Browsing by Subject "Rayleigh-Taylor Instability"
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Item Experimental and Numerical Study of Molecular Mixing Dynamics in Rayleigh- Taylor Unstable Flows(2010-01-16) Mueschke, Nicholas J.Experiments and simulations were performed to examine the complex processes that occur in Rayleigh?Taylor driven mixing. A water channel facility was used to examine a buoyancy-driven Rayleigh?Taylor mixing layer. Measurements of ?uctuating den- sity statistics and the molecular mixing parameter were made for Pr = 7 (hot/cold water) and Sc 103 (salt/fresh water) cases. For the hot/cold water case, a high- resolution thermocouple was used to measure instantaneous temperature values that were related to the density ?eld via an equation of state. For the Sc 103 case, the degree of molecular mixing was measured by monitoring a di?usion-limited chemical reaction between the two ?uid streams. The degree of molecular mixing was quanti- ?ed by developing a new mathematical relationship between the amount of chemical product formed and the density variance 02. Comparisons between the Sc = 7 and Sc 103 cases are used to elucidate the dependence of on the Schmidt number. To further examine the turbulent mixing processes, a direct numerical simu- lation (DNS) model of the Sc = 7 water channel experiment was constructed to provide statistics that could not be experimentally measured. To determine the key physical mechanisms that in?uence the growth of turbulent Rayleigh?Taylor mixing layers, the budgets of the exact mean mass fraction em1, turbulent kinetic energy fE00, turbulent kinetic energy dissipation rate e 00, mass fraction variance gm002 1 , and mass fraction variance dissipation rate f 00 equations were examined. The budgets of the unclosed turbulent transport equations were used to quantitatively assess the relative magnitudes of di?erent production, dissipation, transport, and mixing processes. Finally, three-equation (fE00-e 00-gm002 1 ) and four-equation (fE00-e 00-gm002 1 -f 00) turbulent mixing models were developed and calibrated to predict the degree of molecular mix- ing within a Rayleigh?Taylor mixing layer. The DNS data sets were used to assess the validity of and calibrate the turbulent viscosity, gradient-di?usion, and scale- similarity closures a priori. The modeled transport equations were implemented in a one-dimensional numerical simulation code and were shown to accurately reproduce the experimental and DNS results a posteriori. The calibrated model parameters from the Sc = 7 case were used as the starting point for determining the appropri- ate model constants for the mass fraction variance gm002 1 transport equation for the Sc 103 case.Item Experimental Investigation of the Effect of Initial Conditions on Rayleigh-Taylor Instability(2011-10-21) Kuchibhatla, Sarat ChandraAn experimental study of the effect of initial conditions on the development of Rayleigh Taylor Instabilities (RTI) at low Atwood numbers (order of 10-4) was performed in the water channel facility at TAMU. Initial conditions of the flow were generated using a controllable, highly reliable Servo motor. The uniqueness of the study is the system?s capability of generating the required initial conditions precisely as compared to the previous endeavors. Backlit photography was used for imaging and ensemble averaging of the images was performed to study mixing width characteristics in different regimes of evolution of Rayleigh-Taylor Instability (RTI). High-speed imaging of the flows was performed to provide insights into the growth of bubble and spikes in the linear and non-linear regime of instability development. RTI are observed in astrophysics, geophysics and in many instances in nature. The vital role of RTI in the feasibility and efficiency of the Inertial Confinement Fusion (ICF) experiment warrants a comprehensive study of the effect of mixing characteristics of RTI and its dependence on defining parameters. With this broader objective in perspective, the objectives of this present investigation were mainly threefold: First was the validation of the novel setup of the Water channel system. Towards this objective, validation of Servo motor, splitter plate thickness effects, density and temperature measurements and single-mode experiments were performed. The second objective was to study the mixing and growth characteristics of binary and multi-mode initial perturbations seeking an explanation of behavior of the resultant flow structures by performing the first ever set of such highly controlled experiments. The first-ever set of experiments with highly controlled multi-mode initial conditions was performed. The final objective of this study was to measure and compare the bubble and spike velocities with single-mode initial conditions with existing analytical models. The data derived from these experiments would qualitatively and quantitatively enhance the understanding of dependence of mixing width on parametric initial conditions. The knowledge would contribute towards a generalized theory for RTI mixing with specified dependence on various parameters, which has a wide range of applications. The system setup was validated to provide a reliable platform for the novel multi-modal experiments to be performed in the future. It was observed that the ensemble averaged mixing width of the binary system does not vary significantly with the phase-difference between the modes of a binary mode initial condition experiment, whereas it varies with the amplitudes of the component modes. In the exponential and non-linear regimes of evolution, growth rates of multi-mode perturbations were found to be higher than the component modes, whereas saturation growth rates correspond to the dominant wavelength. Quadratic saturation growth rate constants, alpha were found to be about 0.07 ? 0.01 for binary and multi modes whereas single-mode data measured alpha about 0.06 ? 0.01. High-speed imaging was performed to measure bubble and spike amplitudes to obtain velocities and growth rates. It was concluded that higher temporal and spatial resolution was required for accurate measurement. The knowledge gained from the above study will facilitate a better understanding of the physics underlying Rayleigh-Taylor instability. The results of this study will also help validating numerical models for simulation of this instability, thereby providing predictive capability for more complex configurations.Item Experimental Study and Computational Turbulence Modeling of Combined Rayleigh-Taylor and Kelvin-Helmholtz Mixing with Complex Stratification(2014-06-24) Finn, Thomas PatrickAn experimental study of the combined Rayleigh-Taylor instability (RTI) and Kelvin-Helmholtz instability (KHI) is presented at three different Atwood numbers (0.05, 0.971, 0.147) and multiple velocity ratios to examine the morphological development of the flow field. The Atwood number is the ratio of the difference between the densities of the heavy and light streams to their sum. These experiments were performed using the multilayer gas tunnel facility at Texas A&M University. The tunnel is a convective type system, where gases of different densities flow parallel to one another and are separated by a splitter plate until mixing is allowed downstream in the test section. Three-wire hot-wire probe and particle-image velocimetry (PIV) diagnostic techniques are used to set the velocities in the experiments. Visualization is performed using a high resolution digital camera by injecting fog into one of the streams and collecting scattered light from the illuminated fog particles. Shear effects on the complex stratification are studied. Complex stratification occurs when there is a non-constant density profile in the light fluid mixture, while the constant density profile case is referred to as generic stratification. Transition was found to occur between Richardson numbers of -0.25 and -1.0. Additionally, two different scenarios with and without complex stratification are examined through the mixing layer growth and the non-dimensional growth rate parameter ?_(b,s). Complex stratification was found to produce higher mixing layer growth and larger values of ?_(b,s) than generic stratification. The stratification experiments were also simulated using a one-dimensional, two-equation K-? Reynolds-averaged Navier-Stokes (RANS) model in collaboration with Lawrence Livermore National Laboratory (LLNL). The model is implemented in a hydrodynamics code using a third-order weighted essentially non-oscillatory (WENO) or central differencing method for the advection terms, a second-order central differencing method for the gradients in the source, sink, and diffusion terms, and a second-order implicit Crank-Nicolson (CN) method for the time evolution. Simulations are compared to experiments through the mixing layer and non-dimensional growth parameter ?_(b,s). The overall trends shown by the simulations were consistent with experimental data. Specific values for growth height and ?_(b,s), however, were found to be vastly under predicted.