Browsing by Author "Bertsch, Rebecca Lynne"
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Item Effect of Inhomogeneity and Unsteadiness on the Stability of High-Speed Shear Flows(2014-07-09) Bertsch, Rebecca LynneIn hypersonic flows, turbulence critically influences mass and momentum transport, mixing, heat transfer and acoustic noise generation. In contrast to incompressible flow, in high speed flows pressure is a true thermodynamic variable and flowthermodynamic interactions render the investigations extremely challenging. Most studies to date have been performed on steady, uniform or homogeneous shear flows leading to important insight on the flow physics. In most real world applications,flows of practical importance will exhibit unsteadiness and strong inhomogeneity. To date, investigations of unsteadiness and inhomogeneity in high-speed flows are rare. The goal of this dissertation is to study and understand these non-ideal effects when pertinent to shear flows. Towards this goal, we perform three distinct studies: (a) examination of time reversal characteristics of linear inviscid mass, momentum, energy and state equation in compressible flows; (b) Linear analysis (RDT) of compressibility effects on instabilities in temporally periodic (unsteady) homogeneous shear flow; and (c) Numerical investigation of small perturbation evolution in compressible Kolmogorov (inhomogeneous) shear flow. The first study shows that even with the additional governing equations required in the high-speed regime, the inviscid flow field is still reversible. This justifies the use of temporal periodicity to investigate the effect of unsteadiness. The second study presents a detailed analysis of the pressure equation in temporally periodic homogeneous shear flow. The analysis and numerical results show unsteady uniform shear exhibits two stages of evolution due to the changing behavior of pressure. These stages are analogous to the first two stages of evolution established in steady shear. The third stage seen in steady shear is not achieved by periodic shear flow. The final study shows that the evolution of small perturbations in spatially periodic Kolmogorov flow is influenced by: i) the initial compressibility parameter, M_(g0), ii) the initial perturbation orientation, and iii) the stream normal location. Ultimately, the final study supports the postulate that all shear flows exhibit perturbation stability boundary classifications seen in homogeneous shear flows. The findings of this research further our understanding of the effects of unsteadiness and inhomogeneity in realistic flows, which will aid in the development of improved computational tools.Item Rapidly Sheared Compressible Turbulence: Characterization of Different Pressure Regimes and Effect of Thermodynamic Fluctuations(2011-10-21) Bertsch, Rebecca LynneRapid distortion theory (RDT) is applied to compressible ideal-gas turbulence subjected to homogeneous shear flow. The study examines the linear or rapid processes present in turbulence evolution. Specific areas of investigation include:(i) characterization of the multi-stage flow behavior,(ii) changing role of pressure in the three-regime evolution and (iii) influence of thermodynamic fluctuations on the different regimes. Preliminary investigations utilizing the more accurate Favre-averaged RDT approach show promise however, this approach requires careful validation and testing. In this study the Favre-averaged RDT approach is validated against Direct Numerical Simulation (DNS) and Reynolds-averaged RDT results. The three-stage growth of the flow field statistics is first confirmed. The three regime evolution of turbulence is then examined in three different timescales and the physics associated with each regime is discussed in depth. The changing role of pressure in compressible turbulence evolution shows three distinct stages. The physics of each stage is clearly explained. Next, the influence of initial velocity and thermodynamic fluctuations on the flow field are investigated. The evolution of turbulence is shown to be strongly dependent on the initial gradient Mach number and initial temperature fluctuations which tend to delay the onset of the second regime of evolution. The initial turbulent Mach number, which quantifies velocity fluctuations in the flow, influences turbulence evolution only weakly. Comparison of Reynolds-averaged RDT against Favre-averaged RDT for simulations of nonzero initial flow field fluctuations shows the higher fidelity of the latter approach.