200 Matching Results

Search Results

Advanced search parameters have been applied.

An Imcompressible Rayleigh-Taylor Problem in KULL

Description: The goal of the EZturb mix model in KULL is to predict the turbulent mixing process as it evolves from Rayleigh-Taylor, Richtmyer-Meshkov, or Kelvin-Helmholtz instabilities. In this report we focus on a simple example of the Rayleigh-Taylor instability (which occurs when a heavy fluid lies above a light fluid, and we perturb the interface separating them). It is well known that the late time asymptotic, fully self-similar form for the growth of the mixing zone scales quadratically with time.
Date: September 22, 2005
Creator: Ulitsky, M
Partner: UNT Libraries Government Documents Department

Materials Response under extreme conditions

Description: Solid state experiments at extreme pressures, 10-100 GPa (0.1-1 Mbar) and strain rates (10{sup 6}-10{sup 8} s{sup -1}) are being developed on high-energy laser facilities. The goal is an experimental capability to test constitutive models for high-pressure, solid-state strength for a variety of materials. Relevant constitutive models are discussed, and our progress in developing a quasi-isentropic, ramped-pressure, shockless drive is given. Designs to test the constitutive models with experiments measuring perturbation growth due to the Rayleigh-Taylor instability in solid-state samples are presented.
Date: October 6, 2005
Creator: Remington, B A; Lorenz, K T; Pollaine, S & McNaney, J M
Partner: UNT Libraries Government Documents Department

Experiments showing dynamics of materials interfaces

Description: The discipline of materials science and engineering often involves understanding and controlling properties of interfaces. The authors address the challenge of educating students about properties of interfaces, particularly dynamic properties and effects of unstable interfaces. A series of simple, inexpensive, hands-on activities about fluid interfaces provides students with a testbed to develop intuition about interface dynamics. The experiments highlight the essential role of initial interfacial perturbations in determining the dynamic response of the interface. The experiments produce dramatic, unexpected effects when initial perturbations are controlled and inhibited. These activities help students to develop insight about unstable interfaces that can be applied to analogous problems in materials science and engineering. The lessons examine ``Rayleigh-Taylor instability,`` an interfacial instability that occurs when a higher-density fluid is above a lower-density fluid.
Date: February 1997
Creator: Benjamin, R. F.
Partner: UNT Libraries Government Documents Department

Modeling and analysis of the high energy liner experiment, HEL-1

Description: A high energy, massive liner experiment, driven by an explosive flux compressor generator, was conducted at VNIIEF firing point, Sarov, on August 22, 1996. We report results of numerical modeling and analysis we have performed on the solid liner dynamics of this 4.0 millimeter thick aluminum liner as it was imploded from an initial inner radius of 236 mm onto a Central Measuring Unit (CMU), radius 55 mm. Both one- and two-dimensional MHD calculations have been performed, with emphasis on studies of Rayleigh-Taylor instability in the presence of strength and on liner/glide plane interactions. One-dimensional MHD calculations using the experimental current profile confirm that a peak generator current of 100-105 MA yields radial liner dynamics which are consistent with both glide plane and CMU impact diagnostics. These calculations indicate that the liner reached velocities of 6.9-7.5 km/s before CMU impact. Kinetic energy of the liner, integrated across its radial cross-section, is between 18-22 MJ. Since the initial goal was to accelerate the liner to at least 20 MJ, these calculations are consistent with overall success. Two-dimensional MHD calculations were employed for more detailed comparisons with the measured data set. The complete data set consisted of over 250 separate probe traces. From these data and from their correlation with the MHD calculations, we can conclude that the liner deviated from simple cylindrical shape during its implosion. Two-dimensional calculations have clarified our understanding of the mechanisms responsible for these deformations. Many calculations with initial outer edge perturbations have been performed to assess the role of Rayleigh-Taylor instability. Perturbation wavelengths between 4-32 mm and amplitudes between 8-240 {mu}m have been simulated with the experimental current profiles. When strength is omitted short wavelengths are observed to grow to significant levels; material strength stabilizes such modes in the calculations.
Date: August 1, 1997
Creator: Faehl, R.J.; Sheehey, P.T. & Reinovsky, R.E.
Partner: UNT Libraries Government Documents Department

