Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session G30: Instability: Rayleigh-Taylor I |
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Chair: Ranga Narayanan, University of Florida Room: 408 |
Monday, November 25, 2013 8:00AM - 8:13AM |
G30.00001: Experiments on Effects of Initial Conditions and Material Strength on Rayleigh-Taylor Instability Pamela Roach, Arindam Banerjee The effects of initial conditions on Rayleigh-Taylor (RT) instability in an accelerated elastic-plastic solid were studied. A novel rotating wheel RT experiment that uses centrifugal forces to accelerate a two-material interface was utilized to study the effect of amplitude and wavelength on RT instability with an elastic-plastic solid. The experiment consists of a container filled with air and mayonnaise, a non-Newtonian emulsion, with an initial perturbation between the two materials. Single mode perturbations of various amplitudes and wavelengths were analyzed and results indicated the acceleration required for instability increased for both decreasing initial amplitude and wavelength. Three-dimensional interfaces were found to be more stable than two-dimensional interfaces. Critical amplitude and growth rates were compared with prior experimental results and analytical growth models. Elastic and plastic peak amplitude responses were observed for stable interfaces using a variable acceleration profile where the test section was first accelerated to slightly below the critical acceleration and then decelerated at the same rate. This exercise allowed for verification of the elastic-plastic (EP) transition process before instability was reached. [Preview Abstract] |
Monday, November 25, 2013 8:13AM - 8:26AM |
G30.00002: Mixed-mode instability of a miscible interface due to coupling between Rayleigh-Taylor and double-diffusive convective modes Jorge Carballido-Landeira, Philip Trevelyan, Christophe Almarcha, Anne De Wit In a gravitational field, a horizontal interface between two miscible fluids can be buoyantly unstable because of double diffusive effects or because of a Rayleigh-Taylor instability arising when a denser fluid lies on top of a less dense one. We show here both experimentally and theoretically that, besides such classical buoyancy-driven instabilities, a new mixed mode dynamics exists when these two instabilities act cooperatively. This is the case when the upper denser solution contains a solute A, which diffuses sufficiently faster than a solute B initially in the lower layer to yield non-monotonic density profiles after contact of the two solutions. We derive analytically the conditions for existence of this mixed mode in the (R, $\delta$) parameter plane, where R is the buoyancy ratio between the two solutions and $\delta$ is the ratio of diffusion coefficient of the solutes. We find an excellent agreement of these theoretical predic- tions with experiments performed in Hele-Shaw cells and with numerical simulations. [Preview Abstract] |
Monday, November 25, 2013 8:26AM - 8:39AM |
G30.00003: Buoyancy Driven Mixing By Microwave Volumetric Energy Deposition Adam J. Wachtor, Veronika Mocko, Farzaneh F. Jebrail, Malcolm J. Andrews, Robert A. Gore An investigation of buoyancy driven mixing of two miscible fluids due to volumetric energy deposition by microwaves is presented. The experimental setup is initially Rayleigh-Taylor stable and consists of a light, non-polar fluid at rest atop a heavier, polar fluid. Microwaves preferentially heat the polar fluid, and its density decreases due to thermal expansion. As microwave heating continues, the density of the lower fluid eventually becomes less than that of the upper fluid, thus, the system passes through the neutral stability point and becomes Rayleigh-Taylor unstable, causing buoyancy driven mixing. The evolution of the experimental design from proof-of-concept, to a customized facility designed for enhanced data collection is discussed. In addition, the fluid selection criteria found necessary for experimental success is presented. Single fluid heating experiments were performed to facilitate model development used to predict the neutral stability point and onset of buoyancy driven mixing. Results from the two-fluid mixing experiments demonstrate the capability of this novel Rayleigh-Taylor driven experiment. Particular interest is paid to the onset of buoyancy driven mixing, and atypical aspects of the experiment in the context of typical Rayleigh-Taylor driven mixing. [Preview Abstract] |
Monday, November 25, 2013 8:39AM - 8:52AM |
G30.00004: Miscible and immiscible 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 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 induction motors and accelerated downward causing the initially stable interface to be unstable and allowing the Rayleigh-Taylor instability to develop. Forced and unforced experiments are conducted using both immiscible and miscible liquid combinations. Forced initial perturbations are 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 growth. Comparisons between miscible and immiscible mixing layer distributions are made from the resulting interface concentration profiles. [Preview Abstract] |
Monday, November 25, 2013 8:52AM - 9:05AM |
G30.00005: An analysis of the Rayleigh-Taylor instability of thin viscous layers E.M. de la Calleja, S. Zetina, R. Zenit Recently, Zetina and Zenit (2013) showed that certain textures in the early paintings of D.A. Siqueiros resulted from a hydrodynamic instability. Siqueiros invented the so-called ``accidental painting'' technique, which consisted in pouring layers of different color son top of each other. For the correct color combination, the dual layer became Rayleigh-Taylor unstable and mixed; the density of a paints depends on its color. In this investigation, we conducted experiments to fully understand the instability of thin viscous layers. We varied the densities, viscosities and thicknesses of the layers. We measured the size of the visible blobs and characterized their change in size with the parameters of the flow. We contrasted our observations with the predictions of a linear instability analysis of the flow. We discuss the implications of these results with modern painting techniques. [Preview Abstract] |
