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  • Identification and investigation of local optima in aerospace structural design
    (2006) Shi, Jianming
    Thesis
    This thesis reports on research into the causes of local optima when optimization algorithms are applied to aerospace structural design. A thorough understanding of local optima will enable the engineers to select the algorithm for optimization or to guide the optimization to ensure either global optima or near optimal solutions are achieved. Therefore, a comprehensive literature review has been conducted and several illustrative examples have been identified to help fully understand the cause and importance of local optima. The first application involved the design of the internal structure of a simplified wing spoiler. MSC.NASTRAN was used to optimize each discretized location of an additional rib with the aid of a Patran Command Language (PCL) algorithm. The objective function of minimum weight was approximated as a multimodal function in a 2D smooth curve where the local and global optima were identified. The theory of continuous rectangular plates was used to explain the phenomena. The second problem considered buckling of a wing rib. A PCL code was written to obtain the rib buckling factors as the position of the center of a square cutout was varied within a constrained area. The rib linear buckling factor versus the centre position O(X, Y) of the square cutout was plotted in a 3D surface contour plot. Load path theory and relevant plate buckling theories were used to explain the local and global maxima identified. The final example considered the maximization of the buckling load of a simply supported composite laminated plate under in-plane loading. A conventional Genetic Algorithm was used to examine the local and global optima of the critical buckling load factor. Many local and global optima were identified and explained and many near-optimal solutions were found in a single run. A significant understanding of local optima in aerospace structural design with the optimal utilization of available software and the appropriate selection of optimization algorithms has been achieved. Further work could either include implementing the proposed global optimization strategies or include implementing rapid methods for identifying multiple local optima.
  • Operational modal analysis and model updating with a cyclostationary input
    (2006) Hanson, David
    Thesis
    This thesis addresses the problem of identifying the modal properties of a system based only on measurements of the system responses. This situation is frequently encountered in structural dynamics and is particularly relevant for systems where the in-service excitation is not artificially reproducible. The inherent non-linearities in these systems mean that the modal properties estimated using traditional input/output techniques will be different to those exhibited in operation. A common example from the literature is an aircraft in flight where the modal properties are heavily influenced by the operating point, i.e. the combination of load, speed, altitude etc., at which the aircraft is travelling. The process of identifying the modal properties of systems in-service is called Operational Modal Analysis (OMA). Not knowing the input complicates the analysis. Most of the techniques in the literature overcome the lack of knowledge about the unmeasured excitations by assuming they are both spatially and frequentially white, i.e. of equal magnitude and with a flat autospectrum. This thesis presents a new technique for OMA which relaxes these constraints, requiring only that the system is excited by a so called cyclostationary input with a unique cyclic frequency, and that the log spectrum of the second order component of this input is frequentially smooth, as will be explained. Such systems include vehicles with internal combustion engines as the vibration from such an engine exhibits cyclostationary statistics. In this thesis, the technique is applied to a laboratory test rig and a passenger train both using an artificial input, and to a race car using the engine as the excitation. By combining cyclostationary signal processing and the concept of the cepstrum, the technique identifies the resonances and anti-resonances in the transfer functions between each response and the cyclostationary source. These resonances and antiresonances can be used to regenerate Frequency Response Functions (FRFs) and it is shown how the unknown scaling of the system can be recovered by employing finite element model updating in conjunction with this regeneration. In addition, the contribution made to model updating by the anti-resonances is also investigated. Finally, the potential of OMA to inform a model updating process is demonstrated using an experimental case study on a diesel railcar.
  • An automated micro-grinding system for the fabrication of precision micro-scale profiles
    (2006) Milton, Gareth Edward
    Thesis
    Production of micro-scale components is an important emergent field. One underdeveloped area is the production of micro-scale 3D surfaces, which has important applications in micro-optics and fibre optic sensors. One particular application is the production of micro-lenses. With scales of less than 200 μm these lenses can improve light coupling efficiencies in micro-optic systems. However, current lens production techniques have limitations in accuracy and versatility. Creating these surfaces through mechanical micro-grinding has the potential to improve the precision and variety of profiles that can be produced, thus improving transmission efficiencies and leading to new applications. This work presents a novel micro-grinding method for the production of microscale asymmetric, symmetric and axisymmetric curved components from brittle materials such as glasses. A specialised micro-grinding machine and machining system has been designed, constructed and successfully tested and is presented here. This system is capable of producing complex profiles directly on the tips of optical fibre workpieces. A five degree of freedom centring system is presented that can align and rotate these workpieces about a precision axis, enabling axisymmetric grinding. A machine vision system, utilising a microscope lens system and sub-pixel localisation techniques, is used to provide feedback for the process, image processing techniques are presented which are shown to have a sensing resolution of 300 nm. Using these systems, workpieces are centred to within 500 nm. Tools are mounted on nanometre precise motion stages and motion and infeed are controlled. Tooling configurations with flat and tangential grinding surfaces are presented along with control and path generation algorithms. The capabilities and shortcomings of each are presented along with methods to predict appropriate feed rates based on experimental data. Both asymmetric and axisymmetric flat and curved micro-profiles have been produced on the tips of optical fibres using this system. These are presented and analysed and show that the system, as described, is capable of producing high quality micro-scale components with submicron dimensional accuracy and nanometric surface quality. The advantages of this technique are compared with other processes and discussed. Further development of the system and technique are also considered.