Nonlinear Rayleigh-Taylor instability experiments in Nova

Description: We examined the progression of the Rayleigh-Taylor (RT) instability from an initial multimode perturbation. The RT experiments focused on the transition from the linear to non-linear regimes for perturbation growth at an embedded, or classical, interface. The multimode experiments have attempted to observe the process of bubble competition wherein neighboring structures either continue to rise or are washed downstream in the flow depending upon; their relative size. This competition is predicted to result in an inverse cascade at late times where progressively larger structures will begin to dominate the flow. Experiments to date have shown evidence of coupled modes arising, but have not yet accelerated the interface long enough to produce the several generations of coupling required for a true inverse cascade.
Date: June 16, 1997
Creator: Budil, K.S.; Remington, B.A.; Weber, S.V.; Perry, T.S. & Peyser, T.A.
Partner: UNT Libraries Government Documents Department

Effect of enhanced thermal dissipation on the Rayleigh-Taylor instability in emulsion-like media

Description: Rayleigh-Taylor instability in a finely structured emulsion-like medium consisting of the two components of different compressibility is considered. Although the term ``emulsion`` is used to describe the structure of the medium, under typical fast Z-pinch conditions both components behave as gases. The two components are chosen in such a way that their densities in the unperturbed state are approximately equal. Specific emphasis has been made on the analysis of perturbations with the scale {lambda} considerably exceeding the size of the grains a. Averaged equations describing such perturbations am derived. The difference in compressibility of the two components leads to the formation of temperature variations at the scale a, and increases the rate of the thermal dissipation by a factor ({lambda}/a){sup 2}. The strongest stabilizing effect of the thermal dissipation takes place when the thermal relaxation time is comparable with the instability growth rate.
Date: July 1, 1997
Creator: Toor, A. & Ryutov, D.
Partner: UNT Libraries Government Documents Department

Classical Rayleigh-Taylor instability experiments at Nova

Description: The evolution of the Rayleigh-Taylor (RT) instability at an embedded, or classical, interface is examined in a series of experiments at the Nova laserfacility .[reference for Nova] These experiments focused on the transition from the linear to nonlinear regimes for both single- and multimode initialperturbations. The development of a single mode at the embedded interface is compared to its evolution at an ablation front and the effect of ablativestabilization is experimentally demonstrated. The multimode experiments have shown evidence of the process of bubble competition, whereinneighboring structures either continue to rise or are washed downstream in the flow depending upon their relative size. The experiments with simulations performed with either the LASNEX are comparedcode [G. B.Zimmerman and W. L. Kruer, Comments Plasma Phys. Controlled Fusion 2,51 (1975).], a two-dimensional Lagrangian radiation-hydrodynamics code, or CALE [R. Tipton, reference for CALE], a two-dimensional arbitrary Lagrange-Eulerian radiation-hydrodynamics code.
Date: November 10, 1997
Creator: Budil, K. S.; Remington, B. A.; Weber, S. V.; Perry, T. S. & Peyser, T. A.
Partner: UNT Libraries Government Documents Department

Nonlinear Rayleigh-Taylor and Richtmyer-Meshkov mixing experiments at Nova

Description: The evolution of the Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) instabilities in the nonlinear regime of growth was investigated in indirect-drive experiments on the Nova laser. The RT experiments investigated the evolution of both single- and multimode perturbations at an embedded interface, isolated from the effects of ablation. This ``classical`` geometry allows short wavelength ({lambda} {approximately} 10-20 {micro}m) perturbations to grow strongly, in marked contrast to prior results at an ablation front. The RM experiments studied singly- and doubly-shocked perturbed interfaces in both face-on and side-on geometries. (U)
Date: September 15, 1997
Creator: Budil, K.S.; Remington, B.A.; Weber, S.V.; Farley, D.R.; Murray, S. & Peyser, T.A.
Partner: UNT Libraries Government Documents Department

Rayleigh-taylor instability growth experiments in a cylindrically convergent geometry

Description: Convergent geometry Rayleigh-Taylor experiments have been performed with a 122-point detonation initiation system on cylinders having sinusoidal perturbations on the outer surface ranging from mode-6 to mode-36. Experiments were performed with various perturbation mode numbers, perturbation amplitudes, and ring accelerations. Feedthrough perturbation growth on the inner surface was observed in several experiments, and in one experiment the feed through perturbation underwent a phase inversion. These experimental results were found to be in good agreement with linear, small-amplitude analysis of feedthrough growth in an incompressible, cylindrically convergent geometry.
Date: June 11, 1997
Creator: Weir, S. T., LLNL
Partner: UNT Libraries Government Documents Department

