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Experimental and Numerical Investigation of Flows in Expanding Channels

Description: This is the first year progress report for our grant starting Feb. 1 2004. It describes experimental and theoretical achievements during the first year, lists the articles published during this period, as well as the progress of the graduate students supported by this grant. The timeline for the future is outlined; the current results convince us that the work will be done on time and within the budget.
Date: October 28, 2004
Creator: Vorobieff, Vakhtang Putkaradze Peter
Partner: UNT Libraries Government Documents Department

Simple model for linear and nonlinear mixing at unstable fluid interfaces with variable acceleration

Description: A simple model is described for predicting the time evolution of the half-width h of a planar mixing layer between two immiscible incompressible fluids driven by an arbitrary time-dependent variable acceleration history <i>a(l)</i>a (t): The model is based on a heuristic expression for the kinetic energy per unit area of the mixing layer. This expression is based on that for the kinetic energy of a linearly perturbed interface, but with a dynamically renormalized wavelength which becomes proportional to h in the nonlinear regime. An equation of motion for h is then derived by means of Lagrange�s equations. This model reproduces the known linear growth rates of the Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) instabilities, as well as the quadratic RT and power-law RM growth laws in the nonlinear regime. The time exponent in the RM power law depends on the rate of kinetic energy dissipation. In the case of zero dissipation, this exponent reduces to 2/3 in agreement with elementary scaling arguments. A conservative numerical scheme is proposed to solve the model equations, and is used to perform calculations that agree well with published mixing data from linear electric motor experiments. Considerations involved in implementing the model in hydrodynamics codes are briefly discussed.
Date: December 23, 1998
Creator: Ramshaw, J. D. & Rathkopf, J.
Partner: UNT Libraries Government Documents Department

Film Boiling on Downward Quenching Hemisphere of Varying Sizes

Description: Film boiling heat transfer coefficients for a downward-facing hemispherical surface are measured from the quenching tests in DELTA (Downward-boiling Experimental Laminar Transition Apparatus). Two test sections are made of copper to maintain low Biot numbers. The outer diameters of the hemispheres are 120 mm and 294 mm, respectively. The thickness of all the test sections is 30 mm. The effect of diameter on film boiling heat transfer is quantified utilizing results obtained from the test sections. The measured data are compared with the numerical predictions from laminar film boiling analysis. The measured heat transfer coefficients are found to be greater than those predicted by the conventional laminar flow theory on account of the interfacial wavy motion incurred by the Helmholtz instability. Incorporation of the wavy motion model considerably improves the agreement between the experimental and numerical results in terms of heat transfer coefficient. In addition, the interfacial wavy motion and the quenching process are visualized through a digital camera.
Date: April 1, 2004
Creator: Kim, Chan S.; Suh, Kune Y.; Rempe, Joy L.; Cheung, Fan-Bill & Kim, Sang B.
Partner: UNT Libraries Government Documents Department

Spatially resolved instability measurements with a heavy ion beam probe

Description: A heavy ion beam probe was used to make simultaneous measurements of the amplitude and phase of both density and space potential fluctuations in an energetic arc plasma. Detailed comparison with theoretical predictions identifies a 70 KHz coherent oscillation as a Kelvin-Helmholtz instability localized to a region of strong fluid shear. (auth)
Date: January 1, 1975
Creator: Jennings, W. C.; Hickok, R. L. & Glowienka, J. C.
Partner: UNT Libraries Government Documents Department

Accuracy considerations for implementing velocity boundary condiditons in vorticity formulations

Description: A vorticity formulation is described that satisfies the velocity boundary conditions for the incompressible Navier-Stokes equations. Velocity boundary conditions are satisfied by determining the appropriate vortex sheets that must be created on the boundary. Typically, the vortex sheet strengths are determined by solving a set of linear equations that is over-specified. The over-specification arises because an integral constraint on the vortex sheets is imposed. Vortex sheets determined this way do not accurately satisfy both components of the velocity boundary conditions because over-specified systems do not have unique solutions. An integral collocation technique is applied to a generalized Helmholtz decomposition. This formulation implicitly satisfies an integral constraint that is more general that constraints typically used. Improvements in satisfying velocity boundary conditions are shown.
Date: March 1, 1996
Creator: Kempka, S.N.; Glass, M.W.; Peery, J.S.; Strickland, J.H. & Ingber, M.S.
Partner: UNT Libraries Government Documents Department

