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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.

Progressive collapse of titanium alloy micro-lattice structures manufactured using selective laser melting

Hasan, Rafidah January 2013 (has links)
The starting point for this research was the viability of the Selective Laser Melted (SLM) titanium alloy Ti-6Al-4V micro-lattice structure for applications in Foreign Object Impact (FOI) situations in aerospace sandwich constructions. To this end, the mechanical behaviour of single struts and the compression behaviour of micro-lattice blocks were studied. Detailed characterizations of dimensional accuracy, circularity and microstructure, as well as clarifications of deformation behaviour and failure of single manufactured struts under tensile loading were done. The variability in stress-strain curve of struts which was derived using compliance correction method was found to arise from the variations in strut diameters, due to outer surface roughness of the material. Post-manufacture heat-treatment processes improved the surface roughness and variations of strut diameters as well as the microstructure of the α/β titanium alloy, hence reduced the scatter in the stress-strain curve of single struts. The deformation of the SLM Ti-6Al-4V micro-lattice blocks with Body Centred Cubic (BCC) structure was elucidated using combined experimental studies and computational analysis. Detailed analysis of geometry and diameter variations in struts of the micro-lattice blocks were done and compared to that of single manufactured struts. Node formation and manufactured quality of the micro-lattice structure were revealed from a 45° angle diagonal plane of sectioned block. The compressive deformation behaviour of the BCC micro-lattice block structures was then studied. Effects of different manufacturing routes and parameters as well as post-manufacture treatments in the compressed micro-lattice structures were discussed. Finite element analysis was performed using a validated model of BCC micro-lattice unit cell. The progressive collapse of the micro-lattice block structure was shown to be comparable with the prediction from the finite element model of a unit cell. The numerical simulation was then used to quantify the effect of parent material properties on block collapse. In this way, the relations between SLM manufacturing route, material properties and structural performance are highlighted.

Low mach number CFD for wind turbine analysis

Carrion, Marina January 2014 (has links)
To maximise the amount of energy extracted from wind turbines, the rotor diameter has increased, reaching values of 160m in some cases. Large scale wind turbines are working at high Reynolds numbers and a wide range of flow conditions, with virtually incompressible flow present at the root and mildly compressible near the blade tips, where the Mach numbers can reach locally 0.48 for the largest wind turbines employed to date. In traditional aerodynamics, most CFD methods were designed to cope with high Mach number flows and consequently solve the compressible Navier-Stokes equations. This is the case of the Helicopter Multi-Block (HMB2) CFD method from Liverpool University. The present PhD thesis aims to provide an all-Mach-number capability to the HMB2 method, by implementing modified Roe schemes to account for low-Mach flows. For 2D cases, the modified Roe schemes showed great improvement in the convergence and the quality of the solution, when compared with the Original Roe and Osher schemes, and the Low-Mach Roe scheme showed the best performance. With the low-Mach capability included in the compressible solver, both MEXICO and NREL Annex XX experiments were simulated. A detailed analysis of the velocity field behind the MEXICO rotor was performed, where the low-Mach scheme (LM-Roe) showed less sensitivity on the grid size than the Osher scheme. Accurate prediction of wind turbine wake breakdown is also important for the performance analysis of the turbines and their optimal positioning within tightly-spaced wind farms. Using a fine mesh able to preserve the vortices up to 8R downstream the MEXICO rotor plane, the instabilities on the wake leading to vortex pairing were captured. FFTs of the axial velocity component enabled to identify the main harmonics in the wake. In the stable region, the wake was a perfect spiral and the main frequency was the bladepassing one. An approximate exponential growth was then observed and in the region where instabilities were present, higher frequencies dominated, leading an oscillatory pattern. Simple wake models were also investigated and a combination between a kinematic model to account for the wake initial expansion and a field model to account for the far wake decay was proposed, showing good agreement with the CFD solution. With the correct set of constants, it was proved that this simple model can be used to approximate the behaviour of wind turbine wakes with minimal computational cost. Another consequence of the increased size of wind turbines is that their stiffness lowers and aeroelasticity therefore plays an important role, since the blades can suffer great deformations. To account for the blade deformations, a tightly coupled CFD-CSD method was employed to analyse the MEXICO and NREL Annex XX wind turbines. For the latter, the tower and nacelle were considered as stiff bodies and the blades were allowed to deform. As a result of the aeroelastic calculations, the blades showed deformation in bending (towards the tower). The maximum deflections were present after the blades had passed in front of the tower, and maximum amplitudes of 0.59%R, at 20m/s were observed.

