Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session G19: Rayleigh Taylor Instability I |
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Chair: Jeff Jacobs, University of Arizona Room: 322 |
Monday, November 21, 2011 8:00AM - 8:13AM |
G19.00001: Miscible experiments on the Rayleigh-Taylor instability using planar laser induced fluorescence visualization Matthew Mokler, Michael Roberts, Jeffrey Jacobs Incompressible Rayleigh-Taylor instability experiments are presented in which two stratified miscible liquids having Atwood number of 0.2 are accelerated in a vertical linear induction motor driven drop tower. A test sled having only vertical freedom of motion contains the experiment tank and visualization equipment. The sled is positioned at the top of the tower within the linear motors and accelerated downward causing the initially stable interface to be unstable and allowing the Rayleigh-Taylor instability to develop. Experiments are presented with and without forced initial perturbations produced by vertically oscillating the test sled prior to the start of acceleration. The interface is visualized using a 445nm laser light source that illuminates a fluorescent dye mixed in one of the fluids. The resulting fluorescent images are recorded using a monochromatic high speed video camera. The laser beam is synchronously swept across the fluorescent fluid, at the frame rate of the camera, exposing a single plane of the interface allowing for the measurement of spike and bubble mixing layer growth rates. [Preview Abstract] |
Monday, November 21, 2011 8:13AM - 8:26AM |
G19.00002: Experiments on Rayleigh-Taylor instability with Multi-modal initial conditions at low Atwood numbers Sarat Chandra Kuchibhatla, Devesh Ranjan The water channel facility at TAMU is employed for performing Rayleigh-Taylor experiments at low Atwood numbers ($\sim $10$^{-3})$ using hot and cold water as working fluids. Effects of initial conditions on the onset of instability can be investigated using the novel flapper mechanism as previously presented. This mechanism using a very accurate, repeatable servo motor can induce multi-modal initial conditions consisting of up to 8 component modes. Visualization is performed using Nigrosene dye as marker with a LED backlight setting. Experiments were performed to observe the development of instability and its dependence on initial condition (wavelength and phase). Images of the flow-field for different multimodal initial conditions are presented here, and mixing layer growth estimated using ensemble-averaging technique. A measure of the molecular mixing (B$_{0}$, B$_{2}$ and $\theta $ from the BHR model of turbulence) in these flows was also performed using high-speed thermocouple measurements in the flow domain. Furthermore, bubble and spike velocities for single-mode initial experiments were measured using a seeded flow system and high-speed digital camera imaging. Velocities and growth rates are compared with Goncharov's analytical model. [Preview Abstract] |
Monday, November 21, 2011 8:26AM - 8:39AM |
G19.00003: Effect of shear on R-T mixing at low and medium Atwood numbers Bhanesh Akula, Devesh Ranjan Combined RT and KH instabilities are studied at three different Atwood numbers using the gas channel facility at TAMU. In the experiment, two gas streams of different densities (heavy over light) flowing parallel to each other are initially separated by a thin splitter plate. At the end of the splitter plate the two fluids are allowed to mix and the Rayleigh-Taylor instability develops. Simultaneous 3 wire and cold wire anemometry (S3WCA) is used to measure velocities and densities at different locations along and across the channel. Temperature is used as a marker to identify the streams and density is calculated from the temperature measured using a temperature probe with a constant current anemometer. High resolution digital imaging is performed to measure mixing heights and growth rates by injecting smoke into one of the streams and collecting the scattered light from the fog particles illuminated by the back lighting of the channel. Experiments are performed at Atwood numbers 0.04, 0.1 and 0.3. At these Atwood numbers, effect of shear is studied by varying the velocity of one the top stream. Initial conditions are characterized at the interface right after the splitter plate using S3WCA. Different parameters obtained from these measurements including, molecular mixing parameter $\theta $, u', v', w' rms profiles, velocity correlations, vertical turbulent mass flux $\rho $'v' and their effect on mixing growth rate are discussed. [Preview Abstract] |
Monday, November 21, 2011 8:39AM - 8:52AM |
G19.00004: An Update on Experiments and Simulations of Rayleigh-Taylor Instability in Elastic-Plastic Solids Pamela Roach, Arindam Banerjee When an elastic-plastic plate is accelerated by a fluid of lower density, Rayleigh-Taylor instability (RTI) instability is mitigated by the mechanical strength of the plate. Such instances of RTI is observed in explosive welding, volcanic island formation, and inertial confinement fusion. In contrast to Newtonian fluids, experimental study of RTI in accelerated solids is traditionally hindered by difficult to measure material properties and exceedingly small time scales. Both experiments and simulations are used to define the instability region - moreover, the dependence of RTI on initial conditions in elastic-plastic solids is explored. In the experiment, a horizontally mounted disc is rotated to accelerate the interface between an elastic-plastic solid and air. The instability threshold and perturbation growth rate are captured using backlit imaging and a high speed camera. Two dimensional simulations are performed using finite element model ABAQUS to predict instability for a given initial perturbation and acceleration. Experiment and numerical simulation results are compared to the relevant literature. [Preview Abstract] |
