Mechanical Engineering - Theses
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ItemOptimisation of closed-loop aerodynamic systems( 2018)The design of a missile system is a multi-disciplinary engineering activity that involves structural, aerodynamics, rocket propulsion, guidance, electronic, and closed-loop control engineering to name a few. In modern engineering practice, a systems engineering approach is utilised to manage the design of a missile, but this does not necessarily guarantee that the final design is optimal. The process may also be inefficient, requiring many iterations of design, prototyping and testing in order to achieve the required specifications. In this thesis, multi-disciplinary optimisation frameworks are developed that target the aerodynamics and closed-loop control system of a supersonic tail-fin controlled missile. The aerodynamics and control system are highly coupled systems, but it is rare to see these subsystems optimised together in the literature. This is due in part to the computational requirements of the aerodynamic simulations and in part due to many control system design techniques that tend to treat the missile dynamics as immutable. A model representing a supersonic tail-fin controlled missile is developed. The model utilises computational fluid dynamics (CFD) simulations in order to capture the aerodynamic behaviour and a state-space model for the dynamics of the missile. Control algorithms are utilised to perform the autopilot function of the missile. This model serves as a basis on which the aerodynamic shape and controller gains can be optimised. Aerodynamic shape optimisation problems typically have large computational demands thus making them impractical to be used with global optimisation algorithms. The first optimisation framework developed is based on sample based global extremum seeking. It is shown that under certain conditions, the convergence behaviour of CFD simulations can be viewed as plant dynamics and thus extremum seeking techniques can be applied to find the optimal aerodynamic shape. The results are a step toward obtaining globally optimal solutions within comparative computation times of gradient-based optimisers. While useful for shape optimisation, the previous result would still struggle with combined aerodynamic shape and control optimisation problems. The next framework proposed is an adjoint-based gradient optimisation framework. The adjoint method has previously been utilised for static shape optimisation problems, but the result presented here is an extension for dynamic and controlled missile problems. The result shows that with appropriate time-scale separation between the actuator and slow states of the missile, the gradient of the cost function can be found with just two times the computational requirements of mapping the aerodynamic characteristics of the missile. This computational requirement is independent of the number of shape design variables and thus shows its practicability. An example of a missile tail-fin profile and autopilot gain optimisation problem is presented. There exists limitations of the adjoint based framework which prevent its use for certain missile geometries. Consequently, an implicit filtering framework is utilised in combination with the adjoint framework to cater for general missile geometries while still maintaining competitive computational speeds. This framework shows that general missile problems can be optimised without restriction. A number of optimisation examples involving a missile tail-fin profile and platform, missile nose cone and autopilot gains are presented. Lastly, goal-oriented mesh adaptation which has often been utilised in the CFD community to refine their computational meshes is utilised in non-linear model predictive control (NMPC). Goal-oriented mesh adaptation is a result derived from the adjoint method. The control algorithm that is developed is computationally faster than the standard NMPC and therefore can be utilised in so-called "fast" systems.
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ItemEvaluation of coronary stents and atherosclerosis employing optical coherence tomography and computational fluid modelling( 2017)Atherosclerotic coronary artery disease (CAD) is a major health burden worldwide and percutaneous coronary intervention (PCI) is an established treatment for this condition. Both PCI and invasive imaging techniques have evolved tremendously over the past few decades. Limitations of angiography were largely overcome, first by intra-vascular ultrasound, and then, optical coherence tomography (OCT), which is now recognized as the most sensitive and validated tool to examine the vessel lumen, plaque composition and stent-vessel wall interface. This thesis centers on randomized, OCT trials of coronary atherosclerosis and stents. In-vivo, comparative studies of drug eluting stents were conducted to directly observe their mechanical and healing characteristics. Results of these trials subsequently laid foundation for computational fluid dynamics (CFD) experiments and some illuminating observations were made around the effects of stent malapposition on intra coronary flow dynamics. It is the information of this kind that guides scientists to refine stent designs and clinicians, to improve procedural outcomes.
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ItemNumerical analysis of turbulent heat transfer in pipe and channel flows( 2014)Turbulent heat transfer (THT) for internal geometry (pipe or channel) has a wide range of applications such as heat exchangers, combustion chambers, transport of hot fluid from underground geothermal wells, nuclear reactors, etc. Hence, the understanding of the role of turbulence through direct numerical simulation (DNS) of flow and heat transfer in a canonical pipe geometry will have a significant influence on the designs of many engineering systems. A review of literature reveals that very few DNS studies were conducted for THT in pipe flow. In fact, most researchers prefer to conduct DNS and THT studies for channel flow due to simplified geometry and coordinate system. As a result, there is now plenty of DNS data on channel for a wide range of Reynods and Prandtl numbers. The present work aims to fill a gap in the literature by carrying out DNS of THT in pipe flow for a wide range of Prandtl number (0.025 <= Pr <= 7). It has been found that previous studies only considered either the effect of mesh resolutions or the applied thermal boundary condition on thermal statistics in turbulent flow apart from the influence of governing parameters, Reynolds and Prandtl numbers. Often, it is very difficult to compare those results for making a decision solely based on the dependence of the particular parameter since the computational domain length varies in each simulation. A parametric investigation conducted in this thesis clearly illustrates the influence of domain length on the convergence of thermal statistics eventually explains the reasons for variation of different results in published literature. A review of the scaling properties of the governing equation for passive scalar transport in wall-bounded turbulent flows is also conducted in this thesis. Traditional scaling approaches fail to provide the invariant profiles as the relevant non-dimensional parameters are varied. Based on magnitude ordering and scaling analysis of the mean energy equation, I explore the properties of four distinct thermal balance layers and determine the limiting value of Prandtl number for the onset of the four layer regime. Moreover, the generalized form of the intermediate length scaling has been deduced and verified with existing heat transfer data. Further comparison between inner and intermediate normalizations reveals that the existing and proposed scaling approaches fail to simultaneously and self-consistently reconcile all the profiles, which has become an open challenge for many researchers. A systematic comparison of thermal statistics between pipe and channel flow for different Reynolds and Prandtl number has been presented to provide a better understanding of the similarities and differences of the heat transfer mechanism of these two types of geometry. The intermediate scaling theory matches uniformly for the mean temperature and turbulent heat flux profiles irrespective of pipe and channel flows. However, the inner scaling of turbulence thermal statistics such as mean temperature, thermal intensities, and heat flux profiles shows that there are significant discrepancies between pipe and channel flows unlike the turbulent flow statistics. Finally a set of DNS have been performed for a common engineering application like turbulent flow in a heated wavy-wall pipe. The main objective is to present the flow and thermal turbulence statistics for establishment of a new approach by comparing with available and present scaling framework. Like surface roughness, the corrugation height shows similar tendency to influence the flow and thermal behavior and as a result, the mean momentum and energy balance theory provide a basis to quantify its characteristics to derive the scaling behaviors as a function of roughness scale.