Integration of a web processing service (WPS), GIS and hydraulic modelling (TELEMAC) for geophysical analysis

Zhang, Yicheng January 2014 (has links)
No description available.

Polarisation dependent NUV femtosecond laser inscription of high grade Volume Bragg Gratings in poly(methyl)methacrylate with a spatial light modulator

Ye, Lingyi January 2014 (has links)
Parallel near-ultraviolet(NUV)beam, 387nm femtosecond laser pulses with linear and circular polarisations were used to inscribe high efficiency Volume Bragg Gratings (VBGs) in clinical grade poly(methyl)methacrylate (PMMA) with the aid of a Spatial Light Modulator (SLM). Large, high quality VBGs with dimensions of 5mm x 5mm in size with (1-7)mm thickness and 20µm pitch were created at high speed, fabricated in 36 minutes, showing a first order diffraction efficiency 1 >94% with 4mm thickness. This is the highest diffraction efficiency observed to date in the undoped polymer, PMMA. Linear polarisation produced a higher refractive index contrast than circular polarisation, was found to be due to polarisation dependent non-linear filamentation, initiated through self-focusing. Using the threshold for supercontinuum, the ratio of critical power for self-focusing in PMMA was measured to be Pc lin/Pc circ  1.5, in accord with theoretical expectations. The pulse energy uniformity of the multiple beams was also dramatically improved by a camera based feedback system which was integrated into a LabVIEW based hologram calculation and display system, reducing the non-uniformity of parallel NUV and NIR beams significantly, thus improving the inscription process.

Lattice Boltzmann modelling of immiscible two-phase flows

Zhang, Duo January 2015 (has links)
The scope of the current thesis is the comprehensive understanding of the droplet impact and spreading dynamics on flat and curved surfaces with the aim of simulating high density ratio immiscible two phase flows in porous media. Understanding the dynamic behavior of droplet impingement onto solid substrate can provide significant information about the fluid flow dynamics in porous structures. The numerically study process will be realized by using a high density ratio multi-phase lattice Boltzmann model which is able to simulate multi-phase flows in complex systems. The interfacial information between the two immiscible phases can be captured without tracking or constructing the vapour-liquid interface. A three dimensional lattice Boltzmann model is applied on the study of the impaction of a liquid droplet on a dry flat surface for a liquid-gas system with large density ratio. The impaction of liquid droplet on a curved surface for the liquid-gas system with large density ratio and low kinematic viscosity of the fluid is computed by a two-dimensional multi-relaxation-time (MRT) interaction-potential-based lattice Boltzmann model based on the improved forcing scheme. The dynamics behaviors of the spreading of the liquid droplet on the flat surface as well as the impaction of the liquid droplet on a curved surface are computed, followed by their dependence on the Reynolds number, Weber number, Galilei number and surface characteristics. Moreover, an improved force scheme is proposed for the three-dimensional MRT pseudopotential lattice Boltzmann model which is based on the improved force scheme for the Single relaxation time (SRT) pseudopotential lattice Boltzmann model and the Chapman-Enskog analysis. The validation for the new developed three-dimensional multi-relaxation time lattice Boltzmann model is carried out through Laplace’s law ad by achieving thermodynamic consistency. In addition, the relationship between the fluid-solid interaction potential parameter Gw and the contact angle is investigated for the new developed three-dimensional MRT lattice Boltzmann model. The immiscible two-phase flow in porous media is carried out by a two dimensional MRT lattice Boltzmann model. The porous media structures with different geometrical properties are artificially generated by a Boolean model based on a random distribution of overlapping ellipses/circles. Furthermore, the impact of geometrical properties on the immiscible two-phase flows in porous media is investigated in the pore scale. The lattice Boltzmann model results provide significant information i on the interface between the two immiscible phases in complex systems, it is easy to apply for complex domains with bounce back boundary wall condition and be able to handle multi-phase and multi-component flows without tracing the interfaces between different phases.

