Mechanical Engineering - Theses
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ItemAcoustic microstreaming in the vicinity of oscillating microbubbles( 2012)Acoustic microstreaming has gained increased interest in the past few years due to innovative medical treatments such as sonoporation and sonothrombolysis [1-4]; however, few flow visualizations or measurements of velocity fields and other significant metrics havebeen conducted. In this study, work first conducted by Tho et al. [2] has been further explored. Micro-PIV (Particle Image Velocimetry) measurements and streak photography were used to study the flow field around single bubbles, ranging in diameters of 30-300 um, oscillating in various fluid mediums. The bubbles were attached onto the lower surface of a microchamber (pendant), and held in place via capillary forces. Previous studies have focused on the visible secondary flow of acoustic microstreaming, not the potentially more significant primary flow within the Stokes boundary layer. Since primary-flow velocities are much higher, they may exert stresses on nearby surfaces and be of much greater biological significance. In this study, whilst the key focus has still been at studying the secondary flow fields, a novel approach was used to capture and quantify the velocity/speed of the primary vortices, which were observed to be twice an order of magnitude stronger than the secondary vortices. Surfactants have been introduced into the fluid medium to study the possible effects on microstreaming and if there was a correlation to observed increases in rates of rectified diffusion. The addition of surfactants lead to an increase in the streaming metrics, however the results did not fully correlate with those of observed increased rates of rectified diffusion. This would suggest that the increase in streaming velocity due to the introduction of surfactants is not the primary driver for the increase in rectified diffusion rates.
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ItemDevelopment of an experimental loading and imaging protocol for studying cartilage cell morphology in a mouse model( 2012)A methodology was developed to apply controlled cyclic loads on mouse knees and to investigate resulting changes in chondrocyte morphology in vitro using confocal microscopy. For this purpose, an existing loading device was modified. The legs of wild-type and ADAMTS5-deficient mice were immobilised, and simulated knee joint loading was performed using the loading device. Chondrocytes of the loaded and unloaded knee cartilage samples were subsequently imaged with confocal microscopy, and the volume and sphericity of the chondrocytes, the distance between the chondrocytes, and cell viability were measured. The experimental results showed that changes in chondrocyte viability and morphology were different in the ADAMTS5-deficient mice compared to the wild-type mice after applying 10 N cyclic loading for one hour. The distance between cells was significantly less in the ADAMTS5-deficient cartilage compared to the wild-type. This suggests that deletion of the ADAMTS5 gene in mice may result in the formation of chondrocyte clusters. However, the chondrocyte clusters may not have adverse effects on the resilience of the cartilage tissue, as chondrocyte viability did not decrease in the ADAMTS5-deficient cartilage after loading. Changes in sphericity and distance between chondrocytes were not statistically significant after loading; however, these results must be interpreted with caution due to the small sample size used in the present study. Assessment of changes in cartilage stiffness in the ADAMTS5-deficient mice may explain the differences observed between chondrocyte viability in the loaded ADAMTS5-deficient and wild-type mouse cartilage samples. Such assessments are suggested for future work. The loading apparatus and methods developed in this study may also be applied to characterize microstructural changes in subchondral bone, which is believed to play an important role in the etiology of osteoarthritis.
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ItemLower-limb muscle forces and patellofemoral joint loading in people with and without patellofemoral osteoarthritis during walking and stair ambulation( 2012)Osteoarthritis (OA) is a chronic degenerative joint disease that is the leading cause of musculoskeletal pain and disability in Australia. OA often affects the knee joint and commonly occurs between the patella (knee cap) and femur (thigh bone), known as patellofemoral joint osteoarthritis (PFJ OA). PFJ OA is highly prevalent in elderly populations and is more common than tibiofemoral joint osteoarthritis (TFJ OA). Despite the differences in prevalence, PFJ OA is less studied than TFJ OA. The mechanical loading transferred through a joint is generally thought to play a major role in OA initiation and progression via the lifelong “wear and tear” of a joint’s articular cartilage. Alterations to gait patterns and muscle dysfunction are commonly observed in people with knee disorders. However, little is known on how these changes actually combine to alter patellofemoral joint (PFJ) loading. The use of computational models is becoming a popular approach to quantify in vivo muscle forces and joint contact forces in healthy individuals during common activities. However, these models have rarely been applied to pathological populations, such as PFJ OA. Given the dearth of knowledge in the gait characteristics, in vivo muscle function and PFJ joint loading in the PFJ OA population, the purpose of this study was to investigate the joint kinematics, joint moments, muscle forces and PFJ loading in individuals with and without PFJ OA during three common activities of daily life: (1) overground walking, (2) stair ascent and (3) stair descent. Four specific research questions are addressed in this thesis. 1. Do people with PFJ OA demonstrate altered trunk and lower-limb joint biomechanics in comparison to people without PFJ OA during walking and stair ambulation? 2. Do people with PFJ OA demonstrate differences in hip and knee muscle forces when compared to controls during walking and stair ambulation? 3. Is the patellofemoral joint reaction force different in people with and without PFJ OA during walking and stair ambulation? 4. Are there biomechanical characteristics that distinguish people with isolated PFJ OA from people with concurrent TFJ OA and PFJ OA? Quantitative gait experiments and computational modelling were utilised to address the above research questions. The main findings of this study were that subjects with PFJ OA (with and without concurrent TFJ OA) adopted activity-specific gait modifications to off-load the PFJ. Specifically, participants with PFJ OA chose to alter their sagittal-plane kinematics, such as increased anterior pelvic tilt and/or decreased knee flexion, during stair ambulation to reduce the knee extension moment, quadriceps force and the patellofemoral joint reaction force. PFJ OA participants also walked and descended stairs with lower hip abductor muscle forces and lower knee adduction moments, which may represent a frontal-plane adaptation to off-load the PFJ. The alterations in quadriceps and hip abductor muscle forces observed in the individuals with PFJ OA suggest that hip and knee muscle dysfunction may be characteristic features of the PFJ OA population. These findings indicate the importance of including the PFJ and the dynamics of joints proximal to the knee when investigating the knee OA disease process.