Monday, November 21, 2011 8:52AM - 9:05AM |
G19.00005: Progress on the Two-Wheel High Acceleration Experiment to Study Rayleigh-Taylor Turbulence Aaron Haley, Arindam Banerjee A new two-wheel experiment, scaled by a factor of 4 from the previously presented proof of concept, is used to study turbulent incompressible Rayleigh-Taylor (RT) instability. Two counter rotating wheels are mounted side by side such that axes of rotation are normal to gravity. A test section containing pairs of either miscible or immiscible fluids is attached to the first wheel and rotated so that a stable stratification is formed. The test section is then transferred to the adjacent wheel using a pneumatically actuated transfer mechanism. RT instability is effected by the inverted density stratification relative to the centrifugal acceleration. Late time RT turbulence at buoyancy $Re \approx 230,000$ is achieved. Details of the mixing layer development and growth constants are captured using high speed backlit imaging. A variety of fluid combinations (immiscible and miscible) are utilized to investigate development of RT mixing over a range of Atwood numbers and results are compared with data available in the literature. [Preview Abstract] |
Monday, November 21, 2011 9:05AM - 9:18AM |
G19.00006: Miscible and immiscible liquid experiments and simulations on the Rayleigh-Taylor instability Michael Roberts, Matthew Mokler, William Cabot, Jeffrey Jacobs Experiments and numerical simulations are presented in which an incompressible system of two liquids is accelerated to produce the Rayleigh-Taylor instability. In these experiments, the initially stable, stratified liquid combination is accelerated downward on a vertical rail system in one of two experimental apparatuses: an apparatus in which a system of weights and pulleys accelerates the liquid filled tank (which is affixed to a test sled), or a new apparatus which uses linear induction motors to accelerate the tank (which is attached to an aluminum plate) to produce much greater acceleration levels. Both miscible and immiscible liquid combinations are used. In both apparatuses the resulting fluid flows are visualized with backlit imaging using LED backlights in conjunction with monochrome high-speed video cameras, both of which travel with the moving fluid filled containers. Initial perturbations are either unforced and allowed to progress from background noise or forced by vertically oscillating the liquid combination to produce parametric internal waves. The results of these experiments are compared to numerical simulations performed using the CFD code Miranda. [Preview Abstract] |
Monday, November 21, 2011 9:18AM - 9:31AM |
G19.00007: ABSTRACT WITHDRAWN |
Monday, November 21, 2011 9:31AM - 9:44AM |
G19.00008: ABSTRACT WITHDRAWN |
Monday, November 21, 2011 9:44AM - 9:57AM |
G19.00009: On Accounting for Initial Conditions in Models of Rayleigh-Taylor Turbulent Mixing Bertrand Rollin, Malcolm Andrews For many fluid engineering applications, considering only the fully developed turbulence regime with a rough estimate for the turbulence model initial conditions is not sufficient. If we consider for example Inertial Confinement Fusion (ICF), the turbulence and turbulent mixing induced by the Rayleigh-Taylor (RT) instability are subject to initial condition (IC) effects during the time of interest. The degree of confidence in any prediction of the flow evolution in the turbulence regime is therefore open to question. To improve our predictive capability, we need to capture the evolution of the instability from the initial state until it is appropriate to use a turbulence model, in particular with the treatment of ICs that seed the instability. We present our approach for tracking the growth of the RT mixing layer evolution. We have constructed a modal model based on several existing descriptions for single mode and multimode RT mixing evolution. We also present how to extract profiles of turbulence variables that are used as initial conditions for a turbulence model. Finally, we discuss a metric for switching from our modal model to the turbulence model. [Preview Abstract] |
Monday, November 21, 2011 9:57AM - 10:10AM |
G19.00010: Surfactants and the Rayleigh-Taylor instability of Couette type flows A.L. Frenkel, D. Halpern, A.S. Schweiger We study the Rayleigh-Taylor instability of slow Couette- type flows in the presence of insoluble surfactants. It is known that with zero gravity, the surfactant makes the flow unstable to longwave disturbances in certain regions of the parameter space; while in other parametric regions, it reinforces the flow stability (Frenkel and Halpern 2002). Here, we show that in the latter parametric sectors, and when the (gravity) Bond number Bo is below a certain threshold value, the Rayleigh-Taylor instability is completely stabilized for a finite interval of Ma, the (surfactant) Marangoni number: Ma$_{L}$$<$Ma$_{1}$$<$Ma$<$Ma$_{2}$. For Ma$<$Ma$_{L}$, the instability is longwave: the finite interval of unstable wavenumbers borders on the zero value. For Ma$>$Ma$_{2}$, and also for Ma$_{L}$$<$Ma$<$Ma$_{1}$, the instability is ``midwave'': the interval of unstable wavenumbers is bounded away from both the zero and infinity. By numerical and asymptotic means, we determine typical dispersion curves and also characteristic dependencies such as the critical Marangoni numbers Ma$_{L}$, Ma$_{1}$, and Ma$_{2}$ as functions of the Bond number. We note that (for an interval of the Bond number) there are two distinct criticalities with nonzero (and distinct) critical wavenumbers. [Preview Abstract] |
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