Structural dynamic analysis and model updating for a welded structure made from thin steel sheets

Abdul Rani, Muhamad January 2012 (has links)
Modern large, complex, engineering structures normally encompass a number of substructures which are assembled together by several types of joints. Despite, the highly sophisticated finite element method that is widely used to predict dynamic behaviour of assembled complete structures, the predicted results achieved, of assembled structures are often far from the experimental observation in comparison with those of substructures. The inaccuracy of prediction is believed to be largely due to invalid assumptions about the input data on the initial finite element models, particularly those on joints, boundary conditions and also loads. Therefore, model updating methods are usually used to improve the initial finite element models by using the experimentally observed results. This thesis is concerned with the application of model updating methods to a welded structure that consists of several substructures made from thin steel sheets that are assembled together by a number of spot welds. However, the welded structure with a large surface area is susceptible to initial curvature due to its low flexible stiffness or manufacturing or assembling errors and to initial stress due to fabrication, assembly and welding process of substructures. Nevertheless, such initial stress is very difficult to estimate by theoretical analysis or to measure. This thesis puts forward the idea of including initial curvature and/or initial stress (which have a large effect on natural frequencies) as an updating parameter for improving the performance of the finite element model of a structure made from thin steel sheets. The application of conventional iterative model updating methods which use a full finite element model has been widely practised. However when updating large, complex structures with a very large number of degrees of freedom, this application becomes impractical and computationally expensive due to the repeated solution of the eigensolution problem and repeated calculation of the sensitivity matrix. It is therefore preferable to use a substructuring scheme based model updating which is highly computationally efficient for the reconciliation of the finite element model with the test structure. However, in certain practical cases, where the confidential and proprietary issues of modelling work are of concern between the collaborating companies, in which the finite element models of the substructures could not be revealed and only the condensed matrices of the substructures are used instead, the areas of the substructures having fewer number of interface nodes would always be the first choice as the interface nodes. For welded structures, the nodes in the vicinity of spot weld element models are few and hence are usually taken as the interface nodes for connecting substructures. However, the present MSC. NASTRAN superelement model reduction procedures are known not to allow the nodes of CWELD elements to be the interface nodes of substructure. Prior to the present study, no work appears to have been done to use the nodes of CWELD elements as the interface nodes of substructures in the investigation of dynamic behaviour of welded structures. In this work, the application of branch elements as the interface elements of substructure are proposed and tested. Prior to the present study, it also appears that there has been no work done concerning the adjustment of the finite element model of the welded structure by including the effects of initial curvatures, initial stress and boundary conditions that are contributing to the modelling errors, via the combination between the Craig-Bampton CMS and model updating. This thesis presents two approaches for model updating of the welded structure: the conventional methods which use full finite element model and the substructuring scheme based model updating which uses the Craig-Bampton CMS technique. The accuracy and efficiency of both approaches are thoroughly discussed and presented and are validated with the experimentally observed results.

Optimisation of aspects of rotor blades using computational fluid dynamics

Johnson, Catherine January 2012 (has links)
This work presents a framework for the optimisation of various aspects of rotor blades in forward flight. The literature survey suggests that the quest for such a method is generating much research as more performance is obtainable from current designs. With increasing computational power and efficient methods, this can be of practical use to the helicopter industry. The proposed method employs CFD in conjunction with metamodels such as artificial neural networks (ANNs) and kriging interpolation, and a non-gradient based optimiser, in the form of genetic algorithms (GAs), for optimisation. The approach is demonstrated using several cases, including the optimisation of linear twist of rotors in hover (a steady case) and the optimisation of rotor sections in forward flight (an unsteady case); other cases include transonic aerofoils, wing and rotor tip planforms. For rotor tip planforms, first a simple rectangular rotor in hover was optimised. Then the developed method was used to optimise the anhedral and sweep of the UH60-A rotor blade in forward flight while constraining its hover performance and the final rotor optimisation was for a BERP-like rotor in forward flight, also constraining hover performance. For each case, a parameterisation method was defined, a specific objective function created using the initial CFD data and the metamodel was used for evaluating the objective function during the optimisation using the GAs. The obtained results suggest optima in agreement with engineering intuition but provide precise information about the shape of the final lifting surface and its performance. The results were checked by comparison with the Pareto subset of data and the metamodels were also validated with high-fidelity CFD data. Neither was sensitive to the employed techniques with substantial overlap between the outputs of the selected methods. The main CPU cost was associated with the population of the CFD database necessary for the metamodel. To improve this further, the Harmonic Balance alternative for obtaining the CFD data (as opposed to Time Marching) was used to increase efficiency and reduce clock time for the BERP-like tip optimisation. The novelty of this method is the use of a metamodel in conjunction with high-fidelity CFD data so that high-resolution performance improvements can be captured efficiently using a non-gradient based method.

