurements for validation which demonstrates that the present numerical tool can reasonably predict the manoeuvring motions of a slender ship in regular waves.
49

Preventing marine biofouling : the fouling-release-coating approach

Coward, Rebecca L. January 2005 (has links)
The unwanted build up of fouling organisms on immersed structures has been a problem that has been addressed over the years in many different ways, from tar and pitch on the hulls of vessels to various toxin based ablative coatings and most recently, foul-release coatings that present a non stick surface to which organisms can not adhere strongly. These foul-release coatings have been relatively successful and further investigation into the formulation of siloxane based coatings is a environmentally acceptable and commercially viable concept. The significance of the hydrophilicity of a range of cured siloxane polymers upon the attachment of marine fouling species is presented. The polymers were synthesised from polymethylhydrosiloxane (PDHS) with the grafting of hydrophilic ethoxy based, linear chains of various lengths. Following cross linking, films of these materials were characterised by Nuclear magnetic resonance (NMR), Infrared (lR) spectroscopy, X-ray photoelectron spectroscopy (XPS), contact angle goniometry, topography, thermal analysis, sorption of water, force of adhesion and nano-indentation. The films were tested by bacterial growth and attachment studies, the growth and attachment of various algal propagules and also by static raft trials. Results suggest that there is a maximum hydrophilic content possible when investigating these coatings, due to the intake of water molecules, which causes swelling and subsequent degradation of the stability of the coating. The optimum hydrophilic content for achieving minimum adhesion of fouling organisms was unclear, however, trends in experimental data were identified. The bacterial attachment and growth studies conducted upon Fucus propagules indicated an increase in growth upon the PMHS polymers with the addition of3-{2-[2-(2-methoxy-ethoxy)-ethoxy]ethoxy}- propene groups, while the Sargassum propagules illustrated a reduction in growth during the same conditions. Ulva and Enteromorpha propagules showed no visible trends in growth upon the coatings tested. The surface energy and adhesion results illustrate that the PDMS with 3-{2-[2- (2-methoxy-ethoxy)-ethoxy]-ethoxy}-propene groups were the most adhesive of the coatings teste4 (14.9 oN in comparison to 3-9 oN) but possessed the lowest surface energy (22.46 mJ m2 ). In exposure trials over a 10 month period, the peroxide cured coatings out performed the other curing systems tested, however the colonisation of the range of polymers was inconclusive.
50

Time domain mathematical model for six-degree-of-freedom motion in a wave

Kang, Joonyun January 2009 (has links)
In this thesis a novel controller for providing greater flexibility of operation of wind turbines known as the Power Adjusting Controller (PAC) is presented. The controller takes the form of an augmentation to a wind turbine’s full envelope controller, allowing it to be applied to any horizontal axis, pitch regulated, variable speed wind turbine. Conventional wind turbine control seeks to maximise the power output of a wind turbine whilst minimising the loads on the turbine, controlling on the error in generator speed via demands to the blade pitch actuator and generator torque actuator. The PAC uses additions to the full envelope controller inputs and outputs to alter the power output of the turbine by an additional input value ∆P. It is ensured that the operation of the full envelope controller is not compromised by the PAC. Testing of the PAC using lumped parameter models of wind turbines and full aero-elastic models makes clear a requirement for a wind speed estimator within the PAC that incorporates the effects of dynamic inflow. A novel wind speed estimator that accounts for dynamic inflow by redefining blade element momentum theory solely in terms of the dynamics at the rotor is therefore developed and incorporated into the PAC. Limits are designed to ensure that the operating point of a wind turbine with the PAC is kept within a safe operational envelope, and a system of flags and sub-flags is developed to allow easy integration of the PAC into a hierarchical wind farm control structure. The effect of using the PAC on the wind turbine loads is investigated, with the ultimate loads introduced by operation of the PAC found to be within the range of normal operating loads and the impact of prolonged reduction of the power output found to reduce the lifetime damage equivalent loads in most cases.

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