Browsing by Subject "Rayleigh-Taylor"
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Item An investigation of the influence of initial conditions on Rayleigh-Taylor mixing(Texas A&M University, 2006-04-12) Mueschke, Nicholas JayExperiments and direct numerical simulations (DNS) have been performed to examine the e??ects of initial conditions on the dynamics of a Rayleigh-Taylor unsta- ble mixing layer. Experiments were performed on a water channel facility to measure the interfacial and velocity perturbations initially present at the two-fluid interface in a small Atwood number mixing layer. The experimental measurements have been parameterized for use in numerical simulations of the experiment. Two- and three- dimensional DNS of the experiment have been performed using the parameterized initial conditions. It is shown that simulations implemented with initial velocity and density perturbations, rather than density perturbations alone, are required to match experimentally-measured statistics and spectra. Data acquired from both the exper- iment and numerical simulations are used to examine the role of initial conditions on the evolution of integral-scale, turbulence, and mixing statistics. Early-time turbu- lence and mixing statistics are shown to be strongly-dependent upon the early-time transition of the initial perturbation from a weakly-nonlinear to a strongly-nonlinear flow.Item Effect of Initial Conditions on the Compound Shear- and Buoyancy-driven Mixing(2012-10-19) Placette, BethThe effect of initial conditions in combined shear- and buoyancy- driven mixing was investigated through the use of an implicit large eddy simulation code under active development at Los Alamos National Laboratory and Texas A&M University. Alterations were done over several months both at Los Alamos National Laboratory and at the Texas A&M University campus, and include a transition from tilted rig to convective channel arrangement, introduction of an inertial reference frame, alteration of boundary conditions, etc. This work resulted in the development of a numerical framework with the capability to model various shear and Atwood number arrangements such as those seen in an inertial confinement fusion environment. In order to validate the code, it was compared to three published experiments, one with Atwood number 0.46 (White et al. 2010), one with high Atwood number 0.6 (Banerjee et al. 2010), and one with very low Atwood number 0.032 (Akula et al. 2012). Upon validating the code, pure Rayleigh-Taylor and pure Kelvin-Helmholtz instabilities were modeled along with five intermediate cases of increasing shear and constant density gradient. Plots of mixing width, Richardson number, growth parameter, and molecular mixing were compared in order to determine at what level of shear the minimum amount of mixing occurs. The results of height gradient and Reynolds number were to previous experiments and theory. The least amount of molecular mixing at the centerline was found to be when the system had a low Atwood number (0.032) and a multimode initial interface perturbation. While the increase in modes of the interface perturbation did not result in a significant change in the growth parameter, the level of molecular mixing at the centerline substantially decreased. As shear was increased in the system, the mixing width and molecular mixing subsequently increased. For this reason, the shear in the system should be eliminated, or at least minimized, if at possible so as to prevent any additional amalgamation in the system. Analysis of the Reynolds number revealed that with an increase in velocity difference between the fluid layers, the value consequently increased. This trend matches with theoretical results as the value is a function of the mixing width and velocity, thus further validating the code. Analysis of the transitional Richardson number revealed that it had a smaller value in the computational case over the experiment, but this fact can be attributed the difference in mixing width between the two methods. The development of the numerical framework with the capability to model various shear and Atwood number arrangements offers the platform for future study of hydrodynamic instabilities.Item Effect of initial conditions on the development of Rayleigh-Taylor instabilities(2009-05-15) Peart, Freeman MichaelThere are two coupled objectives for this study of buoyancy-driven turbulence. The first objective is to determine if the development of a Rayleigh-Taylor (RT) mixing layer can be manipulated experimentally by altering the initial condition of the experiment. The second objective is to evaluate the performance of the Besnard, Harlow, and Rauenzahn (BHR) turbulent transport model when initialized with experimentally measured initial conditions. An existing statistically steady water channel facility at Texas A&M University and existing experimental diagnostics developed for this facility have been used to measure the turbulent quantities of buoyancy-driven turbulence. A stationary, bi-planar grid with a high solidity ratio, ?, has been placed immediately downstream of the termination of the splitter plate, perpendicular to the flow direction, to generate a turbulent initial condition. The self-similar growth parameter, ? , for the RT mixing layer has been measured using a visualization technique to determine if the initial conditions affect the development of the RT mixing layer. The self-similar growth parameter, ? , decreased from a value of 0.072 ? 0.0003 with the fine grid to values of 0.063 ? 0.0003 and 0.060 ? 