The di raction of a one-dimensional expansion wave over a 90 corner was explored using experiment and simulation methods. Unlike studies in shock di raction, expansion wave di raction was hardly explored in the literature and therefore is considered as novel. Two independent parameters were identi ed for the present study: 1) the initial diaphragm shock tube pressure ratio, and 2) the position of the diaphragm from the apex of the 90 corner. The experimentation only considered variation in the shock tube pressure ratio whereas the simulation varied both independent parameters. A Navier-Stokes solver with Menters SST k-! turbulence model was found to adequately model the ow eld. A number of major ow features were identi ed, that occurred in the vicinity of the 90 corner. The ow features identi ed were: a shear layer which originated by ow separation near the apex of the 90 corner, a separation bubble that remained attached to a wall boundary in absence of rig-dependent e ects, and a re ected compression wave due to perturbation signals generated by di raction of the expansion wave. For a narrow-width expansion wave existing prior to di raction, it was found that after di raction a re ected compression wave developed which would steepen into an outwardly propagating, weak, cylindrical shock wave. Other major ow features identi ed were a strong indication of an oblique shock located near the separation bubble and a large wake region immediately downstream of the separation bubble. The wake region, through schlieren imaging, was found to consist of two distinct layers. The experimental results through shadowgraph and schlieren imaging have indicated large-scale turbulent structures within the separation bubble and the shear layer. Shear layer instability and vortex shedding o the separation bubble were also evident. The Navier-Stokes solver was found not to resolve the experimentally observed turbulence, the wake region and the vortex shedding.

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No description available.

The properties of a crossed grating of square pyramids and a crossed grating with hemispherical cavities to eliminate specular reflection from a conducting surface are studied experimentally. Measurements were made in the microwave range of 35 GHz. The best performance is that 99.94% of the power of a TM-polarized incident wave can be scattered into a single spectral order by a pyramidal crossed grating, while for TE polarization the reduction in specular reflection can be as high as 98%. Anti-reflection properties of a crossed grating with hemispherical cavities near normal incidence are also observed. Comparison between the behavior of triangular and pyramidal gratings of the same profile is made. Effects of the profile parameters are investigated. Basically the experimental results agree with the theoretical predictions. This investigation provides a set of experimental data to assist further numerical study. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

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No description available.

Efficient and accurate solutions of acoustic wave diffraction by a rigid step discontinuity and a curved half-plane are derived by the uniform geometrical theory of diffraction. These solutions can be used in seismic data processing to evaluate and, eventually, to improve the existing data processing procedures. They can also find applications in electromagnetics, microwave antenna design, acoustic design and sound engineering. The rigid step discontinuity solution given in this thesis is more accurate than the existing solutions which are based on Kirchhoff theory of diffraction. This solution removes the previous restriction on the source and the receiver arrangement. It also provides high efficiency by the use of ray theory. This solution is further generalized to two offset half-planes and an inclined wedge. Solutions for more complicated structures can be obtained by superposition of these solutions with added interactions. The complex source position method is used to extend the omnidirectional point source solution to a beam source solution. The effect of changes of the directivity and orientation of the beam source is studied. Time-domain single and double diffraction coefficients are determined through direct Fourier transforming and convolution. An infinite impulse response filter is applied to the time-domain direct computation of single diffraction. This combination achieves a total saving of 75% of computing time over the frequency-domain approach. Diffraction by a curved half-plane is analyzed with the inclusion of creeping wave diffraction and second order edge diffraction. An acoustic model of a curved half-plane is designed to verify the theory. The experimental results obtained by Mellema have verified the existence of the creeping wave diffraction and weak traces of the second order edge diffraction. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

This work describes an implementation of an iterative algorithm to design two-zone binary phase diffractive optical elements (DOEs) which produce a specific irradiance distribution in both the axial and transverse directions under high numerical aperture focusing. The full vector character of a polarized beam is rigorously accounted for using the vector diffraction integrals. The axial and tran verse irradiance distributions are calculated for applications involving multiphoton direct la er writing, specifically, a novel method known as resolution augmentation through photo induced deactivation or, RAPID. In the RAPID method two beams are used, one for excitation and the other for de-excitation. By having both the excitation and de-excitation beams, the resolution achievable using the RAPID method is maximized.

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Micro-cylinders with the diameters spanning the 1-20 μm range are growing in importance, for realizing devices with new functionalities. For custom functionalities, their device-designs impose, tolerance-related constraints on their critical dimension. To meet the challenges for the associated online micro-metrology, new methods for the micro-cylinder diameter measurement, are currently receiving considerable attention. „Optical diffraction under Fraunhofer Approximation‟ is one of the most viable experimental techniques for cylinder diameter measurement, in the laboratory as well as Industrial environment. In 1-20 μm diameter range, however, the cylinder-diffraction is not well understood. The reliability of the current models/formulations for this range is far from satisfactory in respect of speed, accuracy, resolution etc., and need a re-examination. The present thesis concerns with a few investigations on the „Optical Diffraction by Micro-Cylinder‟. It highlights both the theoretical and the experimental aspects of the investigations on micro-cylinders with diameters in the range of 1-50 μm. The results of the investigation are organized into two categories. The first of them details a pair of new analytical models obtained from the principles of „Geometrical Theory of Micro-Cylinder Diffraction‟ while the second category highlights another pair of new analytical models obtained from the principles of the „Customary Fraunhofer Theory of Micro-Cylinder Diffraction‟. The model from the „geometrical theory‟ is based on the hypothesis that the ‘ray-paths relevant to the location of ‘diffraction minima’, facilitate to construct, a geometrically-equivalent triangle’. The solution of such a triangle provides the new formulation for the micro-cylinder diffraction. The model from the „customary Fraunhofer theory‟, instead, relies on the on the fact that „the diffraction pattern for a micro-cylinder is essentially, a chirped-interference pattern modulated by a diffraction envelope’. The functional form of the formulation depends upon, the type of triangle constructed for geometrical theory and the type of illumination used in the customary Fraunhofer theory. The thesis highlights, four new formulations (two from each of the approaches) to describe the micro-cylinder diffraction. The principal conclusions of the investigations are as under. - All the new formulations for the micro-cylinder diffraction facilitate, enhanced diameter inversion accuracy, in the hitherto esoteric diameter range of 1-20 μm. For the reported experimental data on 3 μm diameter micro-cylinder, the models proposed in the present investigation improve the accuracy of diameter-estimation from 16.5% known from earlier models to less than 1%. - The investigation also brings out for the first time, the hitherto unnoticed difference between slit-diffraction and the micro-cylinder diffraction: When the micro-cylinder diameter approaches the wavelength of the illumination, the first order diffraction angle approaches nearly 200. It may be noted that for a slit of same width, the corresponding diffraction angle approaches 900. When the critical dimension of the cylinder and the slit decreases from λ to 5λ, the difference in the corresponding diffraction angles reduce from nearly 700 to nearly 1.50. - The investigation also highlights that in the micro-cylinder diffraction for the said range of interest, the absolute intensity at the zero-order interference- maximum provides a new signature for the distance of separation between the diffraction minima. The consequence of this new finding is a considerable simplification in the apparatus and algorithm for diameter inversion from a diffraction experiment. The function of an array detector can be replaced by a point detector at a fixed point for all the diameters in the range of interest. - The two formulations proposed from the geometrical theory are suited for diffraction- minima search based algorithm, while those from customary Fraunhofer theory are well suited for intensity minima search based method for diameter inversion.