Mechanical Engineering - Theses
Permanent URI for this collection
Search Results
1 - 10 of 22
-
ItemGeometric properties of streamlines in turbulent wall-flows( 2019)Streamline geometry has been studied in case of turbulent wall flows. Complex but coherent motions form and rapidly evolve within wall-bounded turbulent flows. Research over the past two decades broadly indicates that the momentum transported across the flow derives from the dynamics underlying these coherent motions. This spatial organization, and its inherent connection to the dynamics, motivates the present research. The local streamline geometry pertaining to curvature $(\kappa)$ and torsion $(\tau)$ has apparent connection to the dynamics of the flow. The present results indicate that these geometrical properties change significantly with wall-normal position. One part of this research is thus to clarify the observed changes in the streamline geometry with the known structure and scaling behaviours of the mean momentum equation. Towards this aim, the curvature and torsion of the streamlines at each point in the volume of existing boundary layers and channel DNS has been computed. The computation of $\kappa$ and $\tau$ arise from the local construction of the Frenet-Serret coordinate frame. The present methods for estimating $\kappa$ includes components of curvature in the streamwise, wall-normal and spanwise direction. The analysis shows that even though the mean wall-normal velocity is zero (e.g., for channel flow), the wall-normal curvature component shows a notable positive peak close to the wall. This arises from the strong wallward flow followed by a weak movement of the streamlines away from the wall. The correlation coefficient and the conditional average of the wall-normal velocity corresponding to the wall-normal curvature exhibit an anti-correlation between them. The probability density function of the curvatures have been calculated at some wall-normal locations of interest and compared with a scaling of the exponent of $-4$ for both total and fluctuating field. This scaling of curvature values describes the geometric features of the length scales that are smaller than the Kolmogorov scale. The onset of this scaling with wall distance has a potential connection to the three-dimensionalization of the vorticity field and the stagnation points structure in the inertial domain. In this region, the mean radius of curvature scales like Taylor microscale. The probability density functions of the wall-normal curvature show that high curvature regions similar to those in isotropic flow begin to appear outside the viscous wall layer. The standard deviation for torsion exhibits a decreasing effect with distance from the wall. The torsion to curvature ratio reveals the intensity of out of plane motion of the streamlines relative to their in-plane bending. The joint pdf of curvature with velocity magnitude supports the notion that large curvature values correspond to the region near a stagnation point. Furthermore, the joint pdf results between curvature components reveal the orientation of the streamlines at different wall-normal locations. Overall the curvature and torsion statistics examined thus far point to intriguing correlations with the four layer structure associated with known structure of the vorticity field in turbulent wall-flows.
-
ItemAdaptive cruise control with collision-avoidance guarantees( 2018)An adaptive cruise control (ACC) is designed to schedule the velocity of a vehicle whilst maintaining an adequate distance to a vehicle in front. Subject to this distance requirement, the velocity control problem can show benefit in fuel economy and/or emissions performance, although the overall problem is complicated by uncertainties in the vehicle models and the complex behaviours of other vehicles on the road. In this project, a simple velocity prediction algorithm for the future traffic conditions is used but its error is characterised in combination with the errors arising from low order vehicle models. The vehicle control problem can then be posed as a robust economic model predictive controller (MPC), guaranteeing the distance separation constraint in the presence of a range of disturbances, such as prediction error and model mismatch, which is critical to driving safety. Meanwhile, it allows explicit consideration of fuel economy for ACC. The performance of the proposed approach is compared with a conventional ACC system in high-fidelity simulations, and was found to offer slight fuel economy improvements relative to a PID controller, whilst providing collision avoidance guarantees.