Numerical analysis of slender vortex motion

Description: Several numerical methods for slender vortex motion (the local induction equation, the Klein-Majda equation, and the Klein-Knio equation) are compared on the specific example of sideband instability of Kelvin waves on a vortex. Numerical experiments on this model problem indicate that all these methods yield qualitatively similar behavior, and this behavior is different from the behavior of a non-slender vortex with variable cross-section. It is found that the boundaries between stable, recurrent, and chaotic regimes in the parameter space of the model problem depend on the method used. The boundaries of these domains in the parameter space for the Klein-Majda equation and for the Klein-Knio equation are closely related to the core size. When the core size is large enough, the Klein-Majda equation always exhibits stable solutions for our model problem. Various conclusions are drawn; in particular, the behavior of turbulent vortices cannot be captured by these local approximations, and probably cannot be captured by any slender vortex model with constant vortex cross-section. Speculations about the differences between classical and superfluid hydrodynamics are also offered.
Date: February 1, 1996
Creator: Zhou, H.
Partner: UNT Libraries Government Documents Department

Computational and experimental studies of hydrodynamic instabilities and turbulent mixing: Review of VNIIEF efforts. Summary report

Description: The report presents the basic results of some calculations, theoretical and experimental efforts in the study of Rayleigh-Taylor, Kelvin-Helmholtz, Richtmyer-Meshkov instabilities and the turbulent mixing which is caused by their evolution. Since the late forties the VNIIEF has been conducting these investigations. This report is based on the data which were published in different times in Russian and foreign journals. The first part of the report deals with calculations an theoretical techniques for the description of hydrodynamic instabilities applied currently, as well as with the results of several individual problems and their comparison with the experiment. These methods can be divided into two types: direct numerical simulation methods and phenomenological methods. The first type includes the regular 2D and 3D gasdynamical techniques as well as the techniques based on small perturbation approximation and on incompressible liquid approximation. The second type comprises the techniques based on various phenomenological turbulence models. The second part of the report describes the experimental methods and cites the experimental results of Rayleigh-Taylor and Richtmyer-Meskov instability studies as well as of turbulent mixing. The applied methods were based on thin-film gaseous models, on jelly models and liquid layer models. The research was done for plane and cylindrical geometries. As drivers, the shock tubes of different designs were used as well as gaseous explosive mixtures, compressed air and electric wire explosions. The experimental results were applied in calculational-theoretical technique calibrations. The authors did not aim at covering all VNIIEF research done in this field of science. To a great extent the choice of the material depended on the personal contribution of the author in these studies.
Date: December 31, 1994
Creator: Andronov, V.A.; Zhidov, I.G.; Meskov, E.E.; Nevmerzhitskii, N.V.; Nikiforov, V.V.; Razin, A.N. et al.
Partner: UNT Libraries Government Documents Department

Kelvin-Helmholtz-like instability of a shear layer subject to free boundary conditions

Description: For free boundary conditions a shear layer with linear velocity profile supports irrotational disturbances, one mode being unstable when its wavelength lambda > 2.619 times the thickness h of the shear layer. For long wavelengths the dispersion relation of the mode approaches Kelvin-Helmholtz form.
Date: January 2, 1979
Creator: Mjolsness, R.C.
Partner: UNT Libraries Government Documents Department

Kingfish striations and the Kelvin-Helmholtz instability. Part 1

Description: The role of the Kelvin-Helmholtz instability in initiating the formation of the density striations observed in the Kingfish fireball is examined. Two idealized models are proposed for the velocity shear layer on the sides of the fireball, each of which includes essential characteristics of the Kingfish event insofar as the development of Kelvin-Helmholtz instabilities is concerned. A complete linear analysis is presented for each model.
Date: October 1, 1985
Creator: Hunter, J.H. Jr.
Partner: UNT Libraries Government Documents Department

Simulation of the Kelvin-Helmholtz instability of a supersonic slip surface with the Piecewise-Parabolic method (PPM)