Instability Control in a Staged Z-pinch

Description: A \Staged Z-Pinch” is a fusion-energy concept in which stored-electric energy is first converted into plasma-liner-kinetic energy, and then transferred to a coaxialtarget plasma [H. U. Rahman, F. J. Wessel, and N. Rostoker, Phys. Rev. Lett. 74, p. 714(1996)]. Proper choice of the liner and target materials, and their initial radii and mass densities, leads to dynamic stabilization, current amplification, and shock heating of the target. Simulations suggest that this configuration has merit as a alternative inertial-confinement-fusion concept, and may provide an energy release exceeding thermonuclear break-even, if tested on one of many newer pulsed power systems, for example those located at Sandia National Laboratories.
Date: April 22, 2011
Creator: WESSEL, Frank J.
Partner: UNT Libraries Government Documents Department

Investigations of the Rayleigh-Taylor and Richtmyer-Meshkov Instabilities

Description: The present program is centered on the experimental study of shock-induced interfacial fluid instabilities. Both 2-D (near-sinusoids) and 3-D (spheres) initial conditions are studied in a large, vertical square shock tube facility. The evolution of the interface shape, its distortion, the modal growth rates and the mixing of the fluids at the interface are all objectives of the investigation. In parallel to the experiments, calculations are performed using the Raptor code, on platforms made available by LLNL. These flows are of great relevance to both ICF and stockpile stewardship. The involvement of four graduate students is in line with the national laboratories' interest in the education of scientists and engineers in disciplines and technologies consistent with the labs' missions and activities.
Date: March 14, 2008
Creator: Bonazza, Riccardo; Anderson, Mark & Oakley, Jason
Partner: UNT Libraries Government Documents Department

Investigation of the Rayleigh-Taylor and Richtmyer-Meshkov instabilities

Description: The present research program is centered on the experimental and numerical study of two instabilities that develop at the interface between two different fluids when the interface experiences an impulsive or a constant acceleration. The instabilities, called the Richtmyer-Meshkov and Rayleigh-Taylor instability, respectively, adversely affect target implosion in experiments aimed at the achievement of nuclear fusion by inertial confinement by causing the nuclear fuel contained in a target and the shell material to mix, leading to contamination of the fuel, yield reduction or no ignition at all. The laboratory experiments summarized in this report include shock tube experiments to study a shock-accelerated bubble and a shock-accelerated 2-D sinusoidal interface; and experiments based on the use of magnetorheological fluids for the study of the Rayleigh-Taylor instability. Computational experiments based on the shock tube experimental conditions are also reported.
Date: November 3, 2006
Creator: Bonazza, Riccardo; Anderson, Mark & Oakley, Jason
Partner: UNT Libraries Government Documents Department

Direct drive heavy-ion-beam inertial fusion at high coupling efficiency

Description: Issues with coupling efficiency, beam illumination symmetry, and Rayleigh-Taylor instability are discussed for spherical heavy-ion-beam-driven targets with and without hohlraums. Efficient coupling of heavy-ion beams to compress direct-drive inertial fusion targets without hohlraums is found to require ion range increasing several-fold during the drive pulse. One-dimensional implosion calculations using the LASNEX inertial confinement fusion target physics code shows the ion range increasing fourfold during the drive pulse to keep ion energy deposition following closely behind the imploding ablation front, resulting in high coupling efficiencies (shell kinetic energy/incident beam energy of 16% to 18%). Ways to increase beam ion range while mitigating Rayleigh-Taylor instabilities are discussed for future work.
Date: May 16, 2008
Creator: Logan, B.G.; Perkins, L.J. & Barnard, J.J.
Partner: UNT Libraries Government Documents Department

A Graphical Representation of Temporal Data from Simulations

Description: The analysis of extremely large data sets is time-consuming and tedious. In this project, we create two tools, the Image Inspector and the Video Inspector, to aid in the automated analysis of 3D temporal data from simulations. Our problem is the 3-dimensional time varying data of the Rayleigh-Taylor instability in a fluid mix problem. We examine the bubble dynamics due to the acceleration of gravity in an initially perturbed interface between a heavier and lighter fluid. Through the use of OpenGL and C++, we automate the capture of the temporal dependence of selected features along a chosen path in a time-dependent 3D simulation. The selected feature, e.g., the height of a bubble, is displayed graphically as a function of position and time. The path can be chosen arbitrarily; differing from previous projects which required the use of horizontal or vertical orientation.
Date: October 13, 2005
Creator: Eder, E F & Harrison, C D
Partner: UNT Libraries Government Documents Department