  • Lifetime monitoring of appliances for reuse
    (2006) Mazhar, Muhammad Ilyas
    Thesis
    Environmental awareness and legislative pressures have made manufacturers responsible for the take-back and end-of-life treatment of their products. Therefore, manufacturers are struggling to find ways to recover maximum value from returned products. This goal can best be achieved by promoting multiple reuse programs as reuse is one of the most effective ways to enhance a sustainable engineering economy. Since the essential goal of the reuse strategy is to reuse parts, the reliability of used parts becomes a core issue. Research indicates that reuse is technologically feasible, associated with a significant manufacturing cost saving, and it does not compromise product quality. However, it is not easy to be applied in reality. There are several uncertainties associated with reuse, the most common is the uncertainty of the product’s quality after use. A widespread implementation of the reuse strategy could be triggered, subject to the availability of reliable methods to assess the useful remaining life of parts. The evolution of such a methodology would play a pivotal role in making decisions on the supply chain process and the recovery value of returned products. Reliability assessment by life cycle data analysis is the basis of this research. The proposed methodology addresses the problem of reliability assessment of used parts by considering two important aspects. It performs statistical as well as condition monitoring data analysis for decision-making on reuse. The analysis is carried out in two stages. Firstly, a wellknown reliability assessment procedure, the Weibull analysis, is applied to analyse time-tofailure data to assess the overall reuse potential of components. In the second stage, the used capacity (actual life) of components is determined by analysing their operating history (condition monitoring data). The linear and nonlinear regression analysis, Kriging procedures and artificial neural networks (ANN) are employed in this stage. Finally, the Weibull analysis and ANNs are integrated to estimate the remaining useful life of components/assemblies of a product at the end of its first life cycle. The model was validated by using life cycle data from consumer products.
  • Experimental and numerical investigation of noise generation from the expansion of high velocity HVAC flows on board ocean going fast ferries
    (2006) Neale, James Richard
    Thesis
    This thesis details a study of strategies used to limit the flow generated noise encountered in the outlet diffusers of high velocity heating, ventilation and air conditioning (HVAC) duct systems. The underlying noise rating criterion is drawn from the specifications covering ocean going aluminium fast ferries. Although directed primarily towards the fast ferry industry the results presented herein are applicable to other niche high velocity HVAC applications. Experimental tests have been conducted to prove the viability of a high velocity HVAC duct system in meeting airflow requirements whilst maintaining acceptable passenger cabin noise levels. A 50 mm diameter circular jet of air was expanded using a primary conical diffuser with a variety of secondary outlet configurations. Noise measurements were taken across a velocity range of 15 to 60 m/s. An optimum outlet design has been experimentally identified by varying the diffuser angle, outlet duct length and the termination grill. A 4 to 5 fold reduction in required duct area was achieved with the use of a distribution velocity of 20 to 30 ms-1, without exceeding the prescribed passenger cabin noise criteria. The geometric configuration of the diffuser outlet assembly was found to have a pronounced effect on the noise spectrum radiating from the duct outlet. The development of a numerical model capable of predicting the flow induced noise generated by airflow exiting a ventilation duct is also documented. The model employs a Large Eddy Simulation (LES) CFD model to calculate the turbulent flow field through the duct diffuser section and outlet. The flow-generated noise is then calculated using a far field acoustic postprocessor based on the Ffowcs-Williams and Hawkings integral based formulation of Lighthill’s acoustic analogy. Time varying flow field variables are used to calculate the fluctuating noise sources located at the duct outlet and the resulting far field sound pressure levels. This result is then used to calculate the corresponding far field sound intensity and sound power levels. The numerical acoustic model has been verified and validated against the measured experimental results for multiple outlet diffuser configurations.