Description: The Piecewise-Parabolic Method (PPM) has been used to study the nonlinear development of the Kelvin-Helmholtz instability of a Mach 2 slip surface in both a gamma-law gas and in an isothermal gas. A simplified version of PPM appropriate to this and other problems with only weak shocks is described. The instability calculations demonstrate the usefulness of discontinuity steepening in PPM and they illustrate the complexity in a flow problem which this method can treat accurately on Cray-I-class computers. The simulations also bring to light characteristic combinations of nonlinear waves which arise from finite-amplitude perturbations of the slip surface and which exhibit an approximately self-similar growth. After passing through a fairly chaotic phase of development, the mixing layer generated by the instability achieves a relatively ordered state which does not appear to depend greatly upon the nature of the initial perturbation, but which does depend upon the length scale over which strict periodicity is enforced in the simulation.
Date: March 19, 1984
Creator: Woodward, P.R.
Partner: UNT Libraries Government Documents Department

Gamma ray bursts from comet neutron star magnetosphere interaction, field twisting and E sub parallel formation

Description: Consider the problem of a comet in a collision trajectory with a magnetized neutron star. The question addressed in this paper is whether the comet interacts strongly enough with a magnetic field such as to capture at a large radius or whether in general the comet will escape a magnetized neutron star. 6 refs., 4 figs.
Date: January 1, 1990
Creator: Colgate, S.A.
Partner: UNT Libraries Government Documents Department

Possible mechanisms of macrolayer formation

Description: The high heat flux nucleate boiling region, also called the vapor mushroom region, has been shown to have a thin liquid layer on the heater surface under the large mushroom-shaped vapor bubbles that grow from the heater surface. The name given to this liquid layer is the macrolayer to differentiate it from the microlayer that exists under the discrete bubbles found at lower heat fluxes in the nucleate boiling region. Typical thicknesses of this macrolayer range from 50 to 500 {mu}m for water on a flat horizontal boiling surface and depend upon the heat flux. Thus, the macrolayer is thicker than the wedge-shaped microlayers, found under discrete bubbles, which range in thickness from 1 to 10 {mu}m. Although the mechanism of microlayer formation and its evaporation is conceptually simple that of the macrolayer is still not understood. This paper critically compares the potential mechanisms proposed for macrolayer formation. These mechanisms include the Helmholtz instability applied to the vapor stem above active nucleation sites, liquid trapped by lateral coalescence of discrete bubbles that initially form during the mushroom bubble's waiting period, and the limitation of liquid resupply at mushroom departure as a result of vapor flow from the active nucleation sites.
Date: January 1, 1992
Creator: Sadasivan, P.; Chappidi, P.R.; Unal, C. & Nelson, R.A.
Partner: UNT Libraries Government Documents Department

Effect of the radial electric field on turbulence

Description: For many years, the neoclassical transport theory for three- dimensional magnetic configurations, such as magnetic mirrors, ELMO Bumpy Tori (EBTs), and stellarators, has recognized the critical role of the radial electric field in the confinement. It was in these confinement devices that the first experimental measurements of the radial electric field were made and correlated with confinement losses. In tokamaks, the axisymmetry implies that the neoclassical fluxes are ambipolar and, as a consequence, independent of the radial electric field. However, axisymmetry is not strict in a tokamak with turbulent fluctuations, and near the limiter ambipolarity clearly breaks down. Therefore, the question of the effect of the radial electric field on tokamak confinement has been raised in recent years. In particular, the radial electric field has been proposed to explain the transition from L-mode to H-mode confinement. There is some initial experimental evidence supporting this type of explanation, although there is not yet a self-consistent theory explaining the generation of the electric field and its effect on the transport. Here, a brief review of recent results is presented. 27 refs., 4 figs.
Date: January 1, 1990
Creator: Carreras, B.A. & Lynch, V.E.
Partner: UNT Libraries Government Documents Department

Lattice gas simulations of one and two-phase fluid flows using the Connection Machine-2