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ItemA critical assessment of Detached Eddy Simulation in the prediction of airfoil stall( 2012)The flow around an airfoil at different orientations exhibits a variety of complex flow phenomena that has traditionally been difficult to replicate numerically due to the constraints of computational cost and numerical accuracy of conventional numerical methods. The recent introduction of hybrid RANS/LES methodologies aim to collectively address these constraints. Although preliminary investigations reported in the literature have been favorable, a detailed assessment is still lacking. This thesis aims to throughly asses the accuracy of a hybrid RANS/LES numerical method known as Detached Eddy Simulation, particularly in its ability to accurately simulate the dynamic and static stall phenomena of a spanwise homogeneous NACA0012 airfoil. In performing this assessment, emphasis have been placed on the investigation of features that have not been previously reported in the literature. In order to aid in the investigations, experiments were conducted to gather turbulence statistics against which the numerical simulations can be validated. Streamwise velocity statistics in the wake of a NACA0012 airfoil at various configurations were measured using thermal anemometry. Novel experimental techniques were also developed to overcome some of the unique challenges encountered in the experiments. For the dynamic stall simulations, although DES has produced a physically more realistic flowfield, it has failed to show a significant improvement over URANS in terms of accurately predicting the evolution of the airfoil surface pressures as it undergoes dynamic stall. For the static stall simulations, the success of DES has been selective. DES was shown to be more accurate than URANS in the massive flow separation case, especially in the prediction of turbulent fluctuations. However, this accuracy is not displayed for the shallow flow separation case, where the performance of DES was only comparable to that of URANS. Evidently, DES displayed general weakness in the prediction of flows that are at the onset of separating from the airfoil.
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ItemThe relationship between performance on a single leg squat task and clinical measures of strength, balance and functional performance( 2012)The single leg squat task is frequently used in clinical sports medicine to provide a simple and convenient assessment of lower-extremity dynamic control. It is commonly used to screen for abnormal hip and knee motions, which have been implicated in the subsequent development of lower limb injuries. The single leg squat is thought to be a particularly important assessment tool when examining people with patellofemoral pain syndrome; a common overuse injury often found in runners that is frequently associated with altered lower-extremity control. Clinicians often assume that poor functional task ability indicates underlying deficits in lower limb strength, balance or functional capacity in general. However, such subjective visual assessments rarely undergo investigation and this relationship has not yet been empirically studied. This study investigated the relationship between performance on the single leg squat task using a qualitative rating scale of Good, Fair and Poor categories, and a range of specific outcome measures, including hip and trunk strength, hip range of motion, lower limb alignment, as well as sports-specific tests of functional capacity and balance. 79 healthy runners were recruited, as runners demonstrate a high incidence of lower-limb injury, and screening for altered hip and knee movement patterns may help to identify those considered most at risk for developing overuse running conditions such as patellofemoral pain syndrome. Single leg squat performance was videotaped for subsequent visual assessment by a panel of physiotherapists. Participants were tested on several objective clinical measures including hip strength (via hand-held dynamometry), hip rotation range of motion (with a gravity inclinometer), lower-limb alignment (static knee valgus and foot posture), sports-specific tests of functional capacity (one-leg rise, side hop and hop for distance measures) and balance (measured as centre of pressure excursion on a Wii Balance Board). The main findings were that female participants with Poor performance on the single leg squat task had significantly reduced hip abduction strength (p = 0.011) and functional capacity on the one-leg rise (p = 0.043) and side hop (p = 0.007) measures than those rated as Good. This difference was not found for male participants. Hip range of motion and lower limb alignment measures were not associated with single leg squat performance for either men or women. For a subset of participants (n = 49), Poor single leg squat performers displayed significantly worse balance as indicated by greater mediolateral postural sway on the Wii Balance Board, compared to the Good group (p = 0.001). Identification of impairments associated with Poor single leg squat performance can aid clinicians in planning prevention strategies for athletes at risk of patellofemoral pain. Further research is required to investigate whether similar associations exist within different sporting participants and for individuals with lower limb injury in particular patellofemoral pain. As clinical expertise and emerging research into patellofemoral pain syndrome indicates that treatment strategies can result in greater improvements if these treatments are targeted towards the subgroup of people who exhibit the underlying impairments, early identification of people with abnormal lower extremity dynamic control via a simple clinical tool, could help to guide practitioners in the prescription of preventative treatments.