Novel electrospinning techniques with nano-materials

King, Simon G. January 2015 (has links)
Modern society is ever in demand for higher performing materials, with increased efficiency. Recognising this need, the work discussed here details the steps taken to develop and engineer a cost-effective manufacturing process, which could be easily commercially scalable for the production of large-areas of aligned carbon nanotubes. These aligned carbon nanotubes can then be directly applied in areas such as advanced ‘multi-functional’ composites. Of the available routes, the electrospinning technique demonstrated to be one of extreme promise towards achieving this goal. This thesis guides and justifies the investigative steps taken in scientifically engineering a suitable electrospinning method to achieve high-aligned arrays of carbon nanotubes. This includes the design and development of a novel, large-area high-throughput needleless electrospinning system, which is capable of not only producing nano-fibres in excess of 160 g per hour (700 times faster than conventional single needle electrospinning), but also in an aligned orientation, using purely aqueous based polymeric solutions. This success has led to the successful production of the World’s first large area sheets of highly aligned arrays of single walled carbon nanotubes by electrospinning. The analysis of these sheets found substantial increases in both mechanical and electrical performance. For the aligned nanotube-loaded nano-fibres, the tensile strength increased up to 320%, ductility increased up to 315% and Young’s modulus increased up to 430% (compared to the original polymer performances). The realisation of the significant enhancements CNTs pose on a composite material, led to an investigation into the chemical interactions that lead to these results. This resulted in the discovery of a new small angle X-ray scattering peak, which we attributed to a crystalline interface between the polymer and carbon nanotubes, giving rise to the enhancements seen during mechanical testing. In addition to mechanical performance, there was also a significant increase in electrical conductivity of 108 S/m, an improvement of 8 orders of magnitude compared to the original polymer. These results, combined with the realisation of industrially viable throughput, provide promise for impressive application into advanced multi-functional composites. While the primary objectives of this research focused on large area electrospinning, the work outlined in this thesis also discusses investigations into other important aspects, and significant scientific discoveries. These scientific achievements include the introduction of a novel, micro-centrifugal dispersion assessment method, for the efficient surfactant functionalisation of nano-materials. This method allows for a fast and effective assessment of a material suspension, without the need for any equipment other than a simple centrifuge and a balance. This process leads to fast and efficient use of surfactants, producing greater loadings of nano-materials which can be suspended within a solvent for further processing. As a method to recover the nanotubes once they have been processed and aligned, this thesis also explores post processing of the aligned nanotube-loaded sheets using steam purification. This led to the complete recovery, and purification, of the high quality aligned CNTs, which were found to significantly increase the resulting nanotubes resistance to oxidation, increasing their oxidation temperature in excess of over 900° C, a previously unreported achievement. The mechanisms behind the underlying chemistry were further probed using Raman spectroscopic analysis, this revealed how selective oxidation of CNTs was limited to that of metallic CNTs, leaving the remaining material as only semi-conducting species. This selective oxidation process could lead to selective manufacture of specific CNT species, allowing for better suited application in electrical devices.

Detecting freeplay in mechanical systems : for the development of autonomous sensing devices

Hewitt, Daniel January 2016 (has links)
This thesis is a study of freeplay - gaps between notionally connected components in mechanical systems. For some systems any amount of freeplay can complicate closed loop control design and introduce energetic impacts between bearing components. The work was motivated by freeplay present in the control systems-of rotorcraft and explores the viability of developing sensing devices to detect freeplay during operation. In this work a generic freeplay model is introduced from first principles which predicts potential measures of a system that could indicate the level freeplay. To validate these measures a mechanical experiment is constructed which represents a minimum working example of a freeplay system. It is designed to exclude as many sources of variability as possible and offers unparalleled completeness and acuity of generated data. A novel mathematical model is derived (and found to be in good agreement with the gathered experimental data) which validates the theorised damage sensitive features. Freeplay sensitive features are used as detection metrics in two real systems. First, on data gathered from the experiment; A freeplay detection system was created which determined the level of freeplay to a high accuracy using data gathered from a single accelerometer. Second, to strain measurements taken from a rotorcraft in flight; It was found that some metrics derived from earlier work were not good predictors of freeplay on rotorcraft data, but, using a selection of data processing techniques, a bespoke freeplay detection system was successful for many flight conditions. The work in this thesis is a strong indicator that detection of freeplay is possible for certain classes of systems using autonomous devices. Positive identification of freeplay in a simple and a complex freeplay system have been shown and a critical review is offered of the conclusions, limitations and some opportunities for future study.

Control of industrial multivariable systems

Jennings, Geoffrey Jesse January 1969 (has links)
No description available.

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