0.0003 with the medium and coarse grids, respectively. With the results from the first objective, a unique opportunity arose to evaluate the performance of the variable density, RANS-type, BHR turbulent transport model. Measurements of velocity statistics necessary to initialize the model accurately have been obtained using particle image velocimetry (PIV). The performance of the BHR model was evaluated through comparison of the experimentally measured and BHR modeled self-similar growth parameter, ? , from the penetration height of the bubbles/spikes and the self-similar growth parameter, K ? , of the turbulent kinetic energy at the centerline of the low Atwood RT driven turbulent mixing layer. When initialized with the experimentally measured initial conditions, the BHR model did agree with the experimental measurements of the penetration height growth parameter, ? , as well as the centerline turbulent kinetic energy growth parameter, K ? , in the self-similar portion of the flow.Item Effects of Single Mode Initial Conditions in Rayleigh-Taylor Turbulent Mixing(2011-02-22) Doron, YuvalThe effect of single mode initial conditions at the interface of Rayleigh-Taylor(RT) mixing are experimentally examined utilizing the low Atwood number water channel facility at Texas A&M. The water channel convects two separated stratified flows and unifies them at the end of a splitter plate. The RT instability is attained by convecting a cold stream above a warmer stream. Average density calculations are based on long time average optical measurements. The water channel was modifified with a flapper fin like device at the end of the splitter plate which was actuated by a computer controlled servo motor. Other modifications to the experiment were implemented resulting in reduced uncertainty. The experiment examined five different modes in addition to the baseline: 2 cm, 3 cm, 4 cm, 6 cm, and 8 cm wavelengths. The mixing width growth rates were shown to be dependent on initial conditions. Additionally, it appears that the growth rates commence with terminal velocity and are observed to line up with the baseline case.Item Experimental investigation of a stratified buoyant wake(Texas A&M University, 2004-11-15) Kraft, Wayne NealAn existing water channel facility at Texas A&M University is used to experimentally study a stratified, buoyant wake. A cylindrical obstruction placed at the centerline of a developing Rayleigh-Taylor mixing layer serves to disturb the equilibrium of the Rayleigh-Taylor mixing layer. The development of the near wake in the presence of unstable stratification is examined, in addition to the recovery of the buoyancy driven mixing layer. Planar laser induced fluorescence (PLIF) is used to visualize the mixing layer / wake interactions, and qualitative observations of the behavior have been made. Also, quantitative measurements of velocity fluctuations and density fluctuations in the near wake have been obtained using particle image velocimetry (PIV) and a high resolution thermocouple system. These experimental measurements were used to investigate how the wake and buoyancy driven mixing layer interact. Finally, a mathematical model has been used to describe the decay of vertical velocity fluctuations in the near wake due to the effects of buoyancy.Item On the dynamics of Rayleigh-Taylor mixing(Texas A&M University, 2004-09-30) Ramaprabhu, Praveen KumarThe self-similar evolution of a turbulent Rayleigh-Taylor (R-T) mix is investigated through experiments and numerical simulations. The experiments consisted of velocity and density measurements using thermocouples and Particle Image Velocimetry techniques. A novel experimental technique, termed PIV-S, to simultaneously measure both velocity and density fields was developed. These measurements provided data for turbulent correlations, power spectra, and energy balance analyses. The self-similarity of the flow is demonstrated through velocity profiles that collapse when normalized by an appropriate similarity variable and power spectra that evolve in a shape-preserving form. In the self-similar regime, vertical r.m.s. velocities dominate over the horizontal r.m.s. velocities with a ratio of 2:1. This anisotropy, also observed in the velocity spectra, extends to the Taylor scales. Buoyancy forcing does not alter the structure of the density spectra, which are seen to have an inertial range with a -5/3 slope. A scaling analysis was performed to explain this behavior. Centerline velocity fluctuations drive the growth of the flow, and can hence be used to deduce the growth constant. The question of universality of this flow was addressed through 3D numerical simulations with carefully designed initial conditions. With long wavelengths present in the initial conditions, the growth constant was found to depend logarithmically on the initial amplitudes. In the opposite limit, where long wavelengths are generated purely by the nonlinear interaction of shorter wavelengths, the growth constant assumed a universal lower bound value ofItem On the Effect of Initial Conditions on Rayleigh-Taylor Mixing(2014-12-16) Kuchibhatla, Sarat ChandraAn experimental investigation of the effects of Initial Conditions (ICs) on Rayleigh-Taylor Instability (RTI) is performed using theWater Channel facility at Texas A&M University. Hot and cold water (with a temperature difference of ~ 5-8 ?C) selected as working fluids are unstably stratified initially. The resulting Atwood number for this instability is of the order of 10^-3. In this dissertation, effect of the composition of the initial perturbations generated by a flapper mechanism is studied. Using the servo controlled flapper system, initial wavelengths are varied between 2-8 cm and phase angles within 0-180?, and the dependence of ensemble averaged mixing width in the linear and nonlinear stages of growth on low Atwood number Rayleigh-Taylor mixing is studied. The Interaction of multiple (up to 11) modes of the IC is studied by varying the wavelengths and phase angles that are generated at the interface of these fluids. High resolution Planar Laser Induced Fluorescence (PLIF) images of the flow field indicate that changing the phase angle results in the leaning phenomenon, i.e. the leaning of bubbles and spikes with respect to gravitational acceleration, and relative to each other. Experimental measurement of total mixing width, quality of molecular mixing and scalar dissipation rate are performed using ensemble averaging technique. The results indicate that nonlinear mode coupling of the initial modes affects the rate of mixing as well as the transition to turbulence. Molecular mixing measurements indicate that the molecular mixing rate depends upon the ICs. Particle Image Velocimetry (PIV) data indicates that late-time velocity profiles of the mixing layer depend upon ICs. While the density power spectra indicate independence of ICs in the inertial range, the turbulent velocity statistics indicate that increasing the number of modes of the IC quickens the transition to turbulence. It also results in greater mixing, which reduces the density gradients as the flow evolves with time. Thus, the memory of the ICs is lost sooner with increasing number of initial modes.