A system for recording the trajectories of non-planar shocks and particle tracers within a shock tube flow has been developed. The optics consists of a double-pass schlieren system with a multiply pulsed ruby laser as light source. The laser is synchronized with a high speed framing camera. A grid of ammonium chloride tracers is injected into the flow field, and the motion of the tracers behind the Mach reflection of intermediate strength shocks has been recorded. Analysis of the trajectories has yielded the space and time variation of the physical properties within the flow field. / Graduate / 0605

The aim of this thesis is to explain the change in the debate about police accountability in Britain that took place in the 1980s. In seeking such an explanation in the reinvention of police accountability over this period, a four dimensional analysis of accountability is presented. This is used to examine, in turn, the history of police governance in London, the debates about police accountability that took place in the 1980s, and the implications of the growing influence of community policing that culminated in the introduction by the Metropolitan Police of a new style of ‘sector policing’. A series of questions about whether and how police accountability was reinvented in the 1980s are posed, and the implications of the reconceptualisation that took place are assessed in their historical and theoretical contexts. Use is also made of empirical data drawn from a study of the implementation of sector policing on an inner city police area in North London. It is argued that far-reaching changes took place in the conceptualisation of police accountability during the 1980s on all four of the dimensions identified, and that this reinvention of the relationship between police and people made policing in London neither more democratic nor more consensual.

Transport properties of the two-dimensional electron gas (2DEG) in high magnetic fields are used to investigate scattering processes affecting the resistivity of GaAs-GaAlAs and GaInAs-InP heterojunctions and quantum wells: especially coupling of electrons to acoustic and optic phonons; and transitions between electric subbands. The experiments fall into two groups: A systematic study of magnetophonon resonance (MPR) between 30K and 300K. Resonance positions indicate a coupling substantially below the LO phonon energy, expected from 3D measurements. GaAs-GaAlAs hetero junctions show amplitudes varying smoothly with electron density (n_s) and closely related to the 4K mobility. On rotation in magnetic field they decrease rapidly as the resonance position returns to the LO value. In modulation doped structures the damping factor is determined by remote impurity scattering. As n_s is increased in GaInAs-InP the coupling frequency decreases dramatically from the GaAs-like LO at 272cm^-1 to the InAs-like TO at 226cm^-1. At higher electric fields the 'normal' MPR maxima invert, starting at low magnetic fields, to form 'hot electron' MPR minima, with maximum amplitude at ~60K. This is the first direct observation of HEMPR in 2D and is explained in a diffusion picture. At lower electric fields, additional resonances are identified with resonant cooling by inter-subband scattering. Comparisons are made with calculations and explanations sought including consideration of interface phonons; coupled plasmon-phonon modes; and shifts of the resonance positions due to the shape of the density of states. Low temperature magnetoresistance measurements in GaAs-GaAlAs heterojunctions with more than one occupied electric subband. Shubnikov-de Haas oscillations in perpendicular magnetic fields contain non-additive terms at electron temperatures > 2K where acoustic phonon mediated inter-subband scattering is comparable to intra-subband scattering. Subband separations and greatly enhanced g-factors [largest for electrons in the upper subband ] are deduced from the oscillations. Damping of the oscillations in field, gives values for quantum lifetimes (τ_s), much smaller than τ_tʼ, deduced from mobility. With two subbands occupied τ_s is always largest for the upper subband, while relative sizes of τ_t depend on sample quality. Study of electron energy loss rates, from thermal damping of the oscillations, shows enhancement in the region kT_e ~ ħω_cʼ, which is evidence for cyclotron phonon emission. Depopulating subbands in parallel fields causes the resistance to drop, by up to 60%, due to suppression of inter-subband scattering. Systematic studies show this scattering rate is independent of n_s.

This dissertation investigates the problem of estimating a kernel over a large graph based on a sample of noisy observations of linear measurements of the kernel. We are interested in solving this estimation problem in the case when the sample size is much smaller than the ambient dimension of the kernel. As is typical in high-dimensional statistics, we are able to design a suitable estimator based on a small number of samples only when the target kernel belongs to a subset of restricted complexity. In our study, we restrict the complexity by considering scenarios where the target kernel is both low-rank and smooth over a graph. Using standard tools of non-parametric estimation, we derive a minimax lower bound on the least squares error in terms of the rank and the degree of smoothness of the target kernel. To prove the optimality of our lower-bound, we proceed to develop upper bounds on the error for a least-square estimator based on a non-convex penalty. The proof of these upper bounds depends on bounds for estimators over uniformly bounded function classes in terms of Rademacher complexities. We also propose a computationally tractable estimator based on least-squares with convex penalty. We derive an upper bound for the computationally tractable estimator in terms of a coherence function introduced in this work. Finally, we present some scenarios wherein this upper bound achieves a near-optimal rate. The motivations for studying such problems come from various real-world applications like recommender systems and social network analysis.

A numerical analysis is performed for the three-dimensional, turbulent flow of air in a thyristor valve hall located at the Dorsey Converter Station, owned and operated by Manitoba Hydro. The goal of this analysis was to determine the configurations that result in increased air-side cooling effectiveness in the valve hall. The governing equations are solved using the Computational Fluid Dynamics (CFD) code CFX-5. This computer code uses a finite volume method of solution and is based on a finite element approach for representing the geometry. The effects of inlet and outlet opening geometry, inlet air mass flow rate, and inlet air angle on the thermal performance for the air-side cooling of the thyristor valve hall geometry are examined.