Monday, November 25, 2013 9:05AM - 9:18AM |
G30.00006: Rayleigh-Taylor instability under a curved substrate Hyoungsoo Kim, Naima Hammoud, Howard Stone The instability of a thin film under a curved substrate is studied experimentally. A thin film layer is uniformly coated inside a concave surface. We investigate the evolution of the liquid layer by varying the film thickness and the radius of curvature. Two typical perturbation patterns are observed; a flow perturbation in the angular direction and a periodic wavy pattern in the axial direction. These modes are observed at different Bond numbers. Although the classic Rayleigh-Taylor instability of a thin film under a flat substrate is unconditionally unstable, our experimental study highlights that the thin film is conditionally stable due to the curvature. If the aspect ratio between the thin film thickness and the radius of the curved substrate is small enough, the upside-down thin film is always stable. We compare our results with theory (P. Trinh and colleagues). [Preview Abstract] |
Monday, November 25, 2013 9:18AM - 9:31AM |
G30.00007: Experimental measurements of velocity and density statistics in Rayleigh-Taylor instability at High Atwood numbers Devesh Ranjan, Bhanesh Akula, Tom Finn Velocity statistics are measured in a Rayleigh-Taylor mixing layer at Atwood number 0.6 using the multilayer gas tunnel facility at Texas A{\&}M University, which is capable of achieving mixing Reynolds numbers around 30000. Particle Image Velocimetry (PIV) and hot wire anemometry are used to measure the instantaneous velocities inside the mixing layer. The techniques are validated for small Atwood number and plane mixing layer experiments. The velocity statistics obtained including $u_{rms} $,$v_{rms} $, $\overline {u'v'} $, $\overline {\rho 'v'} $ and $\overline {\rho '^{2}}$ are presented and their variation across the mixing layer is also discussed. The probability density functions of the velocities, densities and their spectra are also presented. [Preview Abstract] |
Monday, November 25, 2013 9:31AM - 9:44AM |
G30.00008: Experimental investigation of combined Rayleigh-Taylor instability and Kelvin-Helmholtz instability at different Atwood numbers Bhanesh Akula, Tom Finn, Malcolm Andrews, Devesh Ranjan Combined Rayleigh-Taylor instability (RTI) and Kelvin-Helmholtz instability (KHI) is studied at three different Atwood numbers using the multilayer gas tunnel facility Texas A{\&}M University. S3WCA (Simultaneous 3 wire and cold wire anemometry) and Particle Image Velocimetry (PIV) are used to measure instantaneous velocities and densities at different locations along and across the mix layer. High resolution digital imaging is performed during the experiments 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. Different parameters obtained from measurements including, molecular mixing parameter $\theta $, $u_{rms} $, $v_{rms}$ velocity profiles, velocity correlations, vertical turbulent mass flux $\overline {\rho 'v'} $ and their effect on mixing is discussed. [Preview Abstract] |
Monday, November 25, 2013 9:44AM - 9:57AM |
G30.00009: Efficient mixing in stratified flows: Rayleigh-Taylor instability within a stable stratification, experiments and computation Megan Davies Wykes, Andrew Lawrie, Stuart Dalziel When a Rayleigh-Taylor unstable interface is confined within an otherwise stable stratification the resulting mixing efficiency can be higher than 0.75. This process has been investigated through the use of laboratory experiments, computational numerics and a simple theoretical model. Boussinesq laboratory experiments will be presented, which examine three distinct initial stratifications. Computational experiments using an ILES code have also been performed. The mixing efficiency of laboratory and computational experiments agrees very well. A theoretical model, developed to predict the size of the turbulent mixing region that grows at the unstable interface, matches the results of laboratory experiments. [Preview Abstract] |
Monday, November 25, 2013 9:57AM - 10:10AM |
G30.00010: Experiments on the expansion wave driven Rayleigh-Taylor instability Robert Morgan, Oleg Likhachev, Jeffrey Jacobs Experiments are presented in which a diffuse interface between two gases is accelerated to generate the Rayleigh-Taylor instability. The initially flat interface is generated by the opposing flow of two test gases at matched volumetric flow rates exiting through small holes in the test section. This interface is then accelerated by an expansion wave generated by the rupturing of a diaphragm separating the heavy gas from a tank evacuated to $\sim$ 0.1atm. The expansion wave generates a very high, O(1e3g0), but non-constant acceleration acting on the interface causing the Rayleigh-Taylor instability to develop. Planar Mie scattering is employed using a planar laser sheet generated at the top of the apparatus, which illuminates smoke seeded in in a small amount of air in the heavy gas. Scattered light is then imaged using a CMOS camera operating at 12 kHz. Shadowgraphy is also used to visualize the instability using 200 mm diameter f/6.0 parabolic mirrors along with three CMOS cameras operating at 10 kHz with exposure times of 1e-6s. Perturbations are introduced by either horizontally or vertically oscillating the fluid interface to generate single-mode or random-mode perturbations respectively. Instability amplitude and growth rates are extracted and will also be presented. [Preview Abstract] |
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