-
ItemDisturbance attenuation in tree-structured interconnected systems( 2018)Tree topologies are a subset of networked systems that can be used to represent many large-scale applications ranging from transportation networks to irrigation systems. One important aspect of network design for these systems is to certify a specific controller design with desired disturbance propagation characteristics. Most existing literature cast desirable disturbance propagation characteristics as a stability property, which guarantees the boundedness of states rather than disturbance attenuation. Given the fact, however, that in most real-world systems the states are expected to stay within a specific set, it is highly desirable that the disturbance is attenuated when propagating through the network. The benefit is that the largest effect of the disturbance is always caused in the subsystem where it originally occurs, and state constraints can be satisfied for all subsystems as long as the origin subsystem is well controlled. This research proposes computationally scalable sufficient conditions for tree topologies to provide guarantees of disturbance attenuation. Only neighbouring information is required by these conditions, which enables distributed controller design. Under these conditions, a centralised controller synthesis method is then developed for tree networks with controlled interconnections between subsystems. Distributed design of controllers is then presented by breaking up the centralised controller synthesis problem into sub-problems for subsystems. Subsequently, a relaxation of the requirements of network topology to "tree-like" systems is proposed to widen the scope of the results. Specifically, an approach to approximate a system with an arbitrary topology by a tree is developed, facilitating the application of disturbance attenuation results in more general cases. Finally, demonstrations via a water supply system and a traffic simulation are used to validate the theoretical findings for continuous-time and discrete-time systems, respectively. Results show that the developed controller can effectively attenuate disturbance. Subsequently, the time efficiency of the proposed sufficient conditions are demonstrated. Moreover, the efficacy of the tree approximation method is shown by comparing various tree extractions.
-
ItemAnalysis of resolvent method for turbulence inflow generation( 2018)High-fidelity simulations of turbulent flows aim to accurately reproduce the statistical and structural properties of real-life turbulence. Such simulations rely on accurate inflow boundary conditions. A novel turbulent inflow generation method for high-fidelity DNS/LES has been developed which utilizes reduced order modelling (ROM) and evolutionary algorithms. The core idea behind this method is the classical view of turbulence which represents it as a collection of coherent structures. A low-rank approximation approach known as the resolvent analysis, developed by McKeon and Sharma [Journal of Fluid Mechanics, Vol. 658, 336-382 (2010)], is used to represent the governing equations as a linear input-output system. The non-linearities in the governing Navier-Stokes equations are the driving force behind the ow. This forcing of the linear system produces a response which represent the velocity perturbations. The resolvent analysis is performed at different wavenumber-frequency combinations which are selected in a manner to represent a variety of energetic coherent structures like the near-wall longitudinal streaks, hairpin vortices, Large Scale Motions (LSMs) and Very Large Scale Motions (VLSMs). A Singular Value Decomposition (SVD) of the linear operator in the input-output system is performed to generate a set of orthonormal basis functions for the forcing and response fields. A major advantage of the resolvent analysis is its reduced dependence on external data. It requires only the mean statistical quantities as an input which are readily available for various ow problems or can be easily obtained from cost-effective RANS simulations. The amplitudes of the selected modes were linearly scaled such that the turbulence kinetic energy (TKE) of the resolvent modes is equal to the target TKE. This technique successfully resulted in a fully developed turbulent field, although with a large development length. A further improvement of this method is obtained by optimizing the amplitude of each resolvent mode, which represents the energy content of the associated coherent structure. A genetic algorithm approach has been used to optimize the resolvent modes to represent the target Reynolds stress profiles. This modified process results in a significantly improved development length.
-
ItemThe direct injection of liquefied petroleum gas (LPG) in an optical, spark ignition engine( 2017)Given concerns with energy security and the environmental impact of anthropogenic climate change, spark-ignition (SI) engine research is intensely focused on improving fuel efficiency and reducing emissions outputs. The solution requires a synergistic approach that utilises multiple technological innovations, including fuelling strategies (e.g. efficient fuel injection system and engine operation regime) and cleaner, alternative fuels. One promising combination is DI liquefied petroleum gas (LPG); DI and LPG both enable more fuel efficient operation and LPG-fuelled engines produce a lower quantity of harmful emissions than a gasoline equivalent. However, the commercial realisation of DI LPG in an SI engine requires a thorough understanding of the in-cylinder fuel spray behaviour. To this end, an investigation is conducted in a modern, production engine converted for optical access in one cylinder, using propane as a surrogate for LPG. Planar laser Mie-scattering is employed to assess the extent of the liquid phase of the fuel spray over a range of engine operating conditions. These results are compared against those of iso-octane, a common surrogate to approximate gasoline. It was found that DI propane is sensitive to its initial condition in the fuel rail before injection, as well as the in-cylinder pressure at the start of injection. DI propane is also susceptible to severe flash-boiling and spray collapse at all DISI operating conditions. These experimental results are further analysed using a previously developed framework that employs simple thermodynamic and geometric information to predict flash-boiling spray structure in a multi-hole injector. Though this model was validated using spray image data from a quiescent vessel and an experimental injector, it was shown to adequately describe the overall spray structure in the optical engine, which uses a production injector. The overall spray structures predicted by the flash-boiling criterion calculations were consistent with flashing spray structures observed in the engine. Because the time-scales of flash-boiling are significantly faster than those of the flow field in the engine, the formation of flashing fuel sprays is dominated by flash-boiling phenomena, i.e. multi-phase, underexpanded flow, spray collapse due to plume interaction and increased vaporisation rates.