Bubble Counts for Rayleigh-Taylor Instability Using Image Analysis

Description: We describe the use of image analysis to count bubbles in 3-D, large-scale, LES [1] and DNS [2] of the Rayleigh-Taylor instability. We analyze these massive datasets by first converting the 3-D data to 2-D, then counting the bubbles in the 2-D data. Our plots for the bubble count indicate there are four distinct regimes in the process of the mixing of the two fluids. We also show that our results are relatively insensitive to the choice of parameters in our analysis algorithms.
Date: January 24, 2007
Creator: Miller, P L; Gezahegne, A G; Cook, A W; Cabot, W H & Kamath, C
Partner: UNT Libraries Government Documents Department

Analysis of Rayleigh-Taylor Instability: Statistics on Rising Bubbles and Falling Spikes

Description: The analysis of coherent structures in Rayleigh-Taylor simulations is a challenging task as the lack of a precise definition of these structures is compounded by the massive size of the datasets. In an earlier work, we used techniques from image analysis to count these coherent structures in two high-resolution simulations, one a large-eddy simulation with 30 terabytes of analysis data, and the other a direct numerical simulation with 80 terabytes of analysis data. Our analysis indicated that there were four distinct regimes in the process of the mixing of the two fluids, starting from the initial linear stage, followed by the non-linear stage with weak turbulence, the mixing transition stage, and the final stage of strong turbulence. In this paper, we extend our earlier work to focus on only the rising bubbles and the falling spikes. We first consider different ways in which we can constrain the bubble and spike definitions and then extract various statistics on them. Our results on the rising bubble and falling spike counts again show that there are four regimes in the process of fluid mixing, each characterized by an integer-valued slope. Further, the average bubble heights and spike depths are related to similar results obtained using a threshold-based definition. Finally, the ratio of the rising bubbles to all bubbles is very similar in character to the ratio of the falling spikes to all spikes, with near constant values over part of the simulation.
Date: October 30, 2007
Creator: Kamath, C; Gezahegne, A & Miller, P
Partner: UNT Libraries Government Documents Department

Bell-Plesset effects for an accelerating interface with contiguous density gradients

Description: A Plesset-type treatment [J. Appl. Phys. 25, 96 (1954)] is used to assess the effects of contiguous density gradients at an accelerating spherical classical interface on Rayleigh-Taylor and Bell-Plesset perturbation growth. Analytic expressions are obtained that describe enhanced Rayleigh-Taylor instability growth from contiguous density gradients aligned with the acceleration and which increase the effective Atwood number of the perturbed interface. A new pathway for geometric amplification of surface perturbations on an accelerating interface with contiguous density gradients is identified. A resonance condition between the density-gradient scalelength and the radius of the interface is also predicted based on a linearized analysis of Bernoulli's equation, potentially leading to enhanced perturbation growth. Comparison of the analytic treatment with detailed two-dimensional single-mode growth-factor simulations shows good agreement for low-mode numbers where the effects of spherical geometry are most manifested.
Date: December 20, 2005
Creator: Amendt, P
Partner: UNT Libraries Government Documents Department

Nonlinear mixing behavior of the three-dimensional Rayleigh-Taylor instability at a decelerating interface.

Description: We report results from the first experiments to explore the evolution of the Rayleigh-Taylor (RT) instability from intentionally three-dimensional (3D) initial conditions at an embedded, decelerating interface in a high-Reynolds-number flow. The experiments used {approx}5 kJ of laser energy to produce a blast wave in polyimide and/or brominated plastic having an initial pressure of {approx}50 Mbars. This blast wave shocked and then decelerated the perturbed interface between first material and a lower-density, C foam. This caused the formation of a decelerating interface with an Atwood number {approx}2/3, producing a long-term positive growth rate for the RT instability. The initial perturbations were a 3D perturbation in an ''egg-crate'' pattern with feature spacings of 71 {micro}m in two orthogonal directions and peak-to-valley amplitudes of 5 {micro}m. The resulting RT spikes were observed to overtake the shock waves at the undisturbed, ''free-fall'' rate, and to subsequently deliver material from behind the interface to the forward shock. This result is unanticipated by prior simulations and models.
Date: March 19, 2004
Creator: Robey, H; Remington, B; Edwards, M; Perry, T; Wallace, R J; Louis, H et al.
Partner: UNT Libraries Government Documents Department

