Στην εργασία αυτή θα πραγματευθούμε προβλήματα με βάση τη γενικευμένη θεωρία των Lord και Shulman και αυτή των Green και Lindsay. / --

Experiments are presented in which the diffuse interface between two gases is made Rayleigh-Taylor unstable by interaction with a rarefaction wave. The apparatus used consists of a test section where the counter-flow of light and heavy gases generates a diffuse, stably stratified interface. A tank attached to the bottom of the apparatus is evacuated, and when an appropriate pressure is reached, the interface is perturbed using either a horizontal or a vertical oscillation technique to produce 2D, 3D, and multi-mode perturbations. A solenoid plunger fires an arrowhead which ruptures a membrane, generating a rarefaction wave that travels upward toward the interface. When the rarefaction wave impacts the interface, the interface accelerates down toward the vacuum tank, and the Rayleigh-Taylor instability and mixing develop in the test section. The instability evolution and mixing are recorded using high-speed CMOS cameras and planar laser Mie scattering of smoke particles seeded in the heavy gas. Additional visualization is undertaken with a high-speed shadowgraph system using three CMOS cameras. Interface diffusion thicknesses are recorded using the Rayleigh scattered light of an unexpanded laser beam. Simulations are conducted using a 1D numerical characteristics code based on the method of Hoskin (Meth. Comp. Physics, 3, 1964), and using the LLNL research hydrodynamics code Miranda (Cook, Phys. Fluids, 19, 2007). This 1D code produces Lagrangian interface trajectories while the 2D and 3D simulations using Miranda calculate the growth of perturbations. The theory of Chandrasekhar (Chandrasekhar, Proc. Camb. Phil. Soc., 51, 1955) is extended to capture the effects of diffuse interfaces while including viscosity, and dispersion curves are solved for numerically using a Riccati technique. These solutions show that the method of Duff et al. (Phys. Fluids, 5, 1962) may not accurately describe the growth of single modes for large wavenumbers. For large wavenumbers, when the interface has a large diffusion thickness, perturbations are found to grow with the linear growth rate n = 2Ag/(√πv₀δk²), where A is the Atwood number, g is the acceleration, v₀ is the average kinematic viscosity, δ is the thickness of the interface, and k is the wavenumber of the perturbation. Flat interface experiments exhibit predictable acceleration profiles, but the tail of the rarefaction wave appears at late times reducing the duration of acceleration. Single-mode experiments are conducted for four Atwood numbers including CO₂/SF₆ with A = 0.49, Air/SF₆ with A = 0.63, He/CO₂ with A = 0.82, and He/SF₆ with A = 0.94. Early time results compare well with linear stability theory when non-constant acceleration and diffusion thickness are accounted for. Simulations show good agreement with experiments into the non-linear growth phase. The CO₂/SF₆ and Air/SF₆ experiments show terminal velocity behavior where buoyancy is balanced by drag, but produce Froude numbers larger than those predicted by the Goncharov model (Phys. Rev. Lett., 88, 2002). Using the Mikaelian model (Phys. Fluids, 21, 2009), improved asymptotic Froude numbers are found. The He/CO₂ and He/SF₆ experiments exhibit free-fall behavior, accelerating freely without external forces, with spike amplitudes proportional to the displacement of the unperturbed interface. Single-mode experiments conducted with 3D perturbations using CO₂/SF₆ and Air/SF₆ show good agreement with linear stability theory when non-constant acceleration and diffusion thickness are accounted for. Simulations and the model of Mikaelian predict the growth of the spikes up until late time, while the 3D bubbles reach a terminal velocity more quickly than in simulations. Multi-mode experiments were conducted using Air/SF₆. Multi-mode experiments exhibit nearly t² growth at early times which decays. Using extraction techniques that account for variable acceleration, alpha values are found between ɑ = 0.02 and ɑ = 0.04. These alpha values are lower than are seen for most experiments, but are similar to ɑ values seen in miscible experiments.

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No description available.

Demand for capacity in wireless communication systems has been rapidly growing world- wide. This has been driven by increasing data rate requirements of cellular mobile systems, and demand for wireless Internet and multimedia services. As the available radio spec- trum is limited, higher data rates can only be achieved by designing more efficient signaling techniques. In this thesis, we propose a new transmission scheme, which benefits from the advantages of conventional space-time trellis codes (CSTTCs) but does not have the disadvantages, especially for larger signal constellations. We achieve this by developing a new class of codes, called Multilevel Space-Time Trellis Codes (IMLSTTC). The new scheme provides a scalable and promising alternative to CSTTCs, by providing the system designer with the flexibility to choose any desired balance between code performance, complexity and throughput. The proposed scheme outperforms layered schemes at high SNRs, using a smaller number of antennas.

In a recent study, an environment-friendly material, corrugated cardboard, was used as a building block for the vibration isolator with a preliminary study. The present thesis was motivated to advance technology for improving the design of such a corrugated cardboard vibration isolator with a focus on the modeling of its stiffness and damping. In particular, this study has performed the following works: (1) improving the FE (finite element) model of the stiffness of the corrugated cardboards by more accurately identifying the material parameters in the cardboard material constitutive equation; (2) analyzing the effect of the error in geometry of the corrugated cardboards in the FE model; (3) developing the Rayleigh damping model of the corrugated cardboards and evaluating its accuracy. Several conclusions were drawn from this study: (1) the parameter identification procedure based on the inverse analysis is feasible for improving the accuracy of the model of the stiffness of the cardboard. (2) The FE model of the cardboards with a greater in-plane geometrical deflection has less vertical compressive stiffness. The geometrical deflections of the corrugated cardboards also change the condition of the contact friction stress and the compressive deformation. (3) Rayleigh damping model is accurate enough for calculating the damping of the corrugated cardboards. The contributions of the thesis include: (1) provision of a more accurate model for the compressive stiffness the corrugated cardboards, (2) finding that the friction between the cardboard and the vibrator and the geometrical error of the cardboards have a significant influence over the accuracy of the FE model, (3) finding that in practice the foregoing influence can significantly degraded the performance of the cardboards as a vibrator isolator, and (4) provision of a model for the compressive damping of the corrugated cardboards.

The application of laser generated ultrasonics was first demonstrated in the mid-seventies and has shown good potential when applied to isotropic materials. However, its use with composite materials is still in the early stages of development. This study explores the potential for application of laser generated Rayleigh and Lamb waves in graphite/epoxy composites. Numerical results are obtained by the solution of the wave equations using assumed solutions, and enforcing the boundary conditions. Experimentally, Rayleigh and Lamb waves were generated by a Q-switched ruby laser in the ablation regime and detected by piezoelectric pinducers which permitted accurate phase velocity measurements. The Rayleigh wave velocity was measured at various directions relative to the fiber direction and results were found to agree closely with numerical predictions. The increase of surface wave velocity using thin plates could be useful for the application of delamination detection in thick composites and an increase of Rayleigh wave attenuation could indicate damages caused by impact. Also, surface waves can reflect from small surface cracks. Therefore, laser generated surface waves, particularly along the fiber direction, have high potential for application in non-destructive testing. Lamb wave experiments were conducted in aluminum plates and gave distinctive signals, but there were some difficulties in detecting the precise arrival of each Lamb wave mode for the graphite/epoxy composite plates. / Graduation date: 1992

Thesis (Ph. D.)--Ohio University, November, 1995. / Title from PDF t.p.

Thesis (M.S.)--Oregon State University, 2009. / Printout. Includes bibliographical references (leaves 57-58). Also available on the World Wide Web.

Thesis (M. Eng.)--Carleton University, 1993. / Also available in electronic format on the Internet.