-
ItemExtremum seeking for spark advance calibration under tailpipe emissions constraints( 2016)Engine control parameters are calibrated on a test rig laboratory by a series of experimental methods. The resulting parameters are obtained using fuels with a fixed composition and tested for compliance with the emission standard on legislated drive cycle. As fuel composition and driving behaviour may vary in the real world, there is motivation in considering methods of continually calibrating online for optimal performance (in some sense) subject to emissions legislation. In this regard, extremum seeking (ES) is a potential non-model-based adaptive control strategy to achieve the online calibration of automotive engines. The technique has been used in tuning the engine’s spark timing to minimize fuel consumption. Spark timing also plays a role in emission formation, which has not been considered previously. This research proposes an approach to extend extremum seeking control for online optimisation of dynamic systems by explicitly considering output constraints. The proposed controller formulation required a slight relaxation to provide average constraint satisfaction in the limiting case. The stability of the proposed approach is investigated under a range of circumstances. The novel formulation is then applied to the problem of fuel consumption optimisation subject to emissions constraints in a high-fidelity engine model with a three-way catalytic aftertreatment system. The manipulated input was the spark timing, brake specific fuel consumption was chosen to be the metric function, and the distance-based NOx tailpipe emission was treated as the constrained output. Results showed that it is possible to obtain the optimal spark timing whilst satisfying on average the Euro-3 emission limit for NOx under different operating points.
-
ItemImproving the safety of forklifts through automation of task execution( 2016)Forklift vehicles are used throughout the world, in many industries to move stock from location to location. Unfortunately these vehicles are prone to toppling, possibly causing stock damage, injuries and loss of income. There are fewer safety interventions applied to forklifts than would be expected of other wheeled vehicle, with the industry relying on operator training and experience to prevent accidents. This thesis proposes a method of measuring the toppling propensity, and a trajectory generation algorithm to allow the autonomous use of forklift vehicles. The zero moment point and a barycentric coordinate system will be used to measure the dynamic balance. The measure will allow analysis of forklift behaviour and may be used to provide information of the state of balance to the operator with the intent of aiding judgement and to improve accountability. The obtained dynamic balance measure will then be used as an inequality constraint during the construction of a non-holonomic trajectory. This will be used to ensure the trajectory complies with safe manoeuvring of the forklift vehicle. The autonomy of the vehicle will allow the operator to distance themselves from the vehicle, interacting only through high level decisions as opposed to low level handling of the vehicle, with the intent of removing the human element of error from the toppling problem. An analysis of forklift behaviour given differing loading scenarios will be discussed, with similar scenarios computationally simulated to produce sample trajectories of the new algorithm. The results are compared to intuitive notions of movement and to safety literature.
-
ItemMusculoskeletal model for gait analysis in people with partial foot amputation( 2015)Partial foot amputation is the most common amputation performed in the world today. In Australia, over 74% of all lower limb amputations are partial foot amputations. Diabetic patients are 15 times more likely to have an amputation than people without diabetes. Despite the prevalence of partial foot amputation, the influence of different levels of amputation on the biomechanics of gait is not well understood. This research project aimed to develop subject-specific musculoskeletal models to simulate and analyse gait in people with partial foot amputation, including quantification of the forces developed in the lower limb muscles during walking. One subject with transmetatarsal amputation and another with metatarsophalangeal amputation were selected based on inclusion criteria. In the transmetatarsal subject, the effects of compromised metatarsals on spatiotemporal data, kinematics, kinetics and individual muscle forces were compared with those of a control subject with metatarsophalangeal amputation through toes. The validation of developed models showed that predicted joint angles and joint moments were comparable to those reported in another study so developed musculoskeletal models for subjects were used for individual muscle force prediction of lower limb muscles during stance. For some muscles, the timing of forces predicted by the models and the timing of their EMG data were comparable so the forces predicted for muscles were reliable so far. The results showed that once the metatarsals are compromised through transmetatarsal amputation, some gait abnormalities were observed. These changes in gait were caused by an inability to generate power across the ankle joint. In the residual limb with transmetatarsal amputation, the reduction in the magnitude of the net ankle joint moment was associated with the reduction in the calf muscle forces during terminal stance and pre-swing of the gait. Compromised metatarsals through transmetatarsal amputation may impair the biomechanics of gait significantly through the reduction in the capacity of calf muscle forces to plantar flex the ankle and generate the necessary ankle torque to propel the residual limb.