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ItemPerformance of a spark ignition, lean burn, natural gas internal combustion engine( 2012)Relative to gasoline and diesel, use of natural gas as a transport fuel can produce significantly lower emissions of particulate matter and greenhouse gases. Future emission standards for commercial transportation, combined with projections in transport demand and gas and oil production, are resulting in increased interest in natural gas use in road vehicles. Lean-burn, natural-gas fuelled spark ignition engines have particular potential in terms of both regulated emissions and increased thermal efficiency. This work therefore first presents a comparison of an engine’s performance fuelled with gasoline and natural gas. Particular emphasis is placed on the natural gas engine’s lean burn behaviour. Analysis of the in-cylinder pressure traces for the natural gas engine is then undertaken. This analysis is used to explain how the observed variations in emissions and efficiency of the natural gas engine vary with air/fuel ratio via the turbulent flame propagation. In particular, it is argued that the commonly observed optimal efficiency at a given lean condition is due to a trade-off between reducing heat losses and increasing flame quenching with increasing air/fuel ratio.
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ItemIntegration of computational fluid dynamics and control system for a missile-shaped body( 2012)The simulation of missile systems plays an important role in the development of technological improvements for defence forces worldwide due to the considerable cost advantages. Within this field, aerodynamics and control continue to be main focal areas that can greatly enhance mission critical parameters such as range and manoeuvrability. Computational Fluid Dynamics (CFD) software has evolved over several decades as an engineering tool for performing flow analysis around complex aerodynamic bodies. From CFD data, an analysis can be performed to characterise the forces and moments that a bluff body experiences in a uniform flow. The data gathered from a CFD simulation can be used as the basis for a low order aerodynamic model, and subsequently used in control design. As missile path tracking represents a constrained control problem, Linear Quadratic Regulation (LQR) presents a challenging approach to optimal control system design. This thesis examines the steady state aerodynamic characteristics of a missile-shaped bluff body in a supersonic free stream and uses these characteristics as the basis for control system design. Confidence in the Reynolds Averaged Navier Stokes (RANS) simulation fidelity is developed systematically by using classical aerodynamic examples, a sphere and an ogive-cylinder, before adding fins for the full missile model. Numerical data from the simulated bluff bodies is found to compare well with published experimental data. The aerodynamic coefficients are supplied to a control-oriented, nonlinear, Six Degree of Freedom (6DoF) model. Derivation of the control system addresses the inherent computational complexity of the nonlinear dynamics and kinematics through linearisation of the 6DoF model using a Linear Time Varying (LTV) approach, coupled with full state feedback. The control system is subsequently tested by perturbing the missile mid-flight trajectory and measuring the resultant deviation from a nominal flight path. Under the proposed approach, the control system counteracts positional and angular error within an appropriate time frame, even in the presence of actuator saturation.
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ItemGround-contact friction estimation and slip prevention in bipedal robots( 2012)The foot-ground contact force is one of the important factors affecting the gait behaviour, balance and safety of a bipedal robotic mechanism. This force is constrained by the coefficient of friction of the foot-ground contact. This coefficient of friction is thus of high importance, as non-sliding contact and/or the knowledge of the coefficient of friction of the foot-ground contact are common assumptions among bipedal robotics literature. Failure to keep the ratio of the friction force to normal force of the foot-ground contact below the coefficient of friction causes the foot to slip, resulting in a large external disturbance being applied to the biped and causing the bipedal robot to lose balance and fall. In this work an online estimation technique for estimating the coefficient of friction of the foot-ground contact is developed. It is expected that the estimation algorithm converges to the value of the coefficient of friction of the contact before the ratio of the friction and normal forces exceeds the coefficient of friction, thus allowing the robot control strategy to avoid slipping altogether by altering the forces exerted by the robot actuation. The proposed estimator is validated experimentally using real-world materials. The estimator is shown to be able to determine the coefficient of friction of the materials ahead of the applied force ratio exceeding the coefficient of friction. Finally a 6-link bipedal robot is modelled and a slip prevention algorithm based on the estimator is implemented to illustrate how a bipedal robot would be able to avoid slipping while walking on a surface of unknown coefficient of friction. The bipedal gait is achieved using a previously developed control algorithm and walks on two flat surfaces of unknown coefficients of friction under ideal simulated conditions. The resulting simulations are compared to demonstrate the effectiveness of the proposed approach. The bipedal robot is shown to be able to walk on a surface of coefficient of friction of 0.4 without slipping and without prior knowledge of this value.