In dimensional measurements, it is required to specify uncertainty in the measurement as the range of possible deviation for the measurement result. In this thesis, a computer program is developed for evaluating uncertainty in measurement of commonly used dimensional measuring devices like vernier callipers, micrometers, comparators, and gauge blocks. In evaluation of the uncertainty in measurement, some uncertainty sources like temperature difference between the measured part and the instrument, uncertainty in reference gauge block&rsquo / s dimension, mechanical effects, etc. are considered. The program developed, employs the EAL, NIST and GUM uncertainty evaluation equations as standard equations. However, the program can also be used for other measuring instruments and the users can define their own uncertainty equation. In the evaluations, for the standard uncertainty of the variables considered, symmetric distributions are used. The program gives the uncertainty budget and to compare the contribution of each variable on the overall uncertainty of the measurement, the uncertainty effect ratio is also given. In this thesis the evaluation process for uncertainty in measurement, the difference between the measurement error and uncertainty in measurement and the structure of the program are discussed. Also, a set of experiments has been made to illustrate the application of the program for evaluating the measurement uncertainty of vernier callipers with 1/50 and 1/20 resolutions, digital vernier calliper and 25 mm micrometer.

In this thesis, assuming that higher spatial dimensions existed only during the inflationary prematter phases of the universe, we construct a (1+D)-dimensional (D&gt / 3), nonsingular, homogeneous and isotropic Friedmann model for dimensional reduction. In this model, dimensional reduction occurs in the form of a phase transition that follows from a purely thermodynamical consideration that the universe heats up during the inflationary prematter phases. When the temperature reaches its Planck value Tpl,D, which is taken as the maximum attainable physical temperature, the phase of the universe changes from one prematter era with D space dimensions to another prematter era with ( D-1) space dimensions where T_pl,D is higher. In this way, inflation gets another chance to continue in the lower dimension and the reduction process stops when we reach D=3 ordinary space dimensions. As a specific model, we investigate the evolution of a (1+4)-dimensional universe and see that dimensional reduction occurs when a critical length parameter l_4,3 reaches the Planck length of the lower dimension. Although the predictions of our model for the cosmological parameters are beyond the ranges accepted by recent measurements for closed geometry, for a broad range of initial conditions they are within the acceptable ranges for open geometry

A large number of studies have concentrated on golf swing biomechanics, ranging from planar rigid-link models to 3D kinematic analysis. A promising technique, instantaneous screw axis (ISA) theory, has not been covered in the literature and could provide a better true segment rotation approximation. The objectives of this study are to identify ISA location and orientation, as well as segment angular velocity, of the major body segments involved in the golf swing. For all subjects, it was found that the magnitude of maximum angular velocities increased from the most proximal segment (the pelvis) to the most distal segment (the left arm), in accordance with the summation of speeds principle. Furthermore, most subjects achieved their maximum angular velocities in the desired kinematic sequence, where the first maxima was achieved by the most proximal segment and followed by the more distal segments in the kinematic chain.

Early detection is one of the most important factors in the survival of patients diagnosed with breast cancer. For this reason the development of improved screening mammography methods is one of primary importance. One problem that is present in standard planar mammography, which is not solved with the introduction of digital mammography, is the possible masking of lesions by normal breast tissue because of the inherent collapse of three-dimensional anatomy into a two-dimensional image. Digital tomosynthesis imaging has the potential to avoid this effect by incorporating into the acquired image information on the vertical position of the features present in the breast. Previous studies have shown that at an approximately equivalent dose, the contrast-detail trends of several tomosynthesis methods are better than those of planar mammography. By optimizing the image acquisition parameters and the tomosynthesis reconstruction algorithm, it is believed that a tomosynthesis imaging system can be developed that provides more information on the presence of lesions while maintaining or reducing the dose to the patient. Before this imaging methodology can be translated to routine clinical use, a series of issues and concerns related to tomosynthesis imaging must be addressed. This work investigates the relevant physical processes to improve our understanding and enable the introduction of this tomographic imaging method to the realm of clinical breast imaging. The processes investigated in this work included the dosimetry involved in tomosynthesis imaging, x-ray scatter in the projection images, imaging system performance, and acquisition geometry. A comprehensive understanding of the glandular dose to the breast during tomosynthesis imaging, as well as the dose distribution to most of the radiosensitive tissues in the body from planar mammography, tomosynthesis and dedicated breast computed tomography was gained. The analysis of the behavior of x-ray scatter in tomosynthesis yielded an in-depth characterization of the variation of this effect in the projection images. Finally, the theoretical modeling of a tomosynthesis imaging system, combined with the other results of this work was used to find the geometrical parameters that maximize the quality of the tomosynthesis reconstruction.

This project explores systematic strategies for establishing a definitive mode of representation in drawing. These strategies are considered in the context of the subjectivities and contingencies of perceptual experience, the mediation of that experience and inherent limitations in representational language. The aim is to explore the possibilities of constructing two-dimensional artworks which serve to define or investigate perceptual processes, perspective devices and spatial relationships.