-
ItemDirect numerical simulation of turbulent natural convection bounded by differentially heated vertical walls( 2013)Using new, high-resolution direct numerical simulation (DNS) data, this study appraises the different scaling laws found in literature for turbulent natural convection of air in a differentially heated vertical channel. The present data is validated using past DNS studies, and covers the Rayleigh number (Ra) range between 5.4 × 10^5 to 1.0 × 10^8. This is followed by an appraisal of various scaling laws proposed by four studies: Versteegh and Nieuwstadt (77), Holling and Herwig (34), Shiri and George (63) and George and Capp (23). These scaling laws are appraised with the profiles of the mean temperature defect, mean streamwise velocity, normal velocity fluctuations, temperature fluctuations and Reynolds shear stress. Based on the arguments of an inner (near-wall) and outer (channel-centre) region, the DNS data is found to support a −1/3 power law for the mean temperature in an overlap region. Using the inner and outer temperature profiles, an implicit heat transfer equation is obtained and a correction term in the equation is shown to be not negligible for the present Ra range when compared with explicit equations found in literature. In addition, I determined that the mean streamwise velocity and normal velocity fluctuations collapse in the inner region when using the outer velocity scale. A similar collapse is noted in the profiles of temperature fluctuations with increasing Ra when normalised with inner temperature and length scale. Lastly, I show evidence of an incipient proportional relationship between friction velocity and the outer velocity scale with increasing Ra. The study is extended to the spectrum of turbulent kinetic energy and temperature fluctuations of the flow. The one-dimensional streamwise spectra collapse onto the −5/3 slope, coinciding with the standard Kolmogorov form of the power spectra reported in literature. This collapse is found to occur in the outer region of the flow in the bounds between the peaks of the mean streamwise velocity. In spectrogram form, I find evidence that the spectral peaks correspond to energetic velocity structures in the channel — the structures of streamwise velocity fluctuations appear to stretch half of the streamwise domain and occur at a quarter intervals in the spanwise direction. From 2-dimensional autocorrelations, the structures of spanwise velocity fluctuations are found to be organised in a hatched pattern in an inner location (z^× i ≈ 7) and at the channel-centre. The respective pattern angles are \theta_ i ≈ 54◦ and \theta_ o ≈ 48◦, both measured from the horizontal. For the temperature spectrum, the −5/3 collapse is also observed in the same bounds as the velocity spectrum. In pre-multiplied form, the spectral peak is found to occur at the wall-normal location which coincides with the peak temperature fluctuations in the channel. With increasing Ra, the wall-parallel isocontours of temperature are found to show standard features of turbulent pressure driven boundary layers — streaks with spanwise length of 100+ units.
-
ItemA computational approach to investigate subchondral bone adaptation during the development and progression of osteoarthritis( 2013)Osteoarthritis (OA) is a degenerative joint disease widespread in the general population –particularly the aged. Advanced OA is most notably characterised by the gradual degradation of the protective cartilage tissues of articulating bones that can lead to severe joint pain and immobilisation. There is emerging evidence to suggest that observable changes in the structure and material properties of underlying subchondral bone (bone directly beneath the joint’s cartilage) play critical roles in the aetiology of OA. Factors driving these subchondral bone changes are largely unknown but it is believed to occur as a response to either stabilise or accelerate OA due to altered biomechanical loads. This study quantified the extent of subchondral bone adaptation from an experiment assessing two cohorts: mice with induced knee OA by surgical destabilisation of the medial meniscus and a sham surgery group of mice without OA. Time-dependent subchondral bone adaptations were quantified using microtomography imaging techniques prior to surgery and 4, 8 and 12 weeks post-surgery. Three-dimensional finite element (FE) models were created from the baseline image data and a bone (re)modelling algorithm was developed to iteratively update the FE models, simulating subchondral bone adaptation to load. Simulations were compared with the post-surgery image data for validation, and potential osteoarthritic load changes were identified. Simulated osteoarthritic bone adaptations matched the experimental results, where localised subchondral bone density changes occurred in response to altered biomechanical loads, verifying that the simulation was a physiologically accurate model.