Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Includes bibliographical references (p. 123-130).

Thesis (doctora)--München, 1980.

Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (leaves 134-145).

Thesis (Ph.D.)--Montana State University--Bozeman, 2007. / Typescript. Chairperson, Graduate Committee: Joseph A. Shaw. Includes bibliographical references (leaves 191-198).

With the development of manufacturing techniques such as the Electron Beam Free Form Fabrication (EBF3), a metal deposition technique which deposits metal in complex shapes on a metallic base plate, it has become easy to manufacture complex shapes such as panels with curvilinear stiffeners. Designing and optimizing stiffened panels with predefined structural and acoustic response is the focus of this dissertation. Researchers have dealt with sizing optimization of panels with straight/curvilinear stiffeners for many years and it has been proven that in some cases the mass of a panel with curvilinear stiffeners is lesser than the mass of a panel with straight stiffeners for a complex loading such as bi-axial compression with shear and transverse pressure. The research work in this dissertation addresses the sizing as well as placement optimization of panel with straight and curvilinear stiffeners for desired structural as well as acoustic response. For acoustic optimization, point-excited stiffened panels are designed for minimal sound radiation given the constraint on total mass of the structure. To reduce the computational expense of structural-acoustic optimization, a new methodology for the objective function evaluation is also proposed and optimal design for minimum radiated acoustic power is discussed. The developed framework, named EBF3PanelOpt for structural acoustic optimization of point excited stiffened panels is extended to multi point excitation to capture the realistic excitations such as turbulent boundary layer (TBL) pressure fluctuations. The Corcos model of representing TBL is used to capture correlation of TBL pressure excitation. Validation of the approach using Corcos TBL model, implemented in EBF3PanelOpt is performed using fast multi-pole boundary element method in FastBEM and a conventional boundary element code, HELM3D. The optimal designs are obtained for a panel with two and four stiffeners, respectively. The minimization of both, the mass and the acoustic response during structural-acoustic optimization is conflicting in nature. Therefore, a multi-objective design optimization using non-dominated sorting genetic algorithm-II (NSGA-II) is performed. The Pareto optimal designs, obtained using multi-objective design optimization approach has reduced the acoustic response significantly with a minor mass penalty of the structure when compared to a baseline design while meeting all the constraints such as buckling eigenvalue, von Mises, and crippling stresses. A multi-objective design optimization framework is also developed for design optimization of diffuse field excited panels with straight and curvilinear stiffeners. The Pareto optimal designs of panel with six stiffeners are obtained using developed framework and a comparative study is performed with a baseline design with six straight stiffeners. The developed framework is also extended to perform multi-objective design optimization of point excited complex structures such as curved panels with straight or curvilinear stiffeners. A fast multi-pole boundary element method is used to calculate the acoustic response of the curved panel with stiffeners and design optimization results of a curved panel with two and four stiffeners are discussed. Experiments are also performed at Sound and Acoustic Load Transmission (SALT) facility of Langley Research Center to measure the sound radiation and transmission loss for two panels with straight and curvilinear stiffeners, respectively. The stiffened test panels with six stiffeners have been designed using multi-objective design optimization framework for TBL excitation. / Ph. D.

Nonlinear ultrasonic waves have demonstrated high sensitivities to various microstructural changes in metal including coherent precipitates; these precipitates introduce a strain field in the lattice structure. The thermal aging of certain alloy steels leads to the formation of coherent precipitates, which pin dislocations and contribute to the generation of a higher harmonics in an initially monochromatic wave. The objective of this research is to develop a robust technique to perform nonlinear Rayleigh wave measurements in metals using a non-contact receiving transducer. In addition a discussion about the data processing based on the two-dimensional diffraction and attenuation model is provided in order to calculate the relative nonlinearity parameter. A precipitate hardenable material, 17-4 PH stainless steel, is used to obtain different precipitation stages by thermal treatment and the influence of precipitates on the ultrasonic nonlinearity is assessed. Conclusions about the microstrucutural changes in the material are drawn based on the nonlinear Rayleigh surface wave measurement and complementary measurements of thermo-electric power, mircohardness and ultrasonic velocity. The results show that the nonlinearity parameter is sensitive to coherent precipitates in the material and moreover that precipitation characteristics can be characterized based on the obtained experimental data.

The present work aims at studying the coupling between hydrodynamic fingering instabilities and chemical reactions at the interface between two miscible solutions. Hydrodynamic deformations of interfaces between two reactive fluids as well as flows induced by chemical reactions at the front between two initially steady fluids are encountered frequently in combustion, petroleum, chemical and pharmaceutical engineering. Most of the time, concrete applications imply a very large number of variables so that an understanding of the fundamental processes of chemo-hydrodynamic coupling is out of reach. Our goal is here to analyze a much simpler model system in which only one mechanism of hydrodynamic instability is at play and for which the chemical reactions can be modeled by a one or two-variable model. Buoyantly unstable, autocatalytic chemical fronts, are one such model system, which can be used as prototype to study the effects of the coupling between chemical reactions and hydrodynamic fingering instabilities. Fingering processes occur whenever a fluid of high mobility displaces a less mobile one in a porous medium. The initially planar interface looses then stability and a cellular fingering deformation of the interface is observed. Such an instability has been observed, for instance, in the iodate-arsenous acid and chlorite-tetrathionate reactions, autocatalytic redox reactions known to produce a change of density across a traveling front. Fingering happens there when the heavier solution lies on top of the lighter one in the gravity field. Our theoretical contribution to the analysis of fingering of chemical fronts focuses on different points which we detail in this thesis along the following outline. In chapter 2, we introduce fingering phenomena occurring in porous media and distinguish the situation of viscous and density fingering of pure non reactive fluids. Chapter 3 reviews the literature on coupling between fingering and chemical reactions before studying the linear stability conditions as well as nonlinear dynamics of density fingering of isothermal iodate-arsenous acid fronts. This prototype nonlinear redox reaction is the first one on which experimental results on fingering in spatially extended set-ups have been obtained. We next analyze in chapter 4 the density fingering of another front producing autocatalytic system i.e. the chlorite-tetrathionate reaction in order to address the influence of the chemical kinetics on the dynamics observed. The influence of the exothermicity of the reaction is then presented in chapter 5. Eventually, chapter 6 analyzes what happens if the kinetics is now bistable and further compares the situation of both viscous and density fingering of bistable fronts. We then conclude and present suggestions for future work in this subject at the frontier between nonlinear chemistry, hydrodynamics and engineering.

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