Description: In this paper, we report recent lattice gas simulations for single-phase and two-phase flows for two dimensional problems using the Connection Machine-2. For the single-phase fluid problem, we use the standard 7-bit lattice gas model with the maximum collision rules. The velocity and vorticity field of the Kelvin-Helmholtz instability is studied. It is shown that the lattice gas method preserves the main properties of the flow patterns observed in other numerical simulations. Using colored particles and holes, the lattice gas method is extended to simulate immiscible fluids with adjustable surface tension, using a purely local collision scheme. The locality of this model allows us to implement a very fast and parallel algorithm on the Connection Machine-2. Because this new model correctly describes short-range particle-particle interactions between liquids and also particle-solid interactions between the fluid and the wall, cohesion and wettability can be simulated. Applications of the current model to several physical systems including spinodal decomposition, Rayleigh-Taylor flows and wettability in two-phase flows through porous media are discussed. 15 refs., 10 figs.
Date: 1990-09~
Creator: Chen, S.; Doolen, G. D.; Eggert, K.; Grunau, D. & Loh, E. Y., Jr.
Partner: UNT Libraries Government Documents Department

Pattern formation by shock processes

Description: Shock waves in condensed media often produce and leave behind periodic patterns and textures. These patterns have been observed both in real time and in postmortem examination. In many cases the patterns can be related to analogous Pattern-forming mechanisms in classical fluid dynamics, such as the Rayleigh-Taylor and Helmholtz instabilities. In other cases, the textures arise from peculiarities in the dynamic stress state immediately behind the leading edge of the shock wave. Periodic waves in the interface between two shock welded metals have a close resemblance to the classical Helmholtz instability. From a practical point of view, these waves are crucial to the formation of a good bond. Impulsive acceleration of an interface can result in the Meshkov instability, which forms patterns qualitatively similar to the Rayleigh-Taylor instability driven by continuous acceleration. However, the patterned stress state left behind after a shock crosses a perturbed interface can result in perturbation growth for shock propagation in either direction across the interface. Even in homogeneous media, the non-hydrostatic component of the stress behind a shock can drive a pattern forming instability. Adiabatic shear banding has been proposed as a mechanism to explain both the patterns observed in shock-compressed and recovered metal samples and the apparent loss of macroscopic shear strength of shocked ceramics. New optical photographs of shocked quartz support this mechanism. 27 references.
Date: January 1, 1983
Creator: Shaner, J.W.
Partner: UNT Libraries Government Documents Department

Nonlinear aspects of the Rayleigh-Taylor instability in laser ablation

Description: We report on our investigation of the Rayleigh-Taylor (R-T) and Kelvin-Helmholtz (K-H) instabilities in laser ablatively accelerated targets for single mode perturbations for a series of wavelengths in the parameter regime 1/2 less than or equal to lambda/..delta..R less than or equal to 10, where lambda is the wavelength of the perturbation and ..delta..R is the cold foil thickness. We find linear growth rates well below classical values (by a factor on the order of 3 to 4). We also find a cutoff in the growth rates for wavelengths less than the foil thickness. The striking result is the dominance of nonlinear effects; i.e., the K-H instability, for short wavelength perturbations. Although the linear growth rates increase as k/sup 1/2/ up to the cutoff, the K-H rollup dominates at large k, drastically reducing the penetration rate of the dense spike below its free fall value and effectively doubling the aspect ratio of the foil. In other words, it is the long wavelength perturbations that are most effective in destroying the symmetric implosion of the shell.
Date: September 3, 1982
Creator: Emery, M. H.; Gardner, J. H. & Boris, J. P.
Partner: UNT Libraries Government Documents Department

Interfacial instabilities leading to bubble departure during film boiling on vertical surface

Description: Bubble departure from a vapor/liquid interface has been estimated from prediction for the wave length and frequency of the most unstable disturbance along the wavy interface. The Orr-Sommerfeld equation was solved for two extreme cases of the vapor momentum equations, i.e., viscous force dominated and inertia force dominated. The most unstable wave length, which determines the axial location where bubbles depart from the interface, and the wave growth rate, which determines the flow rate of the bubble departure were found to be very sensitive to the vapor-film velocity and thickness. A similar analysis also was applied to an inviscid parallel flow of high-temperature melt and water separated by a thin plane sheet of steam that could occur during a molten-jet/water interaction. The vapor velocity and the vapor film thickness were solved by taking into account the effects of the entrained water droplet and the dispersed jet material in the vapor film. A similar character of the interfacial instabilities was found to exist at the melt/vapor and vapor/liquid interfaces. 12 refs., 12 figs.
Date: January 1, 1989
Creator: Wang, S.K.; Blomquist, C.A. & Spencer, B.W.
Partner: UNT Libraries Government Documents Department