National Ignition Facility Target Design and Fabrication

Description: The current capsule target design for the first ignition experiments at the NIF Facility beginning in 2009 will be a copper-doped beryllium capsule, roughly 2 mm in diameter with 160-{micro}m walls. The capsule will have a 75-{micro}m layer of solid DT on the inside surface, and the capsule will driven with x-rays generated from a gold/uranium cocktail hohlraum. The design specifications are extremely rigorous, particularly with respect to interfaces, which must be very smooth to inhibit Rayleigh-Taylor instability growth. This paper outlines the current design, and focuses on the challenges and advances in capsule fabrication and characterization; hohlraum fabrication, and D-T layering and characterization.
Date: December 10, 2007
Creator: Cook, R C; Kozioziemski, B J; Nikroo, A; Wilkens, H L; Bhandarkar, S; Forsman, A C et al.
Partner: UNT Libraries Government Documents Department

Strong stabilization of the Rayleigh-Taylor instability by material strength at Mbar pressures

Description: Experimental results showing significant reductions from classical in the Rayleigh-Taylor (RT) instability growth rate due to high pressure effective lattice viscosity are presented. Using a laser created ramped drive, vanadium samples are compressed and accelerated quasi-isentropically at {approx}1 Mbar pressures, while maintaining the sample in the solid-state. Comparisons with simulations and theory indicate that the high pressure, high strain rate conditions trigger a phonon drag mechanism, resulting in the observed high effective lattice viscosity and strong stabilization of the RT instability.
Date: November 19, 2009
Creator: Park, H S; Lorenz, K T; Cavallo, R M; Pollaine, S M; Prisbrey, S T; Rudd, R E et al.
Partner: UNT Libraries Government Documents Department

Comparison of two- and three-dimensional simulations of miscible Rayleigh-Taylor instability

Description: A comparison of two-dimensional and three-dimensional high-resolution numerical large-eddy simulations of planar, miscible Rayleigh-Taylor instability flows are presented. The resolution of the three-dimensional simulation is sufficient to attain a fully turbulent state. A number of different statistics from the mixing region (e.g., growth rates, PDFs, mixedness measures, and spectra) are used to demonstrate that two-dimensional flow simulations differ substantially from the three-dimensional one. It is found that the two-dimensional flow grows more quickly than its three-dimensional counterpart at late times, develops larger structures, and is much less well mixed. These findings are consistent with the concept of inverse cascade in two-dimensional flow, as well as the influence of a reduced effective Atwood number on miscible flow.
Date: February 23, 2006
Creator: Cabot, W
Partner: UNT Libraries Government Documents Department

EXPERIMENTAL TESTS OF VANADIUM STRENGTH MODELS AT HIGH PRESSURES AND STRAIN RATES

Description: Experimental results showing significant reductions from classical in the Rayleigh-Taylor (RT) instability growth rate due to high pressure material strength or effective lattice viscosity in metal foils are presented. On the Omega Laser in the Laboratory for Laser Energetics, University of Rochester, target samples of polycrystalline vanadium are compressed and accelerated quasi-isentropically at {approx}1 Mbar pressures, while maintaining the samples in the solid-state. Comparison of the results with constitutive models for solid state strength under these conditions show that the measured RT growth is substantially lower than predictions using existing models that work well at low pressures and long time scales. High pressure, high strain rate data can be explained by the enhanced strength due to a phonon drag mechanism, creating a high effective lattice viscosity.
Date: March 2, 2010
Creator: Park, H; Barton, N R; Becker, R C; Bernier, J V; Cavallo, R M; Lorenz, K T et al.
Partner: UNT Libraries Government Documents Department

On the geometry of two-dimensional slices of irregular level sets in turbulent flows

Description: Isoscalar surfaces in turbulent flows are found to be more complex than (self-similar) fractals, in both the far field of liquid-phase turbulent jets and in a realization of Rayleigh-Taylor-instability flow. In particular, they exhibit a scale-dependent coverage dimension, D{sub 2}((lambda)), for 2-D slices of scalar level sets, that increases with scale, from unity, at small scales, to 2, at large scales. For the jet flow and Reynolds numbers investigated, the isoscalar-surface geometry is both scalar-threshold- and Re-dependent; the level-set (coverage) length decreases with increasing Re, indicating enhanced mixing with increasing Reynolds number; and the size distribution of closed regions is well described by lognormal statistics at small scales. A similar D{sub 2}((lambda)) behavior is found for level-set data of 3-D density-interface behavior in recent direct numerical-simulation studies of Rayleigh-Taylor-instability flow. A comparison of (spatial) spectral and isoscalar coverage statistics will be disc
Date: March 20, 1998
Creator: Catrakis, H.J.; Cook, A.W.; Dimotakis, P.E. & Patton, J.M.
Partner: UNT Libraries Government Documents Department